I started a couple of threads on the 'DIY CNC Wood Routers' forum some time ago but now I've started building I realised I should be posting on this forum and Paul (CNCAdmin) suggested I start a new thread.

Those wishing to read the history of the project can look at:

http://www.cnczone.com/forums/showthread.php?t=2849
http://www.cnczone.com/forums/showthread.php?t=2971

To recap: I teach Design Technology in the UK and was responsible for introducing CNC into the school's curriculum some six years ago. We have a small (300mm x 250mm x 90mm) Isel Automation machine which is OK for small items and as an introduction tool for key stage 3 (11-14), but when it comes to GCSE and A level work it is somewhat lacking in the work area department - some of our students think big! :)

It was apparent we needed something bigger. However, the phrase " I want to spend big sums of money" and the word 'Bursar' do not sit comfortably together and the cash could not be made available to finance a machine outright and by the time the department had saved enough, I would be looking for early retirement. (Don't want to be teaching much past 55, as there are plenty of things I want to do with my time)

The solution is that I will build the machine and finance the major items and the school will have use of it till I retire at which time the machine retires with me to my workshop at home. Bursar's happy - I'm happy.

At this point, it must be said that I would not have contemplated this had it not been for CNCZone where I had lurked for some time before realising that it was possible to think the unthinkable. I will be leaning heavily on you guys over the coming months, particularly when it comes to the electrics.

Below is an unfinished CAD rendering of the proposed design and I will follow this with progress photos. This could be a long journey.


Mike

Items bought to date:

2200mm THK Linear guides and blocks
700mm THK Linear guides and blocks
300mm THK Linear guides and blocks
2000mm THK C7 Precision rolled 25mm ballscrew & preloaded nut
600mm THK C7 Precision rolled 20mm ballscrew & preloaded nut
300mm THK C7 Precision rolled 16mm ballscrew & preloaded nut
Fixed and supported end bearings for above ballscrews.

That little lot from THK at a supposed educational discount was £2,500.00 :eek:

Welded, 60mm x 40mm x 4mm box section, steel frame made by local firm £160
Facing, drilling and tapping of frame for linear guides £250 (essential for getting faces in the same plane and guides parallel)

Photos of the THK goodies and frame are on the other threads and will be updated here as and when progress is made. Next job is to mount the frame on the wall and get it trued.

Hi Mike,

My first question would be the possible cantilever problem when the axis that carries the z reaches it's greatest distance from the long axis. I am by no means a design person as far as machines go but I currently have a machine that quite a bit of overhang and has much more vibration than I care about.

Mike

Sorry if i missed it in your writing, but is this to be mounted to a wall ?
If so, why?

I also agree with Turmite, you may get a sort of gate effect happening. I would recomend some serious bracing.

Good luck with your unique project Mike

Termite & Ynneb,

Thanks for the posts.

Yes, the whole thing is attached to a wall. The idea is that, with limited space, the table can be removed, gantry parked at one end and the floor area used for other purposes. I know it's not an ideal arrangement but the need for a larger machine outweighed the other details. A bonus is that any size of material can be placed under the machine - I even had thoughts of students driving their cars under it and doing some serious artwork on the bonnets (trunks?) :D

I appreciate that the worst case is with the Y axis at its furthest point and that there might be cause for concern with movement but there are a couple of design details that minimise the problem. One, the Y axis motor, and hence its mass, is placed as close to the X axis as possible giving very little moment about the X axis. Two, the intention is to make the whole gantry out of carbon fibre. Composites is one of my interests and it struck me as an ideal material for this type of work as it has very low thermal expansion and is far more rigid than aluminium. Another plus is that the expansion ratios of steel (ballscrews and linear guides) and carbon are closer than steel and aluminium, the two materials most people are using.

I have built a computer controlled oven for baking pre-preg carbon and am confident I can make this work. My initial thoughts are going to be tested out using MDF but will then be replaced as the design gets verified. I know I will have problems with the MDF but at least I can test the structure.

If you have read the other threads you will be aware I have already got the frame made and X axis rails mounted. Just waiting to screw it to the wall now.

Mike

Mike, we have a commercial router at work. A Routech 220, made in Italy that is designed almost exactly like your machine. Its not mounted to a wall, but the concept is similar.
They have a wider spread on the X axis bearings for stability (horizontally, not vertically) and use a large tubular frame for the for cantilevered part.
I'm sure that your idea will work, just allow double, maybe triple your anticipated build and debug time.
I hope that you are going to add some support for your X axis ball screw. That thing will whip like crazy. If you can do it, maybe mount it stationary and spin the nut.

Keep us posted, as this is going to be really spectacular.
Trent

Buscht,

Distance between rails on the X axis is 640mm and the distance between the blocks is 350mm. THK assure me that at this spacing, there should be no skew at all and so far, with the MDF board in place, there is no discernable movement.

I'm a bit concerned about the build time too, especially the electrics as this is definitely not my forte. Will have to lean heavily on you guys for info. I want to use servos but have no idea of size needed for this machine. Any thoughts?

Working volume should be about 1800mm x 600mm x 100mm.

As for the whip on the X axis ballscrew, if only I could find a way to mount everything in fresh air it would save an inordinate amount of time :-) Seriously, THK claim that with both ends supported the 25mm screw should be stiff enough.

Mike

Things are progressing slowly. Problem with teaching is that pupils tend to get in the way of personal projects :) and progress can only be made while I am in school.

Managed to machine the stand-offs to go between the wall and the frame. This will allow me to finely tune the parallelism and trueness of the rails once mounted to the wall. Below are pictures of the stand-offs and CAD rendering of how they will be positioned and used. The nuts either side of the frame will allow very fine adjustment and although the whole leveling and truing process is likely to be lengthy, at least I know I can get it right.

Well, by the skin of my teeth I have made my first deadline and that was to have the frame mounted on the wall by the end of this week - don't forget, I can only work on this project during school time, when not teaching.

The photos show the wall mountings and the frame in place. Not yet got it perfectly true but have thought of a good way of doing it. I am going to mount a laser line to the X axis plate and place an 8' x 4' board of MDF some 20' infront of the machine on which is drawn a perfectly straight line. As the X slide is moved along the rails the laser line should follow the line. With the board being so far infront of the machine, it will exagerate any misalignment of the rails. If the laser deviates from the line, the rails can be adjusted to bring it back on track. This method should true the rails up for parallelism and twist.

I'll report back as to how successful I am with this method.

Mike

Hope to start on the MDF test version of the gantry next week.

Mike, this is a fantastic project. Great job and thanks for sharing.
Trent

I love alternative thinking like this. Thats how new inventions are formed.
Keep up the great work and the pictures.

Buscht & Ynneb,

thanks for the encouragement, I am going to need plenty before this little beast is up and running. Still unsure about motor sizes for this machine. Any help will be most welcome. The intention is to use servos and with having linear guides and ballscrews all round, I am assuming I can get away with smaller motors i.e. less friction to overcome. I would like it to be capable of machining aluminium in light cuts - time not really an issue.

I keep hearing of 100, 200, 300 - 800 oz in motors but have no way of getting an idea of what this means in actual cutting and operating terms. For example, would the cheap servos, often mentioned on this site, be suitable? I believe Jeff Davies of HomeCNC still sells them.

Hope someone can help.

Thanks,

Mike

Mike,

check the article I wrote (PDF) here: http://www.cnczone.com/forums/showthread.php?t=5866

It tells you how to calculate what linear force you can expect to get from your motors. Then there's another matter to know if that will be enough or not, but at least it should be easier to get a "feel" for than motor torque.

Arvid

Arvid,

Thanks for that. I've read through your article and printed it for further digestion- bed time reading, I'm at that age now :)

It's interesting that you use the same screw pitch and speeds I am considering for my project so it was pretty easy to translate. However, I don't know what a cutting force of 200N equates to in terms of cutting speed, depth of cut and material. I appreciate there are many other variables too like spindle speed/power, tool diameter etc.

I am really quite worried about the electrics as it is something I have always fought shy of and I don't want to go to all the trouble of making a good, accurate machine only to make an expensive mistake with the motors/drivers and all. Having said that, I do have some local expertise and I am sure a call to the zoners will bring the right results.

Many thanks again,

Mike

For ballpark comparison only ....
My table uses Jeff's -Homecnc servos, the 430 oz in jobbers, with Gecko 320s on a 2700 mm x 1270 mm table, they work fine; the rack and pinion X axis with a 80 lb (36 kg) gantry does 2500 mm/min rapids; the Y with cheesy Allthread (temporary :rolleyes: ) will do about half that speed. Still testing the limits for cutting speeds, but relatively aggressive cutting in wood can move at 700 mm/min.
Ninewgt sells plans for a 4' x 8' table he made that uses 470 oz in steppers, I think :confused: ...Says it is all he needs...
Today I was rewiring the control box into something less rat's nesty and was struck by how long it took to do it the first time and how straight forward it seems the second time. So anyway no need to worry about the electrics...lots of helpful souls here.

Nice project you have going there! :)

Sol,

Thanks very much for the info - quite reassuring. It is apparent that the quality of the drive system plays a large part in motor selection. Your reply has definitely given me a clearer picture of what I should be looking for - I am more of a hands-on type, empirical designer/worker and sometimes have difficulty working purely theoretically. Once I can visualise something, the numbers become much more meaningful.

Managed to get round to trying the laser line technique for truing the rails. The photos show how it was done. The close up of the pencil marks on the board show how far out the top rail was to begin with - there was some 10mm difference from top to bottom. By placing the board well away from the rail, it exaggerated any discrepancy. In fact the amount the rail was out was immeasurable by any other means.

The rail was then adjusted till there was no discernible deviation of the laser line. The full board was then placed on the rails and was further tested and everything was spot on. By using just the one block to begin with, I was able to find larger errors, as the distance between two blocks tends to even out some of the minor errors.

The next job is to check the two rails for perpendicularity and I will do this with the laser pointing away, perpendicular to the frame, aimed at a board some 6 metres away.

More anon,

Mike

Good use of the laser line. Smart thinkin'

Frame work looks good, nice "factory" looking paint job too. No one will know you built this yourself :)

What a bummer! :( Once I had trued the top rail, I began checking the bottom one and to my horror the two top surfaces of the rails were not in the same plane - this despite them being supposedly trued on a large CNC machine, for which I paid a not inconsiderable sum! Placing a straight edge on the top rail I measured a 3mm gap between the straight edge and the bottom rail surface, with the rails being 640mm between centres.

Problem, how do I fix this? I decided the error was too small to shim behind the rails so eventually opted to shim the blocks. A few calculations later and it was realised that I needed shims that tapered 0.115mm over the 50mm block width. I pressed our little Isel machine into service and produced the four shims as seen in the photos. Now when the board is mounted, it is true - job sorted!

Shame it had to be done though.

What are the dimensions of the THK rails you bought? I am looking at a set of rails 1300 mm long, x 20 mm x 15 mm.

Mícheál.

BTW, really nice job, and fascinating to read about your alignment problem. All credit to you to be able to make up these precise shims, that's the kind of stumbling block that can make or break a project! Best of luck through to completion!

Mícheál,

I bought 2 off 2200 x 25 GSR linear guides for the X axis, 2 off 700 x 20 GSR guides for the Y axis and 2 off 300 x 15 for the Z axis. The THK rep was very helpful and said that customers usually order linear guides that are far too big for the anticipated loads (a comment I have also read somewhere on this site) and he assured me that at the sizes I ordered, they were more than enough to meet my specification of a machine capable of cutting aluminium.

I have always kept an eye on eBay for such components but here, in the UK, eBay carries nowhere near the quantity of useful stuff than the American site and many of them will not ship to UK - besides which shipping costs and possible duties make it difficult to assess the 'real' cost.

At this moment in time, I cannot put my hands on the costs of the rails separately but can let you know if you need that kind of information.

Progress is a little slow at the moment - too much teaching to do :)

Mike

Got some work done over the last couple of days and managed to get the rails to within 0.0007" using an old, imperial DTI. The purists among you will no doubt cringe at the mounting technique - but when means must. :nono: I think this is probably OK but with no terms of reference, it is difficult to know for certain. It took some considerable time to tweak the linear guide mounting bolts to this level of accuracy and the simple application of a little more torque on the allen key, sent the dial spinning.

The GSR blocks have been mounted against a reference edge on small plates that will then be linked with a large plate that will carry the gantry. The last pic shows this whole assembly held together with cramps to check for sliding smoothness. Movement is very smooth with no discernable play though I am quite surprised at how noisy the balls are in the blocks - they rattle away like mad when traversing, not excessively though.

I must stress again that I am testing the layout of the system by using MDF that I know will not be stiff enough for any serious machining but I will then use the MDF version to machine the patterns for making the composite version that will have all the rigidity I will need.

Oh well, the bell has just gone for afternoon school so it's back to the grind.

Mike

As you can see in the pictures, I have managed to make up most of the gantry in 18mm MDF. Although it is only clamped to the slides, it moves very smoothly - can't wait to get it powered but I realise I am a long way off that just yet.

I am beginning to see the wisdom of all those posts on this site that say, 'build in as much adjustment as possible.' Already I am at the stage where being able to adjust something to a fine degree, makes all the difference. Thanks to all those who offered that advice and woe betide those that don't heed it.

I will shortly have to consider how I am going to mount the X axis ballscrew and bearings but I can put this off a while by mounting the Y rails and slide and sorting out the Z axis.

More anon,

Mike

Nice work!

What kind of screws did you use in the MDF? Did you pre-drill for the screws in the horisontal "support bars"? If so, any tip on how to get those holes straight? I guess the drill likes to wander when drilling into the edge of an MDF sheet?

Arvid

Arvid,

The screws are called 'Confirmat screws' and are specifically designed for use in MDF. You need also to purchase the special bit that drills the pilot hole and the clearance hole in both pieces at the same time. The drill is stepped to accommodate this operation. The pieces of MDF are clamped together accurately before drilling through both, once drilled, the screws are inserted before the clamps are removed thus maintaining accuracy.

The screws have a very coarse, thin and sharp thread that anchors well in the MDF 'end grain' and to my knowledge are the only screws that do not allow the MDF to delaminate. Because you drill through both pieces at the same time, the drill bit does not wander in the 'end grain' as it is supported by the hole through the face of the outside piece.

Hope that makes sense,

Mike

Mike, thanks, that makes perfect sense and will be great to know when my build starts! I've been wondering a bit on how to do this...

Arvid

I don't know whether anyone else has had similar problems but when I tried to bolt everything together, the gantry did not want to slide at all freely and it seemed as though it was riding on square balls. Individually, the blocks move extremely freely and smoothly - so nothing wrong with the blocks. However, when bolted to the MDF carriers - square balls.

On checking the alignment of the blocks they were found to be slightly out of plane yet the MDF had not been touched. I figured that MDF is nowhere near accurate enough for my linear guides. Has anyone here used THK linear guides on an MDF machine? Have you too found a problem with alignment?

I have now machined some 50mm x 25mm x 4mm box section steel to mount the blocks on which will hopefully be accurate enough for the tolerances of the THK blocks.

MDF is a wonderful material but perhaps I was expecting too much of it.

Mike

Is it worth doing the final tightening as you would on a cylinder head, just add a little on each screw and go around checking that it is not binding until all is tight and hopefully not sticking.

Graham

Good point Graham.

I did in fact try adjusting the tightness of each screw and yes, there was a point at which it all rode smoothly but unfortunately, it was not holding the gantry at all well. One of the things I noticed was that after tightening the bolts reasonably well, I could go back a few minutes later and have to re-tighten them. It appears the MDF is compressing under the load and obviously needs some oversized washers on both sides to spread the load better.

I have concluded that if you need the highest accuracy, because of the nature of the running gear, you cannot make do with MDF. Hence the move to steel for gantry attachment.

Thanks for the reply.

MDF is actually pretty soft. Even with big washers it will compress easily. One thing you can do is to drill a counterbored hole, and epoxy in steel washers. But, the opposite side will also compress too. This will happen with any type of wood panels, and even with hardwoods to some extent.

Mike, I'm pretty interesed in the composite stuff you talked about. I've been reading a lot of composite fab info for the last few months, and would like to make a composite gantry machine. Can't wait to see how that goes for you.

MDF shrinks and swells a lot over time with temp and humidity.
Recently I noticed how a once smooth MDF work surface had dimpled at every screw head; the wood had become thicker even though it was very well sealed with multiple layers of quality paint.
Yes, and I had the same problem with bearing tensioning...MDF has its place but an application of high precision isn't it.

Gerry & Sol,

Thanks for your observations, it is surprising just how much MDF does move and expand with moisture. If allowed to, and exposed to high humidity, it can double in thickness quite easily and then becomes quite useless. I don't know whether the waterproof versions fare much better? Attached are photos of the replacement steel holders. I still have to face the edges and bolt on a shoulder piece for the blocks to sit against.

Gerry, composites is one of my main interests and I build competition model gliders with prepreg material. In fact I recently became World Champion in S8E which is rocket propelled, R/C gliders. The whole model only weighs 200g and launches to well over 1000' on a 40Ns motor!

I have to build the MDF version of the router first so I can use it to manufacture the patterns for the composite version. This is going to be a fairly lengthy project so I hope you have the staying power - me also :p

Mike

I have to build the MDF version of the router first so I can use it to manufacture the patterns for the composite version. This is going to be a fairly lengthy project so I hope you have the staying power - me also :p

Mike

I started my first router about 18 months ago. A lot of things have kept me from finishing it. My second router will be the composite one. I already have some THK and IKO rails for it, but it's probably 2 years away.

Gerry,

You must be an early riser to respond to my post so quickly. It's only 12:40pm over here!!!

Mike

Not by choice. :D

This is probably the wrong forum but you are talking about using MDF and moisture issues... so here goes... I was just curious if 5/4 Composite Decking material could be used in place of MDF. I'm thinking of using it on my first router so that moisture isn't an issue. You can find it all over the web and at most lumberyards. I think that in most applications you would still need to use some MDF or "plastic plywood". Unfortunately the plastic plywood is outragous in price. I found some that was 3/4 inch thick [4 x 8 foot sheets] for around $100 US. That didn't include shipping. :p

"3/4 inch thick [4 x 8 foot sheets] for around $100 US"

If this is HDPE thats a cheap price, here in Louisville KY a 4' x 8' sheet of 3/4 HDPE costs 280.00 plus tax

"3/4 inch thick [4 x 8 foot sheets] for around $100 US"

If this is HDPE thats a cheap price, here in Louisville KY a 4' x 8' sheet of 3/4 HDPE costs 280.00 plus tax

I was incorrect on the price... it is $131.00 plus shipping. As far as being HDPE... well... I am not for sure. Here is the link:

[I am not affiliated with this website in any way.]

http://www.epsplasticlumber.com/plasticplywoodsheets.shtml

Check it out. :)

****UPDATE*** incorrect on the material.. it isn't HDPE it is a product called Durapoly. I don't know the difference.

The material at $131 is re-cycled plastic and, in my experience, would not be suitable as it is difficult to machine and join - too plastic (plastic used in its true sense). If it is the same as similar material over here, it is also fairly flexible. I did note that the HDPE was considerably more expensive. To choose between the re-cycled plastic and MDF, it would be MDF every time despite its drawbacks.

Mike

I built a composite deck this summer. That stuff is like a wet noodle. It won't hold screws, and I don't think it's as stable as MDF. They recommend 1/4" gaps for expansion, although I haven't noticed it really expand or contract since July.

Instead of that plastic "plywood", why not use Baltic Birch. Very strong, lighter than MDF, holds screws better. Less dust when cutting. Won't swell like MDF. Twice the price.

Hi
as long as you guys are dicussing alternative materials, does anyone have any experience with HDU High Density Urethane, I think it is used in sign making for outdoor use?
Boxwood

Sorry Boxwood, HDU is something I know nothing about. I have used plenty of the expanding urethane in sandwich structures and urethane block for machining patterns/plugs from, but not the high density stuff. The filled urethane block material could be useful in its higher densities, maybe as a table surface but not for structural purposes.

Mike

I hate to chime in ...since I'm not sure what you're trying to accomplish....seems that all the alternatives to MDF are very much in the same price range as some surplus aluminum plate....I would use the aluminum for the table surface...the rest can be 80/20, aluminum plate or MDF if you so desire.

I cannot believe the trouble it has been trying to get the THK blocks to run freely when the bolts are tightened. Despite my latest efforts of milling flats on 50mm x 25mm x 4mm steel box section, the blocks still ran with lumpy balls. On closer inspection the flats were not in the same plane which can only mean that the 'crappy', cheap milling machine's spindle is not perpendicular to the bed.

What now? - I superglued some aluminium shims to the flats and stuck two bits of wet and dry paper to an expensive surface plate. After some considerable time the two shims had been flattened and the whole lot tested on the surface plate - bingo! - perfectly flat.

With added enthusiasm I tightened up the blocks and placed them on the guides only to find they still were not right - better, but not perfect. At least the movement was smooth but it still did not sound right. I have now given up for the day and will look again tomorrow.

Has anyone else experienced this problem?

Mike

Mike, Your problem might be the alignment of the rails. You mentioned earlier that the rail mounting surfaces were not in the same plane and you shimmed the carriages to match.

From my experience, the rails need to be parallel in several planes for everything to work right. Not just the obvious, being in line along the length, but they can't be tilted vertically out of alignment across the cross section. It could be that when you tighten up the carriages you are introducing a bind in the rails.

Try loosening up the lower rail, but keep the upper rail tight and the carriages tight to the plate. See if you can get a smooth action that way. Then slowly start tightening up the lower rail. You might have to shim behind it, or fill in with some kind of bedding compound.

If that works, great. If not, just reverse the procedure with respect to the top and bottom rails.

Good luck
Trent

Trent,

I am only attaching the blocks to one rail - the top one! I can appreciate that if I were bolting the carriage across the two rails I would have a problem if they were out of plane. If you look at the second photo of post #34 you will see the arrangement - the two blocks are bolted to the box section then the box section hung on the top rail and it is this simple arrangement that is not sliding as it should. Woe betide me when I try to tie the two rails together if I cannot get this right!!!

Mike

So, a technique that Thomson (darn I always forget...Thomsen) anyway.....what they recommend when mounting their round rails to the t-bar rail support...was a bed of metal epoxy to help the alignment...so you would only tighten the end bolts until the epoxy had set then you'd tighten the others. In your case you are setting the carriages on the surface plate and then placing your machined plate on top of the carriages...Is that correct?

Mike, it's always tough the first time. THK has good information about mounting rails and carriages in their catalogs and website. If you haven't done so, I highly recommend reading it.

I might have missed something in an earlier post. Do the individual carriages move smoothly on the rail? I do know that you can impart a twist or bow in the rail if it isn't mounted to a perfectly flat surface. The problem has to be in rail mounting, or the flatness of the rectangular tubing.

Are your bolt holes sufficiently oversize so you let the carriage align themselves? Tighten one carriage and leave the other just slightly loose. Move the assembly back and forth and check for smoothness. Then slowly tighten up the other carriage trying to see when it starts binding.

Another wild idea, get your THK distributor to come in and help you out. He might be willing to help out of the goodness of his heart, and the good publicity.

Another wild idea, get your THK distributor to come in and help you out. He might be willing to help out of the goodness of his heart, and the good publicity.

That idea has certainly crossed my mind! All the other suggestions I have tried. I also have the THK General Catalogue with all the mounting information in and have followed it as closely as I can with the limited machining available to me.

My hair is getting thinner by the day but I will crack this one and that's a promise.

Thanks for your interest. This site is brilliant.

Mike

Trent,

Sorry, missed one of your points - the individual blocks glide as beautifully as Torville & Dean (sorry showing my age and colours there :) )

Mike

Finally got it sorted today. :banana: These things are so fussy. One of THK's boasts is that their systems of linear guides are quite forgiving of alignment discrepancies. Obviously not the kind of discrepancies I was able to build in :)

Well on the way to finishing the second carriage for the bottom rail after which I have to tie the two together with the gantry and probably find a new set of misalignments to sort out.

It is such a great feeling when something actually works after you begin to doubt your ability to do it.

Mike

Mike, after its all together and running, you wonder what the big deal was. Until then every little thing is a major hurdle.

Be prepared for more alignment issues once you go and try to attach the ball screw. I fought that for an entire weekend.
Trent

Mike, what did you do to get it working?

Arvid

Arvid,

Back in post #19 I attached a photo of the aluminium shims made to correct the out of plane rails. These were to be placed between the blocks and the mounting system. These shims were superglued to the box section steel shown in post #34 - that was the easy bit. Because the machined flats, on either end of the box section, were not in the same plane (dodgy, out of perpendicular milling machine), these had to somehow be corrected.

I used some peelable spray on adhesive to stick some 150 grit wet and dry paper to a surface plate then painstakingly rubbed the two shims down till they were perfectly level. This worked very well indeed though was time consuming.

Next, to make sure the top of the box section was flat this too was milled and a piece of aluminium screwed to it, against which the blocks are aligned. (see photo) You can just make out the aluminium strip at the top of the assembly. The blocks were clamped to this while they were bolted through the box section. Everything was now square and true - phew!

What I have learned from this is that when THK say their system of linear guides are tolerant of misalignment, they are not talking the same kind of figures that I am :)

Mike

Well, progress at last, the clamps are off and the whole gantry is self supporting and moving freely - I've had a good day today after solving the problem of the square balls yesterday. The second block carrier went together first time and ran beautifully. I just had to get the gantry mounted that ties the two block carriers together. I was a little nervous as I was expecting the whole assembly to bind up and jam again but, to my surprise, it didn't. It ran almost perfectly, slightly stiffer than before but perfectly acceptable. The rails now need some slight adjustment to take out some very slight binding in one or two places.

Now I can press the laser line into action again to test for rail perpendicularity. I'll post some more pictures as I do it.

Trent, I've not yet worked out how I am going to mount the ballscrews but I am sure you are right about more alignment problems.

Mike

A bit more progress.

The gantry was disassembled, cleaned up and lacquered prior to attaching the Y axis rails. This was done to stop the oil from the rails staining the MDF and the whole thing becoming grubby from handling. The lacquer used was a two part, acid curing cellulose that was sprayed on. Two coats were given, rubbing down after the first. This stuff is good as it goes hard very quickly and can be rubbed down with wet and dry paper (or very fine glass paper) after just half an hour, without the paper clogging.

Our small, Isel machine was again pressed into service to machine a drilling jig for the Y axis rail mounting. This was simply a piece of 6mm MDF that was accurately drilled with the appropriate spacing. The jig was clamped to the gantry side and the holes drilled through - perfect alignment :)

I had to work out how to attach the rails, as this time they would be fixed to MDF, not steel as for the X-axis. I figured it would not be wise to screw into the MDF so I drilled and tapped a piece of 700mm x 9mm x 5mm brass strip to take the 5mm bolts. You can see one of them in the photo below that shows one rail attached and the other not. This works well and spreads the mounting load much more evenly and there is no chance of it pulling through the MDF under operational loads.

The other photos show the gantry re-assembled and a close-up. I hope to get the Y-axis, Z carrying plate in place before we break for Christmas at the end of this week. Only two days to go!

Mike

For some reason the photos did not appear with the previous post :confused:

Mike, great job! I can't thank you enough for sharing all this information.

I have had good luck screwing into MDF with something that we call a EURO screw. They are used in 32mm systems. You drill a 5mm hole in the MDF and these screws are designed for cabinet hinges and drawer slide mountings.

Also, threaded tee nuts work pretty well but aren't as pretty as your solution.

Thanks again
Trent

Trent,

I was actually thinking of rebating the brass strip into the back of the MDF which would have been even neater but decided it would probably weaken the structure too much. MDF has two quite hard surfaces which take all the bending loads whereas the centre is really mushy and if you break one of the surfaces, you end up with a very weak member.

My advice to anyone using MDF would be not to break the load bearing surfaces if at all possible.

Mike

Trent,

I was actually thinking of rebating the brass strip into the back of the MDF which would have been even neater but decided it would probably weaken the structure too much. MDF has two quite hard surfaces which take all the bending loads whereas the centre is really mushy and if you break one of the surfaces, you end up with a very weak member.

Mike

Go ahead and cut the rabbet (american sp.), just epoxy in the brass strip. The epoxy will solidify the soft core and still give you a neat and clean installation.

Are you sure your x-axis blocks are spread out far enough? It looks to me like you may get some flex if you do any heavy cutting. I guess that's where the composite comes in. :)

Gerry,

According to the THK rep I have more than enough spacing - he quoted a spacing of just one block width, between the blocks, as being adequate, I have four block width spacing so I should be well within limits.

I know the MDF version will flex somewhat more than the composite version but will beef it up where necessary. I have already spotted an area that needs some attention - the left hand side of the back plate flexes slightly when I swing on the end of the gantry or push it up with some force - a bit of vertical, buttress style bracing should fix it. Also the open side of the box structure at the bottom of the gantry will be filled in with a removable panel. This too will brace the whole structure, making a decent torsion box.

I don't plan on doing too much heavy cutting with this machine, just enough for it to cut its replacement :)

Mike

I guess I really didn't mean the spacing itself, but if the spacing was a bit farther, you could brace the gantry better.

Not a huge amount of progress this week but the Y axis rails are in place and the Y axis carriage almost complete. The MDF looks real rough compared with the nicely finished gantry but it will look similar eventually.

The Y carriage plate was small enough to be machined in full on our small Isel machine so I am hoping there will be no alignment problems. The shoulder that the blocks are indexed against are made from top quality oak, cut slightly oversize and the edges machined on the Isel to be perfectly parallel. The mounting holes for both the blocks and the Z axis rails were drilled at the same time.

I'm going to have to start thinking seriously about mounting the ballscrews soon and finding some suitable servos. So difficult to find bargains here in the UK. Does anyone know the approximate cost of shipping motors to the UK from USA? I'm thinking, if I buy say three motors and encoders for $500, what will that be in actual GBP by the time they are in my hands?

There won't be much activity on the construction for the next three weeks - school workshops are locked up!!

Mike

Mike,
I've been enjoying reading this as you progress along. I too can't wait until you get to the carbon fibre gantry stage. Keep the pics coming and enjoy your Christmas break.

Phil

Phil,

I hope you've got plenty of staying power - this could end up being a veeeeeery long thread. I am not the fastest worker in town even when I have enough time. I am hoping the first, MDF, version will take no more than twelve months to complete and starting from zero knowledge (apart from professional interest in operating our small, Isel machine), it is also a steep learning curve. This will be particularly true of the electrics which I keep putting off.

New year's resolution maybe to get stuck in :)

Thanks for the Christmas wishes and I extend mine to all those reading this thread. May you all get what you deserve :)

Mike

Mike,
Keep it coming. Your endeavor is certainly helping as I design and build my machine. I am also thinking about a composite gantry solution, although I think my gantry is going to be more conventional (not cantelevered). Because of this I was thinking about just purchasing some pultruded carbon fiber wide-flange I-beam and using it. It seems to me that while you generally want weight throughout the machine to combat vibration, having a lighter gantry would ease some of the inertia loads put on the machine as the gantry stops and changes direction along the x axis. Anyways, keep it coming Mike...I'm really enjoying your progress.
-Eli

Eli,

Thanks for the comments. The carbon fibre approach is very logical in one sense but it does throw up this quandary about mass. My reasoning has always been that the lighter the machine, the less force required to move it and the stiffer the machine, the greater the proportion of that force getting to the tool. I do however take the point about vibration and I have been thinking along the lines of somehow damping the movement with some kind of viscous fluid - haven't quite worked out how yet but it will come.

Have you worked out how you will tie the beams together? I think there may be a problem if you use a lot of aluminium in conjunction with carbon fibre because of the huge differential in coefficients of expansion.

By using prepreg materials I can use the facilities I have already built for my modelling i.e. computer controlled oven and vacuum bagging equipment. As an interesting aside I have attached some photos of my World Championship winning model. There's probably another thread there somewhere. The whole model is constructed using prepreg glass and carbon and weighs in at just 200g complete with five channel radio, ready to fly.

Have a great Christmas

Mike

Mike,
As far as tying it all together, I would most likely be tying steel to the carbon fiber. I would bolt and use epoxy to join all of the pieces. Being a boatbuilder, I have quite a bit of confidence in the solidity of epoxy as a structural adhesive. Also, wherever there is an interface between steel and carbon fiber, I will make sure to put a layer of fiberglass so as to isolate the carbon from the steel. Carbon is more noble than steel, and consequently will corrode it quite quickly. I know this is more of a problem whilst on salt water, but it can still be an issue if the machine is located near som salt air (mine will be a stones throw from Boston harbor here in the states). Cool looking plane!

Eli,

I agree about the merits of epoxy - if it will hold military aircraft together, I am sure it will do the business for us. However, I was not aware of any electrlytical (?) problems between carbon and steel. Am I to assume these are similar to the dissimilar metals problem? If so, does it not need some kind of electrolyte before problems occur? I appreciate that even the humidity in the air is enough for some metals to behave badly but with the carbon, supposedly, integrated in a resin matrix, is it really going to be a problem? I know that glassfibre in a polyester matrix is hygroscopic but not sure about carbon in an epoxy matrix. Any thoughts?

Mike

Mike,
Even if the carbon is integrated in the epoxy matrix it is still a problem, particularly if you are tapping screws. Obviously in this situation you will have steel contacting carbon directly. Yes, you will need an electrolyte to start corrosion problems, but this is a "better safe than sorry" situation for a precision piece of equipment like a router. Whenever I am building a boat or component out of carbon fiber I am very careful to isolate steel/aluminum from the carbon with either fiberglass or aramid. The effect is exactly the same as dissimilar metals if you do not isolate the materials. As far as epoxy being hydroscopic, to a very small degree it is....epoxy being 99% waterproof. If you coat and saturate a piece of wood with epoxy, and then leave it in a tub of water for a year, a small amount of water will have penetrated the epoxy and migrated into the wood fibers. In an epoxy glass matrix water will not penetrate so as to cause osmotic blistering like you sometimes see in boats that are built with polyester resins. Vinylester resin is the most waterproof. At the end of the day though this really doesn't have much to do with water penetration, but more to do with electrolytic action caused by dissimilar metals (carbon not really being a metal, but being more noble than steel).
-Eli

Eli,

Thanks for that insight - I will certainly now bear that in mind when planning the construction of the gantry. I was thinking of laminating steel into the structure, to take the assembly bolts, which will then be drilled and tapped. Maybe now I'm thinking - mould the holes in place for the various mounting bolts for linear guides, ballscrew bearings etc. then bond the gantry sections together with reinforced epoxy.

The disadvantage of this approach, and a big one at that, is that it becomes difficult if not impossible to make substantial changes or modifications once assembled.

Going back to the bolted together sections - does anyone know if nylon bolts would be strong enough to hold a machine together? Or, am I now worrying about something that is not going to be too much of a problem? Maybe someone out there has experience of dissimilar metals/materials being a problem or not?

One of the great things about this forum is the ability to bounce ideas off others - so much easier than re-inventing :)

Mike

Mike,
One thing to think about is bushings. You could epoxy in nylon bushings where steel bolts will be in way of the carbon. I don't think there will be any problem here since the compressive strength of the nylon is quite good. I wouldn't want to use nylon bolts if I had the choice. Also, thickened epoxy set in larger holes and then drilled to suit whatever bolt you pass through works well too.

*snip* Does anyone know the approximate cost of shipping motors to the UK from USA? I'm thinking, if I buy say three motors and encoders for $500, what will that be in actual GBP by the time they are in my hands? *snip*

I have ordered stuff from Ebay (US) before to Sweden. I've used the United States Postal Service (USPS) Global Express Mail (EMS). You can calculate the shipping cost on their homepage if you know the weight - 20 pounds to UK would be $83.75. This is for "3-5 day" delivery - which usually drags out up to two weeks including customs clearance and local delivery. Also expect someone to charge $10-$20 for doing the customs clearance, and a few percent of actual customs duty.

Arvid

Arvid,

It's the 'small' customs duty that is annoying me. I think it will be at least 17.5% here in the UK which is crazy when you consider that most of what you buy has already had tax paid on it some two or three times since manufacture! Somebody once told me that I would probably have to pay about the same in GBP as US$ i.e. if I buy something for $500 I should be prepared to pay £500 and with the dollar close to 2/GBP, I find that hard to bear.

What is more, even if the goods are second hand, the customs officials will work out their cut on their perceived value of the goods.

Thinks - I wonder if I know anyone flying out to the States?

I'm going to have to bite the bullet shortly and, with there being virtually nothing on offer this side of the pond, will probably have to purchase from the States.

Mike

Mike

Arvid,

It's the 'small' customs duty that is annoying me. I think it will be at least 17.5% here in the UK which is crazy when you consider that most of what you buy has already had tax paid on it some two or three times since manufacture! Somebody once told me that I would probably have to pay about the same in GBP as US$ i.e. if I buy something for $500 I should be prepared to pay £500 and with the dollar close to 2/GBP, I find that hard to bear.

What is more, even if the goods are second hand, the customs officials will work out their cut on their perceived value of the goods.

Thinks - I wonder if I know anyone flying out to the States?

I'm going to have to bite the bullet shortly and, with there being virtually nothing on offer this side of the pond, will probably have to purchase from the States.

Mike

Mike


Mike,

Can't you have someone here buy the item in question, ship it to you... and give them a small service charge? I don't see how you'd have to pay the "customs" charges then.... how could they make you do that?

Jimmy,

I've just contacted Jeff Davies of HomeCNC www.cnczone.com to see what he can do for me. He is still offering the 430oz/in servos on his site. Noticed he also has some very nice looking 360oz/in servos on ebay - cheaper too, but I think I will need the more powerful ones. I was really looking for something around the 600-800oz/in but can't find anything within a reasonable price range.

Mike

Jimmy,

I've just contacted Jeff Davies of HomeCNC www.cnczone.com to see what he can do for me. He is still offering the 430oz/in servos on his site. Noticed he also has some very nice looking 360oz/in servos on ebay - cheaper too, but I think I will need the more powerful ones. I was really looking for something around the 600-800oz/in but can't find anything within a reasonable price range.

Mike

If he has to ship them to you with some type of customs slip from our postal system then have him put the value at $20 US dollars and the description of the motors as "used something or other..." That should keep them off of your back. I have no experience with sending stuff to the British Isles, but when I send stuff to Canada they do have me fill out a slip that I mentioned above. Make the seller aware of your situation and I am sure he can try to help you out. If not... contact an individual *like myself or others on the board* and see if you can't work something out by having them purchase the item themselves and then sending it to you via the mail as mentioned above. :cheers:

Thanks for the offer, Jimmy. I'll wait and see what Jeff can do for me first but may well be in touch.

Cheers,

Mike

Well, I've been and gone and done it! I've ordered three 430 oz/in servo motors from Jeff Davis at HomeCNC. He was very helpful and answered all my questions by return email - thanks Jeff.

I now have to decide between Gecko and Rutex drivers. The advantage with Gecko is that Jeff uses these and knows the set up for his servos however, Rutex offer mother boards and IO boards for easily setting up home switches, spindle on/off etc. but I'm not sure how this is done using Geckos. I have emailed Gecko and await their response.

Any help from others would be much appreciated.

Mike

A lot of people using Geckos use the breakout boards from http://www.campbelldesigns.com

Gerry,

I've just looked on the Campbell Designs site and added up the costs of both systems and there really is nothing in it. However, there is something you may be able to help me with. On the Rutex it has a step multiplier (with Gecko it is an extra) and I was wondering exactly how does this work? Does it multiply the steps from the software or does it multiply from the encoder? For example; if my software indicates 100 steps and I use the 10x multiplier, does my motor move 1000 steps or 1? In other words, does this improve the resolution of the system or detract from it?

Am I right in assuming this is not the same as stepper motor microstepping where one step is divided into many, smaller steps and that a servo motor can only be moved by whatever the resolution of the encoder is (x4 with quadrature)?

Many more questions to come.

Mike

Mike, I was getting ready to answer, and got confused just thinking about it. The multipliers multiply the steps sent from the computer. The step size is determined by the encoder resolution. I THINK this is right: Like you said above, if you use the 10x multiplier, when the software says 100 steps, the motor will turn 1000 steps. The difference is that you have to tell the software that the step size is 10x bigger than without the multiplier. Say your not using the multiplier, and your encoders give you 1000 steps per inch. The step size is .001. If you want to move 1 inch, the computer sends 1000 steps. With the 10x multiplier, you only need to send 100 steps. The step size would be .01. (The step size you tell the controller is .01. Your actual step size would still be .001, based on your encoders)

The only reason you should be using the multiplier is if your computer can't output steps fast enough. Your motors should have 250 cpr encoders, which should give you 1000 steps per revolution. If you use a 10 tpi leadscrew, that's 10,000 steps per inch. With a good computer, Mach2 can output 45,000 steps per second. That's 4.5 inches per second, or 270ipm (MAX, you'd probably want to stay a little under that). If you need to go faster, I'd use either a higher lead screw, or maybe try DeskCNC to control the machine, which I think can output 125,000 steps per second. Within the next year or so, Gecko should have a product that can produce even faster step rates (G2005?).

Gerry

So, if I am understanding this correctly, there is absolutely no difference in accuracy, just speed. The purpose of the multiplier is simply to overcome the problems of slower processors? Therefore, with a good PC, multipliers are not worth having?

I'm still not quite with it. Suppose I am machining a circle and I use the x10 multiplier. Does the machine interpolate these extra 10 steps or does it simply go in a straight line between the PC's given steps?

I'm still confused - I think :)

Mike

Gerry

So, if I am understanding this correctly, there is absolutely no difference in accuracy, just speed. The purpose of the multiplier is simply to overcome the problems of slower processors? Therefore, with a good PC, multipliers are not worth having?

I'm still not quite with it. Suppose I am machining a circle and I use the x10 multiplier. Does the machine interpolate these extra 10 steps or does it simply go in a straight line between the PC's given steps?

I'm still confused - I think :)

Mike

I'm not really sure, but it seems like it would go in a straight line. The PC just says to the controller - turn the motor 1 step. So the Gecko or Rutex would just turn it 10 steps. Like I said, from what I understand, the sole reason for their existence was that not long ago, you couldn't get affordabe software to produce steps fast enough. That's changing now, although servos tend to need higher step rates than steppers. Even with servos, I couldn't see the reason to use more than 2x or maybe 5x multipliers, unless you use very high resolution encoders. The multiplier basically makes your higher res encoder a lower res encoder. Nothing more.

What leadscrews are you going to be using? Any gearing? Do the math, figure out the steps/ second you'll need, based on the speed you want to go, and determine if you'll need to use it. If not, I think the Gecko's are quite a bit cheaper than the Rutex.

I forgot to mention the accuracy issue. If you really needed the 10x multiplier, the resolution of your machine would be 1/10, ie, if your step size was .0001, with the multiplier it would in reality be .001. BUT, if your step size was .0001, can your machine really move .0001 at a time? Not very likely. Your likely to have more backlash than that, and it most certainly will flex more than that. (At least the MDF version :) )
But like I said above, you really shouldn't need the 10x multiplier.

The best thing you can do is keep reading, try to learn and understand everything you can, and make your choices based on what you think YOU need.

Cheers Gerry, Happy New Year - just leaving for the festivities. I'll post again next year !

Mike

WOW - doesn't time fly - 2005 already and may it be a good one for all zoners.

Gerry,

I am using 5mm pitch ballscrews with preloaded nuts, all riding on THK linear guides so I am hoping there will minimal, if any, backlash. As one of my main interests is in model aircraft, I wanted to make a machine capable of machining wing profiles as accurately as possible. The preferred material for machining the patterns is an epoxy block, so this should be perfectly OK without too much strain on the machine. I hope also that the composite version will be capable of machining aluminium.

The motors I have bought from Jeff Davis have 250cps encoders on board which will give me 2000 steps at the ballscrew after quadrature and 2:1 reduction via timing belts. If my calculations are correct this should give me a theoretical resolution of 5/2000mm = 0.0025mm or 0.0001" - just as in your example!

Mike

.... which should give you up to 240+ ipm, if you can get a 45 Khz step rate from Mach2..... with no need for the step multiplier. :)

OK, so now I'm confused again because the motors are rated at 1700rpm which will only give me, after reduction, 850 x 5 mm/min = 4250mm/min approx 170 in/min or 70mm/sec 2.8 in/sec, OK but not excessively fast. Are you saying that these motors can/will be driven faster than their rated rpm if the software/PC can output the steps at 45 khz?

What are the limiting factors on speed? I understand, from the literature, that I should not drive my X axis ballscrew faster than 900rpm despite being fixed one end and supported the other. I would love to be able to have 240 inch rapids but is it realistic? Will the motors/drivers get excessively hot at these speeds?

Too many question - aaaaargh :)

Mike

Sorry, Mike. I didn't realise the motors are rated at 1700rpm. I haven't used servos, but I wouldn't try to run them faster than their rated speed, regardless.

Some trial and error will probably be needed on your part. First, I'm assuming you'll use Mach2 to get the 45Khz step rate. Be aware that you'll possibly need a 2Ghz PC for this. (Not entirely sure). The rapid speed (Max RPM) of the motors will be set in the Mach2. This will take into account your encoders resolution (1000), screw lead (.2), and the gearing you use (2:1?). As you mentioned, you may well be limited by the screw whipping at higher speeds. Here's what I'd probably do. If you think you'll be limited bya screw speed of 900 rpm, I'd gear it to spin maybe 1200 rpm (+30%)with the motor spinning at around 1600 rpm (3:4). Once up and running do some tests to see how fast you really can go, and set your top speed accordingly. If 900 rpm is really your maximum speed you feel it can run at, then just set this in Mach2. You will sacrifice a bit of torque from the lower gearing. But you could always replace the pulleys to maximize torque once you determine your max speed. This way you allow for a higher rapid speed. Also, the shorter the screw, the faster you can spin it, so your Y axis may have faster rapids than the x axis, which you may want to use different gearing on.

Make sense? Or even more confused? :)

What are the lengths of screws you'll be using? And where did you get the rpm info? Looking at the critical speed charts from Nook http://www.nookind.com , it looks like a 26" long screw can spin about 2000 rpm or more, but an 80" screw only 500 rpm.

Gerry,

X axis is 25mm diameter x 2000mm long, Y axis 22mm diameter x 700mm and Z axis 16mm x 300mm

I'm sure one of the sites I visited had a critical speed calculator on it, as does nookind.com, and it gave critical speed as 888rpm which I rounded up to 900. However, checking out the THK manual, with all its complex formulae tends to agree with you at around 550rpm for the X axis.

Your previous post makes a lot of sense and I like the idea of trying different reduction ratios to find the optimum speed for my setup. It would also be possible to make the X axis ballscrew fixed and have a rotating nut - this would allow much faster speeds would it not?

The murk is slowly clearing, many thanks.

Mike

Mike,

Interesting machine and posting.

With the critical speed calculations Nook and THK use different bearing configurations which affects the final number. The nook calculator gives four options of end 'fixity' (don't you just love the american language).

Simply put, what ever speed you end up with I'd suggest that you look at putting two support bearings on each end of your 2m screw, with a space of at least 25mm between. Effectively a cantilever support at both ends this could be incorporated into the thrust bearing arrangement. That would stiffen the screw immensely and so push up the safe rotational speed.

A rotating nut would solve the whip problem and some machines use it. The problem comes in the engineering. If you could machine a replacement for the ball nut flange to incorporate a bearing and timing drive pulley and mount the servo that would be possible.

Another idea to reduce whip in a rotating leadscrew which has worked for me is to put bearings either side of the lead nut. This reduces whip and takes the loads off the nut. Down side is you need to have a bearing that will not mar the screw but ride on it both rotationally and axially. I've used some plastic bearings from Igus for this. It's a bit of a bodge to fix a too small a diameter screw at too fast a speed and only if you're getting too much movement.

hth

Andrew

Andrew,

Thanks for the reply.

Wow, by using the double fixed with spacing between the bearings (Fixity (ugh) D) in the Nook calculations, it more than doubles the critical speed! It goes from 671rpm to 1496rpm. I used the correct measurements from the THK catalogue i.e. 22.2mm minimum thread diameter at 2000mm spacing.

I have heard of adding a preload to double fixed bearings by, effectively stretching the ballscrew between the two, fixed bearings. The problem I face if wanting to increase the speed this way is that the ballscrew is already machined to accept one fixed and one supported end and I am not sure I have the necessary equipment to alter this.

For this first machine I will probably make it as it is, see what the problems are, then alter and modify if necessary.

Thanks for the input.

Mike

Mike, disregard my critical speed calculations. I mistakenly thought you said you were using 5/8 (16mm) screws. :(

Looks like you have a handle on it now.

Yes, thanks Gerry and Andrew for your input. Having you guys to call on when posed with a problem is so reassuring.

Please keep watching - there will be plenty more questions before I'm through.

Should be able to get back to the machine soon. It seems so long, I'm getting withdrawal symptoms :)

Mike

You're assuming I know what I'm talking about :D

My motors arrived today, all the way from Oregon to Liverpool in just over two weeks - not bad. I have to credit Jeff Davis of HomeCNC for some excellent packaging, each motor was separately packed and all three well insulated against the vagaries of all the postal systems they went through.

They look the business and I am pleased they are relatively light in weight which is just what I need for my rather unconventional, cantilevered gantry.

Just need to make the decision between Gecko or Rutex now and I'll soon have these beauties singing.

Mike

Now I have the motors, I need to think about the drive train and I intend driving the screws through 2:1 timing belt reduction. Looking on the internet for possible options leaves me somewhat puzzled. There are hundreds of different belts and pulleys. Can anyone suggest the type I should be looking for and where I could purchase some from. I don't mind buying online (in fact I prefer it) - would prefer to buy from UK but not fussy if cheaper elsewhere.

I am using the 430oz/in servos shown in the previous thread.

Work has started mounting the ballscrews on the X and Y axes. Hope to be able to post some pictures next week.

Mike

I used HTD belts and pulleys on my machine. They have a rounded tooth form and seem to engage better than the square type. RS and Farnell have them. Seem quite expensive tho'. I got mine from work (free). I thought ondrives in the UK stocked them, but looking at their website, it seems not.

Regards Terry.....

Mike, a good source of belts (not UK, though :( ) is http://www.sdp-si.com They have a lot of good info, too.

WHile HTD belts are better than the square tooth styles, GT2 (Gates brand) are supposed to be the best for backlash. The info is available at the site I mentioned.

Gerry & Terry (got to be a joke there somewhere :) ) thanks for the replies. Still a little confused though - On the SDP-SI site I can select metric measurements and pick 16mm T5 belts, there are no matching pulleys. If I then select metal timing pulleys and select HTD 5mm pitch, 15mm width, there are only neoprene, fibreglass reinforced belts available. Will these do the job or should I really be looking for polyurethane, steel reinforced belts?

Should I be looking for something other than 5mm pitch?

Will plastic pulleys be OK? I have only been looking for aluminium ones so far.

Ooooh so many questions - but I need to get these things right.

Mike

I've done some Google-ing and Gerry is correct, in that GT2 does have less backlash that HTD.

Neo+fibreglass belts should be OK. I read that steel reinforced is only very slightly better and is mainly used for harsh shock and impulse loads.

Regards Terry.....

SDP-SI website has pulley and belts for GT 5mm pitch, 15 & 9mm widths.

Not sure you ever got a clear answer on motor speed.
Your servos ( I have them too) run comfortably at 1360 rpm (80% of 1700).

When ordering pulleys I was glad that I bought extras that allowed for different reduction ratios...2:1, 3:1 and 3:2. That is 12 teeth, 24 teeth, and 36 teeth; this permitted trial and error tuning for speed vs. brute force. Sure, there will be extra pulleys laying around, but at this point what is another $50- £30.

I am using the fiberglass reinforced belts. So far so good.

Oh yes, the belts can be purchased in increments of 5 teeth elsewhere; it may be worth sizing things to the nearest 5 rather than the exact number so that things are easier at replacement time...but then I'm on the other side of the pond.... :)

Gerry & Terry (got to be a joke there somewhere :) ) thanks for the replies. Still a little confused though - On the SDP-SI site I can select metric measurements and pick 16mm T5 belts, there are no matching pulleys. If I then select metal timing pulleys and select HTD 5mm pitch, 15mm width, there are only neoprene, fibreglass reinforced belts available. Will these do the job or should I really be looking for polyurethane, steel reinforced belts?

Should I be looking for something other than 5mm pitch?

Will plastic pulleys be OK? I have only been looking for aluminium ones so far.

Ooooh so many questions - but I need to get these things right.

Mike
They seem to only stock the T5 in 10mm width.

Mike,

Do not use the T style belts/pullies. These are made to have some backlash built in. I believe the same is true for HTD. Use the AT or GT2 instead.

(I am having the same problems/confusion with timing belt reductions as you are. There's also a lot of different choices for hubs - setscrew, taperlock etc.)

Arvid

Wow - lots of response, thanks.

OK, it seems that AT or GT2 are the best choices for no backlash, from Arvid, probably go for 2:1 and 3:1 to have a choice of reductions. Max speed of my X axis ballscrew is in the region of 600 - 800 rpm with motors giving 1700 rpm max, from Sol and from Terry it looks like most of the common reinforcements will do.

Gerry, I am thinking that 10mm is probably too narrow and am looking for 15-16mm.

Arvid, I like the idea of the taperlock pulleys - no annoying grubscrews or filing of flats :) Problem is finding sources that will sell small quantities. Good luck with your searching. Maybe we can keep each other posted about progress?

Right, a bit more surfing in order.

Mike

Gerry, I am thinking that 10mm is probably too narrow and am looking for 15-16mm.

Mike

I've got a feeling the 9mm would be more than strong enough, but if it were me, I'd buy the 15mm. :) The prices aren't much different, are they?

I agree. Like I said. I got my belts & pulleys for free, but having now used the machine a little, if I were buying some, I'd go for 15mm GT type, 5mm pitch.

Regards Terry.....

Right then, that seems to be sorted - 15mm GT it is. Now all I've got to do is find a supplier :)

Thanks for all the information, I'm sure you all realise how much time and effort you have saved me and that is the beauty of this forum - everyone is so keen to share their hard earned experiences. I only hope I can be as useful once I've gained the knowledge.

Mike

Hey Mike F. Glad to see the motors got to you in one piece :) I'm a little behind in seeing this post.

Mike, Are you going to be able to find the parts to build the controller box OK?

Jeff,

Not really thought that far into the electrics yet but I will be press ganging a friend into service who is an electronics genius - works on top secret MOD stuff!! Not sure I should be saying this :) He will want to know why I am not building the controllers myself instead of buying Geckos or Rutex :) I am not anticipating too much of a problem here.

I have spent the last three days scouring the web for compatible pulleys and belts but keep drawing blanks. The problem is that the motors are 1/4" shafts and the ballscrews are 10mm, 12mm and 15mm shafts and trying to find a firm that can supply all these sizes, preferably with taperlock hubs, is proving difficult, especially when I am also specifying GT type belts.

Still, if this game were that easy, everyone would be doing it.

Mike

Mike, try emailing Gates europe and see if they can hook you up with a distributor. While I was just looking at their site, I noticed they now offer an even better belt, the GT3.

Cheers Gerry, I'll do that right now.

Mike

I have spent the last three days scouring the web for compatible pulleys and belts but keep drawing blanks. The problem is that the motors are 1/4" shafts and the ballscrews are 10mm, 12mm and 15mm shafts and trying to find a firm that can supply all these sizes, preferably with taperlock hubs, is proving difficult, especially when I am also specifying GT type belts.

Still, if this game were that easy, everyone would be doing it.

Mike

Mike, for the 1/4" motor shaft. Don't worry about getting taperlock hubs for it. Just get a pulley with the correct number of teeth that has a small hole that you can drill out to 1/4" and then drill a small cross hole through the entire pulley and motor shaft and pin them together. This will keep the pulley on the shaft and it will never slip. This is what I do because of the shaft being so small.

Mike,

I've used misumi in the UK for some mechanical components including timing belts in the sizes you're looking at. They carried both metric and imp including gates and the pully to match, but it was a while ago so don't hold me to it!

UK presence used to be in Staines, and the number was 01784 465530, sales@misumi-europe.com Their catalogs where worth the read alone since there was loads of engineering data.

Andrew

Jeff,

How can you be sure there is no backlash after the pin is inserted? Surely after time, with the motor changing direction quickly, the pin must wear loose and have play in it. For this same reason, I have read that keyways and grubscrews are not recommended. Am I being over cautious? By the way, how many teeth and what pitch are your small pulleys?

Andrew,

I have the Misumi catalogue and most impressive it is too. Maybe I am not looking hard enough but I could not find exactly what I wanted. I have emailed Gates UK and had a promising response. Their project engineer reckons he can sort me out with just what I want - hope so.

Thanks for the input.

Mike

You are only talking 430 oz/in maximum. You get a metal pulley and put a 1/16" roll pin in and you will not see any slop develop. If you were talking some real horse power then a pin can wear out.

For my 2:1 setup I have 12 teeth on the server motor and 24 teeth on the ball screw.

Thanks Jeff, that's brilliant. I can now get down to some serious work without the nagging doubt.

Cheers,

Mike

I received a quote today for three small timing belt pulleys (15 teeth) and three large pulleys (30 teeth) and nearly collapsed - couldn't decide whether to laugh or cry. I had asked for the belts too but they were missing from the quote but I don't think I will be pursuing this one anyway. The quote was for 413.44 GBP :eek: that is about $780 :eek: :eek: These were just grub screw hubs too and I had asked for taperlocks!!

Where do these companies get their pricing from? That is not too far off double what I paid for three servo motors with encoders - which is more difficult to make? I am dumbfounded and more than a little upset. :(

Oh well, carry on searching I suppose.

Mike

Boy, that price seems a bit excessive... unless they are solid gold... I'm sure you can find much better. If nothing else hunt the web. I just did a quick check and found prices to be much lower at Mcmaster-Carr. Don't know what size your motors are but checked 3/8in shaft 14 teeth and was $15.00 us. Shipping etc can't be too bad considering your last quote. Good luck.

http://www.mcmaster.com/

-marc

Marc

They say they are specials because I wanted the small pulleys to have 0.25" bore and the large ones to have 10mm, 12mm and 15mm bores. I need to contact them and ask if they produce them with no bores, and then bore them myself. Be interesting to know what their price will be for unbored pulleys.

It is looking more and more likely that I will be purchasing from the USA again but what an endictment on British industry!

Mike

Marc

It is looking more and more likely that I will be purchasing from the USA again but what an endictment on British industry!

Mike

The Swedish industry :boxing: is not any better. I have discovered that thretening to buy from the US often gets their attention and a better price. :)

A little more information from the firm who quoted me ridiculous. I asked them to quote me for standard pulleys from their catalogue, without specific bores. That I did and noted they have a zero backlash option which was obviously the one to go for. The quote came back and the rep said that zero backlash was also a special and quoted me £43 GBP for the 16 teeth pulleys and £70.80 GBP for the 32 teeth pulleys. When totalled and VAT included it comes to an impressive £401 GBP. That's a massive saving of £12.44!!!!!!!!

Can someone please tell me whether the GT2 or AT are zero backlash?

Thanks,

Mike

I don't think any pulleys are truly zero backlash. All I can tell you is from what I've read, GT2 is the one to get.

I have managed to get the Y axis ballscrew mounted in a rather temporary manner just to test the arrangement. I was pleasantly surprised to find the Y axis plate moved quite smoothly throughout its entire length. It is rather tedious though having to turn the ballscrew by hand.

The great thing about this design is that the whole gantry assembly detaches from the X axis rails with just four screws and can then be worked on easily from a bench.

Next stage is to dismantle the Y axis, tidy up the mounting arrangements, clean up the pieces, spray where necessary then re-assemble. Just hope everything goes back together as well as the first time :)

Mike

Mike, beautiful job!
Could you hook the end of the ball screw to an electric drill and use that to move the carriage?
Trent

Trent,

Yes, that was the intention but when it is mounted on the wall I cannot get at the drive end :( For the next stage of fine adjustment I will drill an access hole through which I can use an electric drill but this will have to be done off the wall and for now, I wanted to see what it looked like in situ. I bet more time is wasted on projects by just standing back and looking at progress than by actual progress itself :) :)

Mike

Finally got my pulleys :banana: :banana:

I ordered them through www.sdp-si.com at a cost of $136, approx 72 GBP, just a little better than the 413 GBP quoted over here!!! However, there was a little sting in the tail as the postage, advertised as "Freight amounts to be determined using standard published list rate." , turned out to be $69. I am still a happy bunny though as the total cost was still just about a quarter of the original quote. It certainly pays to shop around.

I ended up getting 3 x 16 teeth, 3 x 32 teeth, 1 x 48 teeth pulleys and 3 x 60 teeth belts - all are GT2 specification as recommended here. The 48 teeth pulley is to allow a 3:1 reduction on the X axis if the 2:1 gives whipping problems on my 2000mm ballscrew.

I could not get the bores I required so will have to bore these out to suit. One of the ballscrew, 32 teeth pulleys will need a sleeve but should not be a problem.

At last I can start working again. :)

Mike

Finally got my pulleys...
This is all a bit over my head at the moment, but from what I can make out, you are going with the GT2 belts based on a trade off of backlash versus cost.

...First, they recommend the AT tooth profile with polyurethane belts to get minimum backlash. If I understand correctly, HTD is worst in this respect, followed by T, GT, and AT, in that order.
For my design I was considering going with the T2.5mm profile 6mm wide stuff that rswww sells. For a similar shopping list to yours, this would come out around £56 ex VAT. These would be more than ample spec for my bodgy design. Is the GT - T difference that great or are there other considerations?

BTW - great looking machine!

10bulls

I too was very confused by the huge number of different spec belts and pulleys, hence my questions to the zone. As far as I can make out, the reason GT are so good is the profile of the teeth, they are rounded giving much quieter operation. I don't know whether I have gone over the top by specifying 15mm belts but that decision seems to have been supported here by others.

Reliance, a company in the UK, offer backlash free versions of GT pulleys but these are the ones they quoted me silly for (£413) Obviously there is a very real cost consideration here.

I have yet to get the motors mounted so cannot comment yet on how well the system performs but they look pretty good and I cannot distinguish any backlash when fitted over the pulleys by hand. Incidentally, Jeff Davis who supplied my servo motors, says he uses standard pulleys and belts and has no problems with backlash.

I have seen your thread but cannot recall the size of your machine. Mine is fairly large which is why I went for the 15mm x 5mm pitch belts but 6mm x 2.5mm pitch seems awfully small, but there again, I'm a beginner too so what do I know?? :)

Mike

I am now at the point where I really do have to make the decision between Rutex and Gecko. Although price is not a fundamental issue I still do not want to pay more than I need to. As I see it, the advantage of Rutex is that I, as a complete electronic moron, can simply buy a mother board and IO board and plug it into the system. On the other hand, Geckos are cheaper but require a third party IO board and home/limit switch board.

Now the question. Bearing in mind my total incapacity for all thing electronic, how easy is it to interface Geckos with the necessary gizzmos for home/limit switches and IO functions such as spindle, coollant etc.? Is there something I can simply plug in?

Mike

Mike, If you've made it this far I don't beleive the Geckos will be a problem. Go take a look at the PMDX breakout boards at http://www.pmdx.com/ .
Something like the PMDX120 should do everything you need and has all of the pinout/in connectors and relays built on the board. You can set things to your softwares preferences (like pulled low or high, charge pump settings, etc) There are other brands so look around and read the manuals (PMDX has them online for download). They will make connections a snap.
I only mention PMDX because I have one and am happy with it.
-marc

Also llok at this breakout board. http://www.campbelldesigns.com/

Marc & Gerry,

like pulled low or high, charge pump settings, etc

This is what is worrying me - I don't understand the terminology so don't know exactly what I will need but thanks for the replies, I will be investigating the sites suggested. I just hope they cater for electronic dunces :)

Mike

The high or low are software settings, not a big deal. The charge pump is a safety circuit that basically keeps your motors from spinning when the software doesn't have control yet.

The breakout boards really make setting up Geckos (or Rutex) drives almost plug and play.

Mike, Take a look at this guys products for Break-out-boards and charge pump board. He sent me a full set of his products to test. He is also working with Art and the Mach 2 software for full compatability with it.

http://www.cnc4pc.com/

Mike, Take a look at this guys products for Break-out-boards and charge pump board. He sent me a full set of his products to test. He is also working with Art and the Mach 2 software for full compatability with it.

http://www.cnc4pc.com/

Nice price on that board. Think it'll be comparable to the PMDX board?
Gotta let us know how the testing goes!

Most software like Mach 2 will let you select either pulled active low or high, so it's not a big issue except that if it's not built into the board and you manually wire it, you need resistors and a specific voltage to pull the pin(s) high or low. Basically, the software is either reading a low or high voltage at a specific pin as either active (on) or inactive (off) and just does whatever the software is programed to do (like turn a spindle on with an M3 g-code) when the active signal changes from low to high or high to low.

Charge Pump is simply a software safety feature that will work with any software that will send a signal via the parallel port (normally pin 17). If for any reason the software driving your machine burps, bumps, locks or quits (windows error, power bump, driver error whatever) it makes an emergecy stop and the breakout board stops all motors, turns off all the relays etc. It's a very nice safety feature.

The reason I choose the PMD over Campbells was it had the relays built in and avoided the cost of additional "daughter boards" or wiring relays.
I think PMD 120 board has 4 a/c relays to power things like spindle for on/off, coolant pump for on/off or whatever you want. You only have so many pins available for stuff like limit/home switches and relay output signals. Doesn't matter what breakout board, as you are limited by the 25 pins per parlallel port and only so many allow signals in and so many allow signals out. Any board will basically do the same thing, but others may have 1 or more relays built on. Mainly breakout boards keep things isolated from your computer motherboard LTP port and ensures that you don't damage your computer when a problem occurs with a driver board (and visa versa). You can also get a cheap PCI multi I/O card that will give you another LPT port effectively doubling available pin in & outs. Of course if you do that you may need another breakout board to use that ports effectively also. That way you could use a number of limit, home or reference switch setup alternatives. For example I'm currently setting up a plasma table with a torch height controller and a torch height sensor and since I already have a number of limit and home switchs connected, I required more pin in/outs than one parallel affords. You'll have no problem setting up/wiring the stuff you're talking about... and the cost is minimal compared to motors, drivers, linear bearings and the like... the hardest part will be running the wires and mounting the switches! (sorry for the long post and blabbing on about stuff you may already be aware of, but hope it helps a little)
-marc

To all those who have recently replied - many, many thanks. I am so glad I found this site after contemplating a self build. Unfortunately my mechanical skills far outweigh my electrical skills and, as I have said previously, I will be leaning heavily on the good, patient, expertise here.

Marc - long winded or not, it was just what I needed and I am sure it is probably not the first time you have explained this detail. Thanks. If I want both home and limit switches on a three axis machine plus spindle and coolant, does this mean I need to occupy eight additional pins of the parallel port? If so are there that many available to me without the added complexity of another parallel port?

Presuming I will be using Mach II or III, does the software give me enough control to negate the need for limit switches and thereby save the need for using pins I probably haven't got? The small Isel machine we have at the moment has no limit switches, you just tell the software what the cutting limits are.

Jeff - When are you likely to be able to report on the CNC4PC boards?

So, bottom line is that there is definitely something out there that is comparatively idiot proof that can just be plugged into Geckos to do the necessary additional functions other than driving the motors. That being the case it looks like Gecko may well be getting a sale.

Mike

Jeff - When are you likely to be able to report on the CNC4PC boards?Mike

The items are still in the mail. I will let everyone know how they test out.

I kind of like the modular method that cnc4pc has used. If you don't want releays you don't have to buy them. If you don't want the charge pump option you don't have to get that as well. Get the options you want and only pay for the options you want.

To all those who have recently replied - many, many thanks. I am so glad I found this site after contemplating a self build. Unfortunately my mechanical skills far outweigh my electrical skills and, as I have said previously, I will be leaning heavily on the good, patient, expertise here.

Marc - long winded or not, it was just what I needed and I am sure it is probably not the first time you have explained this detail. Thanks. If I want both home and limit switches on a three axis machine plus spindle and coolant, does this mean I need to occupy eight additional pins of the parallel port? If so are there that many available to me without the added complexity of another parallel port?

Presuming I will be using Mach II or III, does the software give me enough control to negate the need for limit switches and thereby save the need for using pins I probably haven't got? The small Isel machine we have at the moment has no limit switches, you just tell the software what the cutting limits are.

Mike

Usually 8 pins are used for 4 axis of step and direction. From a website I just looked at, there are 5 more input pins (limits, home, probe) and 4 more outputs (relays, coolant). If you're only using 3 axis, you have 2 of the first 8 available as well. Usually limit switches are all wired to one pin, either in series or parallel. Each home switch gets it's own pin. If you need more, you can always add a parallel port via PCI card.

From what I've read, limit switches can be pretty important on a servo machine, because if an encoder fails (or wire breaks), the machine will take off with nothing to stop it until it crashes. You could always stand there with your hand on the E-stop button (another pin for that) :)

Gerry,

To help adjust my ignorance, do you know of a web site that explains this detail in non-technical terms? It is obviously something I have to get my head round if I am to consider any kind of success. I understand the concept of how the printer port works in terms of each pin capable of being either on or off but your post suggests that some are dedicated as input only and some as output only - is this correct?

I would like to consider a fourth axis at some stage so would preferably like to keep those pins spare. I will need spindle on/off, extraction on/off (is it possible to put these two together?) and possible coolant along with both home and limit switches. I take your point about servo runaways. Could be disastrous. :eek: I take it that if all limits are wired to one pin that any one will trip the system which seems to make eminent sense.

Thanks,

Mike

Mike.. go to the PMDX site and download the PDF manual (It's not too big) for the PMDX120 breakout board. Take a look at table 12. It shows what the pins are used for. Basically, it uses 2 thru 9 for step/dir control. (so 4 axis's posssible) It uses 10, 11, 12, 13, & 15 for switches (so you could use one for all the limits wires parallel or series and 4 for home switches). It uses 1, 14, 16, 17 for relay control, so you could connect up to 4 devices like spindle, coolant pump etc. This card also has gecko 300 series servo driver fault signal input. This card or (or simular) would do everything you need with one parallel and no external relays etc. If you ever need more reference switches or specific table setups or part fixtures, then plugging in another lpt port card is cheap and very simple and would double things. Simple!
-marc

Eli (Spank), when you say you build boats, do you mean model or full size?
Marc (Coherent), Mike wasn’t the only one helped by your explanation. Me too.
Gerry (Ger21), thanks for the links to the fibreglass articles some weeks ago. They sure kept me busy for some time. I read the Corsair article twice over.
Hi Mike, your machine’s looking great. I just re-read this thread from the start. I realise you are past the screw in MDF stage however for others reading this thread I just wanted to explain that when Buscht was talking about Euro screws, these are the special screws used in for example, Ikea furniture. (Countersunk allen head requiring a special drill, produces a very strong joint) Well I spent about 2 hours tonight searching the internet for the article below on aluminium honeycomb sandwiched by carbon fibre. I couldn’t figure out why I couldn’t find it until I realised that I had it in the physical form (i.e. a magazine article) as opposed to virtual. The only reason I linked your project to the one in the article is because they are both using flat 2D sections bonded together to make a 3D shape. Although this method might not have the same strength/weight ratio of a true monocoque, it is however achievable by people like us and doesn’t cost trillions. Just out of curiosity, I wonder whether you would be better off to use a foam core (PE or PU) and just plain old fibreglass instead of carbon fibre? If with fibreglass, you had to use one or two more layers than you otherwise would have had to with carbon, so what! If it weighed a few more kilos but cost one tenth of that of carbon, then good! I must admit however that carbon looks the piece. Anyway, this is for you to decide as you are more experienced in composites than I am.
Skippy

Thanks Marc for a very informative answer. I like the idea of a single solution that caters for all my perceived needs - I will be taking a longer, better look at the PMDX site.

Skippy - I think that Euro screws and the Confirmat screws I used are probably one and the same thing. (see picture)

The idea of using flat sections was to simplify the construction and yes, a monocoque would be much stronger but I figured that a well thought out alternative would be much easier to make. I am definitely thinking of a sandwich structure but not yet determined what the core material will be. I use Rohacell, an acrylic foam, for the models. It is extremely light weight 31k/M^3, 51k/M^3 and 71k/M^3 available yet it has very good compressive strength. If the core is reinforced locally where the pieces will be bolted together, I think it will work OK. However, I am a long way off that yet. :)

When considering composites, it is worth noting that you really ought to use just one type of fibre and not mix them. Glass and carbon have a quite marked difference in coefficients of expansion and considering I am using prepreg material, baked in an oven, when mixed the results can be quite alarming. I know because I have done it - if you want panels like bananas, laminate one side with carbon and the other with glass - you will be quite amazed at the outcome.

Carbon prepreg is about twice the price of glass weight for weight but you have twice as much fibre in the carbon so there works out to be not too much significant difference if you compare strength to strength.

Mike

All of the pin configurations described by Coherent are now a standard, set by the research and development of EMC that was paid for by a government grant many years ago. Thus all the step and direction controller software that use the LPT port has followed this standard for pin usage. As a result of that, all the break-out-board manufactures use these pins too. For a very full description of the pin usage for two (2) LPT port options, you can look at the latest Mach 2 manual from Arts site.

Here's a Euroscrew. This might be just US terminology. In Europe, it might be called something else.

Typically Euroscrews like Trent shows are used for attaching cabinet hardware, usually in 5mm holes.

Right then, Euro screws are for fixing things to MDF whereas the confirmat screws are designed for fixing MDF to itself. Looking at the diagram from Trent, it looks like the Euro screw would not be as good as the confirmat screw for screwing MDF boards together, as someone had previously suggested they had done. I have to say that I am very pleased with the way the confirmat screws work and they can be assembled and disassembled many times without fear of compromiseing the thread.

Jef - Thanks for the confirmation and insight into the standardisation of the pin outs for the parallel port usage. It looks like I've got a lot of reading to do.

Mike

I don't want to contradict anyone but to me, Mike's picture and Buscht's picture are one and the same. i.e. Euroscrews (Europe/US) and confirmat screws (UK) are the same thing. Just that in UK the brand name is used instead of the generic name. (same way as acrylic sheet becomes perspex and backhoe becomes JCB in the UK. Absolutely no critisism intended) The idea being that they act as both screw and dowel (the shank area) at the same time, for holding mdf or particle board to itself. Buscht's image is a little deceptive because it looks short in the screw area but that's only because they are showing "Lange" (length) to say that it can be bought in multiple lengths. To work effectively they need a special drill bit that has the shank diameter at the top and the diameter suitable for the screw at the bottom.
Skippy

The carbon does't seem to be too bad if it's only double the price. I was looking at Fibre Glast Dev. Corp.'s prices of normal looking weaves and they are between $32 and $45 per yard. I wonder how many yards you'll need for yours? Sorry Mike, it's not my intention to distract you as I realise that the composite gantry subject is about the furtherest thing from your mind at the moment.
Skippy

I don't want to contradict anyone but to me, Mike's picture and Buscht's picture are one and the same. i.e. Euroscrews (Europe/US) and confirmat screws (UK) are the same thing. Skippy

No, they're not the same. Like Mike says, Confirmat screws are for screwing Partical board and mdf together. To work properly, a special drill bit is required. And like I said, Euroscrews are used to screw hardware into 5 mm holes. They do look similar, though, now that you mention it.

Skippy,

No offence taken, we will just have to differ on the screw thing. Now onto some real issues - when I write about the glass and carbon, I am comparing prepreg, mould making materials that I use for my model making activities and the heavy weight glass cloth works out at around £36/M^2 and the equivalent carbon at approximately £70/M^2. These cloths are 600 - 800g/M^2 and I use seven layers on the moulds to give a thickness of 5-6mm. These are, of course, woven cloth materials but I am seriously considering the bulk of the structure being uni-directional carbon, with the fibres being oriented at the most beneficial angle for the expected loads. The final layer will probably be a reasonably lightweight cloth to give that, all important, sex appeal :) :)

However, I must not get carried away with what will be and must concentrate on the here and now and get the prototype working. It is a huge struggle at the moment, trying to get some time to work on the machine. For my sins, I am the examinations officer at my school and this takes up an awful lot of time and is not as seasonal as some might think. Add to that a full teaching commitment and my working hours are pretty dammed full :(

Mike

When you're wrong, you're wrong and in this instance I'm wrong twice over! Firstly, if you have a look at Buscht's image and see the two vertical lines above the head of the screw that denote the head diameter. Well it was 1am in the morning and I quickly looked at those two lines thinking they were the shank area of the screw and that the head was above but not shown in the diagram. After seeing Gerry's response I looked again today and immediately saw my mistake. (sorry Gerry) Secondly,