Not sure I'm ready for this - I've started building!

[arvidb]

Mike,

check the article I wrote (PDF) here: What motor, screw and gearing should I choose?

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

[Mike F]

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

[sol]

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 ) 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 ...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!

[Mike F]

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.

[Mike F]

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

[InventIt]

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

[Mike F]

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.

[mícheál]

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.

[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!

[Mike F]

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

[Mike F]

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. 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

[Mike F]

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