I just won my x-axis rails on eBay, so I thought it was about time that I start a thread on my router project. I listed the cost of everything I currently have in my signature line, as I acquire more parts ill up date the list, as well as post more pictures in my zone gallery.


My goal is to have a machine that is capable of 3d wood work and the occasional 3d aluminum work and maintain an accuracy of .01”. Here is a sample of the occasional 3d aluminum work I want to do. It’s a mold used in the casting of lead model yacht ballast.
I believe the gentleman from Finland that milled this particular mold is a member of this forum.

http://www.cnczone.com/gallery/data/500/2085cncmolds.jpg

My next major purchase will be ball screws and nuts, though I think I might need to consider some bigger rails for my z-axis. At the moment I’m leaning towards the 5/8” stuff from McMaster-Carr, like wjbzone did on his machine, they seem to be the best deal around for quality ball screws. Any one found a better deal than this?



So for my first question:

Is it possible to use software to compensate for inaccuracies in the ball screw? For example 5/8” stuff from McMaster is quoted as having an accuracy of .004”/ft. So theoretically can I change the tpi setting in the software so the accuracy becomes .000”/ft? I have seen several places on the forum where it was alluded to but never actually stated.

Dan:
I don't think you can compensate for screw inaccuracy in software because the error can vary from plus to minus. That is, if a lead screw is 4 thou short in a given length of 1 foot it may pick that up again in the next foot, so that at 2 feet the error would be 0.
Software can compensate for some backlash in the drive nuts because it is consistant from one reversal to the next. Although backlash does grow over time due to wear on the parts.

robotic regards,

Tom
= = = = =
"Those who question the Bible have whys for the word."
(an example of implied chiasmus, reversing "word for the wise")
- - Frederick J. Ernst

My thinking or should I say hoping was that whatever inaccuracy the screw had it was linear throughout the length of the screw, that is to say that if it was off by -.005” for a one foot move then it would be off by -.015” for a 3 ft move, then just adjusting the software would fix the inaccuracy. I might just have to wait till I get a machine going and test it or call an actual manufacture.

Dan,
With fancy software like Camsoft's cnc (only one I am familiar with), it is possible to "map" the errors at various positions on the screw. This requires some kind of independent assay of the screw's performance in actual use.

If the screw has a few thou backlash, I wouldn't waste a lot of effort on mapping it, because the table will still float by the backlash amount anyways. The only way to combat this float, is to counterbalance or otherwise preload the table in one direction so that it always stays against the one side of the nut.

Hu

I’m planning to use 2 opposing ball nuts per axis, so that should drop the backlash to zero.

Maybe I will just have to bite the bullet and see if I can find a good deal on precision ground ball screws.

Dan, over what length are you trying to hold .01" Is that +-.01" or +-.005".

I think that Toymaker's description is correct and you would have to have some sophisticated software to handle that type of variation.

But if you are correct that its a consistant error build up, then almost any control software can compensate. You just have to tweak the travel per step in the control parameters.

Now, IMO with the type of work that you are showing in the picture, I would think that a rolled ball screw will be plenty accurate enough. Especially with 3D woodwork over a long distance, the wood will grow and shrink so much that .01" is not important.

I use that same McMaster ball screw, but my wood parts are usually under 6" in length. I cannot measure that accurate to see any variation. Another option, might be to use this screw on your Y and Z axis, but only put out more money for the longer X axis.

Good luck and keep up posted on your progress.

Trent

Trent,

I want to try and hold a tolerance of +-.01” over the length of my longest axis. So with the parts I currently have that would be about 34”. So If I ask for a 34” move I’d like to get a move of 34” +-.01”. I know +-.01” is going to be a hard goal to reach, but I would rather aim high and miss, than aim to low.

I don't think .01 is very far out of reach. You can feel a lot less with your fingers.

I believe I read that EMC can do leadscrew mapping, but I'm not sure. You might want to look into it, if you're concerned. But I think a lot of homebuilt machines can get within .01. If you have no backlash, then you should be pretty consistent.

Gerry

I am unfamilar with EMC,what is it exactly?

EMC is a machine control that runs under linux OS

http://www.linuxcnc.org

Dan, if the ball screw is +-.004" per foot, then the most you could be out over 34" is +-.0113" which is very close to what you want anyways.

There is a very good statistical chance that the error is not cumulative and the total error over 34" will not exceed +-.0069". If I remember my statistics correctly, its the square root of (3 * .004^2).

But either way, you could bring this in tighter with what you suggested. I was hung up on being mathematically perfect. You could just measure any inaccuracy over the 34" and adjust for that, any small move distance would only be off the adjusted distance. Half the distance, half the error.

That being said, there are other factors that add to the ball screw error. Backlash, flex, dropped steps, etc. These have to be addressed seperately.

But I would feel comfortable recommending using a rolled ball screw for a 34" travel machine with +-.010" accuracy.

Trent

:banana: :banana: :banana:

My x axis arrived today

http://www.cnczone.com/gallery/data/500/2085xrails4.jpg

2 46” NSR25TB rails with 4 blocks.
I picked them up on eBay for $275 shipping included.

I was like a little kid at Christmas time when the mail man dropped them off. They where still in there original un-opened wrappers from THK. I had to stand on top a chair just so I could get them in the picture.

The only thing I noticed was that they must have been in storage for a long time, because on the end of each rail was some dried out grease. It looks to me like the stuff they coat the rails with right before packing.

OK guys I have run into my first little hiccup. Yesterday I was 3D modeling my parts in rhino. When I got to my x-axis rails I had a little problem. My rails are NSR25TBA’s, so I went out to THK’s technical website to grab the 2d cad drawings. I get to the NSR section and all I can find are NSR25TBC. So I contact THK’s help desk and they tell me that the ‘A’ model has been discontinued, I figured as much.

What I don’t understand is that I was told cad drawings are no longer available for the ‘A’ Model. So what the deal with that, are they lazy, or just don’t want to help because I’m not a big customer?

Does anyone have any, or know where I can get cad data on these Blocks or maybe an old catalog?

I think I have fixed my little problem. I spent about an hour last night reviewing specs for other blocks and noticed that THK like to keep things in increments of .5mm. So I broke out the calipers today and went to work measuring. Every thing was within thousands, so I’m good to go.

My new z rails came today.

http://www.cnczone.com/gallery/data/500/2085zrails2.jpg

So this is what I have for rails now.

X-axis 46” NSR25TBA
Y-axis 30” SR20TB
Z-axis 25” SR20W

This leads me to my next question.

What kind of z travel is reasonable for a home built?
I was thinking of trimming my z rails so that I was left with 9” of travel.


The Tim Allen in me keeps saying “more power”. Before I listen though I want to know what I’m getting myself into.

The bigger the travel, the more flex you'll have. You need to figure out, with the shortest cutter you'll be using, you'll want to be able to cut down to your table. And, with the longest cutter you'll use, you need to be able to lift it up to clear the thickest material you'll be cutting. This is how much travel you'll need. I'd add an inch to each end from there, maybe 2 to 3 at the top.

Gerry

Thanks for the advice. I’m hoping to use a quick change setup like HomeCnc did so all my bits will be the same length. I’m thinking the maximum length is only going to be around 4 or 5 inches, because much more than that, and I think I will have to worry abut hitting the work peace with the bottom of the router. So if I add 2 or 3 inches I get 8 inches. So I was close. :D

Say you want to cut thin (1/4" )board, with an 1/8" bit. Typically, an 1/8" bit is about 1-1/2" long, with maybe 3/4" sticking out of the collet, so you need to be within about 3/4" of your table.

Then, say you want to cut or carve 3-4" material, with a 5" long 1/2" ballnose bit, which might stick 3-1/2" out of the collet. This would put the collet up to 7-8" above your table, which would be about 7" of travel, plus a few inches like I said before.

Even with s simiolar setup to Jeff's, there is no way you can get very small bits setup the same length as very large bits, like the scenario I describe above. Just keep that in mind, because it's awfully hard to make those rails longer after you cut them.

If he makes the collets like I did he can make the smaller ones a little longer to help bring them up to the lenght of the longer bits. You can also purchase larger bits that are shorter than normal. I would set up your Z so you can bring down the router with no bit in it and be close to the table. Then plan for the longest bit you might use. If you plan to drill with it and want a small chuck, that will be something that is long after you hold the drill as well.

I have used professional tool holders with router bits. (Similar to HomeCNC's) but on a larger scale.

The value is not that you can make all of the tools the same length. (It's impractical, if you use a wide variety of tools.) The benefit is that when you put the tools back in the router, you know exactly what the length is. Most programs, including TurboCNC, have a tool offset table. Use the program capabilities to easily accomodate tool lengths.

Ger21 gave good advice on how to size your gantry Z axis.

I forgot to mention, you should be able to lift a long tool above the bottom of the gantry, so you're work is only limited by the gantry clearance.

Thanks for the advice everyone.

It’s good to get a dose of know how from people who actually use a cnc machine. This just reinforces one of the rules I laid down before I started this little project. “Get all the parts in front of you then design the machine”.

While I’m thinking about it, does anyone know where I can pick up some quick change collets, for the porter-cable 89X series routers? I would need 1/16” to 1/2”. If I have to, I will have them made, but it would be cheaper if I can find someone that sells them.

Jeff I’m glad you reminded me about a chuck. Is it possible to drill with a router? I just need some special high rpm bits right?

You can't put a drill chuck on a router. Not very safe. And no one makes special size porter cable collets. (except Jeff). Sears does make quick change collets in 1/4" and 1/2". Most router bits come in 1/4" and 1/2" shanks.

I found a high speed chuck that has a 1/2" shaft at the end. It works fine on the varaible speed Porter Cable that I have using the lowest speed of 10,000 RPM. The chuck is small and I believe the reason is because you don't want to turn large drill bits fast. I think it will hold up to 1/4" drill.

Do you have a link?

Jeff

Do you have a link to the place that sells the drill chuck?

Are you selling any of the quick release collets, or perhaps do you have any plans that could be shared with the group.

I think someone could make a killing selling theses for like $10 each.

Except it probably costs more than $10 to make them.

I posted the collet drawings to the zone awhile ago. You may need to do a search for Porter Cable collets to find it again. WMS offered to quote me a cost to CNC make them but I just did not feel that enough people would be interested in them. I would need to purchase them in lots of 50 (for each collet) to make the cost lower.

I can't remember where I got the drill chuck. It was some time ago. I will continue to think about it and let you know if I remember or find it again.

ADDED: I did the search for you. http://www.cnczone.com/forums/showthread.php?s=&threadid=2549&highlight=Porter+Cable+collets

I think it's post #13 on the link above.

Jeff

thank you, i now have some pics stored to the hard drive.
:banana:

Just wanted to show everyone what I have been up to. I’m all moved into the new place, and started sitting up my shop. I just finished my work bench 96” wide by 40” deep. I was amazed at how much the mdf weighed. The next project is some casters for the table saw, and then some shelving.

Dan - good on 'ya mate! She who must be obeyed Insisted that similar space was her's - Washer Drier! You've got 3 sides done for an enclosure! Cheers - Jim

Just wanted to show everyone what I have been up to. I’m all moved into the new place, and started sitting up my shop. I just finished my work bench 96” wide by 40” deep. I was amazed at how much the mdf weighed. The next project is some casters for the table saw, and then some shelving.

Hey Dan, Is that a CNC lawn mower?

with your z axis travel, can you accomodate keeping the rails at full length, and then just limiting travel with your ballscrew, or even better in software via the limit switches? that way if you decide to change anything down the track you still have full length rails to play with. as others have said, it's pretty hard to stick the extra bits back on again :)

My transformer arrived on Saturday, is should give me just over 36 volts and 20 amps.

http://cnczone.com/gallery/data/500/2085transformer.jpg

ynebb

it's remote control :cool: j/k


happy milling for all