Dan's cnc Router

[Dan S]

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.



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.

[ToyMaker]

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

[Dan S]

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.

[HuFlungDung]

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.

[Dan S]

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.

[buscht]

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

[Dan S]

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.

[ger21]

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.

[Dan S]

Gerry

I am unfamilar with EMC,what is it exactly?

[InventIt]

EMC is a machine control that runs under linux OS

[ger21]

http://www.linuxcnc.org

[buscht]

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