backlash compensation

[ychap]

Hi,
I have a 4by8 rack and pinion CNC running open loop stepper motors which is running Centroid acorn,
I purchased it new from an American factory.
Since I received it on day one it had issues with accuracy, if I would dial it in at 12 inches perfect that it would be off on other values up to .026 of an inch, which is obviously not something we can work with, so eventually the factory sent out a tech to determine what is going on and he couldn't figure out why is was doing that, and their proposal to fix the issue was to use centroids backlash compensation software, I just wanted to know if you can help me out by giving me an opinion if you think this would fix the issue on an open loop rack and pinion system with such big discrepancies, or if its an hardware issue I should continue trying to get it resolved through replacement parts.
Thank you so much!

[suntravel]

0.026 backlash is something that must be fixed mechanically. I would not accept that on a new machine.

Uwe

[ychap]

thank you appreciate your response, I wanted to make sure it wasnt just my opinion that thats too much of a variance to accept on a 25000 dollar machine, Im being told by the company now that thats sort of what they would expect out of this level machine which doesnt really make much sense to me

[avp]

Wow, that's not an answer that I would accept. That is a lot. I would think if it is that much it would be fairly easy to find a mechanical issue that is causing it.
Bill

[ychap]

Thanks, I did put a straight edge on the guide rails on the X and saw a small amount of play, Im thinking that may be the issue, but again they said they want to fix it through software, from my understanding the software compensation software is for when you have a mechanically sound machine and you just need to dial it in to the thousandths not hundreds

[tblough]

The rack is not continuous. If the segments are not properly registered, you could easily see a 0.025" jump at the segment change.

First thing to do is verify what your backlash, and not your part variation, actually is. Measure the backlash on a continuous segment of rack and see what it really is.

[ShawnM]

Yikes, .026 backlash? That certainly is a mechanical issue that CANNOT be fixed with backlash comp. Check the pinion to rack spacing and adjust. I'm guessing you can grab the gantry, wiggle it and feel (or hear) the gear spacing as it bangs side to side.

It isn't an "Industrial CNC" brand router is it?

[ychap]

thanks for all the responses, yes it is advertised as an "industrial machine for small shops",
the tech came down and changed all the transmissions so everything is tight and the issue is still there,
I guess it would make sense that there is something wrong with the rack itself.
I just told them again that there must be mechanical issues, as per all the responses I'm receiving and they responded again that they want to fix it through software.

[ShawnM]

I'm asking if the brand is "Industrial CNC" not how they market it. What brand is your machine? Post a pic please.

When you say they changed the "transmission" what do you mean? Does it have gearboxes and that's what they changed? I don't think I've ever seen a gearbox go bad. Does it have an intermediate gear on the rack and pinion? Those have a concentric bushing adjustment to mechanically minimize backlash.

You are not taking out .026" of backlash with ANY software.

Without pics we really can't give you a plan of attack as we can't see what you see. Please start a Google album and post pics of the machine and the rack and pinion.

[tblough]

OP didn't mention backlash. He stated an accuracy problem that could be caused by wrong turns/inch, steps/turn, or even lost steps since this an open-loop stepper system. Without seeing a report, and actually testing backlash, and fine tuning calibration, everyone is just guessing.