Z travel increasing as the cut proceeds.

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jarm2
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Z travel increasing as the cut proceeds.

Post by jarm2 »

I have been having a problem I need help with. I have a servo driven table with a Command CNC (linux based) control software.
On a recent 2.5 D carving the piece was nearly done, about 75% when the z axis began going down with each pass until it cut through the piece, about .75 inches. The roughing pass ran fine with a .25 em and the finish pass was on a 45 degree diagonal with a .125 ball nose cutter at about 8% stepover. I have cut hundreds of similar files with this setup and have not had this problem before. The z travel increase was very small per pass, maybe a few thousands per pass, but seemed to get larger forming a curved decent to cut through.
I have tried two more files, starting each one from scratch. In all cases the simulation looked perfect so I am not suspecting the software. The roughing passes seemed to work okay, but when I ran the finish pass, it started where it should, and as the bit traveled back and for and up and down, it slowly increased depth until I had to stop it. The depth increase seemed linear until I had to stop it.
It is possible it has to do with the post processor, but that has not changed. I have triple checked all mechanical components on the z axis, including motor coupling and encoder wheel and all are tight. The spindle is tight in the holder and isn’t moving. The bits are tight in the collet. I believe something in the Command CNC or the control hardware is causing the z axis to gain depth with each pass with a very small amount.
I would appreciate any thoughts anyone might have that would get me a new place to trouble shoot.
Thank you,
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Re: Z travel increasing as the cut proceeds.

Post by gkas »

Does the Z distance measure the same before and after the cut? If you measure from the spoil board, the Z=0 should be 0 at the spoil board both before and after the cut.

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jarm2
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Re: Z travel increasing as the cut proceeds.

Post by jarm2 »

That is something I thought about after I had cleared the system and shut down. I didn't measure or check that. My dgital readout and the actual measurement would have told me something, but I didn't think of it soon enough. I may set up a test cut and see if I can get some more definitive data. Thanks for the response.
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Re: Z travel increasing as the cut proceeds.

Post by Rcnewcomb »

The bits are tight in the collet.
I'd replace the collet anyway. Collets aren't that expensive; meanwhile lumber prices have gone up considerably.

FYI, I change collets every 6 months, or after a major collision -- whichever comes first.
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Re: Z travel increasing as the cut proceeds.

Post by martin54 »

If you have cut a lot of jobs with this set up without problem then unless you have changed something I would suspect a mechanical problem of some sort. I would have mentioned electrical interferance as well but that tends to be more random & there seems to be a definate pattern to your increase in z height. Changing collets as Randall suggests is a good idea as they are suppose to be a consumable item & should be changed as part of a maintenance routine. Having said that if the collet were the problem I would expect the z to be decreasing in height rather than increasing.
If you have checked everything is tight then the next thing I would do is check for binding on the z axis, you could also try backing off your acceleration settings in your control software a little as well, if you are close to your limits then any slight sticking of the linear bearings could affect your holding torque :lol: :lol:
I actually had a problem like that when I first got my machine because the stepper motors were very old & the magnets didn't have the same power as they did when the machine was new :lol: :lol: :lol:

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Re: Z travel increasing as the cut proceeds.

Post by jarm2 »

Thanks, Martin for your reply. I have checked all the parts on the z axis several times. The servo connects to a ball screw with a split coupling clamped to both parts and that is tight. The only other thing on the z axis mechanically is the encoder wheel for the servo and that is tight as well.
Good thoughts on the collet. I will be checking that and probably replacing it soon. I would expect something loose mechanicaly to be more random and not smooth like this is.
Today I will design and cut a calibration piece that should allow me to see the change and measure it. I will be watching the DRO and measuring the bit clearance as well. All your advice is appreciated.
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Re: Z travel increasing as the cut proceeds.

Post by jarm2 »

I did a test cut this morning with a simple 2.5 D piece with several extra z moves. The z axis again added down movement in a linear fashion as the cut progressed. See pictures attached. When the finish cut started, it was where it should be. As the file progressed, the cut got deeper and deeper as you can see. The roughing pass cut just fine with no variation. The finish pass had the issue. The DRO readout matched the position of the bit, but was nearly .375" deeper than it should have been when I stopped it. I checked again all the mechanical connections and could find nothing loose or with unwanted movement.
Attachments
DRO readout.jpg
End with cut.jpg
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Re: Z travel increasing as the cut proceeds.

Post by dealguy11 »

For what it's worth, whenever I've had collets go bad the bit gets pulled out of the collet making the cut gradually deeper and deeper. This does sound similar to what you're seeing, but do check all the other mechanicals as well.
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Re: Z travel increasing as the cut proceeds.

Post by adze_cnc »

To continue on the collet theme. An easy way to check for bit slippage is to chuck up your bit then mark it with a "Sharpie" at the collet. Run your job. Recheck to see if the line has dropped.

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Re: Z travel increasing as the cut proceeds.

Post by Leo »

Sorry, I'm gonna step away from the collet. That just does not look like a collet issue to me.

I would like to see the preview and / or the CRV file.
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Re: Z travel increasing as the cut proceeds.

Post by adze_cnc »

I don't really think it's a collet issue either. Nor do I think it's a post-processor issue as was hinted at in the original post.

If you do a web search using the terms: candcnc z-axis problems

you get a fair number of hits from candcnc's own forum talking about, sometimes sporadic, motor issues. But since you need to register for that forum I didn't really follow it up.

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Re: Z travel increasing as the cut proceeds.

Post by Leo »

What I see is very uniform, and it seems to be repeating uniformly. It doesn't seem to be a sporadic issue.

It just makes me wonder.

I would like to see more.

Maybe it could be a motor, but I don't think so because it is so uniform.

I am thinking control software issue, but would like to eliminate the Vectric end to be sure.
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Re: Z travel increasing as the cut proceeds.

Post by jarm2 »

The preview for both roughing and finish cuts looks all good. No unwanted travel from the z axis. I have pl;aced the .crv3d file in dropbox so you can see what I did for a test file.
https://www.dropbox.com/sh/2jo7sj7ye0j7 ... kZuKa?dl=0
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jarm2
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Re: Z travel increasing as the cut proceeds.

Post by jarm2 »

Also I have a servo driven table so the issues with the C&CNC motors doesn't apply here. The servos I have were from and earlier vendor. I adapted a Practical CNC table with C&CNC electronics and have been pretty happy with their equpment.
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Re: Z travel increasing as the cut proceeds.

Post by jarm2 »

Additional info is that the roughing cut with a .25" bit cutting .25" deep does not change the z. The finish cut with a .125" ball nose is the one that gives the problems. Down force is minimal with a .02 depth of cut from the roughing passes.

The folks at C&CNC are also looking at my issue to see if they can help. I do follow that forum regularly.
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