Z axis inconsistency

I have an Avid CNC machine that I'm having issues with and I can't figure it out on my own.

We cut 10 different patterns on the machine, over and over. Every once in a while, the machine will cut about a half inch deeper than it is supposed to, destroying the table. I'm not the one operating the machine so it's even harder for me to troubleshoot. From the little I know about stepper motors, I don't know what would cause it to gain steps. Every time I operate the machine, it works correctly.

Any ideas?

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Originally Posted by jet8300

I have an Avid CNC machine that I'm having issues with and I can't figure it out on my own.

We cut 10 different patterns on the machine, over and over. Every once in a while, the machine will cut about a half inch deeper than it is supposed to, destroying the table. I'm not the one operating the machine so it's even harder for me to troubleshoot. From the little I know about stepper motors, I don't know what would cause it to gain steps. Every time I operate the machine, it works correctly.

Any ideas?

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Every once in a while during a cut in a series of jobs or every once and a while during the same job?
If it's between jobs, I could see someone forgetting to set the zero. If working off the same zero as the last job, then accidentally pressing zero at some point during setup could set it below the correct zero.
I've also resumed after pausing and forgot to set the travel move to the next position to start again and had the cutter drag through the material on the positioning move.

If the motors were off between a job, I've seen the z axis move down from the weight of it. Only a few times though.
Otherwise, and it would suck to even check this, take the zaxis apart and see if the ballscrew is shifting somehow.

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Every once in a while on a new job, for the whole job the Z axis is lower than it should be.

This machine is set up to not require the user to zero the tool every time.

The motors are not being shut off between jobs. I just rebuilt the Z-axis including replacement of the anti-backlash nut. There's no play in the assembly and even with the motors off it does not descend any.

Mach 3 or 4? I do not have mach 4 so I can't comment on that really.

In mach 3, sometimes I use some scripts I wrote to do some things. In one script it prompts for input. If I use the entire key instead of pressing the ok button, it will bury itself into the table and move in an arc. I've just to figure this out other than it's mach3 running on windows 10 and I have no way of testing other than repeatability of the problem.
I would say that it's possible that noise could cause something like that. However, I would expect that to happen more throughout the job. Also, if you got the spindle setup from avid, it's shielded.

If it's not being zeroed each time and somehow losing position at the end of the last job and you're not homing before the next, maybe something with that. Otherwise, whoever is controlling it may be zeroing even though you don't want that? Maybe something weird with the Ethernet smooth stepper. How long has it been on when this happens? Not that it should matter. Murphy's law though.
Hopefully more people can chime in on this.


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Mach 3. The spindle is indeed from Avid.

I'm not sure they ever turn the machine off. We run two 10 hour shifts and I don't think they currently power off the machine at the end of second shift.

I tried making a macro to run the same code as the "reference all" button, but when I put the macro in my gcode the machine does the reference all but stops after that - still trying to figure out that issue.

Off the top of my head with your macro, it might need to set the offset back to whatever you are using. When ref all happens it is in machine coordinates. You may need to set the work offset back to the recorded one. So, g54 if default.
I will have to look at some of my macros used. I know I made one for my 4th axis that will reference that axis again between some operations to make sure it goes where it needs to on the next operation. It was overkill after I saw how rigid it was though.

I know that the screenshot I use has to switch to machine coordinates and back during the auto tool change touch off. It's based off of bigtex screen set and probably has the answer inside of those macros. I know that fusion 360 always sets the work coordinate at the beginning of the file.
Another way to do it off always cutting the same things in the same order would be to combine it all into one file and add the referencing in between and then have it wait for the command to start cutting again.

On that note, when it stops after referencing, does it then continue where it should of you hit start?

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For the macro I tried using, it will continue after hitting start again. I tried putting in the script a "press" on the start button, but that just causes Mach 3 to skip over the macro entirely.

I'm still struggling with this and haven't figured it out yet.

If the Z motor coupling is slipping, would that cause this issue? Say it slips when plunging - the z would be up higher than Mach thinks it is. When it goes back to machine 0,0,0, it would hit the limit/home switch before it was supposed to. Would that put some kind of offset into Mach, causing it to drive farther down the next run?

When starting again, do you physically click with the mouse or just hit enter to do it? I ask because I have seen weird things every so often with using enter to verify a prompt. I've just gotten in the habit of manually clicking it.

It may be worth opening up the zaxis and checking the connectors. I believe they are the couplings that are 2 part with the hard rubber flex interfaces. I would expect it to happen more than every once and a while and get worse over time as it loosens.

Another thing you might want to do is swap an axis port. If you have the 5th output available then repurpose that port and enter the zaxis information into the settings. Maybe the break out is having issues itself. If not, swap another axis and the settings.

If you do see the same issue on the swapped axis and not the z, then maybe the driver is going bad.

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Originally Posted by jet8300

When it goes back to machine 0,0,0, it would hit the limit/home switch before it was supposed to. Would that put some kind of offset into Mach, causing it to drive farther down the next run?

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This sounds familiar enough I thought I would mention. I spent 1+ years diagnosing odd Z axis drifts on mine since new. It turned out the post in Fusion360 was for "Mach4" When I used the "Avid" post from avid, it fixed the problem permanently, it has been good for about 6 months now. What was strange is that the Z was drifting sometimes, not other times like some kind of analog or mechanical fault... digital systems don't typically behave like this. Anyhow this is food for thought for you.

Secondarily, during the investigation I found a winding inside a stepper motor that was 80% crimped by the factory as a defect. It was not the cause, but it is easy enough to open up and examine yours. They replaced that one for my anyhow.

So I think I may have found my issue.

We started out with our cncrp plug and play kit about ten years ago, and we stayed using the parallel port. At some point we needed to replace the control PC and parallel port is no longer really a thing. So we got an Ethernet SmoothStepper and all was well. We then eventually added a spindle and the PMDX 107 control module. Well when the spindle was installed, the ESS was never powered independently. Apparently the PMDX 126 can not power both the ESS and the PMDX 107 simultaneously. So I have added an appropriate 5v power supply just for the ESS, hopefully that will finally put this to bed.

After supplying independent power to the ESS, I noticed that the X axis is running more smoothly already, as it was kind of jerking along before.

Looking back on the last few months, we have had a lot of really random things occurring and I hope this was the cause.

Thanks everyone for the help!

That's really interesting, a good argument for always using a dedicated power supply for the ESS. I'm just setting one up now, and debated that issue, decided a dedicated PS was so inexpensive there was no reason not to, glad I did!

I have an avid CNC and I was halfway through a cut and it reset the Z access to be a quarter inch less than the full depth. I’m using machine bed on my file as the profile but nothing changed I was on my fifth or sixth cut of the same profile and it reset.

Quote:

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Originally Posted by jet8300

I have an Avid CNC machine that I'm having issues with and I can't figure it out on my own.

We cut 10 different patterns on the machine, over and over. Every once in a while, the machine will cut about a half inch deeper than it is supposed to, destroying the table. I'm not the one operating the machine so it's even harder for me to troubleshoot. From the little I know about stepper motors, I don't know what would cause it to gain steps. Every time I operate the machine, it works correctly.

Any ideas?

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