Z Axis drift during touch plate sequence

Quote:

Originally Posted by Emptywill8182

I have been using my machine for a few months now (PRO4896) and feel like I have a pretty good handle on it. Recently I updated Mach4 to 4809 and the ESS plugin to 283.

While cutting some material I noticed some weird behavior. After homing the machine and setting the soft limits, I triggered the limit switch on the z axis while jogging the machine. This happened a couple times but I didn't think too much of it, just rehomed and moved on.

I recently added a laser to my machine (which prompted the software update) and while going through the touch plate subroutine, I discovered the issue I was noticing earlier.

When I would set the Z height with the touchplate the z axis would return to the 2" position above the work surface, however, it does not go to 2.000", it goes to something more... I grabbed a dial indicator and put it on the spindle as well as some 1-2-3 setup blocks. Each time the touch plate subroutine is run it adds 0.015-0.020" to the Z height setting, it was even wrong right from the first touch off.

I tested several times with similar results, the dimension would deviate increasingly from the 2.000" setting above the material surface. When I close Mach4 and reopen it will reset back to the 2.020" range then grow with each touch plate subroutine. After 5-6 consecutive touch plate routines I am off by 0.050-0.060".

I went back to my last working version of Mach4 and the ESS plugin (Mach4 4612 and ESS 277.1) and ran the same test and the Z height was completely repeatable and accurate every time.

Just wondering if there is a software setting that I can adjust to eliminate the drift. I have been able to get the laser working great but it is imperative to maintain accuracy in the z axis. It is also worth noting that I can run the touch plate subroutine and the machine goes to Z=2.000, disable, go change some settings in the ESS menu that affect Z, go back and enable the machine and the Z height will now show Z=2.020 or some other number, so it seems like the machine is keeping track of where it is but displays incorrectly?? I even measured the delta after I noticed this to confirm.

As a side note, I wrote a program that jogs the machine in the z axis, I made about 20 vertical moves (1.000 - 1.250 - 2.250 - 1.750 - 1.250 - 2.000...) and as long as the touch plate routine is not involved it seems to maintain accuracy in position, I would end on 1.500" and would measure to verify. It is almost as if there is a dynamic offset in the z axis subroutine.

Any help would be greatly appreciated.

Call the factory but I am pretty sure you are just looking at two things;

1. Repeatability
2. Ringing

Repeatability for these machines is ok but the real issue comes into play when the machine is experiencing ringing. Then your repeatability goes into the garbage.

So, let's deal with the Z axis tripping the limit switch first. Set the back off to a larger (negative) distance. The ringing of the machine can trip the Z axis as that is the part of the machine that will experience it the most.

Secondly, move slower when touching off. I find 100mm is way to fast except for first move (which the AvidCNC probe doesn't do). I would do the actually touch off at 50mm per minute.

You will see that error rate when you are jerking the machine around a lot and the ringing will show up in any surface finish as a series of waves. It can be a real pain when cutting metal. I find 99.7% of people who cut wood don't even know it's happening.

Myself, I can see it in wood. Bugs the ever love'n 'you know what out of me.

You got a large format CNC machine for very few dollars. These AvidCNC machines really are wonderful for what you paid for it.

That said, you will also need to learn its limitations. The 8020 is springy. It has a very long period in it's resonance. It literally rings like a bell. Once you understand that you just program around it.

Definitely go ask the factory tech support for help. Double check you don't have something seriously amiss like screws coming loose or the like.

But judging by the numbers you posted, that is as discussed above. Remember your accuracy is 0.002" and your repeatability is 0.005" which means those numbers you posted are almost withing spec.

When planning cuts, just remember your machine has a "spring" in its "step."

Go slow around corners. Keep circle segments as arcs and turn on smoothing. They tell you how to do this in their how-to-vudeos.

Definitely try a slower touch off speed first.