Odd Z axis problem

Hello Forum, I hope someone has seen this before or has a clue, because I’m stumped.

New Elite Journeyman, just got it a couple of weeks ago. PwnCNC 80mm spindle and VFD. So far I’ve mainly been surfacing boards and slabs but I’m getting very odd results. It seems the Z axis will step up or down at certain locations, quite suddenly. Here’s a picture of today’s result, this is doing a standard back and forth path:

At first I though the bit was slipping…but the beginning of each pass is very accurate. If the bit was slipping each pass would be at some random height.

I thought maybe I had a loose coupling since i installed the braking Z motor, but it seems solid. Also, like the bit if the coupling was slipping I wouldn’t expect to see such an exact match between the start of each pass.

I thought maybe wiring, but it’s all brand new and looks it; all the harnesses & wire runs seem normal with no kinks. I tested for continuity back to the Masso board and it all checks out.

It’s odd that it’s jumping up and down by what appears to be a consistent amount. Todays operation failed with an X motor alarm, doubtless my 2" surfacing bit was dropped back into this slab of cherry suddenly and was overloaded.

Ideas? What else can I check?

Did you look at your gcode files to see if z was being changed during the pass?

I did, g-code shows no change in z, other than the ramp-in

Here’s the file.

3flute1Stepover1-8Extension.nc (65.7 KB)

Hey cvcolomb,

just to be sure, if it’s your first ER collet, do you know that they are not mounted the same way as router collets?

ER Collet Essentials. Do You Know? – Haas Automation Tip of the Day - Youtube

The link jumps to the moment in the video where it is shown.

Haha, yes @Aiph5u I am aware of the difference. Mainly because I already made that mistake ;-}

Thanks for checking

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I had a look at your file and description of what is happening.

I use a different spindle - 2.2Kw 80 mm, and have open loop 3Nm steppers run by a Masso G3 on a Woodworker X50.

A few thoughts…

  1. You are running at 5000 RPM, which in my opinion is very low, even for spindles that claim to be able to machine at those speeds. I believe you will have very little torque.

  2. You are using a very large diameter surfacing bit which may add to the challenge.

  3. Perhaps given the above, and the rigidity of the CNC, a higher RPM and shallow depth of cut - feed rate to give appropriate chip load - may help.

  4. Are the problems occurring perhaps when there is a grain or density change in the slab you are surfacing?

  5. Given the above ensuring minimal stick out and proper collet tightening will be important.

Hey Tom, hey @cvcolomb,

This is true. On my spindle, a 2.2 kW 6000–24000 rpm spindle, the manufacturer tells in the manual to program a lower speed limit of 6000 rpm in the VFD. The constant torque range of a typical 24000 rpm spindle begins only at 6000 rpm:

Also when milling wood, you mill at high speeds. This is because the problem with wood is heat. In this popular jazz bass video, everything is milled at 24,000 rpm: DIY High-End Jazz-Bass-Guitar produced with the CNC machine – Youtube. Of course with a large bit diameter you would choose a lower speed, but you’d have to stay inside the constant torque range of your spindle.

Hmm, interesting point on RPM. I’ll try something higher.

These passes are very light, taking 1/16 or less, and the feed rate is slow at 40 ipm.

There does not appear to be a related change in material density or features.

I’ve checked the bit and collet a gazillion times. I’ve had this issue with my current surfacing bit and with a smaller 1/2" mortising bit with a 1/4" shaft.

Hey cvcolomb,

there can be many causes for loosing steps. EMI is one of them. I noticed the PwnCNC VFD housing is neither made of metal nor is the VFD enclosed into a steel control cabinet, despite the fact that a VFD is a device intended to be installed in a VFD control cabinet (which serves as faraday cage). Also all Onefinity cables are not shielded. In industry, all motor and signal cables of a CNC machine are shielded and grounded to prevent EMI.

Have you searched for Z plunge issue in this forum?

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I had looked at the other posts here about Z axis issues without luck.

I had not found that Support article on testing motors and wires, I’ll try that next, thanks @Aiph5u

EMI for the Win boys! (girls?)

I must have gotten power cords mixed up, between VFD, Masso, dust collection, lights…

The VFD and the Masso were on the same circuit. Isolating those did the trick.

Also valuable info on the RPMs, I’ll keep that in mind, not sure how i overlooked it.