Lost Z-Axis By 5/8" In One Movement?

I was cutting this piece and partway through it completely lost the Z, and was ‘zero-ed’ about 5/8" below where I zero-ed it (relative to the bed). The collet was contacting the material, so it was not a case of the bit slipping.

I stopped it on the third burnt hole in the picture and re-zeroed and restarted and it did not happen again, so I do not believe it is an issue with the G-code. Any idea what could have caused this?

Machine : PRO60120
Spindle : 3HP
Bit : 3/8" Compression
Depth Of Cut : 0.25" Per Pass
Feed Rate : 200 in/min
Plunge Rage : 100 in/min
Spindle Speed : 18000 RPM
Material : Spanish Cedar (Softwood)
X/Y : Bottom Left
Z : Machine Surface

Sounds spicy! Is it a real plunge or did you do a helix or ramp in? What are your steppers?

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Here’s another thread on Z-axis issues.

I’ve had this happen a couple of times, very intermittent, which doesn’t help.

If it lost steps, it must have been on a retract. Lost steps from aggressive plunges make cuts shallower. Still, 100 in/min plunge seems a little high.

Did the network cable get bumped?

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It was a real plunge. The height of the upcut portion on the compression bit is ~0.2", so I was trying to get past that to avoid tear-out on the top. The slot width is the same as my bit width (3/8"). I have the AVID NEMA 34 kit.

Did the network cable get bumped?

This is actually a pretty distinct possibility…I’ve been meaning to replace it because the click lock doesn’t work anymore and it can slide out.

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I’ve had this happen several times since owning my cnc. Always in the Z axis, sometimes it goes negative and sometime it goes positive. I’ve never figured out a reason. Mine best guess is that the mach 4 computer has a hiccup and over/under moves the Z axis. Would love to figure out why but it’s so random/rare that I don’t now how to trouble shoot. Let us know if you find the cause.

I spoke with Avid and they said older machine can have a network hiccup. However, my problem was a frayed cable that got pinched going to the motor. Replaced the motor with a new one with a good cable and everything seems to be working fine now.


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you should have checked if you’ve lost steps on your z axis then it would either be software or cables. I always do the same error when a problem occurs it’s like all you thinking about is do the same thing over and see why rather than stopping and troubleshooting what could have happened while the indices are still there.

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Well, everybody does things differently. Myself I would have gone with a smaller end mill and done multiple finishing passes to cover the tear out from a long ramp.

Do check your cables.

Being an aircraft guy myself I have a checklist for turning on the machine, before each cycle start and on shutdown. In there is cable checks and the like. Catch it BEFORE it costs you serious money.

Surprised nobody said to check to the motor/screw coupling. Intermittent slip caused by loose coupling could do this. power it on, and try to turn the motor coupling with pliers. see if it slips?