Z skips on plunge

Update: I had increased the plunge rate to something like 3x the default and f6or problems.
I changed it back to default and now it’s cutting regularly. Thanks everyone!

(can’t upload the example video)
Cutting 4 simple pockets on MDF, .25 endmill, 2 pockets done. On the 3rd pocket when it plunges the Z skips and doesn’t go any deeper. It makes the path (though not cutting anything because now it’s too high), and when it comes time to plunge again, the Z skips again as soon as it touches the stock and doesn’t go down.

Stopped the project, went back to CC and picked the 2 remaining pockets. Re-centered, re-ran, same thing.

Any idea what just happened?

Loose belt or loose pulley.


Yep, loose belt, loose pulley on the stepper (tighten the wee little step screw), or a really dull endmill and a direct plunge.


Z belt needs to be guitar string-tight. Pulley set screw needs to be on the flat part of the stepper motor shaft. I had problems like that, and what solved it for me was putting the set screw in on the flat, tightening it down and then putting a second set screw in behind the first. You might also check your plunge speed, for the 1/4" endmill, it should be about 1/3 - 1/4 of your regular speed.

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