Troubleshooting a change in z height while cutting

CNC Machines
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I am cutting 1/8" alum plates with a 1/4" amana single flute upcut bit on the HDM. I run at .125 DOC and 55IPM (probably around 65IPM as I increased feedrate to mitigate some chips getting rubbed into cuts) RPM is at 20,000.

Seemed to be cutting fine but probably a bit aggressive. Then the Z height seems to be changing and I am eating into the spoilboard. I thought maybe the spindle or collet was slipping so I stopped, and retightened them and reset the Z height.

Z height started slipping down again and I have marked the collet and spindle so I know they are not slipping - I think it is the actual z motor slipping down.

I guess my question is this - Is my speed and feed so agressive with an upcut single flute bit I am producing enough downward force that it overcomes the holding force of the z stepper motor and pulls the carriage down?

I retrospect I think my RPM is too fast? as I am around 1300 SFM above amanas recommended 1000…

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I run my 1/4" O-flute at 12000 - 15000 RPM.
0.125" seems aggressive, unless it’s adaptive milling with a pretty small stepover.
I usually shoot for a 0.010" x 0.100" chip, with around 0.002" chipload.

If you’re slotting, cutting with the entire width of the tool, then 0.125 seems very aggressive.

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Good to know - my slotting cuts are less aggressive - but I think I overshot the RPM here.

Also probably some evidence by some of chips getting rubbed onto the final pass in the pockets.

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