Benchtop mill CNC drilling steel fails

June 10, 2023

This is a short post on some fails on my first attempts at CNC drilling steel... and what I learned.

My benchtop mill is an RF45 clone with an industrial hobbies CNC kit. It is quite an outdated setup but was used and everything was much less than a new benchtop mill of a similar size.

I was drilling 3/8" mild steel under full CNC control. The job is 6 x 10mm holes. It only has a 2hp spindle motor running with a GS2 VFD.

Spindle stalls

On the first attempt the spindle stalled out. The mill must be run in a reduced gear or it does not have enough power. Make sure the pulley is selected in the controller so the RPM is corrected.

Bit slips in chuck

Do not use a regular drill chuck with a carbide bit. If you listen and watch carefully you can hear a slip and notice the bit pushed up in the chuck about a 1/8" inch.

My drill chuck (ROHM) cannot put enough clamping force on carbide so it slips up into the chuck causing the hole to not be drilled completely through.

The fix for this was putting the bit in an ER40 collet. I did not have problems with a HSS drill only carbide.

Also notice the chips after it slipped are all mangled instead of a clean spiral. Decreasing the pecks would probably help (was pecking at the tool diameter).

Motor driver faults

The drill punches through the hole then continues falling..

This was really confusing initially... but after some testing my explaination is that the current limit of the driver was hit causing it to fault out and temporarily disable the driver. The brake does not automatically engage at the moment on an e-stop so the head dropped under it's own weight.

The servos are driving using Gecko G320's. I have a spare driver setup at my desk and this is what happens when I set the current limit too low and override it, the is motor disabled for 3 seconds (according to the manual).

I increased the current limit for now but need to add a relay to enable the brake automatically and probably a temp sensor on the motor. The Z-axis should have something to counterbalance the head weight.

Maybe someday I will get a real VMC but for now I have to workaround the limits of these benchtop machines.

Hopefully you learned something and don't make the same mistakes! Cheers.