770M+ & early 770MX Z-Axis Over travel

The mystery of the Lost Z

So I was minding my business and all of the sudden the Z decides to go nuts and overshoot it’s target.

Well as a reader you probably go yea, I would proceed to change my shorts :poop: , after cursing :face_with_symbols_over_mouth: at the sky for 5 minutes and to be honest you wouldn’t be wrong :wink: .

My reaction however was different, I was quite staring at my part :astonished: wondering what in the world I just witnessed. Needless to say it was on the chamfer tool operation where I first witnessed this and my chamfer tool turned into a drill bit…yea go ahead and laugh :joy: thinking back at the stunned look on my face I’m sure I looked pretty funny.

Cause of the issue

I will spare you all the non-issues and jump right to it the Servo drives are so AWESOME that the older design of coupling couldn’t handle the torque of the new drive. In my experiments I found that the Z only had an issue when traveling in the negative Z not when returning to a G30 and only when going full rapid. My hypothesis which was later confirmed by the support guru’s at Tormach HQ is that the coupling would slip just a little and the motor would try to catch up before finally erroring out. If the velocity was at 50% the issue would go away so basically if the machine was running at stepper speeds no issue.


So after trying to tighten the screws on the coupling and then a second time with LocTite I found that the issue came down to a worn coupling. I proceeded to replace the coupling and while I was at it coated the inside of the mating surfaces with a light coat of BLUE Loctite which is considered removable and should you need to replace the coupling in the future you will thank me.

Video of the process

To document it I decided to record all the experience via video and upload to Rumble.

Please note the purpose of the video wasn’t to make a massive presentation but rather simply to document the process so it is a little crude but shows all the steps taken to troubleshoot and resolve the issue.

#Mill #770M #770MX

This fix has been in operation for over a month now and I can say with confidence it was the correct resolution to the problem.

If you are experiencing this issue the correct item to purchase is listed below:

Hi @Richard_Borek when you say “early 770MXs”, how early is early? I would be interested in knowing if my early 2021 machine is susceptible. Thanks

So the easiest thing to do to verify is bring the head down estop and pull the 4 screws off the coupling cover the next time you are doing maintenance on your machine. The process to check will only take 5-10 minutes and if you have the 4-screw coupling just order yourself one and replace it.

Thanks very much, I shall have a look.

1 Like