Creeping z-axis - a cable screen was grounded at both ends

[Barrie]

Copied from elsewhere:

. . . . . we came to run a program to level the table bed and for some reason the Z axis seems to drop when the spindle motor is running. we put it in one position and started the spindle, you can hear the Z axis and it does move but very slowly, stop the spindle and its ok. do you think this could be pickup from the high frequency spindle motor?
I look forward to your reply.
Kind Regards
Barrie.
UK

[Gerald D]

A creeping Z axis is unfortunately a common problem with CNC routers. In our case, and with many ShopBotters, it is caused by grounding problems. We solved it by running a thick wire ground from frame to frame, all the way back to the control box. See Grounding or Earthing (http://www.mechmate.com/forums/showthread.php?t=4)

The spindle motor also needs its own ground/earth wire to run in a cable with its power wires.

Can you tell us more about your grounding/earthing?

Where is your VFD located? How close to everything else?

A photo inside your control box could help.

[Barrie]

Hi Gerald
Thanks again for the quick reply and the things to look for regarding earthing.
I checked all my earth connections and they are all fine, every earth point gives 0.5 Ohms back to the main earth. I still had the same problem so I wired the spindle motor direct (not going through the cable track, still yhe same I then disconected the screen from the motor (only 1 end connected) and the problem went away I checked the screen to earth with the motor running and had a reading of 36v ac which was being generated from the high frequency from the inverter, every time i touched this point to earth it caused problems to the Z axis. so I have not connected the screen but all the machine and motor are still earth via the main bonding.
Thanks again for your help. we have now leveled the full bed and run several deisigns all without problems.
Kind regards
Barrie

[jhiggins7]

. . . . . . Tying only one end of the screen to ground is a STRONG recommendation of Mike Richards. I don't pretend to understand it, but given Mike's expertise, I certainly follow it.
. . . . . .

[Gerald D]

Thanks for the feedback Barrie and glad to hear that you are on your way again.