[hp2703]

I use 18i-TB Fanuc controlled CNC grinder with absolute motors' pulscoders. The machine has two sides (NC1 and NC2). NC1 controls grinding wheel position (X, Z axis) whereas NC2 controls the loader arm (U, W axis). The loader arm moves left/right (W axis) and up/down (U axis) passing a workpiece to be ground.

After setting loader W axis motor pulscoder zero postion (home) about 7mm from the left mechanical axis end, the machine runs for some time with no trouble. However after a few days I can hear some kind of noise and W axis motor current increases at the left end position. Then I check the axis zero point position vs. mechanical end and it appears that home point shifts itself a little beyond left mechanical end (collision!) about -0.7mm but it should be set about 7mm from the left mechanical end). So it moves about 7.7mm within a few days with no warning or alarm!

What may be wrong? I haven't changed any parameter or program of the machine.
The problem started a few months ago. Before that, the machine had been working correctly.

I replaced W axis pulscoder as well as cable between pulscoder and servo amplifier. Pulscoder cable connectors cleaning didn't help as well. Do you think I should replace W axis motor? I suspect that W axis servo amplifier controll PCB may be also damaged in such case.

At the moment, I'm forced to set zero point of W axis motor pulscoder from time to time when overcurrent (noise) is observed. It's burdensome.

Thx for help in advance.

[Shoprag]

It sounds like the coupling between the motor and ballscrew is slipping.

[hp2703]

Thank you for your advice. I checked the coupling and screwed it tightly to the ballscrew.
So far so good (no slipping).

[neilw20]

Encoder faulty? Tiniest amount of dirt in them plays havoc.
Don't clean it yourself (after being sure that is the problem) unless you really know what you are doing. It is clean room stuff.
Put a mark on the shaft with a marking pen at your zero point, and see what is drifting.

[nautomation]

is the pulsecoder fitted with coupling ?

[hp2703]

Yes, there is an Oldham coupling.