Unprogrammed Z axis down feed

[pbarratt]

I've searched the forums for my problem but can't find any similar ones so here goes.  Every once in a while (usually less than once every several days), my mill does a z axis ramp down while cutting a part.  This has happened with G code that has been used successfully several times before and not altered since.  The same code will work fine again later.  The result is a ruined piece and often a broken cutter.  The error never occurs upwards, always down into the piece.  The DRO value does not change.  The ramp rate appears to be at the specified plunge rate for the cutter.  The ramp will stop on it's own, then may start again further on.

Any help would be appreciated.

[ERP]

If the DRO is not changing, mach doesn't think it's sending pulses to the Z axis.
I'd start by looking at the wiring.

[pbarratt]

Thanks ERP.

I'm thinking it's a noise problem on the step input to the driver.  Probably related to power line conditions as it's very intermittent.  Since the last direction for the Z axis when cutting is down, any noise on the step input big enough to trigger movement is going to result in down motion.  I'll search the forum for advice on power line noise suppression.

The lack of change in the DRO value does seem to vindicate MACH3.

I was kinda hoping someone else out there had a similar problem and a cure.

Peter

[hawk3000]

I had a similar problem. I checked my progamming and it was fine. I then looked at the controller board and found my fan had stopped and the board was overheating. I would check that out.

Jon

[Tweakie.CNC]

I'm thinking it's a noise problem on the step input to the driver.  Probably related to power line conditions as it's very intermittent.  Since the last direction for the Z axis when cutting is down, any noise on the step input big enough to trigger movement is going to result in down motion.  I'll search the forum for advice on power line noise suppression.

Hi Peter,

What you say here makes sense but, depending on your steps per unit and microsteps, you would need hundreds of individual noise pulses on the step pin just to move the Z Axis 1mm. That being the case, that amount of noise would be extremely unlikely to affect just the Z Axis.

My suggestion would be that you check all your wiring thoroughly and if possible try another LPT parallel port cable and if everything checks out OK then suspect the Z Axis driver.

Tweakie.

[pbarratt]

Everyone,

Thank you for your help.  I've solved the problem.

I have a high speed engraving motor attached to the spindle of my mill.  Everything works great and is nice and solid.

While pondering the latest nasty, I looked more closely at the cutter protruding from the spindle chuck.  It seemed to be sticking out further from the chuck than I remembered.  Sure enough, the cutter had 'pulled' its way partly out of the chuck!  Obviously I had not tightened it sufficiently.  I am astonished at how forcefully the cutter clawed it's way down, even making inroads into a metal surface underneath!

I think Homer put it best when he said "Doh!"

[Tweakie.CNC]

 

Pleased that you found the problem.

Tweakie.