Belt slipping vs stepper skip?

I’ve got a job I’ve probably run 100 times, but yesterday started having a z-axis problem. The tool jumps up a bit and finishes the job maybe a 16th inch high.

So I’m thinking either a belt slip or a stepper skip right?..

Regarding belt slip: After I stop the job and recheck z-axis zero. It is still perfectly zeroed. This would rule out belt slip wouldn’t it? If the belt slips, wouldn’t it change the z-axis zero point after it happens?

Regarding stepper skip: As I said, I’ve run this job 100 times with no issue until now. I am using a 0.25" ball I had my plunge set at 32 inch/min machining wood. After the failure yesterday, I was thinking that must have been too aggressive and my luck finally ran out, so I backed the plunge down to 15 inch/min. But the same thing happened. Tool jumped a 16th high and finishes the job too high. Then I check z-axis zero, and it’s still perfect.

Any ideas? If my belt was slipping it would change my zero, right?

Thanks for any help…

Have you checked your Z-motor pulley for tightness? A loose pulley might allow the position to slip in one direction and then move back again later causing your zero to appear to be good.

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I did check the pulley. It’s tightened solid, and I replaced the set screws for real screws a while back on recommendation. I can tell it hasn’t moved because it remains on the flat spot on the shaft.

Loose eccentrics? It could be loose enough that the gantry flexes a bit (1/16th of an inch is not a lot), but once the force is gone, in idle situation when you jog to verify zero, the problem might not show?

I just thought of one thing I verified has changed, the sharpness of my ball mill. It may finally be toast.

Could that be the cause? Could simple tool dullness cause stepper skip in the z-axis on plunging? Anyone else seen that?

Guess I was waiting to see an effect on the resulting cut before changing it out, but it’s definitely seen it’s better days.

If the stepper is skipping during the job, would it affect zero after the job or would it be the same?

Yes, endmills aren’t very good at plunge cutting (roughly 1/4th as good as a drill), so a loss of sharpness could easily result in that.

Oh good, and zero wouldn’t be effected right? Just skips a step during the job and cuts high. But once I shut it down and recheck zero, will it still be the same even though it skipped as step during the job?

Zero will be restored when you home the machine again, until then, it’s off by the number of skipped steps/teeth.

Hmm… Oh wait! When I stop the job in Carbide Motion it re-homes automatically after I hit stop. Could that be how I’m not loosing zero but still skipping steps during the job?

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Yes. Losing steps. The belts won’t skip unless they break or come completely loose.

Thanks everyone! As an engineer trying to teach myself to be a machinist, this is not the first lesson I’ve learned the hard way, nor will it be my last I’m sure.


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