Z Zeroing Failure on Second Plunge

[indraastra]

We've been plagued by various Z zeroing problems but in the last couple of months, a new strain has developed on our Shopbot PRS Standard 96x48.

Here is what happens during the Z zeroing routine (C2) about 25% of the time:

1. the first plunge happens
2. the bit makes contact with the plate
3. Z is set to the thickness of the plate (.120")
4. the router moves up to a safe Z height (1")
5. the second plunge happens
6. the bit stops JUST before making contact or JUST after touching on the plate, but the Shopbot software says "Zeroing failed"!

Sometimes the green light above input #1 is on indicating contact, and sometimes it is off.

Based on the code, one of two things should happen first during the second plunge:

1. The bit reaches its target depth (-.120", or negative the thickness of plate)
2. The plate makes contact with the bit

Neither one of these happens!! If the target depth was reached, the bit would have driven into the plate AND the bed. If the plate had made contact, then execution wouldn't have reached the failure condition in the G-code. Some mysterious thing seems to detect that contact either has been made, or that the Z depth has reached where it was on the first plunge, because it is ALWAYS on the second plunge, and ALWAYS stops just short of the plate yet reports a failure.

It isn't consistent, but it is frequent enough to annoy the heck out of everyone who uses the bot. It seems to be related to the firmware/hardware/software and not any kind of grounding issues, like with other failures we've had, but I could be wrong.

I set the second Z zero plunge rate to be 1/2 the original value, and in the few times I've tried it since, the problem didn't happen at all. I don't know what that would have solved the problem. I've seen some similar experiences shared on the forum, and at least one person reverted to an older ZZero.sbp without any further issue. Has anyone else experienced this?

Thanks!
Vishal

[steve_g]

I have also experienced the "fail" on Z-zeroing but it has always worked on the second attempt. It's hard to trouble shoot something that won't repeat! It has been suggested in other threads that cleaning and tightening all connections involved in the zeroing routine will correct this problem...

SG

[indraastra]

Thanks for the reply, Steve. This has been a big source of frustration.

We have taken some measures to tighten and fix various things related to grounding issues. We've even tried attaching the clip to the bit directly. I don't know how much more direct we could get.

[jerry_stanek]

I have had that also and I ended up doing the UR and resetting the software also after that I will reinstall the firmware that seemed to help it.

[steve_g]

I haven't had a failure since I cleaned the connection at the Z-zero plate itself... Aluminum and other dissimilar metals are notorious for bad electrical connections! I don't know if this solved my infrequent problem or not but here's hoping!

SG

[Bob Eustace]

Clean the plate with scotchbrite and also the mouth of the clamp. This is a common problem. If you are clamping onto a spindle clean that too. I do it weekly and now dont ave tat problem. Others have permanently connected to the spindle to minimise it.

[curtiss]

One other habit to get into is to ALWAYS put the clip away BEFORE lifting the z plate off the table...

Spinning clips do not spin for very long.

[frank134]

I had a problem like this a awhile. It took me some time to figure it. It was the bit I was using. The company I got it from put a hard coat of metal on the end of the bit to extend it life. Sometime it would zero and other it wouldn't. Needless to say I don't used those bit any more. but just a thought.