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verhoevenc

Drifting Axis: Help!

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So this is actually a continuation of an issue I had awhile back (and believed I had fixed). That thread (now being continued on the Shopbot forum as well), can be seen here: http://www.talkshopbot.com/forum/showthread.php?23276-Drifting-Skipping-Axis&p=193636#post193636

However, seeing as how that's purely Shopbot folks, not general CNC'ers, I figured asking around here couldn't hurt.

Background: Awhile back I was milling a neck... got to the last 20 seconds of the last cutting operation and it decided to (as I finally figured out) lose some steps on the Y axis like this:

728DE89C-278F-4632-B24F-B247F5C95192_zps

It was kind of hard to figure out because due to my table size I mill my necks on a diagonal line... so this could have been lost Y steps or jumped X steps.

The best solution I found on the forum was that my dust collection hose (which ran very near my stepper motors) wasn't grounded and might have led to some static discharge messing with the steppers. So I grounded the hose, moved it's location, and all seemed to go well since then.

Enter my lunch break today: I set the replacement neck for the one that the machine destroyed above on the table to radius a gorgeous african blackwood board and head out to get my lunch. I come back and low and behold the machine has lost a bunch of Y steps leading to this:

DEAC1CAD-FB6A-44A0-A43F-311356D217A8_zps

It's really funny that both times this has happened it's been to the same guitar's, attempted, necks... and also when I've finally regained enough trust to not sit and watch it work. It's like it knows, laughs at me, and screws stuff up just to teach me a lesson in hubris. This is frustrating because having a CNC I can't trust to do what it's supposed to do without me watching it really defeats the purpose of having a CNC! Now I'm scared to put anything on it!
It feels like I have Schrodinger's CNC: it has both destroyed and not destroyed my work until I check on it.
Please help!

Chris

PS: As you can see here, I went ahead and reran the fretboard radius path and it worked fine (albeit I should have probably lowered the Z a bit). Makes sense as this path has worked on an identical neck before...

D8F4F7E7-7EDF-49A6-A575-9558D5006E5E_zps

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I'm not familiar with how the Shopbot software works, but for LinuxCNC the timing of all the stepper pulses is handled entirely via the PC. Anything that can interrupt the stream of pulses can cause loss of steps - processes running in the background, virus checkers, system notifications, emails, network activity, screen savers, power management. Might be something to consider?

Static from your dust collector seems dubious. Stepper motors are high current/low voltage devices. Static discharge is high voltage/low current. I would've thought a static discharge big enough to cause loss of steps would be enough to permanently damage the motors or drive circuitry, which seems unlikely. I guess you could test this by running something with the dust collector switched off and see if it loses its marbles again? Or run something with the dusty on and both the Shopbot and dusty earthed to prevent the possibility of static discharge from one to the other?

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I finally talked to a person there... seems the one year my machine was made they used some linear bearings that a few folks had issues with. I've pulled them and tested them one-by-one as suggested and am thinking we may have found the issue. Some seem to be binding, which may exert just enough back-force for some losses.

Chris

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Sorry for the delay in response. Most loss of steps come from 2 things one being resistance/friction such as you describe from binding somewhere or it comes from harmonic resonance in the stepper motor.. HR will occur more likely in smaller steppers. nema 23 and smaller.. I had this problem with mine when I started out. Depending on the microsteps settings these may occur at different speeds and accelerations.or combinations of simultaneous x,y  xz y,z yx xyz moves.  Many of the smaller steppers have a rear shaft. I made dampers to absorb the harmonics and problem solved.

Hope the bearings are you problem Chris.

Mike.

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