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Topics - motopreserve

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General Mach Discussion / Incorrect Z-Axis zero/home position
« on: June 20, 2017, 01:40:33 PM »
Hey folks,

First post here on the forum.  New to Mach3 and had no luck with a search - although I've had great results researching here in the past.  Hoping someone might point me in a direction for checking the issue I'm having.

I am running a DIY built machine with THK linear rails/trucks, ballscrews and Nema 23 motors.  I have measured backlash to be in the vicinity of about .02mm (no compensation applied in Mach3).  The machine is set up with homing switches on the X,Y & Z axis.  When I first start the machine, I home it using the "ref all" button, with all axis traveling to the home switches and backing off correctly.  I then jog to a position and set/zero the work coordinate G53 position (Z-axis done with a touch plate).   

The problem I have is when I jog away from the WCS zero, then hit "Go to Zero" button the Z-axis returns to a location slightly lower than it should (Go to Zero move always includes the Z-axis traveling up to the machine zero first, then back down to the WCS zero).  I narrowed down the issue by doing this repeatedly - each time hitting the "Go to Zero" button - triggering the Z-axis to travel to the machine zero and then back to the WCS zero.  Eventually I noticed that each time it returned to the WCS zero, it was a little lower than the last time.

I tested the issue by measuring the discrepancy of distance traveled at both the bottom (by locating to WCS zero) and top (by locating to machine zero) of the Z-axis with a dial indicator.  I made sure to remove any backlash before beginning to travel up to the machine zero.  It appears to be incrementally off by about .07mm each pass.  By repeatedly locating to the work zero and then MDI G28 Z0, back and forth, the axis is clearly not reaching the same machine zero location at the top, always stopping shy of the previous location that the machine zeroed to during the Homing operation.  So it seems that traveling between the WCS zero and the machine zero is pretty consistently the same distance, but always offset by a bit.  This puts the cutter too low - and even lower each time the machine raises up and back down to the WCS zero.  Hope this makes sense. 

If you need clarification, please let me know.  Any thoughts on what to test next?  Is this a software or hardware issue?

Thanks in advance for any help, tips and/or tricks,

Scott

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