I’ve been attempting to carve the topography of Yosemite valley. Part way through the finishing pass, the z axis starts to get out of alignment and is too high for the later portion of the cut. This has happened for nearly every 3D cut that I’ve done. The Z axis motor feels hot. I have the plunge set to 25 in/min and it’s 2-3 hour finishing cut. Is the motor bad or not able to handle this kind of work? I do a rectangular relief for the roughing pass and a zero model boundary relief for the finish pass (so the finish bit doesn’t run down the sides).
thank you for supplying all of the photos. I immediately see the cause of the issue.
The Z axis assembly needs to be lowered. The Z axis is reaching the lower limit of the movement and it begins to loose steps, which throws the z position off. Check out this video I put together to address the issue…
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I can drop the z axis to see if that solves the issue. However, I can reach the spoil board with my shortest v bit, and the long ball nose bit that I’m using here can easily reach past its lowest path in the material (the finishing pass only goes about half way through this 1.5” butcher block.
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Hmm, not sure why your z would be loosing steps upwards then
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Well, it seems to me you’re not losing steps. You have a piece of material that’s 13/4" thick and a tapered ball nose cutter that’s 2 1/4 " down from the collet of your Makita router plus your safety height of travel (I use .2 of an inch). Just your cutter and material add up to 4" of Z travel plus “Z” Safety travel.
Possibly your “Z” is over traveling and topping out when retracking and then some causing your Z to lose steps.
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I lowered the z axis, and dropped the model by .1 inches for the finish pass. Over the course of the cut I lost more than the one tenth of extra depth. Picture shows the cut, begins on the right and ends on the left.
Is there a chance it gets mixed up when going through a steep portion of the profile?
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I’d only expect that if youve modified the stock grbl settings for the z acceleration OR max plunge rate.
With the stock settings they are quite conservative to cause this fault. Ohh and even with that sort of fault, the defacto result is gravity pulls it down.
Hmm. One thing weve seen reported on a small amount of brand new machines is that Z axis coupler which connects the stepper shaft to the lead screw. Some ppl have reported that coming loose, to the point the whole thong falls down. Maybe yours isnt that loose but it could be sometimes not sticking well and slipping 
Also make sure the bit is secur in the collet and not backing itself up into the collet.
I ended up disassembling a significant amount of the z axis (threaded screw, coupling, removing the motor, and taking apart the motor), reassembled everything back, and the current cut appears to be running as it should. I’ll report back tomorrow as I’ll be running the same cut again from scratch.
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i am having the same issue. did you figure out what was causing this?