Z Axis Unpredictable

X-Carve Troubleshooting
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I’m running into frustrating issues with my Shapeoko 2 Z-axis, that have been persistent since I upgraded to a 611 Router. When I use the manual jog function, Z-axis movement is perfect within a couple thousandths. When I carve a design, it’s so unpredictable that each feature has an equal chance of either dragging the bit through the work piece, or missing the work piece entirely.

I just carved this test project:
CNC Test Holes

Holes were designed at depths of .6, .5, .4, .3, .2, .1", with a depth of .060" per pass. What I got was .330", .245", a drag at a depth of .135" when I stopped the cut. The .600 (.330) hole made the first 2 passes were in the air missing the work piece. The .500 (.245) hole made 1 pass in air, then seemed to cut normally, except the Z-axis raised for 1 or 2 passes.

CNC Test Piece
CNC Test Piece2048×1536 1.46 MB

Here’s a video of a 2nd botched attempt where the Z axis hits it’s limit due to losing retraction steps:

Edit: I just noticed that when my Z Axis moves, all 3 axis lights come on. I wonder if my gshield is suspect:

If you’d like a history of my setup for troubleshooting reference:
My machine started as a 500x500 with an ACME screw and NEMA 17 Direct Drive motor with a Dremel which worked well. When I upgraded to the 611 router with mount, I used the new mount, inverted the 17 and added the belt drive. I would have issues where the Z axis seemed to be losing retraction steps, and would end up cutting deeper and deeper after each move. I assumed the NEMA 17 was underpowered so I upgraded to the NEMA 23 from Inventables. The issue seemed to get better, but the Z axis now seems to lose or gain steps in both directions. I can’t tell which. I thought maybe my power supply was limiting, so I upgraded to a 24V 5A power supply. This hasn’t seemed to fix anything. At the same time, I expanded the machine to 500x1000, and mounted it to an optical breadboard (giant metal sheet with tapped holes), which should be immaterial to the issue, but worth mentioning. The Z-axis components are all the same, with the exception of longer leads on the stepper motor. Again, when manually jogged in 1" or .1" increments, I get repeatable moves within the limits of my calipers measuring ability, so I’m really scratching my head here.

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Have you tried adjusting the pot on the Z-Axis? It could be that the pot is set too low and is limiting the driver’s current to the stepper motor. If you are also using an upcut bit then this could also exacerbate the issue due to the bit pulling downward into the material.

Another thing to check, probably first, would be any setscrews that you might have on the pulleys for the stepper motor and lead-screw.

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Brandon Parker

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I used the upcut bit for this test, but I’ve had issues with V bit and Burr bit as well.

I haven’t checked the set screws, I’ll do that in the morning.

I watched a video on setting the pot by sound, so I centered it between the “stalls out” setting and the “thermal protection” setting. I’m not sure if that’s good enough or not. I’d love to know a more deterministic approach to seeing that correctly.

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Also I just edited the original post. I just noticed all 3 LEDs light up when the z Axis moves

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I cannot say that I have seen that behavior before while only moving the Z-Axis …

I would first check that all of your wiring is correct, although I do not suspect anything there.

Next I would try reflashing GRBL onto the Arduino and any specific settings you need and then give it another go. The green LED’s for each axis are driven from the AOUT2 port on the stepper driver chip and barring anything crazy the drivers normally just stop working when they fail (or let out the magic smoke). The green LED ceasing to light up when the axis is commanded to move is normally the size that the axis in question has failed. I can’t see that strange of a failure occurring on the Arduino/GShield setup, but magic does happen.

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Brandon Parker