Weird z-axis issue

I’m in the process of cutting several identical relief carvings. The first three went fine, no problems. Then, on number four I noticed changes in z height as the carving progressed.

Design worked up in VCarve Pro. Part is 32” wide and 4” tall, just fits in my XL. Relief is a maximum 3/4” deep.

3D roughing toolpath is a raster on the x-axis, .25 doc, .25 inch ballnose. As I said, first three ran fine. Now, about a third of the way thru the job the z depth decreases about 0.1 inch. Then, about 2/3 of the through the job it decreases another 0.1 inch. So, a stair step from the bottom to the top. Fortunately the 3D finish toolpath with .125 ball nose works ok, cleans up all the leftovers.

On the last piece I changed the roughing toolpath to raster on the y-axis, and…got the same stair steps except right to left rather then bottom to top! What the heck?

Hi Griff,

I’m assuming when you are saying the depth decreases, you mean that on subsequent passes, the tool does not dig as deeply into the material. If this is the case, then this is likely due to belt slippage on the Z-axis. If the belt slips a tooth or more, especially when plunging into material, it will throw the Z-axis off and every subsequent cut or plunge will be less deep, as the machine “thinks” it is moving the Z-axis lower than it actually is.

So the first item to check is the tension on the Z-Axis belt. Make sure it is fairly tight.

I had a similar situation with a job that started with a deeper plunge than I probably should have had, and it was making my loose Z-axis belt slip, throwing the rest of the job off (at one point, the mill was carving nothing but air, about 5 millimeters over the surface of the material).

See if the above helps.



I suspect you are correct, it’s the only thing I could think of. Just confused that the first three were ok.

Possibly a bit of stretch in the z belt, have to check that out.


As @chefjuke mentioned it may be belt slippage, but one thing you mentioned brings something to mind. When your Z axis raises on retract could it be hitting the top of the Z axis? Mine did that with a really thick piece of material and it hit just hard enough to cause a perfectly tensioned belt to slip. Like above, the machine “thought” it was one place, but it was somewhere else. If that’s the case you may be able to set the retract height a little lower and solve the issue.


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I cut another, smaller, 3/4” deep relief last evening. Only 4” wide vs 32”.

I also reduced doc from .25” to .15”. No steps, changes in z.

When I have some extra time I’m going to change out the z-belt rather then mess about trying to adjust for stretch. I have a 10 pack of spares as recommended by Sir Will Adams😃. Then try .25” doc again.

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The other thing to check would be the wiring — is it possible that at some point in the movement there’s a loss of connectivity for the Z-axis stepper?

Is there any chance the bit your using is sliding up slightly in the collet? I had this when I ordered 1/8 bits but was sent 3mm, same has happened with 1/4 bits.

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Loose set screw on the pulley?

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The cutter in question measures 0.2495 so I wouldn’t expect it to slip. Good thing to keep in mind though, will check diameters on all others just to be safe.

Could static cause a loss in connectivity?