Inconsistent bit dragging across material

Hello all. My latest project was going great. I had one step of a 2 stage carve that attempted to drag across the material but I stopped it in time. I reset the machine and easel. Restarted and everything worked fine. The very last step of cutting out the material it decided to drag across and ruin it. I attempted to stop it but even returning to home it continued to drag. The safety height seemed fine at .15 but is there another reason that this randomly happens? It’s only until I reset everything that it will work properly. It doesn’t help that easel is running extremely slow either.

What model of machine are you running? If it’s the non upgraded pre-2021 xcarve I’d take a look at the z gantry, small belt, v wheels and eccentric nuts - belt in particular in case it is skipping or dropping steps.

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It’s the newer model. I bought in November 2021

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check that coupler below the Nema motor at the screw drive. it comes loose at times, some
Lock-Tite(thread-lock) will help.

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I have had the bit drag across the material a couple times usually after I set the xy0 on a two bit carve. When the carving program goes to the next page the “raise bit button” is not there. I back up and hit the XY0 button again then the next page will have the raise bit button on the very top. I am not sure if that this is what you are experiencing

This could be the Z axis dropping lower and lower due to a physical issue (like the one Ken mentioned, this coupler is accessed only from the backside of the Z directly coupling the stepper to the lead screw)…
OR the bit slipping out of the collet and falling down.
OR it could be that at the rear of your machine the workpiece is higher than the front.
Have you surfaced the wasteboard? Do you surface the workpiece first as well?

Thank you all for the suggestions! It looks like in this particular case the bit did drop from the collet slightly. I did however get this bit tight during this 2 bit stage carve. Why this actually dropped down is still a mystery. when removing the bit i need grab it with a pair of pliers and tap it to get it out. The waste board is surfaced and the material is always planed prior to carves. This is the first time this has happened but it just so happens that this time it was the bit that dropped down out of the collet.

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One thing i see sometimes is the bit getting iserted fully, until it bottoms out and this prevents the collet from lifting as the nut is tightened, so not sure if this could be the issue. Make sure to leave some gap at the back of the bit… Just wanted to share just in case.

And one more thing, if youre not removing the collet after evert few carves its a good idea to remove the collet fully and tap out or blow out with compressed air, sawdust that may become packed in between the slits of the collet… this can prevent it from tightening fully.

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