Getting precise z-depths for carving

Hey guys,

First off I got the new bitsetter, and holy cow has it transformed and streamlined my workflow. I feel so much more confident that I can keep precise z-depths when changing bits, especially since my art projects tend to require a lot of changes throughout the job.

I am having one weird thing though, and maybe you guys can help me figure it out. The bitsetter really does seem to be very precise, especially when I am using regular endmills. However, when I change to a v-bit, I find that I will often need to manually measure and adjust the carving depth in aspire.

For example: I cut a .3 inch pocket, with a .015 finishing pass so I know the depth is nice and accurate. Now I want to carve into that pocket with a 60 degree v bit. If I just carve at a depth of .3, it will often carve just a tiny bit too high.

My workaround is to manually zero the v bit to the new surface using the paper method, look at the z coordinate in Carbide Motion, convert to inches, and then save the file with that new z depth (it’s often something like .307, which is close, but those thousands of an inch can make a big difference with fine details and small lettering. It’s been as high as .1 in the past.)

Now, one thought I had is that maybe it’s because I just installed a new z-belt, and maybe there is some stretching out in these initial runs? It seems like I was getting a much larger variance a couple days ago, which makes me think maybe this is the issue. Or maybe I need to support the center of my work bed because it’s warping as I work? But I’m only carving at depths of .1 - .2 inches, so I can’t imagine I’m putting that much force on the machine.

The cuts are always a little bit deeper than they should be, not shallower, which tells me I’m not losing z-depth, I’m gaining it somehow. Or that the bitsetter is somehow measuring my v bits slightly differently than it does my regular endmills, but that doesn’t really make any sense. Maybe the bitsetter button is tapered a tiny bit, and my v bit isn’t hitting it dead-center?

Anyway, sorry for the long post, just wondering if anyone has had a similar issue.

I have seen some minute differences as well - when it is really important (like a v-carve after a flat area clearance) I have started doing a double check. The way I do this is bring the bit to the Z + 6MM then on the job set it to 1MM movements and lower it 5MM then set it to .1mm and lower it until it grabs the paper. If that takes 10 movements I know its dead on, but often its off by .1 or .2 and the double check helps dial it in further.

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0.007” is pretty small, could be all sorts of things but bed sag comes to mind first. Is your machine an XXL, XL or standard? I could see the endmill while doing your pocket causing a slight flex, that then rebounds a little when you do a bit change and the V bit doesn’t push down as hard? On my XXL I had a lot of bed sag till I did something about it (I think the biggest fix was removing the leveling feet and making a one piece waste board). Just my shot in the dark going by what you’re describing.

Dan

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Yeah I have an XXL, so it’s very possible that there is some sag. My secondary wasteboard is a single piece, but I am still using the leveling feet. Maybe I’ll try to ditch those, re-level my wasteboard, and see if that helps. Thanks for the suggestion!

Is there any disadvantage to getting rid of the feet? I suppose it just makes it that much more important that the surface the machine is on is already nice and level.

I’ve done this too. In one job, the zero height did shift a little bit, along with all the pocket depths. They seemed to compound – it was slightly off at 0, slightly more off at .2, slightly more off at .3.

Then, on a later job, it kept the zero depth, but was a tiny bit off at .2 and .3. Less so than the first run.

So it could be sag. But the weird thing is, it was the same project both times, but the first job (where I saw much more variation) was soft maple, while the second job was white oak. You’d think I’d see more variance with the oak! So maybe it really is a combination of a sagging bed and a new z belt that needed to stretch a little bit.

I built a torsion box table top and set my XXL on that without feet. There can be no flex.

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Is your structural wasteboard mounted underneath the steel frame?

The “structural wasteboard” is at the factory mounting position which is on top of the frame. The frame sits on top of my torsion box top. There are no gaps. (I also have an additional wasteboard for clamping.

Did you shim the unsupported part of the wasteboard? I’m asking because I recently put a structural wasteboard under the frame on a machine I’m piecing together. It was going to be temporary, but it can’t have any sag this way.

I didn’t see the need to support that 14" span between the steel supports. As I go along, I’ll be reevaluating the whole machine.

I have bolts holding the wasteboard to the frame that are sticking out under. Did you make an allowance? I’m still looking at best to do this, probably using 1in of insulation foam that will be sandwiched between my torsion box and the Shapeoko.

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Now that you mention it, I’m sure I created some dimples so the steel sits flat.

I was afraid of the foam changing thickness over time.

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