Hi @Illum-I,
Welcome to the forum!
Interesting usecase. A few “random” thoughts on this:
- what V-bit angle are you using? the larger the angle the more sensitive to minute depth errors the carve is. For this type of cut I would probably use a PCB engraver bit, in the 20-45° range.
- the XXL can potentially have a bit of sag in the center of the wasteboard, so supporting it there could help getting wasteboard deflection out of the picture completely (not that I think it is that, such a shallow engraving produces little downwards force). Have you tried setting up the stock in a corner of the work area ?
- top clamps can have a tendency to warp the material especially if it’s not perfectly flat to begin with. For this usecase I would use tape & glue or double-side tape under the stock, for a very even workholding across the stock surface.
- have you tried probing XYZ on the corner and then re-probing Z only placing the probe in the center of the stock surface? XYZ probing is potentially prone to a very slight tilting of the probe during the Z move, while probing for Z only with the probe completely on top of the stock will prevent that. Also, you would get a Z-zero at a position closer to the cut, so there is variations on the stock thickness, at least you would minimize the effect. You could also setup your project for zeroing in the center, eyeball X0 and Y0, and then use the probe for Z0.
- what you are doing is quite similar (in terms of accuracy requirements) to PCB engraving, which is notoriously sensitive to small Z variations from perfect flatness. Surfacing the wasteboard is a must, but then you are still left with any unevenness of the stock itself (and it is hard to tell by just looking at it). Whenever it is possible, the best solution is to surface the top of the stock…but it does not seem to be applicable here unfortunately. There are advanced techniques like “bed leveling” where the machine will go and probe the surface in various points, create a map of small Z differences, and compensate the loaded G-code commands with that map, to produce a perfect cut depth across the entire stock surface. They usually require a conductive surface though, one trick being to cover the surface with aluminum foil. Another way to probe a surface is to use a mechanical probe such as the one @fenrus used in this thread. As I mentioned these are advanced techniques which require special software (i.e. can’t do that with Carbide Motion), but this is just for you to know that folks who need this kind of precision have developed those techniques to get repeatably accurate engravings.
- I understand you are engraving clear acrylic? have you considered using a spring-loaded diamond-drag bit? they take the Z depth out of the equation completely, due to the spring load mechanism, and can be used to do very accurate engraving (I remember engraving asentence on a coin-sized piece of brass)
- finally, I will note that you are in a “worst case scenario”, doing very shallow cuts on a large (XXL) machine on a material than cannot be surfaced. It may take some or all of the above to get a repeatable, consistent, detailed engraving that you like, but I am confident it can be done.