I am in the process of trying to do some v bit work on a board. I used a surfacing bit for my waste board and I used the same bit to surface my work piece. When I run the v bit file, the letters are losing depth as it goes across the x axis. Until the bit isn’t touching the workpiece at all. I can’t figure out what to look at and fix seeing as how I surfaced both boards and the first half of the cut comes out fine. Any ideas?
Try isolating the problem first by moving the x axis and measuring the bit height along its path to see if the issue exists there. If it does try doing the same along your Y axis at XMin and XMax. If you are consistently higher on on side than the other then its simply a matter of squaring your Z axis with some kind of adjustable stop/block.
If air cutting looks good then something is slipping somewhere…
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Confirm your machine can actually get to the depth you are asking it to. I once was bottoming out my Z without realizing it and my Z kept raising up a little higher each time it retracted. It didn’t occur to me that I was using a shorter bit than usual and it couldn’t quite reach the spoil board without bottoming out my machine.
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Check the damn grub screws. Its counterintuitive because the Z is higher than its supposed to be but like in #1 you are essentially bottoming out then rising up on retraction which puts you higher than you expect.
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Check your Z feed and try lowering it to 2mm/sec or so to rule out skipped steps.
There are a few details missing that might help find the culprit here. We know you leveled the spoilboard, and also faced the workpeice. Did you unclamp the workpeice or otherwise touch it after facing? If you did… you shouldn’t.
If the part stayed clamped the whole time, and you did not change anything else, then for some reason your gantry is getting skewed. There may be a few reasons for that.
My first look would be at vertical flex. A facing mill puts a much larger push/pull on the router than an engraver. This is especially true if it was a spiral endmill. That difference in load can cause an offset in z between the bits. Why would that offset not be consistent? I would look for a reason why the gantry may flex vertically more in some areas of the volume than others. In general the mpcnc design will flex more toward the middle of the volume… but a bad setup might change that.
Another more simple cause… your clamping moved. Larger flatter stock can sometimes bow and lift in between clamps and screws. How much it lifts will vary depending on cut loads… could easily produce the result you see.
(Edit) Oh man can’t believe I didn’t mention this one… make sure you bit is not just slipping up in the collet as it cuts! That happens more often than most admit. Likewise double check router mount slippage etc. Easy fixes first…