I've been looking at the problem forensically today and it appears that it's an issue with the Z-axis direction control; rather than gliding down into the cut, it's moving upwards. All ops thereafter are off axis.This would mean that there's a problem with the controller S/W (Mach 4) or the controller board. However, both of these perform perfectly apart from this particular strategy. I have informed the relevant vendors accordingly.In the meantime, I have replaced the 3D parallel strategy with a 3D pocket and I'll run the machine again tomorrow.

Its extremely unlikely that the code itself is causing the issue, its very common however to have problems on stepper-motor/hobby type machines.

I'm using the 3D Parallel strategy to clear out material from a guitar neck model. I zero the Z-axis at the bottom of the material and run the machine.At some point the machine starts cutting fresh air so I stop the process. When I return the bit to XYZ zero, the Z axis has somehow gained 6.1mm from it's original zero position.

The machine executed a perfect 3D parallel cut on the opposite face of the neck, also, I have tried this process four times and the error is exactly the same each time. So no, it's extremely unlikely my machine is skipping steps, the error would be inconsistent.

its actually very common for it to repeat the error of lost steps. There will be one line where it accelerates too fast, the controller is over buffered, or there is a mechanical issue at one node along your linear motion system. It could even be something overheating, and of course that will show up at the same location as it takes the same amount of time for the heat to rise.

2. Looking at the tip of the blade, measure the exposed amount with a ruler at the same angle as the blade. For standard thickness mat board (4 ply) only 1/8" (0.3175cm) of blade should be seen. To adjust, find the screw head recessed below the bottom of the surface of the cutting head near where the blade is coming out.

3. Turning the screw to the right (clockwise) will decrease the blade depth, the left (counterclockwise) will increase the blade depth. Not all matboard requires 1/8" (0.3175cm). Thicker matboard will require more. Always have the blade depth set so that the backing sheet is scored only slightly. This test and adjustment needs to be done periodically as mat board has many variances of thickness and the depth can increase itself over time with regular use.

4. Another way to test your blade depth is to examine your backing sheet after making a cut. The blade tip should just score into the top of the backing sheet. Cutting even half way through the backing sheet is entirely too much blade.