Realized that there is a fundamental flaw in the setup that completely disqualifies a large portion of the y-axis (long axis). There are a couple bandaid fix options for it, and the best are to just drop the vice onto the gantry plate, reduce the Z axis ballscrew length, or just avoid large swaths of the milling volume. Was able to face the top of a 1x1x3" piece of HDPE within .001 (although that's more a measure of how well I was able to guestimate Z-zero position of the top of the material. The total variation across the part was .000 to .002. It cut well but produced a lot of HDPE dust because I was using a PCB drill instead of an endmill for fear of breaking something more valuable on the first real milling attempt. I was cutting a 30 mm/min and 100 Hz with a 1/8" PCB drill, which corresponds to around 5000RPM. I learned a lot about how to use the MachCNC software. See video.