CNC Lithophanes

I tested out making some lithophanes on the CNC about a month ago; only got around to posting this now since I wanted to do some more iteration on my first tests but never really got around to it.

I just had some scrap 1/4" HDPE squares lying around from awhile back that looked suitable for making the lithophanes out of. The source photo was converted to black and white and the toolpath was generated with a trial of MeshCAM. White was mapped to a depth of 0.22 inches and black was mapped to 0.

First roughing was done with a 1/8" square endmill (feedrate of ~50 IPM, sped up in LinuxCNC so I don't know the exact speed; depth of cut of 0.05 in?, don't remember exactly; stepover probably 90% of tool diameter) and the finishing pass was done with a 1/8" ball end mill (don't remember the feedrate/stepover on this one). I felt the level of detail wasn't enough so I bought a 1/16" ball endmill for my next one.

After roughing.

There seems to be some sort of bug with the white region at the top of the photo where the material wasn't removed at all.

After finish pass.

Not bad except for the sky.

For my second attempt, I clipped the white levels to just under pure white which seems to have fixed the problem with the top of the photo from before:

Some strange discontinuities.

For this one, I had to stop after the roughing operation since my Y axis lost steps somewhere in the middle so I never got to use the 1/16" ball end mill.

HDPE isn't ideal since it leaves behind many little strings after cutting which means the picture isn't very clean. I looked online and saw that many others cut out their lithophanes from Corian, which I may want to try next. Another issue is that cutting a lithophane out takes a long time and has many small movements, which makes the stepper motors very hot.