As someone who has cut a fair bit of UHMW… and plastics more generally… going to add that you should probably climb rather than conventional cut to further reduce the amount of heating friction/rubbing going on in your cuts, especially given how soft the material is.
The alternative suggestions to what people are saying here (go faster) are to reduce spindle speed.
The first principles critical things to balance here are chip load and surface speed.
The faster your surface speed, the more frictional heat you’re generating, the more you get strings instead of chips. You have to limit your feed/speed to respect the surface speed recommendation of the material.
Chip load is limited by the size of your tool, the rigidity of your machine (esp. spindle setup), available torque, and the rigidity of your work holding. The more rigid everything is, and the more torque you have, the more aggressively you can cut without too much vibration/poor cut quality/breaking tools.
The reason people recommend and use O-flute cutters is to increase chip-size and reduce feed rates down to something the machine can keep up with, for the spindle speeds at which it’s optimally running.
- Setting a ~40% stepover instead of 50% to reduce cutter vibration
- Optimize your chip load to be about ~2-4% of your cutter diameter per-tooth on your cutter
- Adjust your feeds/speeds to respect the material. A quick googling shows ~600-1000FPM
- Set your depth of cut for roughing to be ~1:1 to cutter diameter.
- You’re leaving a lot of stock if that’s .254". That’s two roughing passes. Figuring this is a typo, but just in case, you should be looking at about ~10% of the tool diameter left as the height of material remaining for your finishing pass, just as a general guideline.
- If you’re using a bullnose/ball-nose tool consider your step-over widths for finishing passes and what level of “flat” you’re looking for—what’s an acceptable remaining cusp size for the job? Consider the impact of chip-thinning on speeds/feeds, given your effective cutting diameter will be much smaller.
- If you can do helical ramping with your CAM, do that instead of plunging. It helps with chip evacuation.