Thanks gentlemen. I played around with this all day trying to find the sweet spot. I’m sure others have had different results as there are so many variables however…
Using the Makita with a 1/16 single flute carbide at the lowest spindle speed (10k) and a feed rate of 30 per minute offered up pretty good results on .125 inch HIPS sheet. I also set it up to cut through on 3 passes. I started with a much lower feed rate of 10 PM and continually increased it from there. The material chips out very nicely using the 1/4 inch end mill supplied with the machine at those same speeds so I’m inclined to think a very low spindle speed is the most important consideration with this material.