Matt Goodman asked:
"Here’s my ask: I’m looking for advice on an appropriate end mill cutter type for use on plastic. My limited experience with milling has been confined to aluminum and brass, and on projects where surface finish wasn’t a driving factor.
I suspect I would need four flute, relatively small diameter mill to reduce bite and the possibility of tearing? Advice on that and other related guidance is greatly welcome."
There were two articles in the hobby press along these lines to heavily kitbash HO scale model hoppers. The first was Brady McGuire's "PRR H25 'Sawtooth' Quad Hopper" in the August 1981 issue of Railroad Model Craftsman, which detailed kitbashing PRR Class H25 quad hoppers by extensively kitbashing HO Athearn offset quad hoppers by milling the sides smooth, cutting away and correcting the orientation of the hoppers, and adding new side posts and end details. The other article is John Munson's "N&W H-4 Modeling: Athearn Twin to N&W Triple Hopper" from the February 1986 issue of Mainline Modeler and reprinted in Volume 3 of The Best of Mainline Modeler Freight Cars. Munson kitbashed these cars using the Athearn composite twin and used the mill to remove material to reduce the width of the panels to get closer to the correct length of the car.