Machining plastics with CNC

[Ben Mitchell]

I already ran a seach for this topic, and while there is some info there, I'd like to ask my specific question in the hopes that someone has experience machining plastics and can help me.

First, let me explain what I'm trying to do. I need to machine a ridged surface into a 1 foot square piece of plastic. Depth of the surface is at least 15mm - so the plastic needs to be about 3/4 of an inch to one inch thick. I'm doing this using an old boxford CNC 3 axis router.

I've tried using wood, including regular wood, MDF, and particle board, but all woods warp a lot under either the stress of being machined or the intense heat being produced. I've attempted to control the heat of the motor and the bit by running vacuums into the machine (one that travels with the machine pulling air away from the bit and one on the other side of the machine) and opening side panels.

Since wood doesn't work, I've moved on to our synthetic friend, the plastic. I don't have any experience working with plastics. I've done some homework and read the other thread regarding plastics, and i've also checked out wikipedia and other web sites. I've also consulted some catalogues.

I see three main options:

1. UHMW polyethylene
2. PVC
3. Some other plastic such as ABS

Does anyone have any experience working with these materials in quantity, and would know if they warp and melt and machine poorly? Or, does anyone have any suggestions for a plastic I could use? I'd like to spend less than 200 dollars on 8 square feet of this stuff, and I can't have the pieces warp as they need to mesh with one another with some precision.

Different sources say different things, and I don't really know what I'm looking at when sites give me statistics. I need something that won't melt or make burrs to a large degree.

I can buy PVC and UHMWP relatively easily (both cost about the same, ~20 a square foot)

Any suggestions/ideas/condolences?


Thanks,

Ben


Edit #1 - Forgot to mention, the CNC machine I'm using was pretty slick about a decade a go - so it's a little old. I cannot change the speed of the motor, which is (no joke) a hand router motor someone bolted into the machine.

I'm machining this in two stages - a rough pass and a finishing pass. The rough pass I'm running with a .25" straight bit (2 flute) and the finishing pass I'm running with a .125 inch ball-end router bit (also 2 flute). I can adjust the feed rates, to some extent.