Hello Drew
While I was in the R&D shop, (as a research Machinist) we made a lot of mock-ups and etc. from plastics of all types. Thing I learned right away, is that plastic will react, (melt, crack or just explode) if you don’t get the speed and feed correct. This can be harder to do if you are feeding by hand. We used CNC machines. Try the bits you have on a piece of scrap. Too fast an RPM and it will melt, too slow and it will chip. Same applies to your feed rate. We often used WD-40 as a coolant, but I don’t think that’s a good idea if you are using a router. Would be Okay on the drill press. When you drill plastic, don’t let the bit pull you, (when a drill bit nears the bottom of the hole, it will screw it’s way out, rather than cut, which will blow out the hole. With a little practice you can tell by feel and sound when this is about to happen, back off on the feed pressure, be ready to stop if you feel it trying to pull. A “Machinist Hand Book” might help you a little, with the RPMs. Your library should have one. Buy a couple pounds of scrap from your supplier, (ours sell the scrap cutoffs by the pound) and just play with it until you get the feel for it. Best advice anyone can give you is “be patience”, be patience and lastly be patience! Go for It!
Harry

 

Hi Drew, I haven't done anything with PET. Have done some with Lexan, acrylic and whatever cutting boards are made of (senior moment, can't remember which plastic). Harry is right, stuff likes to melt. I think your spiral bit is a good idea but I have also used two flute flush trim straight bits. The straight bits will like to kick back on you if you try to take to large a bite, but 1/8" shouldn't be to bad. 3/8" lexan got a little dicey though.

I keep the rpms turned down all the way, 8000 on my router. Also, keep it moving. When trimming I just take small passes. Trim a little, move the work away from the bit and then take another small pass. I think it gives the bit a chance to cool down.

There are also a lot of plastic retailers on the internet. I have used eStreet plastics. Prices are good but shipping hurts on small quantities, 12x12 and such. I did run into one the other nite that had free shipping on orders over $100 which would work if you need a lot. Hmmm, rats, another senior moment, don't remember which one. Try a Bing or Google search on plastic or, I was looking for UHMW.
You may also want to look at using Lexan instead of acrylic. I don't think it is that much more expensive and will likely mix it up better with the steel balls. :dance3:
Good Luck