I built my own small CNC router ( from a kit ) I'm going to be cutting some 1/4" plywood, basically just cutting shapes out of it, and I was wondering, what is the best router bit for simply making shaped cuts through plywood with as little router width as possible .. is there a specific bit for that?
You need a spiral upcut, solid tungsten carbide cutter. Just make a vacuum skirt for the collet area of the router to remove dust and provide cooling.
Here is the one I use: http://www.amanatool.com/products/cnc-router-bits/spiral-compression-cnc-router-bits/solid-carbide-spiral-plunge-cnc-router-bits/46316-solid-carbide-spiral-plunge-1-4-dia-x-1-1-8-x-1-4-inch-shank-up-cut.html
One bit will cut about 800 linear feet flawlessly at 3/4" penetration (You have to get feed pressure and RPM right). After that quality declines linearly.
I think that for what you're describing, a simple 1/4" shank straight bit is what you want. If you use the up-spiral type, it will clear sawdust better. I don't know if that will make a difference with your setup, you'd have to experiment with your CNC, test to see what speeds work best with which bits for different materials.
YOU WANT A STRAIGHT BIT!! do not listen to people who copy paste "up cut bits make the chips come out better" they don't know what they are talking about.. with plywood up-cut bits causes surface tear not a clean cut. You should be using a vac to clear chips if you don't have a built in one use a shop vac+smallest attachment and hold it up near the bit when cutting.
Down cut- clean top and tear out on the bottom. up cut- clean bottom torn out top. compression- clean top and bottom rough middle straight- clean top bottom and middle