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?
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 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 worked for an industrial CNC tool manufacturer that made tools for commercial wood working shops, plastic product manufacturers, and non-ferrous metal applications like aluminum work. Your choice in bit depends heavily on how aggressively you cut, your machine's capability, and your desired efficiency versus quality. There is no single "best" bit for the reasons stated above.
High speed steel is relatively flexible, inexpensive, and available at most big box hardware stores. It is tolerant of lashing, an issue with belt driven machines. Cons are that it dulls faster than carbide, quality varies greatly from manufacturer, and may not be as rigid as carbides.
Carbides: rigid but brittle, stay sharp longer, not tolerant of lashing, and prone to chipping/fracture. Often times can be used with plastics like laminates.
Carbide tipped high speed steel: usually used on bigger tooling but provide a mix of tool flexibility and sharpness.
Straight fluted tools work fine when chip removal is considered. To avoid packing use less aggressive cutting depth, consider multiple passes for roughing and a final pass for finishing, compressed air to assist removing chips in the channel, and consider direction of cut along with grain of material.
When plunging into work, ramp the tool in, that is move down and across at an even rate to make a ramp.
Climb cutting, or having the tool turn in the direction of the cut can lead to chatter, or uneven cuts. Counter-cutting is more stressful on the tool but generally makes for cleaner cuts.
Moisture, grain size, layer thickness of veneers all should be considered when picking a tool.
Generally you want the tool slightly undersized from the final cut width, and do multiple passes, with a final finishing pass. This eliminates climb cutting in the final pass, allowing for a smoother finish.
Flute: double, triple flutes are fine for most wood, any more and your risk dulling the tool prematurely because smaller CNC machines rarely generate cutting pressure significant enough to use higher flute numbers. Feed speed is important in this regard as too aggressive results in ripping instead of cutting and too slow will mearly polish the wood while dulling the tool and could lead to fire. As the wood heats up it will harden which will chip carbides.
Shank length: Keep the tool length as short as practical since a longer shank means more play at the end of the tool and could result in fractured tools. The top of the flute should not be further from the collet any more than the shank is wide.
Finally the balance of the tool isn't important until your tool width increases. A 1/4" tool and it doesn't matter much but a 2" facing tool and it is critical.
I realize this isn't a recommendation of a specific tool but will be a matter of trial and error. I prefer Diablo brand router bits for my 2hp ballscrew driven CNC, but if you have a machine that is meant for fine millwork on small work, there may be better bits. Stay away from Vermont American, these may be branded generically for some hardware store chains, but are essentially at the low end of quality and are ok for drills and manual routers but not CNC.
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