A Brief Primer on CNC Router Bits
An Introduction to Router Bits Commonly Used With A CNC Routing Machine
If you're getting into CNC routing (or even thinking about it), you're probably focused on the amazing projects you'll be able to do because of the incredible precision of CNC machines. That's understandable: Computer numerical control technology can open all kinds of new creative possibilities, whether you're making signs or cutting out project parts. But, as with any other kind of routing, the magic is made at the tip of a bit — maybe several different bits — and you're going to want to have the right cutters in your CNC toolbox.
Bits used for CNC routing typically have two flutes or cutting edges. Some bits are available with more than two flutes, however. Just keep in mind that, in general, bits with fewer flutes remove more material with each cut and require a slower feed rate. Bits with more flutes leave a smoother cut surface and require a faster feed rate to avoid burning.
As with regular routing, perhaps the most important variable when it comes to bits is shank size. You might find a bit that's the perfect profile for the job, but it'll prove useless if it's got a 1/2" shank and your CNC machine is equipped with a compact router that accepts only 1/4" shank bits.
But, beyond that, which bits do you need? The answer to that question is likely to change over time, as your projects evolve.
Here's a look at some of the most widely used CNC router bits and what they can do.
As their name suggests, these bits all have cutting profiles in the shape of a "V." They're available with different "included angles," however, and this is what determines when they're best used. A 90° bit cuts a wider path and so is better suited to cutting larger or shallower letters or details, while the tighter angle on a 60° bit allows for smaller lettering and details, as well as deeper cuts, more contrasting shadows and crisper definition.
These bits cut straight but feature spiral flutes that clear chips up and out of the recess being machined, leaving a flat bottom. They are used for pocketing, cutouts, groove-making and some drilling, so they are pretty fundamental. A 1/4" diameter spiral up-cut bit is a good place to start, but you also might want to get a 1/8" bit for smaller pockets, cutting/grooving items requiring more detail and drilling cribbage board peg holes. Note that, as a general rule, standard straight-cutting bits are not used in CNC machines.
Tapered ball-nose bits are necessary if you want to do bas relief carving or lithophanes. It's a good idea to get 1/4", 1/8" and 1/16" ball-nose bits. If you use them to clear out material from a wider area, they will leave small ridges where cuts overlap, which can create a scalloped visual effect desirable for some projects. If the scalloped ridges are undesirable, they can be eliminated by reducing the step-over of the toolpath. A core-box bit also can be used to create a scalloped effect.
Fly cutters have cutters that are designed to plane sacrificial tables (or "spoilboards") on the beds of CNC machines so they're perfectly flat, as well as to level workpieces prior to CNC machining.