Whether you have a normal router, or a cnc router, you’ll still need to choose which bits to use in order to cut your material. There are many types of router bits, but most of them can be put into one of two categories. Up cut bits and down cut bits.
Up cut bits
Up cut bits are designed to move material up and out of the material you’re cutting. The spirals on up cut bits are similar to drill bits, in that they are both designed to remove material from an object. Up cut bits are great if you’re routing holes, or if you need to remove a larger depth of material at a time. Because they are so good at material removal, you can typically get away with more aggressive cutting, especially when making pockets. Ramping into the material may not be necessary. Up cut bits aren’t just good for wood, but they are often used for metalworking projects as well. Unlike wood, metal is not compressible, meaning that it is often far easier to mill metal such as aluminum with an up cut bit rather than a down cut bit.
Up cut bits do have a significant downside, however. Because of how wood behaves, when you use an up cut bit, it can and will damage the surface of the wood immediately adjacent to the cut. It doesn’t matter how good you are, what speed or depth the router is, up cut bits do disrupt the surface of your wood. Below is a picture of some wood I cut with an up cut bit, and a down cut bit. You can see the obvious different in how the surface of the wood is. So if you’re using an upcut bit, make sure that the area won’t be outwardly visible in wherever it will eventually end up in your project. Another significant downside with up cut bits is that it really wants to lift your material. When I first read that, I thought it couldn’t possibly be that bad. How much can a little bit really try to life the wood I’m cutting? The answer is a lot! Especially if you have a CNC router, you need to make sure your work is held extra well if you plan to use an up cut bit. Otherwise, the entire piece of wood may vibrate dramatically if you’re cutting in the center but it is only secured at its four corners.
Down cut bits
Down cut bits are pretty much the opposite of up cut bits. Instead of evacuating material being cut, down cut bits actually force the material downwards, compressing it into the object being cut. Because of this, down cut bits don’t work very well for metal because that isn’t compressible. Down cut bits are used primarily for wood, and due to the nature of the bits, only shallow cuts should be performed with down cut bits. A quarter of an inch is generally okay, but any more and you might start to have difficulties routing. Routing holes with a down cut bit is also more challenging. You’ll need to cut a little, retract, cut a little retract. If you try to route a deep hole in one go with a down cut bit, you might start a fire, or the bit may break, neither of which is a desirable result.
So why would anyone want to use down cut bits? For one thing, the front finish won’t be damaged by a down cut bit, so if you’re making a cut that’ll be visible in the final product, you’ll want to use a down cut bit. Work holding is also a lot easier with down cut bits, as they don’t try to rip the wood upwards. Don’t get me wrong, work holding is still essential, but you can get away with a low more if you’re using a down cut bit.