A ‘bit’ is the technical term for the spinning metal item that actually cuts the plywood. As with drills and routers, there are various bits available for use in a CNC router:
- standard bits - these are similar to the bits you might have seen in electric drills. These type of bits come in various thicknesses, and there are depth, material and speed limitations with each thickness of bit. The one I’ve had used most often is the 6.35mm bit, which seems to handle milling of furniture-grade plywood of 12mm, 18mm and 24mm thicknesses quite well
- V-bits - from the side, these look like arrows. They have a pointy tip and their sides are angled at a certain degree off the vertical. They commonly come in 15 degree, 30 degree and 45 degree variants. They result in v-shaped grooves and can be used to create bevelled edges.
- Chamfer bits - these are similar to V-bits but don't have a point. They have a similarly angled side and can be used to create bevelled edges.
- Fillet bits - these are used to create rounded edges
- Dovetail bits - these are almost the opposite of chamfer and v-bits in that they are wider at the bottom than at the top. I have used the dovetail extensively and I’ll go into more depth about this in future posts.
Because CNC routers use spinning bits to mill, it is generally not possible to have precise square or angular internal corners. It’s like using a round sponge to clean a corner of a room - you won’t be able to reach right into the corner. If you wanted to fit a square piece into an internal corner of a CNC-cut piece, then you’d need to cut further into the corner, as shown in the diagram:
The accuracy you can achieve with CNC routing is quite impressive. It is normally accurate down to 0.1 mm. Yes, a tenth of a millimetre. When I’m drawing in CAD (I’ll write about CAD in my next post) I end up having to make decisions such as: “do I want this slot to be 9.2 or 9.3 mm wide?”. This accuracy is one of the reasons why I enjoy using CNC routing so much; the shape I draw in CAD will look exactly like its real life counterpart.
When I was trying to find a company offering CNC routing services, I really had to ring around. Some companies only have a half-size machine or cater for specialised 3D milling or large-scale fabrication of kitchen doors, etc.
I found that most companies are not geared towards people just submitting CAD files for them to use in milling a specified material. My guess is that they don't want to spend the time educating potential customers in how to properly format CAD files for them to use - they'd rather see it as a custom design job than a pure CNC cutting service.
However, I found a great company in Melbourne called Power To Make, and have used them for all of my CNC routing so far. They have created a useful submission guidelines document and have even bought new dovetail bits for me when theirs broke to the sheer amount of dovetail groves I drew in my CAD files.