How to choose a CNC router bit for the job
For most hobby work, five bits cover almost everything: a 1/4 inch upcut, a 1/4 inch downcut, a 60 degree V-bit, a 1/8 inch ball nose and a surfacing bit. Add a 1/8 inch upcut for detail and a compression bit if you cut a lot of plywood.
The starter set that actually gets used
Bit sets sold as beginner kits usually contain twenty bits, of which four get used. The honest short list:
| Bit | What it does for you |
|---|---|
| 1/4 inch upcut spiral | The workhorse. Profiles, pockets, roughing, anything deep. |
| 1/4 inch downcut spiral | Clean top edges, shallow pockets, holding small parts down |
| 60 degree V-bit | All lettering and sign work. Transforms what your machine can make. |
| 1/8 inch ball nose | 3D finishing, and rounded decorative grooves |
| Surfacing bit, around 1 inch | Flattening the spoilboard, which makes everything else more accurate |
Then, as the work demands:
- 1/8 inch upcut for smaller detail and tighter inside corners.
- Compression bit if you cut a lot of plywood or melamine parts.
- Tapered ball nose, 1/16 inch tip when you get serious about 3D relief.
- Single flute O-flute if you cut acrylic.
- Keyhole bit the first time you want a sign to hang properly.
Choosing by operation
| Operation | First choice | Why |
|---|---|---|
| Cutting a part out of a sheet | Upcut, or compression for veneered sheet | Deep cut, chips must clear |
| Shallow pocket with a visible top edge | Downcut | Holds the top fibres down |
| Deep pocket | Upcut to rough, downcut for a shallow finish pass | Clearing first, appearance second |
| Lettering and signs | V-bit | Crisp corners that no round tool can produce |
| 3D relief finishing | Ball nose or tapered ball nose | Blends passes rather than stepping |
| 3D roughing | Flat end mill, the biggest that fits the model | Removes bulk fast; a ball nose is slow and wrong for this |
| Flattening a slab or spoilboard | Surfacing bit | Wide flat cut in few passes |
| Inlay | V-bit, or a small flat bit depending on the technique | See inlay techniques |
| Acrylic and plastics | Single flute O-flute | Chips leave before they melt |
| Softening edges | Roundover or chamfer | The profile is the point |
Size: the rule most people get backwards
The instinct is to use a small bit because the design has small details. That is usually the wrong approach, and it costs you time, finish quality and broken tools.
Use the largest bit that fits the smallest feature you need. A bigger bit is more rigid, removes material faster, and cuts more cleanly. Then use a small bit only for the areas where the big one could not reach, with a rest machining or pencil pass, rather than running the whole job with the small tool.
A 1/4 inch bit cannot cut an inside corner sharper than a 1/8 inch radius. If your design needs a sharp inside corner, either design it out with a dogbone or fillet, use a smaller bit for that area, or accept the radius.
How many bits you actually need
Fewer than you will be tempted to buy, and better ones than the cheapest. A good 1/4 inch upcut that stays sharp is worth more than a twenty piece set of soft carbide. Buy the five above properly, use them until you know why you want something else, and let the work tell you what to add.