In this Bit Guide:
- The "Fuzzy Edge" Problem explained
- Upcut: The Chip Evacuator
- Downcut: The Clean Finisher
- Compression: The Plywood King
- Cheat Sheet: Which bit for which material?
You just ran your first job on your new CNC. The machine moved perfectly, but the top edge of your plywood looks like it was chewed on by a beaver. What happened?
You probably used an Upcut bit on a material that needed a Downcut.
Router bits aren't just about size; they are about direction. The spiral on the bit acts like an Archimedes screw, either pulling chips UP out of the hole or pushing them DOWN into the material. Here is how to pick the right one.
1. Upcut Bits (Standard Spiral)
"The Excavator"
This bit pulls chips UP and out of the cut (like a drill bit).
How it works
It clears debris instantly. This prevents the bit from overheating and burning the wood.
The Downside: Because it pulls UP, it tears the top surface of plywood (veneer lift). It can also lift your part off the table if your vacuum hold-down is weak.
Best For:
- Plastics / Acrylic (prevents re-welding)
- Aluminum (chips must escape)
- Deep pocketing cuts
2. Downcut Bits
"The Finisher"
This bit pushes chips DOWN into the cut.
How it works
Because it pushes down, it shears the top surface of the wood cleanly. It leaves a razor-sharp edge on top.
The Downside: The chips have nowhere to go. They get packed into the groove. If you run this too fast or too deep, friction builds up, and you will start a fire.
Best For:
- Shallow dados / Grooves
- Thin Plywood (keeps it pushed down)
- Finish passes
3. Compression Bits
"The Best of Both Worlds"
This is a magic bit designed for cabinet makers. The bottom 5mm is Upcut, and the top section is Downcut.
Why it wins:
It pulls chips up from the bottom (clean bottom edge) but pushes down at the top (clean top edge). You get perfect, chip-free cuts on both sides of a plywood sheet.
The Catch: You must cut deeper than the upcut portion on the first pass, or you will get tear-out. These bits are also expensive ($40+).
The Selection Cheat Sheet
| Material / Task |
Recommended Bit |
Why? |
| Plywood (Cutting Out) |
Compression |
Clean top & bottom edges. |
| Aluminum / Metal |
Upcut (Single Flute) |
Must clear chips or bit breaks. |
| Plastic / Acrylic |
Upcut (O-Flute) |
Removes heat, prevents melting. |
| Carving Text |
V-Bit / Downcut |
Prevents fuzzies on letters. |
| Drilling Holes |
Upcut |
Acts like a drill bit. |
Conclusion: Start with these three
You don't need a drawer full of 50 bits. To start, you only need three:
- A 1/4" Compression Bit for cutting plywood shapes.
- A 1/8" Downcut Bit for fine details and pockets.
- A 1/4" Single-Flute Upcut for plastics and aluminum.
Bit Speed Calculator?
Calculating "Feeds and Speeds" is hard. We built a downloadable Excel calculator for standard bits. Grab it from the CNC & Fabrication file repository.
Download Calculator ?