Upcut bits pull chips up — clean bottom, rough top. Downcut bits push chips down — clean top, rough bottom. Compression bits do both — clean on top and bottom.
Use upcut for most cuts. Compression for plywood through-cuts. Downcut for shallow work where the top surface matters.
How Each Type Actually Works
The difference comes down to which way the spiral flutes wind around the bit. Here’s what’s happening inside the cut:
Upcut Bits: The Chip Vacuum
The flutes spiral upward. When spinning, they work like a screw pulling chips up and out of the cut.
What this means for you:
- Chips evacuate fast (no packing, no burning)
- Clean bottom edge (chips pulled away from it)
- Rough top edge (fibers get lifted and torn)
- Workpiece gets pulled upward (matters for thin stock)
This is your everyday workhorse. If you only own one endmill, make it a 1/4” two-flute upcut.
Nearly universal recommendation for a first bit. Handles 80% of general CNC work.
Downcut Bits: The Top Surface Protector
Flip the spiral direction. Downcut flutes push chips down into the material instead of pulling them out.
What this means for you:
- Clean top edge (fibers pressed down, not lifted)
- Rough bottom edge (if you cut all the way through)
- Poor chip evacuation (chips pack at the bottom)
- Workpiece pressed to the table (good for thin materials)
The catch: packed chips create heat. A lot of heat. One CNC machinist had downcut chips get so hot they caught his spoilboard on fire.
Use downcut for shallow cuts only. Keep depth per pass to about 1/4 of the bit diameter.
Downcut is the specialist tool, not the second bit you buy. Most hobbyists reach for it on specific projects — sign engraving, laminate trimming, shallow pockets where the top face is the show face.
Compression Bits: The Plywood Specialist
Compression bits combine both geometries on one tool. The bottom portion is upcut, the top portion is downcut. Chips move toward the center, and both your top and bottom edges come out clean.
What this means for you:
- Clean edges on both top and bottom
- Perfect for sheet goods (plywood, melamine, MDF)
- Must cut at full depth to engage both geometries
- Requires ramping (not plunging straight down)
Compression bits cost 2-3x more than single-direction bits, but if you’re cutting plywood regularly, they’re worth every penny.
One critical rule: Always ramp into the material — never plunge straight down. Plunging packs chips at the transition point between the upcut and downcut sections, causing heat buildup.
“Changed my life” comes up constantly when hobbyists talk about their first compression bit on plywood. The main debate is whether they’re worth the premium for occasional use — consensus is yes for regular plywood work, skip if it’s rare.
Straight Flute: The Budget Option
No spiral — the flutes run parallel to the bit axis. Cheap, but the cut quality and speed penalty make them a poor choice for CNC work. Skip these and start with spiral.
The Decision Cheat Sheet
| Bit Type | Best For | Top Surface | Bottom Surface | Chip Evacuation |
|---|---|---|---|---|
| Upcut | Pockets, mortises, solid wood | Rough | Clean | Excellent |
| Downcut | Shallow pockets, engraving, laminate | Clean | Rough | Poor |
| Compression | Plywood through-cuts, melamine | Clean | Clean | Good |
| Straight | Skip for CNC | — | — | Poor |
We don't write in a vacuum. Here's what we studied, what we trust, and why.
Where we found the spoilboard fire story and the clearest explanation of chip evacuation physics. Goes deeper than anyone on the mechanical “why.” Dense but worth it.
Written by an industrial CNC manufacturer — these people build the machines. Most precise explanation of why plunging kills compression bits.
They sell bits, so take product picks with a grain of salt — but the tearout prevention strategies are genuinely excellent. Helped us build our cheat sheet.
The video that made this click visually. Same material, different bit types, edges filmed up close. Ten minutes well spent.
Where the onion-skin trick and 12K RPM tip came from. Real hobbyists, real results. Shaped our recommendations more than any single article.
Not sure which specific bits to buy? We’re putting together a complete beginner’s bit guide — check back soon.