- Upcut = your everyday bit. Pulls chips up, clean bottom edge, handles 80% of general CNC work
- Downcut = top surface matters. Clean top edge, but poor chip evacuation causes heat buildup
- Compression = plywood through-cuts. Clean on both sides, but pricier and requires full-depth passes
- Straight flute = skip for CNC. No chip evacuation direction, poor performance on routers
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. Getting your feeds and speeds right is especially critical with downcut bits because there’s no natural chip clearing. Use our calculator to dial in the right parameters for your specific machine and bit combo.
⚠️ Real story: One CNC machinist had downcut chips get so hot they caught his spoilboard on fire. Packed chips + high RPM + deep slot = genuine fire risk. This is why chip evacuation matters.
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.
Which Bit for Common Projects
| Project | Best Bit | Why |
|---|---|---|
| V-carved signs | V-bit (60° or 90°) + downcut for background clearing | Clean top surface on the show face |
| Plywood boxes / cabinet parts | Compression bit | Clean edges on both sides of the sheet |
| Pocketed serving trays | Upcut endmill | Deep pockets need good chip evacuation |
| Inlays (male & female) | Downcut for female pocket, upcut for male piece | Clean mating surfaces where they’ll be visible |
| 3D relief carvings | Ball nose (upcut) | Chip clearing matters more than edge finish on 3D work — great projects to sell |
| Cutting board outlines | Downcut or compression | Top surface is the show face — keep it clean |
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.
Need specific brand recommendations? Our beginner’s bit guide covers the five endmills that handle 90% of projects.