CNC bits
Upcut vs Downcut vs Compression Bits: When Each One Wins
Updated July 7, 2026
All three look like twist drills to a beginner, but the flute spiral direction decides where your chips go and which face of the board tears out. Pick wrong and you sand for an hour; pick right and parts come off the machine finished. Here is the whole decision in one page.
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The 30-second version
| Bit | Chips go | Clean face | Weakness | Reach for it when |
|---|---|---|---|---|
| Upcut | Up and out of the cut | Bottom | Fuzzes/tears the top face; can lift thin stock | Slots, deep pockets, aluminum, anything where chip evacuation matters |
| Downcut | Down into the cut | Top | Packs chips in slots; heat builds in deep passes | Through-cuts on plywood tops, shallow pockets, small parts on tape |
| Compression | Up at the tip, down above | Both | Costs 2–3×; needs first pass deeper than the up-cut zone (~1× diameter) | Plywood and melamine through-cuts where both faces show |
Upcut: the default workhorse
An upcut spiral pulls chips up out of the kerf like a drill bit. That makes it the coolest-running, fastest-clearing geometry — the right call for pockets deeper than one bit diameter, slots, hardwood profiles where the show face is the bottom, and any aluminum work. The cost is the top edge: the upward shear lifts wood fibers as it exits, so veneered plywood and MDF top faces come out fuzzy.
The lifting force is real, not theoretical. Thin stock held only with painter's tape and CA glue can get pulled off the bed by an aggressive upcut. If a small part shifted mid-job, suspect the bit before the clamps.
Downcut: clean tops, hot slots
Reverse the spiral and chips get pushed down into the cut. The top face shears cleanly — this is why sign makers and anyone cutting cabinet-grade plywood keeps a 1/4 in downcut in the collet by default. The downward force also presses stock into the spoilboard, which makes downcuts friendlier to tape-and-glue workholding.
The trade-off: in a full-depth slot the chips have nowhere to go, so they re-cut, heat up, and can burn or even weld PLA-like plastics. Keep downcut passes shallow (0.5× diameter is a good habit), slow down in deep slots, and never use one in aluminum.
Compression: the plywood endgame
A compression bit is an upcut tip with a downcut body. In one through-pass, the tip shears the bottom veneer upward while the body shears the top veneer downward — both faces clean. Cabinet shops run these all day for a reason.
Two rules make them work on a hobby machine. First, your first pass must be deeper than the upcut section (usually about one bit diameter) or the upcut zone tears the top face exactly like a plain upcut would. Second, they want to cut at full depth in 1–2 passes, which a 4040-class machine with a 400 W spindle can do in 12 mm ply with a 1/4 in bit at conservative feed. On 18 mm material, a Masuter 3S-class machine handles it; a 3018 does not.
A three-bit chip-direction kit
1/4 in upcut spiral (2-flute, solid carbide)
The everyday roughing and slotting bit. Buy a two-pack — this is the one you will eventually snap.
1/4 in shank, ~1 in cutting length
See options on Amazon →1/4 in downcut spiral
The show-face bit for plywood tops, engraved panels, and anything on tape-and-glue workholding.
1/4 in shank, 2-flute
See options on Amazon →1/4 in compression bit
One clean-both-faces pass through plywood and melamine. Worth the premium the first time you skip edge sanding on a batch of parts.
First pass deeper than upcut zone
See options on Amazon →1/8 in upcut + downcut set
Small-format machines with 1/8 in collets get the same physics at small scale; a mixed set covers detail work and small parts.
See options on Amazon →How this plays with feeds and speeds
Chip direction changes heat management, so it changes feeds. Upcuts tolerate aggressive feeds because chips carry heat away. Downcuts want 10–20% slower feed in deep cuts and shallower depth per pass. Compressions want the opposite of caution: a deep, committed first pass. If a bit burns wood at a feed rate that used to work, it is dull — carbide dulls invisibly and shows up as smoke and brown edges before it ever looks worn.
If you are still assembling your first bit drawer, start with our beginner bit guide for the full starter set, then come back here when plywood edges start bothering you.
Frequently asked questions
Do I really need all three types of spiral bits?
For solid wood only, an upcut alone gets you far — the bottom face against the spoilboard usually hides tear-out. The moment veneered plywood or melamine enters your projects, the downcut earns its slot, and batch plywood work justifies a compression. Buy in that order.
Why does my plywood edge look furry on top even with a new bit?
That's upcut geometry, not dullness — the flutes shear fibers upward as they exit the top face. Switch to a downcut for the finish pass, or run a shallow downcut scoring pass first, then finish with the upcut.
Can I use a compression bit for pockets?
Poorly. A pocket shallower than the upcut tip zone puts only the up-shearing section in the wood, so the surface tears like a plain upcut and chips pack under the downcut body above it. Compressions are through-cut specialists; pocket with an upcut or downcut instead.
Are cheap Amazon bit sets worth it vs Whiteside or Amana?
Import carbide sets cost about a fifth as much and cut noticeably less cleanly for noticeably fewer hours — which makes them exactly right for learning, crash budgets, and MDF. Buy one premium downcut for show faces and burn through cheap upcuts while you learn feeds.
What about tear-out on the bottom face with a downcut?
The downcut pushes fibers down into the spoilboard, so bottom tear-out is usually mild — but on through-cuts it is real. A fresh, flat spoilboard surface backing the cut is the fix; a chewed-up spoilboard leaves fibers unsupported.