Why does my CNC leave tearout or fuzzy edges?

Work down the list, most likely first

These are ordered by how often they are the answer, not by severity. Check each in turn and stop at the first one that confirms.

1. 1

   Upcut bit lifting the top surface

   Confirm it's this
   The top edge tears and splinters while the underside is cleaner. An upcut's flutes pull material and fibres upward.

   The fix
   Switch to a downcut bit, which presses fibres down for a clean top edge. Accept that a downcut evacuates chips downward, so clear the slot in shallower passes or with air.

2. 2

   Wrong bit for a two-face part (plywood, veneer)

   Confirm it's this
   You need both faces clean - through-cut plywood or veneered board - and a single-direction bit only ever cleans one side.

   The fix
   Use a compression bit: it is upcut at the tip and downcut above, so it draws the bottom edge up-clean and presses the top edge down-clean simultaneously. It is the standard answer for clean plywood.

3. 3

   Dull bit

   Confirm it's this
   Fuzzing has increased over the bit's life and the edge looks rounded. Dull cutters tear rather than sever fibres.

   The fix
   Replace the bit. Sharp edges sever fibres cleanly; dull ones drag and fuzz, especially in softwoods and plywood.

4. 4

   Feeds and speeds off (secondary)

   Confirm it's this
   Bit type is right and the cut is still rough, or the fuzz tracks feed changes.

   The fix
   Tune the chip load - too light a chip fuzzes softwoods. But change the bit type first; feeds are the secondary lever for tearout, not the primary one.

Stopping it coming back

• Choose bit geometry for the faces that must be clean: downcut for a clean top, compression for both.
• Keep bits sharp; fuzz is often just a worn edge.
• Add a light finishing pass to shear off remaining fibres.

Parts this fix may need

The consumables a fix like this usually calls for. Prices are live; we may earn a commission on a purchase, which never affects what we recommend.

A downcut or compression bit

Tearout is a bit-geometry problem before it is anything else: a downcut gives a clean top edge, a compression bit gives clean top and bottom faces on plywood.

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Frequently asked

Downcut vs upcut vs compression - which stops tearout?

Upcut clears chips well but tears the top surface. Downcut gives a clean top edge but evacuates chips poorly, so it needs shallower passes. Compression is upcut at the very tip and downcut above it, giving a clean top and bottom at once - which is why it is the go-to for through-cutting plywood and veneered panels.

Is tearout a feeds-and-speeds problem?

Usually not - it is mostly bit geometry. Fix the bit type first: downcut or compression for clean edges. Feeds and speeds are a secondary factor that can help with fuzzing in softwoods once the geometry is right, but no feed setting turns an upcut bit into a clean-top-edge cut.

Keep going

Diagnostic patterns are distilled from manufacturer documentation and established tooling references; where the community converges on a real-world tell, we state it. Exact feeds and speeds always come from the calculator, never invented here.