Troubleshooting · Marks on the workbeginner~3 min read

Tearout and chipping on a CNC: causes and fixes

Short answer

Tearout is a fibre being lifted and split ahead of the cutting edge rather than severed at it. Sharpness, bit type and a light finishing pass fix most of it. Cut direction matters, and no direction is right everywhere on a closed shape.

Symptom

Material pulled or split out of the surface rather than cut, leaving a rough patch or a missing chunk. Common at the top edge of a cut, at the exit side of a profile, around knots, and in figured grain.

Check
first

Is the bit sharp? A dull edge cannot sever a fibre cleanly, so it lifts it instead. This is the single biggest factor and it is also the easiest to test: swap in a bit you know is sharp and run the same cut.

Most likely causes

Dull bitMost common
A sharp edge cuts the fibre. A dull one pushes it until it splits.
Wrong bit for the surface that mattersCommon
An upcut lifts the top fibres by design. If the top face is the visible one, use a downcut. See upcut versus downcut.
Cutting against the grainCommon
The tool lifts fibres rather than pressing them. On a closed shape this is unavoidable somewhere. See grain direction.
Taking the full depth in the final passCommon
A heavy cut has enough force to lift a fibre. A light finishing pass does not.
Difficult materialCommon
Red oak's open pores, plywood veneer, figured grain and end grain all tear more readily by nature.
Cut direction wrong for the situationOccasional
See climb versus conventional. It matters, and it matters less than sharpness.
No support under the cutOccasional
Where the tool exits into air, the last fibres have nothing holding them. A sacrificial backer fixes it.
Feed too fast for the materialLess common
Genuinely possible in brittle material, and much rarer than people assume.

Diagnostic steps

Note where it tears. Top edge only means bit type. Bottom edge only means support or bit type. Specific areas of a shape means grain direction.
Look at the grain in the torn area. Tearout follows the grain, and seeing which way the fibres run tells you which way the tool was lifting.
Test in a different material. If MDF cuts perfectly and oak tears, the problem is the wood and you need a strategy, not a settings change.
Try one light pass over a torn area. If it cleans up, the answer is a finishing pass.

Fixes, in order

Sharp bit. Not negotiable, and it is the first thing to change.
Leave an allowance and take a light finishing pass at full depth. This single habit fixes most tearout on most jobs, because a light cut has little force to lift with.
Match the bit to the face that matters. Downcut for a visible top, compression for both faces of sheet goods.
Add a sacrificial backer where the tool exits the material.
Score first. A shallow first pass severs the top fibres before the main cut arrives.
Change cut direction on the specific edges that are tearing, if your CAM lets you.
Reconsider the material. Fine detail in red oak is fighting the wood. See hardwoods.

Why it happens

Wood is a bundle of long fibres. A sharp edge arriving at the right angle cuts through them. An edge that is dull, or moving in a direction that lifts rather than presses, gets under a fibre and pulls. Once a fibre starts to lift, it splits along its length ahead of the tool, taking material with it that the tool never reached. That is why tearout damage often extends beyond where the cutter actually went.

Prevention

Plan a finishing pass into every job where the edge is visible. It costs almost no time.
Keep a sharp bit reserved for final passes.
Choose material for the job. Close grained wood for fine detail.
Orient the design on the board with the grain in mind, especially for long thin features.