Rough or tapered walls
A part that is wider at the bottom than the top, or a pocket that is narrower at the bottom, is tool deflection. A wall with visible pass lines needs a finishing pass. A wall with regular ripples is chatter. All three are improved by the same thing: leave an allowance and take one light pass at full depth.
Cut walls that are rough, show horizontal lines where each depth pass ended, lean rather than standing square, or carry regular ripples.
first
Most likely causes
- Tool deflectionMost common
The tip bends away from the material more than the shank does, so it removes less at depth. An outside part comes out wider at the bottom, a pocket narrower. See rigidity. - No finishing passMost common
Each roughing pass deflects a slightly different amount and leaves a line where it stopped. Nothing came back to clean them up. - ChatterCommon
Regular vertical ripples with a sound to match. See chatter. - Dull bit, or the wrong bit for the grainCommon
Fuzz and tearout on a wall have the same causes they have anywhere else. See fuzzy edges. - Rubbing rather than cuttingOccasional
A burnished, glazed or brown wall is a chipload symptom. See burning. - Z axis not square to the tableOccasional
Every wall leans the same way regardless of load. See squaring and tramming.
Diagnostic steps
- Measure the part at the top and bottom of a wall. Different means deflection or squareness. The same means a finish problem, not a geometry one.
- Cut a light finishing pass over one wall. If it comes out square and smooth, deflection was the whole story.
- Check whether the lean changes direction between an outside cut and a pocket. Deflection flips with the cut; a tilted Z leans the same way on both.
- Look and listen for ripples. Regular vertical ripples with a rattle are chatter and want the stickout and engagement fixes.
| What you see | Cause | Fix |
|---|---|---|
| Wall leans: the part is wider at the bottom on an outside cut, a pocket narrower at the bottom | Tool deflection. The tip bends away from the material and cuts less at depth. | Finishing pass at full depth, less stickout, lighter passes |
| Horizontal lines at each depth pass | Each pass deflected a slightly different amount | Finishing pass at full depth with a small allowance |
| Regular vertical ripples | Chatter | Reduce stickout and engagement, tighten the machine |
| Fuzzy or torn surface | Dull bit, or wrong bit for the grain | See fuzzy edges and tearout |
| Burnished, glazed or brown | Rubbing rather than cutting | See burning |
| Wall not perpendicular to the surface | Z axis not square to the table | See squaring and tramming |
Fixes: the finishing pass, which fixes most of this
Starting point Set the roughing toolpath to leave about 0.02 inches of material on the wall, then run the same profile again at full depth with the allowance set to zero.
It works because the finishing pass removes so little material that the cutting force is tiny, so the tool barely deflects, so the wall it leaves is the wall you asked for. It takes almost no extra time because it is one pass over a small amount of material, and it removes every mark the roughing passes left.
If a finishing pass is not enough
- Reduce stickout. Deflection scales roughly with the cube of it.
- Use a larger diameter bit where the geometry allows. Stiffness rises very fast with diameter.
- Reduce depth per pass so each roughing pass deflects less to begin with.
- Check for machine flex rather than tool flex. See rigidity.
- Climb cut the finishing pass on a machine with no backlash, which usually gives a better surface.
Why it happens
A router bit is a cantilever held at one end. Sideways cutting force bends it, and it bends most at the free end, which is the tip. So the deeper the cut, the further the tip is pushed away from the wall it is supposed to be cutting, and the more material it leaves behind. The roughing passes each deflect by a different amount because each one is loaded differently, which is where the horizontal lines come from. The finishing pass works because it removes so little that there is almost no force to bend the bit with.
Prevention
- Put a finishing pass with an allowance in your default profile and pocket toolpaths, so it is there without deciding each time.
- Minimum stickout, every tool change.
- Use the largest bit the geometry allows. Stiffness climbs very fast with diameter.
- Tram and square the Z once, then re-check after any crash.