Burr at the breakout edge is rarely a bad bit — it is a mismatched speed-to-feed ratio, and fixing that ratio removes most of the defect without touching tooling.
Why Do Burrs Form at the Router Edge?
A router cuts FR4 by shearing fibers with a spinning fluted bit. When the bit advances faster than the teeth clear material, the fiber tears instead of cuts. That tear is the burr. The spindle speed sets how often a tooth meets the material; the feed rate sets how much material each tooth must remove.
Get the ratio wrong in either direction and the edge degrades.
What Happens When Speed and Feed Are Mismatched?
A very high spindle speed with a slow feed lets the bit dwell in one spot, generating heat that softens the resin and smears it across the edge. A low speed with a fast feed starves the teeth, so they rip the laminate.
The clean cut lives in the middle: enough RPM to keep each tooth taking a light, consistent bite, and enough feed to clear chips before they redeposit.
How Much Does the Router Bit Itself Matter?
A sharp bit with the right flute count and coating holds the sweet spot longer. A dull bit forces you to slow the feed to compensate, which drifts you back toward heat and smear. Replace bits on a schedule tied to board count, not to when the edge already looks rough.
How Do You Tune Speed and Feed in Practice?
- Set spindle speed to the bit manufacturer’s recommended window for your board thickness.
- Raise feed rate until edge fibers cut clean, then back off slightly.
- Watch the breakout corner specifically — that is where burrs appear first.
- Log the winning pair so the next lot starts there.