A burr is the sharp, raised lip of metal left on the exit (fracture) side of a sheared, pierced, or blanked edge. As the punch drives through the stock, the material first rolls over, then burnishes, then fractures the rest of the way through; the burr is the thin sliver of metal dragged out at the very bottom of the cut where the fracture breaks free. It sits opposite the rollover, so the burr side tells you which way the punch traveled.
Burr height is one of the most useful diagnostic measurements in a stamping shop because it tracks two things at once: tool sharpness and die clearance. As cutting edges dull, the radius on the punch and die grows, the material can no longer shear cleanly, and burr height climbs. Likewise, as die clearance increases beyond the optimum (roughly 5–12% of material thickness per side), the fracture zone grows, rollover increases, and the burr gets taller and more ragged. A practical rule of thumb is that acceptable burr height runs on the order of 5–10% of material thickness, though the exact limit is set by the part print and downstream use.
For example, if a hole that produced a 0.05 mm burr when the tool was freshly ground begins throwing a 0.20 mm burr, that fourfold growth is a clear signal the punch needs sharpening — well before the burr becomes a safety hazard or a fit problem. Many shops formalize this by setting a maximum allowable burr height and using it as the trigger for scheduled tool maintenance, rather than running tooling to failure.
Burrs matter beyond cosmetics. They are a cut hazard for handlers, they interfere with assembly and seating of mating parts, they concentrate stress and can initiate fatigue cracks, and they create coating and plating defects. Common controls include correct clearance, sharp tooling, shaving or skimming operations, and dedicated deburring (tumbling, brushing, or grinding) when a clean edge is specified.
It helps to keep the four cut-edge zones in mind, because the burr is only the last of them. Working from the punch entry side: rollover is the rounded lip where the material first deformed before cutting; the burnish band is the bright, smooth zone where the cut surface rubbed against the tooling; the fracture zone is the rougher, angled region where the metal finally tore; and the burr is the lip left at the exit. A tall burr almost always comes paired with a large rollover and a big fracture zone, which is why burr height is read as a proxy for the whole edge condition. Tightening clearance shifts the edge toward more burnish and a smaller burr; opening clearance does the reverse. On critical edges, this is why the print may specify maximum burr height directly, making it a measured, controlled quality characteristic rather than an afterthought.