How do you control burr height in stamped parts?

The short answer

What a burr actually is

Every sheared edge has up to four zones, and the burr is the last of them. As the punch enters, the material first rolls over at the top, producing a rounded edge; then it shears cleanly against the die, producing a smooth burnished band; then a crack propagates and the remaining section tears; and finally the torn material at the exit side is pushed out as a thin ridge. That ridge is the burr. Its height depends on where the fracture starts relative to the die edge, which is why burr is fundamentally a clearance problem before it is a tooling-maintenance problem.

The four zones also explain why deburring does not make a conventional blank equivalent to a fine blanked one. Removing the ridge leaves the fracture zone untouched, and that rough, micro-cracked surface is the part that matters for fatigue and for coating adhesion. If the function needs a smooth edge, the process has to change, not just the finishing step.

Clearance is the first lever

For carbon steel, per-side clearance is normally 4 to 8 percent of material thickness, with softer alloys at the tight end. Below 4 percent the material is heavily burnished, the tonnage rises, the die wears faster and a secondary shear band forms at the edge; the burr looks small at first and then grows quickly as the edge dulls. Above 8 to 10 percent the material bends into the die opening before it fractures, so the rollover deepens, the fracture band roughens and the burr is large from the very first hit. Between those limits the zones are balanced and the burr height is predictable, which is what makes it usable as a process indicator. Stainless and high-strength steels sit higher, at 8 to 12 percent, while aluminium and copper run at 5 to 8 percent because they stretch further before tearing.

Tool wear moves the burr

A fresh die and a worn die with the same nominal clearance do not cut the same edge. As the punch and die edges round over, the effective clearance widens, the burnished zone shrinks, the fracture zone grows and the burr climbs. The change is not subtle: a die that produced 0.03 mm burrs when new can produce 0.15 mm after 50,000 hits, and the rise accelerates once the edge radius passes a few hundredths of a millimetre. That is why burr height is the best in-process indicator of tool condition, better than a hit counter, because it responds to what the tool is actually doing rather than to how many strokes it has made. Shops that regrind on a fixed hit count either pull good dies too early or run worn ones too long; shops that regrind on burr growth do neither.

How to write a burr limit on a drawing

"Deburr all edges" is the most expensive sentence in a stamping drawing, because it forces every edge, functional or not, through a finishing operation. What a supplier needs is a number and a direction. ISO 13715 provides a symbol that does this cleanly: a small edge-condition mark with the permitted burr size written beside it, so an edge can be specified as "burr inward, maximum 0.10 mm" or "burr free" individually. That lets the tight limit sit only on the edges that touch a mating part, seal a joint, slide in a connector or get handled, while the rest stay at the general tolerance. It also makes inspection objective, because a burr gauge or an optical comparator at 10 to 20 times magnification can measure a stated number and cannot measure an adjective.

Two practical additions are worth making. State which side the burr is allowed on, since on many parts the burr side is buried in an assembly and the visible side must be clean. And state the measurement method if the limit is tight, because a tactile comparison against a reference set and a cross-section under a microscope will disagree at the hundredth of a millimetre.

Deburring methods and what they cost

When a functional edge genuinely needs to be burr free, the method should match the geometry and the volume. Vibratory or tumble finishing is the cheapest per part and works on external edges and accessible holes, but it rounds edges indiscriminately and cannot reach internal crossings. Electrochemical deburring removes burr precisely from internal edges and cross-holes without touching the rest of the surface, at a higher tooling cost and a per-part cycle that only pays at volume. Thermal deburring puts the parts in a chamber and burns the burr off with a brief, very hot gas pulse; it reaches everywhere, including inaccessible internal passages, but it also affects thin sections and cannot be used on heat-sensitive materials. Robotic abrasive deburring suits large or complex parts and is programmable per edge, but it is a line item in the routing with its own fixture. Hand deburring remains the fallback, and it is the one that varies most between operators.

Control the trend, not the hit count

A workable burr control plan has four elements. Define the limit per edge at the drawing stage, with the direction and the measurement method. Set the clearance correctly for the material at tool design, and record the calculation, because a supplier who cannot show it is guessing. Measure burr on a schedule, at first article and then at defined intervals through the run, and plot it rather than filing it, so the trend is visible before the limit is reached. Pull the die for regrinding when the trend approaches the limit, and re-verify burr on the first parts after the regrind. The single change that makes the biggest difference is replacing the hit counter with the burr measurement as the maintenance trigger; the counter tells you how much the tool has been used, the burr tells you how much life it has left.

See metal stamping for the progressive and compound die work these burr limits apply to, custom metal stamping for parts quoted with an edge condition, and laser cutting for low-volume parts where the burr comes from the cut rather than a die.