Burr direction and burr height: what to call out

Why burr belongs on the drawing

A burr is the thin raised lip left where the fracture that separates a part from the strip stops short of the die side. It is a normal outcome of shearing, and no conventional die removes it entirely.

The trouble starts when burr is left implicit. One inspector runs a fingernail along the edge, another measures under a microscope, and a third compares the part with a sample from a different die. Three people, three verdicts, one delivery held. A direction and a number remove most of that argument. They also tell the die designer where to put the punch and how much clearance to grind.

The four zones of a sheared edge

A clean sheared edge is not a single surface. It is four zones stacked through the thickness. At the top, where the punch first touches, the metal bends before it tears. That curved band is the rollover. Below it the metal is sheared under high pressure and the edge is smooth and bright. That is the burnish or shear zone. Then the crack propagates and the surface becomes rough and angled. That is the fracture zone. The burr is the last remnant at the die side.

The proportions move with clearance and material. As an illustration, a mild steel part cut at about 5 per cent clearance per side might show roughly 15 per cent rollover, 35 per cent burnish, 45 per cent fracture and a burr of a few per cent of thickness. Tighten the clearance and the burnish band grows while the burr shrinks. Those percentages are illustrative, but the direction of travel holds for every material.

Which side the burr lands on

In a conventional blanking or piercing operation the punch enters from one side and the die supports the other. Rollover appears on the punch entry side. The burr appears on the die side, because that is where the fracture finally breaks through. So the burr side is fixed by the tool orientation, not by the strip orientation.

In a progressive die the part may be cut in several stages, and some edges may be pierced from above while others are trimmed from below. That produces burrs on opposite faces of the same part. The drawing needs to resolve this, because a burr under a screw head loses clamp load, a burr on a sliding face wears a mating part, and a burr on a contact face raises resistance and can initiate arcing.

Write the requirement against the function. Notes such as burr away from the mating face, burr on the non-bearing side, or burr direction free but height limited to 0.05 mm all work. What does not work is a single arrow with no explanation, because the die builder cannot tell which face the arrow refers to when the part is flipped in the strip.

What actually sets burr height

The largest single influence is clearance. Too much clearance lets the crack start early and run ahead of the punch, so the final break leaves a taller, more ragged lip. Too little clearance raises the punch load, promotes secondary shear and accelerates edge wear. The clearance that gives the best balance depends on material, thickness and the edge quality the part needs.

The second influence is tool condition. A punch with a rounded edge, a chipped corner or a worn die land will produce a taller burr even at the original clearance. So will a die that has lost its alignment or a stripper that no longer holds the strip flat. In production the burr height is therefore a useful wear indicator: a step change in burr usually means the die needs attention, not that the material has changed.

Material matters, but less than most people expect. Ductile materials such as copper and aluminium tend to produce a taller, softer burr. Harder stainless grades produce a thinner, sharper one that is more difficult to remove and more dangerous to handle. Thicker strip produces a proportionally larger burr for the same clearance because the fracture has further to travel.

• Clearance per side: the primary lever on burr height and edge appearance
• Punch and die edge condition: rounding, chipping and wear land
• Die alignment and stripper pressure: strip lift and lateral movement
• Material ductility and thickness: burr shape more than burr height
• Lubrication and slug clearance: galling and secondary shearing

Four ways to write the requirement

There is no single correct burr note. The right one depends on what the edge does in the assembly and how much the part can afford to cost. The table sets out the options in the order most buyers should consider them.

How to measure burr without an argument

A micrometer closed across a burred edge measures the burr plus the part, and it crushes the lip as it does so. That is not a burr measurement. The height is the distance from the parent surface to the tip of the lip, measured perpendicular to the edge, with the part supported so the lip is not flattened.

The practical methods are a profile projector or optical comparator at a stated magnification, a burr gauge with a sharp stylus, or a laser profile scan. Each has a different sensitivity to the root of the burr, so the method belongs in the inspection instruction. Compare like with like, on the same edge type, at the same magnification, with the same datum face.

Sample across the stroke, not just across the batch. Burr varies around the profile of a complex part because the clearance and the punch condition vary. Measure the corners and the narrow webs as well as a long straight edge, and record which face was measured.

When the burr has to come off

If the function cannot tolerate a burr, the choice is to prevent it or remove it. Prevention means tighter clearance, sharper tooling and more frequent sharpening, which trades die cost for edge quality. Removal means a secondary operation: vibratory finishing, brushing, belt linishing or in-die coining. Each has a cost, a cycle time and a risk of rounding edges that need to stay sharp.

Whatever route is chosen, the drawing should say what the edge must look like afterwards, not just that it must be deburred. A note that says deburr all edges invites a rounded corner on a part that needed a crisp one. A note that says remove burr on the contact face only, radius 0.1 mm max elsewhere tells the supplier exactly where to spend the time.