Blanking clearance and edge quality

What clearance means in a blanking die

Blanking clearance is the gap between the punch edge and the die edge, measured on one side. If a punch of 20.00 mm diameter runs in a die of 20.20 mm, the clearance is 0.10 mm per side, or about 0.20 mm in total. Suppliers quote it both ways, which is why the first question on any die specification should be per side or total.

Clearance is usually expressed as a percentage of strip thickness rather than in millimetres, because the same metal behaves similarly at the same proportion. A 1 mm mild steel part at 5 per cent per side runs 0.05 mm of gap. A 3 mm part of the same steel at the same percentage runs 0.15 mm. The percentage travels with the material; the millimetres do not.

The gap is not uniform in every die. A punch with an inclined face, a small corner radius or a long narrow tongue will deflect under load, so the effective clearance at the tip can differ from the ground number. That is one reason thin webs and sharp corners produce more burr than the rest of the same part.

What clearance does to the edge

Shearing is a controlled fracture. The punch presses into the strip, the metal yields, a crack starts at the punch edge and another at the die edge, and the two cracks meet to separate the part. Clearance controls where those cracks start and whether they meet cleanly.

When the clearance is right, the cracks meet near the centre of the thickness. The edge shows a modest rollover, a long burnish band, a shorter fracture zone and a small burr. When the clearance is too tight, the cracks miss each other and the material between them is sheared a second time. The edge looks bright and square, which people often read as good, but the punch load is higher, the die edges wear faster and the strip can pick up metal.

When the clearance is too open, the cracks start early and run ahead of the punch. The rollover widens, the burnish band shrinks, the fracture zone becomes rough and angled, and the burr grows. The edge also tends to draw away from the intended profile, so the part can be dimensionally correct at the die and still show a taper through the thickness.

Starting points by material

The table is a set of starting points for a conventional blanking die. Treat them as a place to begin a die design, not as a specification to paste onto a drawing. The supplier should confirm the number against the strip, the part profile and the press.

Clearance, punch load and tool life

Punch load rises steeply as clearance closes. The material has to be sheared across a longer distance before the cracks meet, and the press does more work for the same part. That extra work appears as heat at the punch edge, which softens the tool steel and accelerates wear. Over a long run, a die that is 2 per cent tighter than it needs to be can cost more in sharpening downtime than it saves in edge quality.

Opening the clearance reduces load, but only to a point. Once the cracks run ahead of the punch, the edge quality falls and the burr rises. A die ground at 5 per cent per side will not stay at 5 per cent after twenty regrinds unless the die sections are replaced or the punch is re-machined.

Wear changes the geometry in two directions at once. The cutting edge rounds over, which increases the effective clearance and the burr. The wear land on the punch flank reduces the actual gap, which raises load. That is why burr height and punch load are both worth monitoring, and why a maintenance plan built only on stroke count misses the dies that are wearing faster than the count suggests.

When normal clearance is not enough

Fine blanking uses a clearance of roughly 0.5 to 1 per cent per side, a V-ring that clamps the strip around the profile, a counter punch that holds the part and a press with a third action. The result is an edge that is almost entirely burnish, with a small rollover and a very small burr. It suits parts where the edge is a functional surface: gear teeth, pawls, latch plates, contact carriers and parts that must sit flat in a stack.

The process costs more. The tooling is heavier, the press is special, the strip must be flat and the part must be designed for the clamp. It is not a way to rescue a part with a bad profile. It is a way to buy an edge that conventional blanking cannot produce.

Shaving is the middle option. The part is blanked with a normal allowance, then a second pass removes a thin layer from the edge. It gives a straighter, smoother edge and can be run in a progressive die, but it adds a station, a slug and another wear point. For a part with one critical edge, shaving that edge only is often cheaper than fine blanking the whole profile.

Reading the edge in production

The edge is a record of what the die did on the last stroke. These are the patterns to look for.

• Long burnish band, short fracture, small burr: clearance close to the low end, die sharp
• Short burnish band, rough fracture, tall burr: clearance too open or punch edge rounded
• Slug pulled back through the strip: clearance too tight at the punch, or insufficient slug clearance
• Galling or pick-up on the punch face: inadequate lubrication or a soft punch coating

What to put on the drawing and what to ask

Clearance itself is a die parameter, not a drawing dimension. The drawing controls the result: the edge quality, the burr height and the burr side. A note that names the function of the edge is more useful than a number the supplier cannot verify on the part.

Ask the supplier what clearance the die was ground at, per side, and how it is maintained through the regrind cycle. Ask for the strip layout and the number of stations, because a profile that needs four stages to avoid a tall burr will cost more than one that needs two.

For a new part, ask for a trial report that shows the edge at the start of the run and after a representative number of strokes. That single record tells you more about the die than a certificate that says the part was inspected.