Manufacturing Process
Piercing
冲孔
Creating holes, slots or openings in stamped parts with punch-and-die tooling.
Process Steps
- Part located in die
- Punch pierces feature
- Burr checked
- Part advanced
What is piercing?
Direct answer: piercing cuts a hole, slot or opening in a stamped part with a punch and die. It is the mirror image of blanking: in blanking the piece that falls through the die is the part, in piercing the strip is the part and the hole is the feature.
What a pierced edge actually looks like
A sheared hole is not a machined hole. Going from the punch side you get a rounded rollover, a smooth burnish band where the material was compressed past yield, a rougher fracture zone where it tore, and a burr on the die side. The proportions are set by clearance:
| Punch-to-die clearance (per side) | Result |
|---|---|
| Too little | Large burnish band, tall burr, accelerated punch wear |
| Correct | Burnish band roughly a third of the thickness, small burr under about 5 % of thickness |
| Too much | Wide fracture zone, ragged edge, rollover increases |
As a starting point the clearance is 5–10 % of the material thickness per side: towards the lower end for mild steel, brass and copper, towards the upper end for stainless steels and thicker material.
Minimum hole size
| Hole diameter | Tooling |
|---|---|
| 1× material thickness or larger | Standard punch |
| 0.7–1× thickness | Standard but fragile; shorter punch life |
| Below 0.7× thickness | Stepped punch, often with a guided or supported design |
A hole smaller than the material thickness is not impossible — a stepped punch handles it — but every step down costs punch life and adds a maintenance interval to the die.
Hole position tolerance
- Same station: ±0.02–0.05 mm, because every punch moves with the same tool in the same stroke.
- Across stations: ±0.10–0.20 mm, because the strip feed pitch error accumulates.
- With pilots: the pilot pin locates the strip from an existing hole before the next station works, which recovers most of that error and is why critical hole positions are usually pierced after a pilot.
Design rules that keep holes round
- Hole to edge: at least 1× thickness; 1.5–2× thickness if the edge has to stay straight.
- Hole to bend: at least 2× thickness plus the bend radius, or the hole distorts as material flows into the bend.
- Slots: avoid slot widths below the material thickness; the web between holes should not be thinner than the material.
- Order of operations: pierce after forming where the hole has to stay round, or accept that forming will move it.
Common problems
- Slug pulling. The cut slug climbs back up with the punch and embosses the strip. Fixed with shear on the punch face, a slug-ejection pin or a clearance change.
- Punch breakage on small holes. Reduced by a stepped punch, better guidance and a shorter punch body.
- Burr above specification. Changes as the punch wears, so the specification belongs in the maintenance interval, not only in the drawing.
- Hole distortion after bending or drawing. A design-rule problem rather than a tooling problem: move the hole or add a relief.
Frequently asked
What is piercing in metal stamping?
Cutting a hole or opening with a punch and die. The strip stays the part and the hole is the feature — the opposite convention to blanking, where the cut-out is the part.
What is the smallest hole a stamping can have?
Practically, around 0.7× the material thickness with a normal punch, and down to about 0.5× with a stepped punch. Below that, drilling or laser cutting becomes the better answer.
Piercing or drilling?
Piercing is effectively free once the die exists and repeats the same hole thousands of times an hour. Drilling is the answer for a hole below the piercing minimum, a hole on a curved or thick section, or a one-off.
Why is my pierced hole out of position after a later operation?
Because material flowed: a bend or a draw close to the hole moves it. Keep holes at least 2× thickness plus the bend radius away from a bend line, or pierce the hole after forming.
Related: blanking · fine blanking · countersinking · tapping · deep drawing