Minimum hole size stamping: limits by material and thickness

Minimum hole size stamping limits are set at one material thickness, and that rule prevents more tool failures than any other line on a drawing. A 1 mm hole in 2 mm CRCA is half the thickness, and a punch that thin will buckle, snap or wear a burr within a few thousand strokes unless it is guided and replaced on a schedule.

The rule is not about whether the hole can be cut. A laser will cut a 0.3 mm hole in 2 mm steel without complaint. The rule is about whether a press tool can cut that hole at production speed for a volume that justifies the tool, and the answer depends on the punch, the die wall behind it and the ligament of material around it.

This article sets out the limits by material, the arithmetic behind them, the edge and web distances that keep the part from tearing, and the volume at which a small hole should move off the press altogether.

What are the minimum hole size stamping limits?

The minimum is one material thickness in CRCA, 1.2 times thickness in 304 stainless and 0.8 times thickness in 5052 aluminium, measured as hole diameter against sheet thickness.

Three separate limits sit behind that number:

• Punch stress. The punch carries the whole piercing load on its nose. Stress is 4tτ/d, so it rises as the hole gets smaller. For CRCA at 2 mm with a shear strength of 240 MPa, a 1 mm hole sees 1,920 MPa in the punch, which is close to the working limit of hardened tool steel.
• Punch buckling. A punch is a column. Below about 1.5t the body needs a shoulder and a guided stripper, or it deflects and snaps. This is the limit that breaks tools in practice, because the tonnage calculation says the press is fine.
• Die wall. The die needs material around the hole to resist the stripping and lateral loads. A die wall under about 1.5t cracks, and a cracked die section is a tool repair rather than a regrind.

The three limits move together. A 3 mm hole in 3 mm CRCA is at the 1.0t line: punch stress is 960 MPa, the body is stocky enough to run unguided, and the die wall is adequate. A 1.5 mm hole in the same sheet is at 0.5t, and all three limits are breached at once.

Why stainless is different

304 work hardens as it is sheared, so the load rises through the stroke and the punch sees a harder material at the exit than at the entry. Shear strength is about 310 MPa against 240 MPa for CRCA, which is a 29% higher load on the same punch area.

The practical response is to raise the minimum to 1.2t and to increase clearance. A 1.2 mm hole in 1 mm stainless is at the limit; the same hole in 1 mm CRCA is comfortable. Where a stainless part needs a small hole in volume, the answer is often to pierce it under a guided punch and accept a shorter regrind interval, or to laser the hole and press only the form.

The guided punch that makes a small hole possible

A hole under 1.5t can be pierced if the punch is supported rather than left as a free column. A pedestal or shouldered punch carries the cutting diameter on a body two or three times as thick, and a stripper plate with a close-fitting guide bush holds the tip on centre through the stroke.

The guide clearance matters more than the punch material for dimensional stability. A bush running 0.02 mm over the punch body keeps the tip within a few microns; a bush at 0.1 mm lets the tip walk, and a walking punch makes a bell-mouthed hole that plug gauges will pass and a mating shaft will not.

Three tool-design choices follow from that:

• Shoulder the punch below 1.5t and keep the unsupported cutting length under about 3d, so buckling is no longer the governing failure.
• Guide in the stripper, not only in the punch holder, because the stripper is the last support before the sheet.
• Use a die insert in a holder rather than cutting the die wall into the die block, so a cracked or worn wall is replaced instead of re-cutting the whole die.

A guided punch costs more than a plain one and needs a bush that wears. It is still cheaper than a second operation to drill the hole, and much cheaper than a tool that misses its first PPAP because the smallest hole drifted.

The pierce force arithmetic

Piercing force is pi times diameter times thickness times shear strength. For a 10 mm hole in 2 mm CRCA: 3.1416 × 10 × 2 × 240 = 15,080 N, or 15.1 kN, which is 1.54 tonne-force.

A bracket with forty 10 mm holes pierced in one stroke needs 604 kN, or 61.6 tonne-force, before any stripping force or margin. Add a 30% margin for material variation and tool wear and the requirement is 80 tonne-force, which is 32% of the 250 T punching capacity. That is the point of a large punching press: it is not there to make one hole bigger, it is there to make many holes at once.

The same arithmetic shows why small holes are expensive. Halving the diameter halves the force, but it doubles the stress in the punch and doubles the load per millimetre of cutting edge, so the edge wears faster and the punch is more likely to snap. The tooling cost rises faster than the tonnage falls.

Limits by material and thickness

The table is the working set for pierced holes in a progressive or single-stage tool. Values assume a guided punch where the hole is at the lower end of the range, and a die wall of at least 1.5t.

Hole-to-edge and web distances

The hole-to-edge distance is measured from the edge of the hole to the edge of the part, not from the centre. A 3 mm hole in 2 mm CRCA at the 1.5t rule needs 3 mm of ligament, which puts the hole centre 4.5 mm from the edge.

The ligament has to carry the strip load and hold the die wall. As the punch enters, the material between the hole and the part edge is pushed sideways; if the ligament is under about 1.5t it yields, the edge bulges, and the part fails a flatness or fit check even though the hole is the right size. In stainless the same effect appears as a crack at the edge, because the work-hardened ligament has little ductility left.

The web between two holes follows the same logic. Two holes closer than 2t apart share a ligament that is loaded from both sides, and a 1.5t web is the floor at which the web starts to shear instead of the holes cutting cleanly. Where a drawing needs a cluster of small holes, the cluster is usually cheaper as a laser-cut feature than as a row of punches.

Hole-to-bend distance is a separate rule and belongs to a separate calculation, covered in the hole-to-bend distance guide. The short version is that a hole near a bend distorts because the bend line stretches the material around it, and the clearance depends on the bend radius and the V-opening rather than on the hole diameter alone.

Slots, notches and internal corners

A slot is a hole stretched along one axis, and the same punch rules apply across its width. The minimum slot width is 1.0t, and a slot longer than about five times its width is usually cut with two or three hits along the length so the punch stays stocky.

Internal corners follow the same logic as a hole edge. A sharp 90° corner in a blank or a notch concentrates stress in the die and in the part, and it is where a fatigue crack starts in service. Keep an internal corner radius of at least 0.5t in CRCA and 1.0t in stainless, and radius every notch where the drawing allows it.

The corner radius is also a tool life decision. A die block with a sharp internal corner needs a wire-cut insert that is difficult to regrind; a 1 mm radius at 2 mm thickness costs nothing in function on most brackets and removes a recurring tool failure.

Tolerances, burr and what the inspector checks

A pierced hole holds its diameter to about ±0.05 mm while the punch is fresh, and the position to about ±0.10 mm relative to the tool datum. The tolerance does not hold forever: as the punch wears, the hole grows and the burr rises, and the part drifts out of tolerance before the punch fails.

Three checks catch that drift:

• Burr height at the exit, measured with a profile projector or a burr gauge. Above 0.1 mm the punch needs a regrind, and above 0.2 mm the part needs dressing before coating.
• Hole diameter on the first and last part of each shift, with plug gauges at the go/no-go limits. A hole that has grown 0.08 mm is a hole that will not hold a rivet.
• Edge ligament on the first article after every regrind, because a worn die can pull the ligament and still produce a hole within size.

ISO 2768-1 covers the dimensions the drawing leaves untoleranced, which is usually everything except the hole pattern itself. The hole pattern should carry its own positional tolerance, and the burr limit should be stated rather than assumed, because "deburr as required" means something different to every inspector.

What we run for pierced parts

Piercing runs on mechanical presses from 20 T to 300 T, with 250 T punching capacity for the multi-hole stations that share a tool with forming. The tool room designs the punch, the guided stripper and the die insert, and it builds the checking fixture that proves the hole pattern on the first article.

For a part with holes near the limit, the tool room will say so at quotation rather than after the tool is cut. The options it will offer are a guided punch with a shorter regrind interval, a change to a slot or a larger hole, or moving the feature to the laser and pressing only the form. The last option is often the cheapest at low volume, because it removes a fragile punch from the tool.

Tooling is quoted separately from the piece price, and a tool with several small punches carries a maintenance cost that a tool with one large hole does not. That cost belongs in the comparison between two quotations, which is why the low-volume stamping guide is worth reading before the PO. The press tonnage calculator gives the force for a hole pattern, and the stamping capability page sets out the press range and the tool room behind it.

What to put on the drawing

1. Hole diameter and tolerance, with the minimum called out separately from the nominal.
2. Positional tolerance for the pattern, tied to a datum rather than to another hole.
3. Hole-to-edge and hole-to-hole distances, if they are tighter than the 1.5t and 2t defaults.
4. Burr limit and the burr side, because a burr on a mating face is a fit problem and a burr on a hidden face is not.
5. Material grade and thickness with tolerance, since the hole rule is a multiple of the actual thickness.
6. Which holes are pierced and which are drilled, so the second operation is quoted rather than discovered.
7. Surface finish after piercing, especially on stainless where a punch mark on a visible face needs a different tool finish.

When piercing is the wrong route for a small hole

Piercing is the wrong route when the hole is under 1.0t and the volume is under about 20,000 pieces a year. The tooling and the punch maintenance outweigh the cycle time, and a laser-cut hole in the same pass as the outline is cheaper to first part and cheaper to change.

It is also the wrong route when the hole is a fine screen or a filter pattern with hundreds of sub-millimetre openings. Those are etched, laser cut or made from perforated coil, not pierced one at a time.

And it is the wrong route when the hole is really a locating feature that will be reamed or tapped after plating. A pierced hole is a rough hole; if the drawing needs an H7 bore, the press makes the pilot and the machine shop makes the hole.

Send the drawing with the hole list, the material and the annual quantity through the RFQ route, and the tool room will mark which holes are punchable before the quotation is issued. The rest of the press and tooling library sits under the stamping and press hub, including the blanking, piercing and forming overview that explains where each operation sits in the tool.