Sheet-metal punching engine for toolmakers and press shops: cutting force from shear strength and cut perimeter, stripping load, required press tonnage with safety factor, per-side die clearance by material group and thickness band, punch slenderness and minimum-feature checks — every input with selectable universal units and reference-standard values, every report with the full audit trail.
SectorCalc SC-039 computes the force required to punch a hole through sheet or plate, the stripping load that follows the cut, the press tonnage you should specify, and the die clearance that produces a clean fracture with acceptable burr and tool life. The engine works from material shear strength τs, cut perimeter and thickness — the same model used in press-shop estimating and die design handbooks — and adds punch-slenderness and feature-size checks so an undersized punch is caught before it breaks in the die. Results are reference-grade engineering estimates: confirm final tonnage against the press manufacturer's capacity curve and validate clearance on first-article parts.
The punch must shear the material along the entire cut contour. The classical press-working relation is:
Where L is the total cut perimeter, t is material thickness, and τs is the material shear strength. For steels, τs is commonly taken as ≈ 0.8 × ultimate tensile strength; aluminium and copper alloys should use datasheet shear values because the 0.8 rule can over- or under-estimate by 15% depending on temper. SC-039 carries a curated material table and accepts a custom τs with an audit flag.
The equation assumes the punch face is flat and the full perimeter cuts at once. A shear (rake) angle ground on the punch or die reduces peak force — roughly in proportion to how much the effective simultaneous cut length is reduced — but increases side loading on the punch. SC-039 reports the flat-face peak force, which is the conservative basis for press selection.
Perimeter — not hole area — drives force. A 20 mm round hole in 3 mm S235 (τs = 290 MPa) needs π·20·3·290 ≈ 54.7 kN; a 40 mm hole in the same sheet needs twice that, although its area is four times larger. Estimators who scale force by area systematically over-rate small holes and under-rate large ones.
After the cut, the punched material springs back and grips the punch. The stripper plate must pull it free:
When several punches fire in the same press stroke (progressive or cluster die), forces add:
Stripping is frequently forgotten in quick estimates and is the reason presses stall on jobs that "calculated fine". A spring stripper at ks = 0.10 adds 10% to every hit; tight webs or adhesive-coated stock can double that.
The safety factor covers punch-edge wear (a dull edge can raise cutting force 20–30%), material property scatter between coils, and stripper spring preload. SC-039 defaults to SF = 1.25 — standard shop practice. Use 1.15 only with freshly sharpened tooling and certified material; use 1.35 for worn dies or mixed stock. Remember that mechanical presses deliver rated tonnage only near bottom dead centre: check the press tonnage curve if the punch contacts the material well above BDC.
Clearance is the gap between punch and die cutting edges, expressed per side as a percentage of thickness:
Too little clearance: secondary shear, high force, galling, short tool life. Too much: heavy burr, rollover, taper, and slug pulling. The recommended percentage rises with thickness (thicker stock fractures at a steeper angle) and with material hardness. SC-039's automatic mode applies material-group base values with a thickness-band correction; manual mode accepts a direct percentage and validates it against the recommended band.
A punch is a slender column loaded in compression at the moment of shear. Two shop rules catch most failures:
Below smin = t, punching force per unit area exceeds the compressive and buckling capacity of typical tool steels (A2, D2, M2) even at 58–62 HRC — the punch chips or snaps. SC-039 warns when the smallest punch feature falls below 1.5·t and blocks the verdict below 1.0·t. Remedies: guided punch retainer, shorter punch, carbide insert, or process change (laser/waterjet for the small feature, punch the rest).
Automatic-mode per-side clearance values used by SC-039 (engineering screening values from die-design handbooks):
| Material group | t < 1 mm | 1–3 mm | 3–6 mm | > 6 mm |
|---|---|---|---|---|
| Aluminium & soft alloys (5052, 6061, Cu, brass) | 4.0% | 5.0% | 6.0% | 7.0% |
| Mild & structural steel (S235, S355) | 5.0% | 6.0% | 7.5% | 9.0% |
| Stainless & hard stock (304, 316) | 6.0% | 7.5% | 9.0% | 11.0% |
Fine-blanking and shave operations use near-zero clearance by design and are outside this engine's scope. Hardened stock above ~450 MPa yield should be validated with the toolmaker — clearance recommendations diverge between European and US handbook traditions by 1–2 percentage points.
No. Required press capacity is SF × (cutting force + stripping force) × number of simultaneous punches, converted to tons. A 55 kN cut needs roughly 7.8 metric tons of press capacity at SF = 1.25 with a spring stripper — not 5.6 tons as a raw force conversion suggests. Mechanical presses also deliver rated tonnage only near bottom dead centre, so the tonnage curve must be checked for thick material.
Engineering references specify clearance per side as a percentage of thickness (typically 4–12% depending on material and thickness). The die button opening is punch size plus twice the per-side clearance. Confusing per-side with total clearance is a classic first-article failure: a "10% total" shop instruction on 3 mm mild steel yields only 5% per side — acceptable — but a "10% per side" reading of the same note produces heavy burr and slug pulling.
The standard shop limit is punch diameter at least 1.5× thickness for an unguided punch in mild steel. Below that, compressive stress and buckling risk on the punch rise sharply. Guided punch retainers, backed punches, and carbide tooling allow down to about 1.0×t with careful setup. Below 1.0×t, consider laser or waterjet cutting, or drill the hole. Stainless and hard stock are less forgiving than mild steel at the same ratio.
For carbon and stainless steels, shear strength ≈ 0.8 × ultimate tensile strength is the accepted estimating relation. Aluminium and copper alloys vary more with temper — use published shear values where possible (5052-H32 ≈ 140 MPa, 6061-T6 ≈ 205 MPa). When in doubt, enter the higher estimate: press tonnage is a capacity check, and over-sizing the press costs little compared to stalling a job on the shop floor.
A correct cut shows a small rollover, a smooth burnished shear band of about one-third of thickness, then a clean angular fracture with minimal burr. Too little clearance gives a double-shear band, high burr adhesion, and punch edge chipping. Too much clearance gives deep rollover, a tapered fracture, and a heavy ragged burr on the die side. First-article edge inspection against this pattern is the fastest clearance validation on the shop floor.
