Size the press force needed to punch a circular hole through sheet metal. Adjust punch diameter, sheet thickness and material shear stress to see the peak punch force, the effect of a punch shear angle, and the total press tonnage (cutting plus stripping) — ready to use for press selection.
Parameters
Punch diameter d
mm
Sheet thickness t
mm
Ultimate shear stress τ
MPa
Mild steel ≈ 300, SUS304 ≈ 450, Al alloy ≈ 180
Shear angle θ (punch slope)
°
0° = flat punch, 30° cuts peak force by ~40 %
Stripping coefficient
Ratio of stripping force to shear force
Results
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Shear perimeter (mm)
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Shear area (mm²)
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Basic punch force (kN)
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Peak force w/ shear angle (kN)
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Total press force (kN)
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Work per stroke (J)
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Press mechanism — punching cycle
The punch descends, shears the sheet, pushes the slug through the die, retracts, and the stripper plate pulls the sheet off the punch on the upstroke. Colour shows peak load magnitude.
τ: ultimate shear stress, d: punch diameter, t: sheet thickness, θ: shear angle. A punch shear angle cuts the peak force by 30-50 % at the cost of slight hole-edge distortion.
Stripping force (k_s ≈ 0.05-0.20) and punching work per stroke W. The press flywheel must store enough energy to deliver W almost instantaneously.
Punching, Blanking and Press Force
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When a press punches a round hole in sheet metal, it does it in one big "thunk". What sets the force needed?
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The baseline is wonderfully simple: shear perimeter × sheet thickness × shear stress. For a round hole the perimeter is π·d. A 50 mm hole in 3 mm mild steel gives π·50·3 ≈ 471 mm² of shear area, and multiplying by mild-steel shear stress (300 MPa) gives ~141 kN — about 14 tons of force. In a real electronics enclosure or appliance panel, one tool punches dozens of holes in one stroke, so the press grows into the hundreds of tons quickly.
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14 tons! That's more than I expected. So for a bigger hole or thicker plate, you'd need an even bigger press?
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Right — a 200 mm hole in 6 mm steel comes out around 1000 kN, over 100 tons. That's beyond what a typical small-shop press can handle. So our forebears invented three "magic tricks" for cutting the peak force. The first is the shear angle: grind a 5-15° slope onto the punch face so the cut progresses from one edge to the other, like scissors. That alone drops the peak by 30-50 %. Push the shear-angle slider on the left and watch the peak number drop.
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Wow, 30° cuts the peak by 40 %! But does that mean the work is also smaller? Getting force for free feels suspicious.
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Great question. The work (energy) stays about the same. A shear angle stretches the peak in time — the area under the force-vs-stroke curve is unchanged. The press benefits are a softer instantaneous shock to the flywheel and a lower required peak tonnage. The down sides are uneven shear (so the hole edge can be slightly distorted) and a more complicated punch regrind. For something like a precision washer where the hole flatness is critical, designers often keep a flat punch and accept the higher peak.
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There is also a "stripping force" in the results. What is that?
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After the cut, the sheet tends to grip the punch and ride up with it. The force needed to pull it off is the stripping force — typically 5-20 % of the cutting force. A spring-loaded stripper plate takes that load, but the energy comes from the press, so the total press force is peak punch + stripping. Thicker sheets and gummy materials (austenitic stainless) push the coefficient up; a 20 % allowance is the safe default. And don't forget the clearance — the gap between punch and die is normally 5-10 % of the sheet thickness. Too tight gives a rough edge and double shearing; too loose gives heavy burrs.
Frequently Asked Questions
Punching pushes a hardened-steel punch through a sheet to produce a hole — the surrounding sheet is the product and the punched-out slug is scrap. Blanking uses the identical mechanism, but the punched-out piece is the wanted part (a washer, a contact, a coin blank). The press-force formula is the same for both: required force equals shear perimeter times sheet thickness times the material's ultimate shear stress. For example, opening fastener holes in a washing-machine side panel is punching, while stamping coins or round washers is blanking.
The basic equation is F = τ · L · t, where τ is the ultimate shear stress of the material (~300 MPa for mild steel, ~450 MPa for SUS304), L is the shear perimeter (π·d for a circle), and t is the sheet thickness. A 50 mm hole in 3 mm mild steel gives F = 300·π·50·3 ≈ 141 kN (~14.4 ton). Add the stripping force (5-20 %) for the total press requirement, then pick a press whose rated tonnage is 1.3-1.5× the calculation as a working safety margin.
Grinding a 5-15° slope onto the punch face (shear angle) makes the cut progress from one edge across, like scissors cutting paper, and drops the peak force by 30-50 %. This tool uses the approximation k = 1 − 0.4·θ/30°, so a 30° angle gives ~40 % reduction. Total work energy stays roughly the same — only the instantaneous peak is reduced, easing both the press tonnage requirement and the flywheel shock load. The trade-off is slightly distorted hole edges and a more complex punch regrind.
The stripping force is the load needed to pull the workpiece off the punch during the upstroke. It is typically 5-20 % of the cutting force; this tool defaults to 20 % (coefficient 0.20). It is taken by the stripper plate, but the energy ultimately comes from the press, so the total press tonnage is calculated as the peak punch force plus the stripping force. Use a higher coefficient (0.15-0.25) when the sheet is thick, the material is gummy (austenitic stainless), or the punch–die clearance is tight.
Real-World Applications
Appliance and electronics enclosure stamping: Washing-machine side panels, outdoor AC covers, PC cases — thin sheet (0.6-2.0 mm) is punched in single strokes that create dozens of screw, ventilation and connector holes simultaneously. The press is typically 100-400 ton class. Cost-driven shops skip the shear angle and use flat punches for maximum cycle speed.
Automotive body stamping: Door panels, fender openings and chassis attachment holes use punching and blanking through 3-6 mm high-strength steel (HSS) and ultra-high-strength steel (UHSS). The ultimate shear stress of HSS hits 600-800 MPa, more than twice that of mild steel, so the press force scales up just as much. Transfer presses and servo presses combine shear angles with staggered multi-stage feeds to keep peak loads manageable while protecting throughput.
Precision electronics — lead frames and connectors: Lead frames, shielding cans and connector terminals are punched continuously from 0.1-0.5 mm stock with pitch accuracy ±5 µm. No shear angle is used; instead a very tight clearance (3-5 %) gives a 100 % burnished shear surface in the so-called fine-blanking process. The press force per stroke is modest, but die precision and tool life dominate the design.
Metal packaging and container blanking: Beverage-can lids (around the pull-tab), button-cell cases and pharmaceutical blister-pack aluminium foils are classic blanking jobs. Per-part cost is dominated by the press operation, so 100s–1000s of strokes per minute on high-speed presses is the norm. Cycle time and tool life matter more than peak tonnage.
Common Misconceptions and Pitfalls
The biggest pitfall is using the ultimate tensile strength (UTS) directly as the shear stress. Punching needs the ultimate shear stress τ_u, which is roughly 0.7-0.8× the ultimate tensile strength σ_u. For mild steel SS400 with σ_u ≈ 400 MPa you should use τ_u ≈ 300-320 MPa. Plugging in the tensile strength instead overstates the press force by 25-30 % and pushes you toward an oversized press. Conversely, estimating from "hardness HRC" of hardened material can under-estimate the shear stress. Take the shear strength from a material data sheet if available, otherwise use 0.75× the tensile strength.
Next, calculating press force without accounting for clearance. The standard punch–die clearance is 5-10 % of sheet thickness. A clearance that is too tight (2-3 %) creates secondary shear and inflates the required force by 15-25 %. Too loose (15 %+), and the cut becomes more like tearing than shearing: heavy burrs, lower force but poor edge quality. This tool's formula assumes a standard clearance of 5-10 %. For tight-clearance fine-blanking, multiply the result by an additional 1.3-1.5× safety factor.
Finally, treating "total press force = punching force alone". Real press selection sums the punching force (peak), the stripping force (5-20 %) and any cushion force (when combined with deep drawing). And with multi-hole tooling, you sum all simultaneous punches. An off-centred force resultant wears out die guides asymmetrically, so good die layouts deliberately balance the punches around the die centre ("centre-of-load punching"). The rule of thumb in the shop is to spec the press at 1.3-1.5× the calculated tonnage and to reserve flywheel energy of 3-5× the calculated work per stroke.
How to Use
Enter punch diameter (mm) in dNum field; use dRange slider for quick iteration between 10–100 mm
Set sheet thickness (mm) in tNum; typical mild steel ranges 0.5–6 mm via tRange
Adjust shear angle (degrees) in shrNum to model tool wear or intentional beveling (0–5° typical)
Read Basic punch force (kN), Peak force with shear angle (kN), and Work per stroke (J) in output
Worked Example
Punch a 25 mm diameter hole through 2 mm mild steel (shear strength 310 MPa, shear angle 2°). Shear perimeter = π × 25 = 78.54 mm. Shear area = 78.54 × 2 = 157.08 mm². Basic punch force = 157.08 × 310 / 1000 = 48.7 kN. With 2° shear angle, peak force increases to approximately 52.4 kN. Work per stroke ≈ 52.4 × 3 mm stroke = 157 J. A 100 ton mechanical press (981 kN) handles this easily; verify 52 kN fits your press capacity and die set.
Practical Notes
Shear angle creates stress concentration; every 1° adds 2–4% to peak force on brittle materials like stainless steel
Include 15–20% safety margin for tool dulling and variability in sheet thickness across coils
For aluminum (shear strength ≈ 150 MPa), force drops to ~24 kN; for hardened steel (≈ 600 MPa), double to ~97 kN
Stroke length affects energy; deeper dies increase work per stroke quadratically—check press tonnage rating