Press tonnage calculator

Tonnage follows cut length, not part size

A 300 × 200 mm bracket can need more press than a 600 × 400 mm panel. The force is set by the length of edge being sheared in one stroke, so a small part with a long internal cut-out and forty holes outranks a large plain blank. Quote-stage mistakes come from judging a part by its outline.

The shear force equation

F (kN) = L × t × τ / 1000

L is the total cut length in mm, t the thickness in mm, τ the ultimate shear strength in MPa (N/mm²). The result is the peak force at the instant of shearing, before stripping and before any forming in the same stroke.

Convert to tonne-force by dividing by 9.807, which is the unit press nameplates use in India.

Shear strength is not tensile strength

Ultimate shear strength sits at roughly 0.7–0.8 of ultimate tensile strength for most sheet grades. Using the tensile value inflates the answer by 25–40% and pushes a part onto a press one frame size too large.

Take the value from the mill certificate when the grade is critical. A 2 mm CRCA coil certified at 380 MPa UTS rather than 440 MPa changes the answer by 14%, which is often the difference between a 63 T and an 80 T press.

What the shear formula leaves out

Stripping force

The stripper has to pull the strip off the punch after the cut. Plan 10–20% of cutting force for material above 1 mm, and up to 30% on thin sheet with a large blank area, where friction and oil hold the strip down.

Forming in the same stroke

A bend, lance or coin station adds force the shear equation does not see. For air bending inside the tool, F (kN) = 1.42 × σb × L × t² / (1000 × V), with σb the tensile strength in MPa, L the bend line length in mm, t the thickness and V the die opening.

Stepped punches

Grinding the punch face at a small step, or staggering punches across stations, spreads the cut over time. Peak force drops by 20–40%, but the energy per stroke does not. On a mechanical press the flywheel energy, not just the tonnage, has to cover the work: roughly F × 0.4 × t.

Selecting a press at 70–80%

No press should run at its nameplate rating. Frame deflection, off-centre loading, clutch and brake capacity, and the next engineering change all live in the headroom.

required rating = total force / 0.75

The table gives the maximum cut perimeter each frame size can shear in 2.0 mm CRCA at 300 MPa, with a 15% stripping allowance and the press held at 75%.

The largest plate the iron worker handles is 30–40 mm, but that is a different process from press blanking and is not covered by this calculation.

Bed size, shut height and stroke

Tonnage is one of four checks. The die footprint has to fit the bolster, the shut height has to suit the tool stack, and the stroke has to clear the part and the feed. A 300 T press with the wrong bed is no use for a long panel die. Ganesh runs mechanical presses from 20 T to 300 T and designs the tooling in house, so the die can be sized to the frame rather than the other way round.

Where the number goes wrong

1. Perimeter counted twice, once for the blank and once for the part.
2. Holes omitted. Two Ø10 holes add 62.8 mm of cut, six add 188.5 mm, a perforated grille adds metres.
3. Tensile strength used as shear strength, inflating the result by a third.
4. Stripping and forming left out, which is exactly where a progressive tool fails.
5. A press chosen at 95–100% of rating, so the first harder coil stalls the flywheel.

A mill tolerance on thickness is worth quoting in the sum. A 2.0 mm coil allowed +0.10 mm is 5% more force on every stroke, and a batch rolled at the hard end of the tensile range adds another 10–14%. The 75% rule absorbs both. A press specified at 100% does not, and the symptom is a tool that runs for a month and then starts marking the strip near bottom dead centre.

What Ganesh runs

Stamping is on mechanical presses from 20 T to 300 T with a 250 T punching press, backed by an in-house tool room that builds press tools, progressive dies, welding fixtures and checking fixtures. Tooling is quoted separately from the piece price, which is why the tonnage conversation belongs at the tooling stage and not after the purchase order.

See the sheet metal stamping capability page, the press tool and die manufacture page, and how to select press tonnage for a sheet metal part. The stamping and press tool guides cover the route decision.

What to send with the enquiry

• Cut perimeter per stroke, including every hole and notch.
• Material grade with a mill-certified shear or tensile value.
• Number of stations and whether any station forms.
• Annual quantity and release pattern, because that decides single-stage or progressive.
• Whether stepped punches are acceptable, which changes press size but not part quality.

Bracket in 2.0 mm CRCA, τ = 300 MPa. The outside contour is 600 mm, there are two Ø10 mm holes, and two 120 mm flanges are formed in the same stroke.

Step 1 — Hole circumference: 2 × π × 10 = 62.8 mm.

Step 2 — Total cut length: 600 + 62.8 = 662.8 mm.

Step 3 — Cutting force: F = 662.8 × 2.0 × 300 / 1000 = 397.7 kN, or 397.7 / 9.807 = 40.6 tonne-force.

Step 4 — Stripping at 15%: 0.15 × 397.7 = 59.7 kN (6.1 tonne-force).

Step 5 — Forming 240 mm of bend line over a 16 mm V die, σb = 400 MPa: F = 1.42 × 400 × 240 × 2.0² / (1000 × 16) = 34.1 kN (3.5 tonne-force).

Step 6 — Total per stroke: 397.7 + 59.7 + 34.1 = 491.5 kN = 50.1 tonne-force.

Step 7 — Press selection at 75%: 50.1 / 0.75 = 66.8 tonne-force. That rules out the 63 T frame. The next size in the range is a 100 T press, which also leaves room for a revision that adds a hole or moves to a harder grade.

If the punches are stepped to spread the cut, peak cutting force drops about 30% to 278 kN, and the total becomes 278 + 59.7 + 34.1 = 371.8 kN (37.9 tonne-force). At 75% that is 50.6 tonne-force, so a 63 T frame becomes viable — but the energy per stroke and the flywheel still have to be checked before the tool runs.