How to select press tonnage for a sheet metal part

How to select press tonnage for a sheet metal part is a four-input calculation: cut perimeter, sheet thickness, shear strength and a stripping allowance. A 2 mm CRCA bracket with 1,100 mm of cut perimeter needs about 62 tonne-force to shear and roughly 75 tonne-force once stripping is added, which puts it on a 100 T press at 75% of rating.

Both errors are expensive. Under-specify the press and the frame deflects, punches chip and the die gains a reputation for burrs that nobody records as a tonnage problem. Over-specify and the part is quoted against a 300 T machine when a 160 T machine would run it at the same rate, which inflates the piece price for the life of the programme.

What follows is the arithmetic, the checks that catch a wrong answer, and the fields to put on the drawing so the tool room does not have to assume anything.

What tonnage does a 2 mm CRCA bracket need?

About 62 tonne-force of shear force, and about 75 tonne-force with the stripping allowance, so plan on a 100 T press as a minimum. The formula is short enough to keep on a shop-floor card:

F (kN) = P (mm) × t (mm) × τ (N/mm²) ÷ 1,000

For the bracket: 1,100 mm × 2 mm × 280 N/mm² ÷ 1,000 = 616 kN. At 9.81 kN per tonne-force, that is 62.8 tonne-force. Multiply by 1.2 for stripping and snap-through and the peak is 75.4 tonne-force.

Two details decide whether that answer is right. First, the perimeter must include every hole, slot and notch as well as the outside profile. Twelve 8 mm holes add 301 mm of cut, which is 27% more force on the same press. Second, shear strength must come from the grade, not from a generic mild-steel figure, because 350 MPa carbon steel and 550 MPa stainless are not the same job on the same die.

The formula in numbers you can reuse

Material certificates usually list tensile strength and yield strength, not shear strength. For cold-rolled mild steel, shear is about 75–80% of tensile. Use 0.8 on CRCA and 0.75 on austenitic stainless, where work hardening runs ahead of the cut. The table converts that into tonne-force per metre of cut, which is how a tool room describes a blanking die.

Read the last two columns as force per metre at a stated thickness. A 3 mm SS 304 genset panel with 2,400 mm (2.4 m) of cut needs 2.4 m × 135 tonne-force per metre, which is 324 tonne-force — more than any single press in the shop will deliver at one station. That is not a reason to abandon stamping. It is the reason the die is laid out across several stations, so that no more than about 100 tonne-force is cut at any one instant, and the panel then runs on a 300 T press at roughly a third of its rating, with margin.

Adding stripping, draw and blank-holder force

A blanking die needs one force. A drawing die needs three, and they add at the bottom of the stroke.

Draw force for a cylindrical cup follows F = π × d × t × Rm × k, where k is a factor of 0.7–0.9 for friction and blank-holder load. Take a 120 mm diameter cup in 1.5 mm CRCA at Rm 350 MPa: π × 120 × 1.5 × 350 = 197.9 kN, multiplied by 0.7 gives 138.6 kN, or 14.1 tonne-force.

Blank-holder force comes from a pressure of 2–3 MPa on the blank area outside the cup. A 220 mm blank for that cup leaves an annulus of π/4 × (220² − 120²) = 26,703 mm². At 2.5 MPa that is 66.8 kN, or 6.8 tonne-force.

The total is 21 tonne-force, and the press choice falls out of it. A 20 T press at 105% of rating cannot do it. A 40 T press runs at 52%. A 63 T press leaves space for a second draw or a thicker blank without buying a new frame.

How to select press tonnage for a sheet metal part: four checks

Tonnage on its own is a single number against a single limit. Four checks sit around it, and a quotation that skips them usually produces a die that cannot be run at rate.

• Simultaneous cut. Add the perimeters of every station that cuts at the same crank angle. Progressive dies with a common cut line often peak at one station group, not across the whole strip.
• The 80% line. Plan repeat production at or below 80% of rating. The margin absorbs thickness tolerance, a grade substitution and punch wear that raises stripping force.
• Shut height and die height. A 300 T press with a tall bolster may not close on a shallow die, and a small press may not open far enough for the part to clear.
• Bed, bolster and cushion. Blank-holder pressure needs a cushion or a nitrogen spring. A die that needs 6.8 tonne-force of hold-down cannot run on a press with a 3 tonne-force cushion.

Feed length, coil width and scrap removal come next. None of them affect the tonnage number, and all of them decide whether the die runs or sits.

A worked progressive-die check

Take a 1.6 mm CRCA bracket on a four-station progressive die. Station 1 pierces 620 mm of perimeter, station 2 notches 340 mm, station 3 forms without cutting and station 4 trims 900 mm.

If stations 1 and 2 cut together, the simultaneous perimeter is 960 mm. Force is 1.6 × 280 × 960 ÷ 1,000 = 430 kN, or 43.9 tonne-force. The trim at a separate crank angle is 900 × 1.6 × 280 ÷ 1,000 = 403 kN, or 41.1 tonne-force. Add 20% for stripping to the larger figure and then add the forming station, which needs roughly 8 tonne-force for a 12 mm form height on this part. Total peak: about 61 tonne-force, which is 61% of a 100 T press and a comfortable route.

Move the trim onto the same cut line and the simultaneous perimeter becomes 1,860 mm. Force rises to 833 kN, or 85 tonne-force, and with stripping the die needs 102 tonne-force from a 100 T press. The part does not change. The tool layout does, and so does the press. That is why the station split belongs in the tonnage discussion rather than in a separate tool-design meeting.

How thickness tolerance and temper move the number

The tonnage formula uses nominal thickness and a handbook shear strength. Production uses neither.

A 2 mm CRCA sheet to IS 513 carries a thickness tolerance of roughly ±0.10 mm, so the real cut can be 5% heavier than nominal. A coil at the top of the band in a harder temper can add another 10% to shear strength. The two effects compound: 57 tonne-force per metre at nominal becomes 66 tonne-force per metre before any allowance. On a part already running at 90% of rating, that is a stalled press and a broken punch.

This is the practical reason for the 80% line. It is not conservatism. It is the headroom that keeps a die running when the next coil arrives 0.08 mm thicker and 20 MPa harder than the one used at tryout.

What happens when a die runs above its tonnage

Excess load does not usually produce a dramatic failure. It produces a slow one. The frame and ram deflect, the punch enters the die at an angle, and clearance closes on one side of the punch and opens on the other. The result is a burr on one edge, a chipped punch corner after a few thousand strokes, and a part that measures inside tolerance at first-off and drifts by the end of the run.

Snap-through makes it worse. When a heavy section finally shears, the stored energy in the frame releases in milliseconds, the ram jumps and the noise level rises sharply. Dies that run at 95% of rating tend to show punch fatigue long before they reach their expected stroke count, and the tool room ends up regrinding a die that was never overloaded on paper.

What Ganesh actually runs

Ganesh stamping runs mechanical presses from 20 T to 300 T, a 250 T punching press for heavy piercing work, and an in-house tool room that builds press tools, progressive dies and checking fixtures. Tooling is quoted separately from the piece price, so the die cost and the per-piece forming cost stay visible when a buyer compares a die route with a laser route.

The tonnage check happens at quotation, not at tryout. The die layout is reviewed against the press list, the simultaneous cut is totalled, and the press is named on the route sheet before the tool is released. Repeat parts carry a first article inspection, and PPAP documentation is available where the programme needs it. The general sheet-metal tolerance is ±0.5 mm, with tighter features agreed against the drawing.

What the press drawing has to settle

The drawing does not need to state a press. It needs to give the tool room the inputs it cannot infer:

• Material grade and standard, for example IS 513 D at 1.2 mm with the thickness tolerance band.
• Tensile or yield strength where the grade is unusual, high-strength or supplied to a customer specification.
• Minimum hole diameter and hole-to-edge distance, so the die layout can be checked for punch breakage as well as tonnage.
• Full cut perimeter on the flat pattern, including every hole, slot and notch that will be cut in the die.
• Annual quantity and release pattern, which set the choice between a single-stage tool, a progressive die and a laser blank.
• Any feature that must remain uncut until a later operation, because that changes the station balance and therefore the peak tonnage.

If the tonnage is close to the 80% line, say so in the RFQ. A supplier who knows the part is marginal will price a larger press or a different die concept; a supplier who finds out at tryout will quote the difference as a tool change.

When tonnage is the wrong question

Below a few thousand pieces a year, the die does not earn its cost and the tonnage figure is irrelevant — the part goes on the 12 kW fibre laser and the press brake instead. Laser cutting vs stamping sets out where the two routes cross by annual quantity, and tooling cost versus piece price shows how to amortise a die properly before deciding.

Above the crossover, tonnage is the first question and springback is the second. How springback affects pressed components covers the angle correction that follows a tonnage decision on high-strength grades.

Run the perimeter and thickness through the press tonnage calculator to get a first figure, then send the drawing for a route check against the stamping capability and the actual press list. The stamping and press tools hub carries the rest of the die-design material: clearance, station layout and maintenance intervals.