§1Shaping by force
Press working forms sheet metal cold, using a punch and die to cut or bend it to shape in one quick stroke. It removes no metal in bending and drawing, and cuts cleanly in blanking — all by applied force, at high speed and in quantity.
The press supplies the force; the tooling — a punch that descends into a matching die — gives the shape. The operations divide into cutting the sheet (blanking and piercing, §2–3, which shear through it) and forming the sheet (bending and drawing, §4–6, which bend it without cutting). Each is fast, repeatable and suited to mass production: once the tooling is made, a press turns out identical parts stroke after stroke. What an engineer must know for each is the force it demands — so the press is big enough — and the geometric rules that make the part come out right: the bend allowance, the springback, the draw ratio. This page takes them in turn.
Contents§2Blanking and piercing
Blanking and piercing are the same shearing action with opposite intent — one keeps the piece punched out, the other keeps the sheet with a hole in it.
In both, the punch shears the sheet against the die around a closed line, snapping out a slug. In blanking, the piece punched out is the product — a disc, a bracket profile — and the surrounding sheet is scrap. In piercing (or punching), the hole is the point and the sheet is the product, the slug being scrap. The mechanics are identical, so the same force calculation (§3) governs both, and the same tooling clearance — a small gap between punch and die, a few percent of the thickness — gives a clean cut with the right amount of fracture. The distinction is only which side you keep, but it drives how the tool is designed: a blanking die is sized to the part, a piercing punch to the hole.
Contents§3The cutting force
The force to blank or pierce is the length of the cut times the sheet thickness times the material’s shear strength — a direct product that sizes the press.
To blank a 50 mm-diameter disc from 2 mm steel of shear strength 350 N/mm², the cut perimeter is π × 50 = 157 mm, so F = 157 × 2 × 350 = 110 kN — an 11-tonne press at least, and more with a margin. The force scales directly with all three factors (the hero charts it against thickness for two materials): double the thickness or the strength and you double the force; a longer or more intricate cut outline needs more. This is why thick, high-strength sheet needs a big press, and why blanking force is the first thing checked before a die is run — ask a press for more than it has and the stroke stalls or the tool breaks. A shaped punch face (a shear ground onto it) spreads the cut over the stroke to lower the peak force on large blanks.
§4Bending and its allowance
Bending folds the sheet over a die without cutting it — but the metal stretches around the bend, so the flat blank must be cut to a length that accounts for it: the bend allowance.
When sheet is bent, the outside stretches and the inside compresses, with a neutral line in between that neither stretches nor shrinks — and it is that neutral line, sitting a fraction K of the way through the thickness, whose length must be added into the flat blank. For a 90° bend (π/2 radian) of inside radius 3 mm in 2 mm sheet, with the neutral line at K ≈ 0.44, the bend allowance is (π/2) × (3 + 0.44 × 2) = 6.09 mm of material consumed in the bend. Add each bend’s allowance to the flat lengths of the legs and you get the flat pattern to cut, so that after bending the part comes out the right size. Get the allowance wrong and every folded part is over- or under-length — which is why the bend allowance is the heart of sheet-metal layout. A tighter radius or thicker sheet consumes more, exactly as the formula shows.
§5Springback
Sheet metal bent to an angle springs back a little when the press releases, because part of the bend was only elastic — so the tool must overbend to land the right final angle.
A bend is part plastic (permanent) and part elastic (recovers), and when the punch lifts, the elastic part unbends, opening the angle slightly and increasing the radius — the springback. Its size grows with the material’s strength and with a larger bend radius, and it is worse in high-strength sheet, which stores more elastic strain. The cure is to overbend: form the metal past the target so that after springback it settles on the wanted angle — bending to, say, 88° so it springs back to 90°. Alternatives are to coin the bend (press it hard at the bottom of the stroke to set it plastically) or to design in a bead or rib that stiffens the bend. Springback is why a folder is set a touch tighter than the drawing angle; ignore it and every bend comes out slightly open.
Contents§6Deep drawing
Deep drawing pulls a flat blank down into a die to form a cup or box — the most demanding press operation, limited by how much the metal can be drawn in one step.
A punch pushes the centre of the blank into the die cavity while a blank-holder restrains the rim, drawing the metal inward and down into a seamless cup — how cans, sinks, pressings and shells are made. The limit is the draw ratio, the blank diameter divided by the punch diameter: draw too deep in one hit — a ratio much above roughly 1.8 to 2.0 — and the cup wall tears, because the metal cannot flow in fast enough to feed the depth. Deeper cups are therefore drawn in stages — successive redraws each within the safe ratio, sometimes with an anneal between to restore ductility. The blank-holder force matters too: too little and the rim wrinkles as it is drawn in, too much and the wall tears. Deep drawing is a balance of holding, drawing and staging, all set by how far the sheet can safely flow.
Contents§7Quick reference
The working core of the page on one card rack.
Two families
cutting: blank · pierce
forming: bend · draw
Cutting force
F = L · t · τ
Ø50 × 2 mm steel → 110 kN
Bend allowance
BA = θ(r + K·t)
90° r3 t2 → 6.09 mm
Springback
overbend to compensate
Deep draw
draw ratio ≲ 1.8–2.0
deeper → redraw in stages