Press tool design: progressive and compound dies

Why it matters

Washers, brackets, connector terminals, motor laminations and car-body parts are made by the million in press tools at tens to hundreds of strokes per minute. The die type, punch and die sizes, press force and strip layout decide part accuracy, burr height, material cost and whether the press is overloaded.

Key ideas

Shearing operations

• Blanking: the piece punched out is the part (the blank); the strip is scrap. Piercing (punching): the hole is wanted, the slug is scrap. Others: notching, lancing, parting, cut-off, trimming and shaving.
• Shearing proceeds by elastic deformation, plastic deformation (rollover), shear (burnished band) and fracture (fracture zone and burr). The punch penetrates only part of the thickness before cracks from punch and die edges meet.
• Clearance c is the gap per side between punch and die. Too small: secondary shearing, high force and wear; too large: large rollover and burr. It rises with sheet thickness and strength; typical values are a few per cent of t per side (take from a data book).
• Size rule: the die controls the blank and the punch controls the hole. Blanking: die = blank size, punch = blank − 2c. Piercing: punch = hole size, die = hole + 2c.

Die types

• Simple die: one operation per stroke at one station.
• Progressive die: several stations along the strip; at each stroke every station works and the strip advances one pitch. Typical sequence: pierce holes, then use them for pilots to register the strip, then notch/form, and finally blank or cut off. High output from coil stock, but large, costly dies; the relative position of features depends on strip registration (pilot accuracy).
• Compound die: two or more cutting operations (typically blanking and piercing) at one station in one stroke. The blanking die is on the upper shoe and the punch below (an inverted arrangement), so the part is pushed back into the strip and ejected. Hole-to-edge concentricity and flatness are better than in a progressive die; slower, because the part must be ejected each stroke.
• Combination die: a cutting and a non-cutting operation (e.g. blanking and drawing or bending) in one station in one stroke.
• Transfer die: separate stations with mechanical fingers moving the part — for large parts.

Die-set elements: punch holder and die shoe on guide posts and bushes; punch plate; die block; stripper (fixed or spring-loaded) to pull the strip off the punch; stock guides and stops; pilots to register the strip; knock-out or ejector to remove the part from a compound die; backing plates to stop punches sinking into the holder.

Press force and centre of pressure

• The shearing force is perimeter × thickness × shear strength. In a progressive die, once the strip is fully in, all stations cut in the same stroke, so forces add.
• Force can be reduced by stepping punches (different lengths) or by grinding shear on the punch (piercing) or die (blanking) — the slug or the strip, whichever is scrap, gets distorted.
• The centre of pressure (the centroid of all cutting perimeters) must lie on the press ram axis; otherwise the ram tilts, guide posts wear and clearance becomes uneven.
• Stripping force is a fraction of the cutting force (often 10–20 %; take from a data book).

Strip layout: blank pitch = blank length + bridge (scrap web); strip width = blank width + 2 × edge allowance. Material utilisation = blank area per pitch ÷ (pitch × strip width); nest or tilt blanks to raise it.

Formulas

• F = L·t·τ — shearing force, N; L cut perimeter (mm), t sheet thickness (mm), τ shear strength of the sheet (MPa = N/mm²). For a circle L = π·d.
• F_total = Σ L_i·t·τ — total force of all punches cutting in the same stroke.
• Blanking: D_die = D_blank, D_punch = D_blank − 2c. Piercing: D_punch = D_hole, D_die = D_hole + 2c.
• x̄ = Σ(L_i·x_i) / ΣL_i, ȳ = Σ(L_i·y_i) / ΣL_i — centre of pressure, mm (x_i, y_i = centroid of each cut perimeter).
• η = A_blank / (p·W) — strip utilisation; p pitch (mm), W strip width (mm), A_blank area of one blank (mm²).
• Work = F·p_e·t — approximate shearing energy, J (N·m with t in m); p_e = fraction of thickness penetrated before fracture.

Worked examples

Example 1 (standard) — compound die for a washer. A washer of 40 mm outside diameter and 20 mm hole is made from 2 mm steel sheet with shear strength 350 MPa. Clearance per side = 6 % of t. Find the punch and die sizes and the press force.

1. c = 0.06 × 2 = 0.12 mm.
2. Blanking (40 mm): die = 40.00 mm, punch = 40 − 2 × 0.12 = 39.76 mm.
3. Piercing (20 mm): punch = 20.00 mm, die = 20 + 0.24 = 20.24 mm.
4. F_blank = π × 40 × 2 × 350 = 87 965 N; F_pierce = π × 20 × 2 × 350 = 43 982 N.
5. Both act in the same stroke: F = 131.9 kN (add the stripping force before selecting the press).

Example 2 (GATE level) — progressive die layout and centre of pressure. The same washer is made in a two-station progressive die: the 20 mm hole is pierced at station 1 and the 40 mm blank is cut at station 2, one pitch later. Bridge = 3 mm, edge allowance = 3 mm. Find the pitch, strip width, utilisation and the position of the centre of pressure from the piercing punch axis.

1. Pitch p = 40 + 3 = 43 mm; strip width W = 40 + 2 × 3 = 46 mm.
2. η = (π/4 × 40²)/(43 × 46) = 1256.6/1978 = 63.5 % (blank area basis).
3. Perimeters: L1 = π × 20 at x = 0; L2 = π × 40 at x = 43 mm.
4. x̄ = (L1·0 + L2·43)/(L1 + L2) = 40 × 43/60 = 28.67 mm from the piercing punch, towards the blanking punch. The shank must be placed here.
5. Press force is again 131.9 kN, because both stations cut in every stroke once the strip is loaded.

Common mistakes

• Using the hole area (π·d²/4) instead of the sheared area (π·d·t) for the cutting force.
• Applying clearance to the wrong tool: in blanking the punch is made smaller; in piercing the die is made larger.
• Confusing compound (several cutting operations, one station) with progressive (several stations) and combination (cutting plus forming) dies.
• Placing the press shank at the geometric centre of the die instead of the centre of pressure.
• Forgetting that in a progressive die all station forces add at every stroke.
• Grinding shear on the wrong member — shear goes on the tool whose side is scrap.

For GATE PI

• NAT on blanking/piercing force, punch and die dimensions with clearance, centre of pressure, strip utilisation and shearing work.
• MCQs on die types (progressive, compound, combination), the roles of pilots, strippers and stops, and which tool carries the clearance.
• Links to sheet-metal forming in the forming subject; practise mixed questions.

Quick check

1. In piercing a 12 mm hole with 0.05 mm clearance per side, what are the punch and die diameters?
2. Force to blank a 50 mm × 30 mm rectangle from 1.5 mm sheet, τ = 300 MPa?
3. Which die gives better concentricity between a hole and the outside edge: compound or progressive?
4. What does a pilot do?
5. Where must the press ram axis be placed relative to a multi-punch die?

Answers: 1. Punch 12.00 mm, die 12.10 mm. 2. 160 × 1.5 × 300 = 72 000 N = 72 kN. 3. Compound. 4. It enters a pierced hole and registers the strip precisely before the next operation. 5. Through the centre of pressure.