Sheet metal and press tools

In short

Sheet metal work shapes flat stock (roughly 0.3-6 mm thick) by cutting and plastic forming at room temperature, almost always between a punch and a die mounted on a press. Press tools - blanking, piercing, bending, drawing and progressive dies - are the production hardware that makes car panels, two-wheeler fuel tanks, switchgear enclosures, utensils, appliance bodies and electrical laminations in lakhs of pieces at a few rupees each. The topic carries steady marks in Manufacturing Technology / Production Technology papers and in GATE (clearance, cutting force, blank size, drawing ratio, bend allowance), and it is the most practical interview area for press-shop, tool-design and tool-room roles at Maruti Suzuki, Tata Motors, Bajaj, TVS, Hero, Godrej, JBM Auto and the Indo-German / Central Tool Rooms. Unlike bulk metal forming, thickness is nominally unchanged and the sheet deforms mostly by stretching and bending.

Explained

Classification of sheet metal operations

• Cutting (shearing-type) operations - the sheet is fractured along a line; the punch is a cutting tool.
  • Blanking: the piece punched out IS the product; the strip left behind is scrap.
  • Piercing / punching: the hole is the product; the slug is scrap. (Blanking and piercing are the same mechanics, opposite intent.)
  • Notching - removing material from the edge; lancing - a slit with no material removed (louvres); perforating - many small holes; parting - separating by cutting a thin strip between two blanks; cut-off - separating along a single line with no scrap; trimming - removing excess from a formed/drawn part; shaving - a second cut of 0.05-0.1 mm to make a smooth, square edge; slitting - lengthwise cutting of coil; nibbling - overlapping punches on a CNC turret press to cut any profile; dinking - a sharpened steel-rule die for soft materials (gaskets, leather).
• Forming operations - the sheet is plastically deformed without fracture: bending, flanging, hemming (folding an edge back 180° for stiffness and safety), curling (rolled edge on buckets and drums), beading, seaming, deep drawing, ironing, embossing (shallow relief, thickness nearly unchanged), coining (closed-die, material squeezed, thickness changes), bulging, necking, stretch forming, spinning, roll forming.
• Mechanics of shearing - three stages: (1) elastic/plastic deformation and roll-over of the surface under the punch; (2) penetration - a smooth, burnished band is cut; (3) fracture - cracks start from punch and die edges and meet, giving a rough break-out zone with a burr on the exit side. So a punched edge always has four zones: roll-over, burnish, fracture, burr. Percentage penetration (k) before fracture is about 0.25-0.30 for hard material, 0.5-0.6 for soft/ductile material.

Clearance, cut-edge quality and force reduction

• Clearance (c) is the per-side gap between punch and die. Too little: high force, secondary shear, rapid tool wear. Too much: large burr and roll-over, tapered edge, excessive distortion. Typical 4-8% of thickness per side for steel, 2-4% for aluminium/brass.
• Who gets the size - the member that forms the product surface is made to size, and the clearance goes on the other:
  • Blanking: die opening = blank size; punch = blank size − 2c.
  • Piercing: punch = hole size; die opening = hole size + 2c.
• Angular (land) clearance of 0.25°-1.5° per side is given below the die land (3-6 mm straight land) so slugs fall free.
• Fine blanking: a V-ring holder clamps the sheet, clearance is reduced to about 0.5% of t and the punch moves slowly - giving a fully burnished, square edge to about IT7, used for gear plates, seat-recliner parts and brake components.
• Reducing cutting force so a smaller press can be used (energy stays the same, the peak force is spread over a longer stroke):
  • Shear (bevel) on the punch face for piercing, on the die face for blanking - so the flat product stays flat.
  • Stepped punches in multi-punch tools, so punches do not all enter together.
  • Keeping the tool sharp, and using a nominal (not zero) clearance.
• Stripping force (to pull the strip off the punch) is about 2-20% of the cutting force; springs or a fixed stripper plate provide it.

Bending, springback and allied forming

• Types: V-bending (punch and V-die, free or bottoming/coining), wiping / edge bending (sheet clamped on a pressure pad, one side wiped down), U-bending, air bending on a CNC press brake (depth of punch entry sets the angle), roll bending, rotary (Hexa) bending.
• Neutral axis shifts towards the inside of the bend; the outside fibre is in tension, inside in compression. The developed (blank) length is measured along the neutral axis - this is the bend allowance.
• k factor - position of the neutral axis as a fraction of t: use k ≈ 0.33 when R < 2t and k ≈ 0.50 when R ≥ 2t (bottoming/coining pushes it lower still).
• Minimum bend radius - below it the outer fibre cracks; it is roughly 0.5t-2t for mild steel and larger for high-strength or hardened sheet. Always bend across the rolling direction where possible; a bend along the grain cracks sooner.
• Springback - elastic recovery opens the bend after the punch retracts, so the formed angle is less than the die angle and the radius is larger. Remedies: overbending, bottoming/coining the bend zone, applying stretch (tension) during bending, using a pressure pad, or local heating. Springback is worse for high-strength steel, aluminium and spring steels (and is the main reason advanced high-strength steel panels need compensated dies).
• Other forming notes: embossing uses matched punch-die relief with little thinning; coining fills the die fully and produces sharp detail (coins, nameplates); ironing uses clearance less than t to thin and straighten the cup wall (beverage cans, cartridge cases); spinning forms axisymmetric parts over a rotating mandrel (utensils, dish antennae, pressure-vessel heads); stretch forming is used for large, shallow aerospace and bus panels; roll forming makes long constant-section profiles (door frames, purlins, railway coach members).

Deep drawing and formability

• Deep drawing turns a flat blank into a hollow cup deeper than its radius. Elements: punch, draw die with a generous corner radius (4t-10t), blank holder (pressure pad), clearance about 1.1t per side.
• Metal flow: the flange is in radial tension and circumferential (hoop) compression - this is what causes wrinkling; the wall is in tension - excessive tension tears the cup at the punch radius.
• Blank holding force (BHF) is the key setting: too low → flange wrinkles; too high → flange locked, wall tears. Roughly one-third of the drawing force; draw beads and variable BHF are used on large panels.
• Limiting drawing ratio (LDR) = D/d ≈ 2 for a well-lubricated deep-drawing steel, i.e. about 50% reduction in the first draw. Deeper cups need redrawing (reductions typically 40-50% first draw, then 20-30% each), with inter-stage annealing for hard materials.
• Defects: wrinkling, fracture at the punch radius, earing (wavy rim from planar anisotropy), orange peel (coarse grain), stretcher strains / Lüders bands (avoided by using killed, temper-rolled EDD steel), scoring from poor lubrication, and thinning at the punch corner.
• Formability measures: n (strain-hardening exponent - higher n spreads strain, better stretching), r (Lankford normal anisotropy - higher r resists thinning, better drawing), total elongation, the forming limit diagram (FLD) of major vs minor strain, and cup tests such as the Erichsen cupping test, the Swift/Fukui cup tests and the bend test (IS 1599 method); the exact current IS/ISO numbers for the cupping tests should be checked against the latest BIS listing.
• Lubrication: drawing oils, soaps and polymer films; electro-galvanised and pre-lubed coils are common in Indian press shops.

Press tools: types, components and strip layout

• Die types by work per stroke:
  • Simple (single-operation) die - one operation per stroke.
  • Compound die - two or more cutting operations at the same station in one stroke (blank + pierce a washer); gives the best concentricity between hole and outside, but is slower and the tool is weaker.
  • Combination die - a cutting plus a forming operation in one stroke at one station (blank and draw a shallow cup).
  • Progressive die - the strip advances by one pitch per stroke through several stations (pierce → notch → form → blank-out), one finished part per stroke; highest output, highest tool cost, used for laminations, clips, brackets, terminals.
  • Transfer die / tandem press line - individual blanks are moved mechanically between separate dies or presses; used for car body panels.
• Main components: die block, punch and punch holder plate, backing plate (prevents punches sinking into the holder), stripper (fixed or spring), pilots (locate the strip accurately), stock guides and stops, knockout / ejector, pressure pad, die set (bolster, shank, guide pillars and bushes), dowels and fasteners.
• Strip layout fixes the pitch (advance), strip width, scrap web (bridge) and front/back scrap (about 1-1.5t, minimum ~1 mm), and the grain direction; single row, double row, staggered or interlocked layouts are compared on material utilisation, which typically decides part cost because sheet is 60-75% of it.
• Centre of pressure - the resultant of all cutting forces must pass through the press ram axis (the die shank), or the tool tilts and the clearance becomes uneven. It is found as a line-length-weighted centroid of the cut profiles.
• Tool materials: O1 / D2 / D3 cold-work tool steels, HCHCr, and tungsten carbide for long runs (IS 3748 covers tool and die steels); punch and die hardened to about 58-62 HRC; die blocks are often in segments for easy regrinding.

Presses, sheet materials, standards and safety

• Press classification:
  • Drive: mechanical (crank, eccentric, knuckle-joint for coining, toggle for drawing - fast, fixed stroke, rated tonnage available only near the bottom of the stroke) and hydraulic (full force anywhere in the stroke, adjustable stroke and speed - preferred for deep drawing and press brakes); also pneumatic and screw presses.
  • Frame: gap/C-frame, OBI (open-back inclinable, so slugs slide out), straight-side (for heavy off-centre loads), pillar type.
  • Action: single-, double- (separate blank-holder slide) and triple-action for drawing.
  • Rating: capacity in tonnes, plus stroke, shut height, daylight, bolster size, strokes/minute and flywheel energy per stroke - a press can be force-adequate but energy-starved for a thick blank.
• Modern sheet cutting: CNC turret punch press, laser cutting (fibre lasers now standard for job work), plasma, waterjet; CNC press brake with back gauge for low-volume bending - they replace dedicated press tools when volumes are a few hundred.
• Indian sheet materials and specifications: IS 513 - cold-reduced (CRCA) low-carbon steel sheet, grades for D (drawing), DD (deep drawing) and EDD (extra deep drawing); IS 1079 - hot-rolled sheet and strip; IS 277 - galvanised (GI) sheet; IS 737 - wrought aluminium sheet and strip; IS 2062 for structural plate. Thickness is quoted in mm (SWG is legacy). Electrical laminations use CRGO/CRNGO silicon steel.
• Safety and law: power presses are among the most dangerous machines in an Indian factory. The OSH Code, 2020 (in force 21 November 2025; it repealed the Factories Act, 1948, whose Sec. 21 fencing of machinery and Sec. 22-23 work near machinery in motion and young persons are still cited) and the State Factories Rules saved under it require guarding, trained operators and records of examination. Practical controls: fixed and interlocked guards, light curtains, two-hand anti-tie-down controls, anti-repeat/single-stroke devices, scrap chutes, feeding tools and tongs instead of hands, and brake/clutch inspection. The specific guarding standards and inspection intervals are set by state rules and BIS standards that are revised from time to time - verify the current numbers before quoting them.

Key points

• Blanking vs piercing - identical mechanics; in blanking the removed piece is the part, in piercing the hole is the part.
• Clearance - per-side punch-die gap, about 4-8% of t for steel; it controls force, burr and tool life.
• Size rule - blanking: die = part size; piercing: punch = hole size; clearance always goes on the scrap side.
• Cut edge zones - roll-over, burnish, fracture, burr; burr forms on the side the punch exits.
• Cutting force = cut length × thickness × shear strength; it is independent of the shape's area.
• Shear on the tool face cuts peak force but not the work done; put shear on the die for blanking, on the punch for piercing.
• Stripping force ≈ 2-20% of cutting force; penetration factor k ≈ 0.25-0.6 is needed for energy calculations.
• Bend allowance is measured on the neutral axis; k = 0.33 for R < 2t, 0.50 for R ≥ 2t.
• Springback - elastic recovery opening the bend; countered by overbending, bottoming or forming under tension.
• Bend direction - bend across the rolling direction to avoid edge cracking.
• Deep drawing flange carries radial tension and hoop compression - hence wrinkling; the wall carries tension - hence tearing.
• LDR ≈ 2 (about 50% first-draw reduction); deeper cups are redrawn at 20-30% per stage.
• Blank holding force ≈ one-third of drawing force - the single most important process setting.
• Ironing uses clearance < t to thin and smooth the wall; coining changes thickness, embossing does not.
• Compound die = several cutting operations, one station, one stroke; combination adds forming; progressive = many stations, one part per stroke.
• Centre of pressure must sit on the press ram axis, or the tool tilts and wears unevenly.
• Mechanical presses deliver rated tonnage only near BDC; hydraulic presses give full force through the stroke - preferred for drawing.
• r-value governs drawability, n-value governs stretchability; EDD grades of IS 513 are chosen for deep-drawn parts.

Formulas and examples

Example 1 - washer in a compound die (force, clearance, tool sizes). A washer of 50 mm OD and 25 mm ID is produced from 2 mm CRCA sheet, τ = 300 N/mm².

• Blanking cut length = π × 50 = 157.08 mm → F = 157.08 × 2 × 300 = 94,248 N = 94.25 kN (≈ 9.6 tonne).
• Piercing cut length = π × 25 = 78.54 mm → F = 78.54 × 2 × 300 = 47,124 N = 47.12 kN (≈ 4.8 tonne).
• In a compound die both cuts occur together: total = 141.37 kN ≈ 14.4 t; with a 1.2 factor ≈ 17.3 t → select a 20-tonne press.
• Clearance c = 0.0032 × 2 × √300 = 0.0032 × 2 × 17.32 = 0.111 mm per side (5.5% of t).
• Blanking die = 50.000 mm, blanking punch = 50 − 0.222 = 49.778 mm; piercing punch = 25.000 mm, piercing die = 25.222 mm.
• Stripping force at 10% of 141.37 kN = 14.1 kN.
• Energy per stroke (k = 0.5): W = 141,372 × 2 × 0.5 = 141,372 N·mm ≈ 141 J.

Example 2 - cutting a 10-tonne press down to size with shear. The blanking punch alone needs 94.25 kN but only a 60 kN press is free. Grind 1.5 mm of shear on the die face (k = 0.5, so k·t = 1 mm): F_s ≈ 94.25 × 1/(1 + 1.5) = 37.7 kN - now within capacity, while the energy (94.25 × 2 × 0.5 = 94 J) is unchanged and is simply absorbed over a longer part of the stroke. (The formula is approximate; shop practice is shear height ≈ t and a 30-50% force reduction.)

Example 3 - bending a bracket (blank length, force, springback). Mild steel strip 60 mm wide × 3 mm thick, UTS 410 N/mm², bent 90° to outside legs of 40 mm and 30 mm, inside radius R = 4 mm, V-die opening 20 mm.

• R/t = 1.33 < 2 → k = 0.33. BA = (π/2)(4 + 0.33 × 3) = 1.5708 × 4.99 = 7.84 mm.
• Flats = (40 − (R + t)) + (30 − (R + t)) = (40 − 7) + (30 − 7) = 33 + 23 = 56 mm.
• Blank length = 56 + 7.84 = 63.84 mm.
• F_b = 1.33 × 410 × 60 × 3² / 20 = 1.33 × 410 × 60 × 9 / 20 = 14,723 N ≈ 14.7 kN (1.5 tonne).
• If the released radius measures 4.8 mm: K_s = (2×4/3 + 1)/(2×4.8/3 + 1) = (2.667+1)/(3.2+1) = 3.667/4.2 = 0.873, i.e. the 90° die bend relaxes to a 78.6° bend, so the tool must overbend to a 103° bend angle - a 77° included punch/die angle - or bottom the bend.

Example 4 - two-stage deep drawn cup. Cup 50 mm diameter × 60 mm deep, t = 1 mm, UTS = 350 N/mm² (corner radius ignored).

• D = √(50² + 4 × 50 × 60) = √(2500 + 12000) = √14500 = 120.4 mm.
• D/d = 120.4/50 = 2.41 > LDR 2 → cannot be drawn in one stage.
• First draw at 45% reduction: d₁ = 0.55 × 120.4 = 66.2 mm (ratio 1.82 - safe). Second draw at 25%: d₂ = 0.75 × 66.2 = 49.7 ≈ 50 mm → two draws.
• First-draw force: F_d = π × 66.2 × 1 × 350 × (120.4/66.2 − 0.7) = 72,790 × (1.819 − 0.7) = 72,790 × 1.119 = 81.5 kN (8.3 t).
• BHF ≈ 81.5/3 = 27.2 kN; total slide requirement ≈ 108.7 kN → with 1.2 factor, a 15-tonne double-action or hydraulic press.
• t/D = 1/120.4 = 0.83% - a thin flange, so a blank holder is essential or the flange will wrinkle.

Example 5 - strip layout and cost of the washer in Example 1. Single-row layout, scrap web and edge scrap 1.5 mm: strip width = 50 + 2(1.5) = 53 mm; pitch = 50 + 1.5 = 51.5 mm.

• Strip area consumed per washer = 51.5 × 53 = 2,729.5 mm².
• Washer area = (π/4)(50² − 25²) = 0.7854 × 1,875 = 1,472.6 mm² → utilisation = 53.9% (71.9% if the central slug is recovered as a usable part). A staggered double-row layout typically raises utilisation to around 65-70%.
• Steel consumed = 2,729.5 × 2 = 5,459 mm³ = 5.46 cm³ → 5.46 × 7.85 = 42.9 g per washer (part weight 23.1 g). At an indicative ₹60/kg for CRCA, material cost ≈ ₹2.57 per piece, of which ₹1.18 is scrap - sheet prices move with the market, so recheck the rate.

Where it is used

• Automotive press shops: Maruti Suzuki (Manesar, Gurugram), Tata Motors, Mahindra, Hyundai and their tier-1s (JBM Auto, Krishna Maruti, Sundaram Clayton) run tandem and transfer lines for doors, roofs, floor pans and fuel tanks from CRCA, galvanised and high-strength steel coils.
• Two-wheelers: Hero, Bajaj, TVS, Royal Enfield - fuel tanks (deep drawn + seam welded), mudguards, chain covers, chassis brackets in progressive dies.
• Appliances and furniture: Godrej, Voltas, Whirlpool, Blue Star - cabinets, panels and brackets, now largely laser-cut and CNC press-brake bent for low volumes, hard tooled for high volumes.
• Electricals and motors: stator-rotor laminations in high-speed progressive dies; switchgear and panel enclosures; transformer tanks.
• Railways and defence: coach body panels and roof sheets (ICF Chennai, RCF Kapurthala), ordnance cartridge cases (deep drawing with ironing).
• Job roles: press-shop / production engineer, tool and die designer (CATIA/NX/AutoForm die-face and springback simulation), tool and die maker, maintenance of presses and dies, quality engineer on burr, springback and dimensional checks - skills taught at NTTF, the Indo-German Tool Rooms (Aurangabad, Indore, Ahmedabad) and CTTC Bhubaneswar.

Interview questions

• Q: What is the difference between blanking and piercing? A: The mechanics are identical; in blanking the punched-out piece is the product, in piercing the hole is the product and the slug is scrap.
• Q: In blanking, which member is made to the part size and why? A: The die, because the blank is sized by the die opening as it is pushed through; the punch is made smaller by twice the clearance.
• Q: What happens if the clearance is too small or too large? A: Too small gives higher force, secondary shear and fast tool wear; too large gives a big burr, excessive roll-over and a tapered, distorted edge.
• Q: Does cutting force depend on the area of the blank? A: No - it depends on the length of the cut line times thickness times shear strength, so a thin slender shape can need more force than a compact one of the same area.
• Q: Why is shear ground on the die for blanking but on the punch for piercing? A: The sheared face distorts; the product must stay flat, so shear is applied to the member that shapes the scrap.
• Q: Why does blank holding force need to be just right in deep drawing? A: Too low lets the flange wrinkle under hoop compression; too high locks the flange so the wall tears at the punch radius.
• Q: What is springback and how do you compensate for it in a production die? A: Elastic recovery that opens the bend after unloading; it is compensated by overbending, bottoming or coining the bend zone, adding tension during forming, or redesigning the die face from simulation.
• Q: Which sheet property matters more for a deep drawn cup - n or r? A: r (normal anisotropy) - it resists thinning and favours flow from the flange; n matters more for stretch-formed, shallow panels.
• Q: Difference between a compound die and a progressive die? A: A compound die does several cutting operations at one station in one stroke (best concentricity); a progressive die does one operation per station as the strip advances, giving a finished part every stroke at higher output.
• Q: Why is the centre of pressure calculated? A: So the resultant cutting force passes through the press ram axis; otherwise the tool tilts, clearance becomes uneven and punches and guide pillars wear or break.
• Q: Why is a hydraulic press preferred for deep drawing? A: It gives full, controllable force over the whole stroke at a controlled speed, whereas a mechanical press develops rated tonnage only near the bottom of its stroke.
• Q: A punched hole is oval and burred on one side only - what would you check? A: Uneven clearance from a misaligned or worn punch/die, worn guide bushes, or an off-centre resultant force - plus punch sharpness and sheet support at the stripper.

Common mistakes

• Using the blank area instead of the perimeter (length of cut) in the cutting-force formula.
• Putting clearance on the wrong member - making the blanking punch to size, or the piercing die to size.
• Taking blanking force as perimeter × t × UTS without noting that τ ≈ 0.7-0.8 UTS, then quoting the result as exact.
• Forgetting that shear on the tool reduces peak force but not the work done - and so undersizing press energy.
• Computing blank length by adding the outside leg lengths, ignoring bend allowance and the neutral-axis shift.
• Attempting a cup with D/d > 2 in a single draw, or omitting the blank holder for a thin blank and then blaming "material quality" for wrinkles.
• Mixing units: τ in N/mm² with t in metres, die opening in cm, or reporting force in kN while sizing the press in tonnes (1 tonne ≈ 9.81 kN).

Related topics

Metal forming · Welding and joining · Jigs and fixtures · CNC machining and CAM · Metrology and quality control · Theory of metal cutting