Blanking, piercing and forming: what each operation does

Blanking and piercing cut the sheet; forming moves it. A bracket arrives with a 10 mm hole that measures 10.4 mm on the diagonal and will not take its bolt. Nothing was wrong with the punch. The hole was pierced in the first station, the flange beside it was formed in the third, and the bend stretched the metal around the hole by 0.2 mm on each side. Blanking, piercing and forming are three different operations with three different force equations and three different tolerance behaviours, and the order they run in decides whether the part assembles.

This page separates the three, gives the force each one adds to the press, the clearance each one needs, and the point at which a hole has to move to a later station. It is written for the design engineer who has to put a hole in a flange and for the manufacturing engineer who has to sequence the tool.

Blanking and piercing: what does each operation cut?

Blanking cuts the outside shape of the part and piercing cuts a hole inside it. The physics is the same shear action, and the force equation is the same, but the scrap logic is inverted. In blanking the falling slug is the part, so the die must catch it without marking it. In piercing the slug is scrap, so the die can let it fall through and the part stays on the strip.

That difference drives the tool design. A blanking die needs a part catcher, a controlled fall and often a cushion, because a 1.5 mm CRCA panel that drops 200 mm onto a chute arrives with a dent. A piercing die needs slug control: a slug that rides back up on the punch leaves a double-struck hole and, in a progressive tool, a smashed station on the next stroke.

The clearance column is where most tool arguments start. Every figure is per side, and the operation decides the band. Piercing runs 3–5% of thickness per side because the hole is a functional feature: a bolt, a rivet or a locating pin sees the bore, and a tapered bore with a rolled edge changes the grip length. Blanking runs 5–8% per side because a small roll-over on the outside edge is usually invisible and the wider gap keeps punch load and die wear down. At 2 mm that is 0.06–0.10 mm per side for a pierced hole against 0.10–0.16 mm for a blanked edge, and a tool built to the blanking figure will leave a 2 mm hole tapered.

How much force does each operation add?

Shear force is the cut length multiplied by thickness and by the shear strength, and each station adds to the total. A 150 mm diameter blank in 1.6 mm CRCA cuts at about 226 kN; a single 10 mm hole in 2 mm CRCA needs 18.8 kN, so twenty of those holes add 377 kN and usually decide the press. The press tonnage method works the sum station by station, and the press tonnage calculator checks the peak against the press curve rather than against a single rated number.

Why does the order of operations decide the tolerance?

A hole cut before a bend is cut in flat metal and then carried through the bend. The material around the hole stretches, and the hole goes oval in the direction of the bend. The distortion grows as the hole edge approaches the bend line, because the stretch zone around a bend extends roughly one thickness beyond the tangent point.

The working rule is L_min = R + V/2 + t: the hole edge sits at least the inside bend radius plus half the V-die opening plus the thickness from the bend line. For a 16 mm V-die in 2 mm steel with a 2 mm inside radius, that is 2 + 8 + 2 = 12 mm. The older 1.5 × t + r shortcut is a fallback that only works when the die opening is about 8t, as it is on that 16 mm die; on a tighter die it understates the distance. The hole-to-bend distance guide owns the rule and works it through for a range of thicknesses.

Moving the hole, piercing after forming, and piercing undersize and reaming are the three ways out, and they cost different amounts:

• Move the hole further from the bend. Free, and usually the right answer at the design stage.
• Pierce after forming. Adds a station and a cam or a separate tool action, but the hole is cut in its final geometry and stays round.
• Pierce undersize and ream or punch again. Rarely worth it in sheet metal; the second operation costs more than the station.

The same logic applies to slots, weld nuts and clinch features. Anything that has to hold a fastener to a tolerance should be cut after the last operation that moves the metal around it.

What tolerance can each operation hold?

Tool-determined dimensions and machine-determined dimensions behave differently, and the split explains most tolerance arguments. Tool-determined dimensions are set by the relationship between punch and die: hole diameter, hole-to-hole pitch within one station, slot width, notch depth. They repeat from part to part because the same steel cuts them, and they wear slowly and predictably. Machine-determined dimensions are set by the press, the material and the setup: formed flange height, bend angle, draw depth, the position of a hole relative to a formed feature.

That distinction explains a common argument. A drawing that calls ±0.1 mm on a hole-to-hole pitch is asking for something a tool can hold. The same callout on a flange height measured from a bend is asking for something the material thickness variation and the press deflection also control, and it will be quoted as a capability question rather than accepted.

General dimensions that carry no individual tolerance fall under ISO 2768-1, which defines four tolerance classes, f (fine), m (medium), c (coarse) and v (very coarse), for linear and angular dimensions. For a formed sheet-metal part in the 30–120 mm size band, class m allows ±0.3 mm on a linear dimension, and class f tightens that to ±0.15 mm. The angular tolerances for f and m are the same: ±0°20′ where the shorter leg of the angle is between 50 mm and 120 mm, and ±0°10′ between 120 mm and 400 mm. The class belongs on the drawing because it sets the inspection gauge and the acceptance argument before either happens.

Ganesh quotes ±0.5 mm typical on formed sheet-metal features, with tighter features by agreement, and first article inspection or PPAP documentation where the programme needs it. A tolerance tighter than the process can hold is not a quality instruction; it is a cost instruction that will be paid in sorting.

How is a progressive die sequenced?

A progressive tool runs the strip through stations, each doing one part of the work, with the strip advancing one pitch per stroke. The sequence is designed backwards from the last feature that must not move.

A typical sequence for a bracketed panel:

1. Pierce the pilot holes. Two pilots, one at each end, locate the strip in every later station. Everything downstream is measured from them.
2. Notch and pierce. Cut the edge reliefs and the holes that are far enough from a bend to survive forming.
3. Blank the outline, partially. Leave the part attached to the strip by carriers so it can travel to the forming stations.
4. Form. Bend the flanges, with the part still located on the pilots.
5. Pierce the near-bend holes. Cut the holes that would have distorted, now that the metal has stopped moving.
6. Trim and separate. Cut the carriers and let the finished part fall.

Each added station costs tool length, press bed and maintenance. A five-station tool is not automatically better than a three-station tool; it is better when the extra stations remove a real failure. The progressive versus single-stage comparison sets out where the crossover sits.

Where Ganesh Multitech builds blank, pierce and form tools

The press shop runs mechanical presses from 20 T to 300 T with 250 T punching, and the tool room designs and builds the blank, pierce, form and progressive tools, the welding fixtures and the checking fixtures in-house. That matters for sequencing: when a hole has to move to a later station, the change is a tool modification in the same building rather than a new supplier conversation.

Where the volume does not carry a progressive tool, the same part is cut on the 12 kW fibre laser with a 40 ft × 16 ft bed, supported by two 6 kW and two 3 kW machines, and formed on the 200 T 5-axis brake with a 4,450 mm bed to 10 mm. The custom-part range is 0.8–40 mm, formed tolerance is ±0.5 mm typical, and the plant holds ISO 9001:2015 and ISO 14001:2015 across five units in Mumbai (Vasai) and Nagpur. Press work has run here since 1992, and the customer list includes Mahindra, Mahindra Powerol, Godrej & Boyce, TKE, Eros Elevators, Minda, Gunnebo and Indian Railways.

When a single-stage tool beats a progressive die

A single-stage tool beats a progressive die below a few thousand pieces a year and when one feature dominates the part. It is cheaper to build, faster to change and easier to correct after the first article. The progressive versus single-stage comparison sets out where the crossover sits.

We do not run presses above 300 T, we do not fine-blank, and we do not produce parts that need a machined mating face from a press tool. If the drawing calls for a ground pad or a bore tolerance below what a punched hole can hold, the route changes and the quotation should say so.

What to put on the drawing

• The material grade and thickness, with the standard reference, so shear strength and clearance can be set.
• Hole diameters with the tolerance, and a note on which holes are functional rather than clearance.
• The bend lines, inside radii and the hole-to-bend distance for every hole near a bend.
• The burr side, and the burr height limit if the part is handled or coated.
• The general tolerance class from ISO 2768-1, plus any tighter callout on pitch, flatness or angle.
• The station-critical features, marked as such, so the tool sequence can protect them.
• Annual volume and release pattern, because that decides progressive against single-stage.

Send the drawing with the hole schedule and the volume through request a quote. The stamping and press tools hub carries the rest of the design guidance, including the press tonnage method used above.