Sheet Metal Blanking
Calculate blanking force and punch compressive stress for sheet metal blanking and punching operations
Inputs
≈ 80–90 % of tensile strength
Formula Interpretation
Blanking Force
Blanking force equals the shear area multiplied by the shear strength . Shear area is the product of cut perimeter and sheet thickness. For a circular blank of diameter : .
Punch Compressive Stress
The compressive stress on the punch face equals the blanking force divided by the punch contact area . For a circular punch: . Punch material must have yield strength greater than to avoid permanent deformation.
Material Reference
| Material | τf (MPa) | c/t (%) |
|---|---|---|
| Low carbon steel | 320~400 | 6~9 |
| High carbon steel | 550~900 | 8~12 |
| Stainless steel | 520~560 | 7~11 |
| Copper (soft) | 250~300 | 6~10 |
| Copper (hard) | 180~220 | 6~10 |
| Aluminium (hard) | 130~180 | 6~10 |
| Aluminium (soft) | 70~110 | 5~8 |
τf is typically 80–90 % of tensile strength. Die clearance c should be c = (c/t)% × t. Smaller clearance improves cut quality but increases force and die wear.
Knowledge Points
What Is Blanking?
Blanking uses a punch and die — also called the male die and female die — to shear parts or holes directly from flat sheet metal. Because tooling can be expensive, blanking is economical only for large production runs. Typical products include washers, brackets, and electronic connector contacts.
Die Clearance
The radial clearance c between punch and die is a critical parameter. For mild steel it is approximately 6–9 % of sheet thickness. A smaller clearance produces a cleaner shear face but increases cutting force and accelerates die wear. A larger clearance reduces force but may produce a more pronounced rollover edge and larger burr. The optimal clearance depends on material and required cut quality.
Cut Surface Quality
The cut cross-section of a blanked part typically shows four zones from top to bottom: rollover, shear band, fracture zone, and burr. A well-set die produces a wide shear band and minimal burr. Shear strength τf is typically 80–90 % of tensile strength and is the key parameter for force calculation.
Worked Example
A circular blank of diameter is punched from a sheet of thickness . The shear strength of the steel is . Find the blanking force and the compressive stress on the punch face.
Given: D = 100 mm, t = 2 mm, τf = 500 MPa
Step 1 — Blanking force (Formula ①)
Step 2 — Punch contact area and compressive stress (Formula ②)
Result: F = 314 kN; σc = 40 MPa. Since σc (40 MPa) is far below the yield strength of typical tool steel (> 1000 MPa), the punch is safe from plastic deformation.
Extended Knowledge
- •Cut surfaces of blanked parts typically show a rollover edge and burr on the punch side, and a fracture zone on the die side. Burr height increases as the die clearance grows and as the die edge wears. Regular die sharpening (regrinding) maintains cut quality.
- •To minimise scrap, parts must be laid out on the strip with minimal spacing before blanking — this planning is called strip layout. Nesting circular blanks in a staggered pattern can raise material utilisation from around 78 % (single row) to over 90 % (double-row stagger).
- •Progressive dies combine multiple blanking and forming stations in a single tool, producing complex parts in one press stroke. They are standard for high-volume electronic components and automotive stampings. Force requirements for a progressive die equal the sum of all individual station forces.