---
title: "Blanking force, clearance and die sizes"
subject: "Manufacturing and Production Processes"
chapter: 6
key: "semester_4/manufacturing_and_production_processes:blanking-punching-force-clearance-numerical"
source: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/blanking-punching-force-clearance-numerical
site: BCT Engineering
---

# Blanking force, clearance and die sizes

Manufacturing and Production Processes · Chapter 6: Sheet Metal Product Manufacturing Process

- Asked in: year not given, 3+5 marks
- Video: [Calculations of DIE and PUNCH size in Blanking and Punching operations.](https://www.youtube.com/watch?v=VZj03NUP5j4) — Knowledge Spreader
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/blanking-punching-force-clearance-numerical
- Whole chapter: https://www.bctengineer.com/md/semester_4/manufacturing_and_production_processes/chapter-6

## Question

(a) Differentiate between blanking and punching, and explain how clearance affects the cut edge. (b) A circular blank of 40 mm diameter is to be blanked from 2 mm thick sheet of shear strength 300 MPa. The clearance is 6 percent of sheet thickness per side. Determine the blanking force, punch and die dimensions, stripping force (10 percent of blanking force) and work done if the penetration before fracture is 40 percent of thickness.

## Short answer

Blanking: cut-out piece is wanted, punch controls hole. Clearance affects edge quality: correct clearance yields clean edge; too small causes secondary shear; too large gives large burr. Blanking force = π×40×2×300 = 75.4 kN; die = 40.00 mm, punch = 39.76 mm; stripping force = 7.54 kN; work = 75.4×0.4×2 = 60.3 J.

## Answer

#### (a) Blanking and punching

| Point | Blanking | Punching (piercing) |
|---|---|---|
| Piece wanted | The cut-out piece (blank) | The sheet with the hole |
| Scrap | Sheet skeleton | Slug |
| Size controlled by | Die opening | Punch |
| Clearance applied on | Punch (made smaller) | Die (made larger) |

**Effect of clearance:** the clearance is the gap between punch and die, given per side as a percentage of the sheet thickness (5-10 percent for steel). With the correct clearance, the cracks from the punch and die edges meet and give a clean edge, with a burnished zone about one third of the thickness and a small burr. Too small a clearance gives secondary shearing, a higher force and rapid tool wear. Too large a clearance gives a large rollover and burr, and a rough, tilted edge.

#### (b) Numerical

**Blanking force**

The shear area is the cut perimeter times the thickness:

$$
F = \pi D\, t\, \tau = \pi\times 40\times 2\times 300 = 75\,398\ \text{N} = 75.4\ \text{kN}
$$

**Clearance and die sizes**

$$
c = 0.06\times 2 = 0.12\ \text{mm per side}
$$

For blanking, the die decides the blank size:

$$
D_{die} = 40.00\ \text{mm},\qquad D_{punch} = 40 - 2c = 40 - 0.24 = 39.76\ \text{mm}
$$

**Stripping force**

$$
F_s = 0.10\times 75.4 = 7.54\ \text{kN}
$$

Total force the press must supply: $75.4 + 7.54 = 82.9$ kN, so a press of 100 kN (about 10 tonne) is selected.

**Work done**

$$
W = F\times (p\,t) = 75\,398\times (0.4\times 2) = 60\,318\ \text{N\,mm} = 60.3\ \text{J}
$$

| Item | Value |
|---|---|
| Blanking force | 75.4 kN |
| Die opening | 40.00 mm |
| Punch diameter | 39.76 mm |
| Stripping force | 7.54 kN |
| Work done | 60.3 J |

The force can be reduced by grinding **shear** (about equal to the sheet thickness) on the cutting face, which spreads the cut over the stroke. In blanking the shear is ground on the **die**, so that the blank stays flat; in punching it is ground on the punch.

**Answer:** blanking force $= 75.4$ kN; punch $= 39.76$ mm, die $= 40.00$ mm; stripping force $= 7.54$ kN; work $= 60.3$ J.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is correct about clearance placement?
   A. Blanking: clearance on die (die larger); punching: on punch
   B. Blanking: die size equals blank, punch made smaller; punching: punch size equals hole, die made larger
   C. Both on the die
   D. Both on the punch
   Answer: B. The part wanted fixes the controlling tool size; clearance goes on the other tool. (Key point: Blanking vs punching)

2. Too large a clearance gives
   A. Secondary shear and high force
   B. Large rollover and burr with rough, tilted edge
   C. A clean edge with long burnish
   D. Rapid tool wear only
   Answer: B. Too small gives secondary shear and wear; too large gives rollover and burr. (Key point: Clearance effect)

3. Blanking force for 40 mm dia, 2 mm thick, shear strength 300 MPa is
   A. pi x 40 x 2 x 300 = 75.4 kN
   B. pi/4 x 40^2 x 300 = 377 kN
   C. 40 x 2 x 300 = 24 kN
   D. pi x 40 x 300 = 37.7 kN
   Answer: A. The shear area is perimeter x thickness: F = pi D t tau = 75 398 N. (Key point: Blanking force)

4. With 0.12 mm clearance per side, the blanking punch diameter is
   A. 40.24 mm
   B. 40.00 mm
   C. 39.88 mm
   D. 39.76 mm
   Answer: D. The die is 40.00 mm and the punch is 40 - 2 x 0.12 = 39.76 mm. (Key point: Punch diameter)

## Related in Manufacturing and Production Processes

- [Sheet bending allowance, force, springback](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Abending-allowance-force-springback-numerical) — Manufacturing and Production Processes, chapter 6
- [Deep drawing blank size, hydroforming, spinning](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Adeep-drawing-hydroforming-spinning-formability) — Manufacturing and Production Processes, chapter 6
- [Cold upsetting force with friction](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aopen-die-forging-friction-numerical) — Manufacturing and Production Processes, chapter 5
- [Flat rolling draft, roll force and power](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aflat-rolling-force-power-numerical) — Manufacturing and Production Processes, chapter 5
- [Wire drawing stress, force and power](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Awire-drawing-stress-force-numerical) — Manufacturing and Production Processes, chapter 5

## Related in other subjects

- [Blanking force and press capacity for washer](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Ablanking-force-press-capacity) — Workshop Technology, chapter 8
- [Flywheel for a punching machine](https://www.bctengineer.com/question.md?key=semester_7%2Ftheory_of_mechanism_and_machine_ii%3Apunching-machine-flywheel-design) — Theory of Mechanism and Machine II, chapter 2
- [General, local and punching shear failure](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Amodes-shear-failure) — Foundation Engineering, chapter 6, asked 4×
- [Diameter to thickness ratio for stress limit](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Apipe-diameter-thickness-ratio-hoop-stress) — Hydraulics, chapter 4