Page 34 - Module DMV20173

3.0 SHEET METAL PROCESSES

3.2.1. PUNCH FORCE

1. The force required to punch is basically the product of the shear strength of the sheet
metal and the total area being sheared along the periphery.

2. The maximum punch force, F, can be estimated from the equation

F = 0.7TL(UTS)

Where T is the sheet thickness, L is the total length sheared (such as the perimeter of a
hole), and UTS is the ultimate tensile strength of the material.

3. Friction between the punch and the work piece can, however, increase punch force

significantly.
4. Furthermore, in addition to the punch force, a force is required to strip the punch from
the sheet during its return stroke.

Example 3.1

Calculation of punch force

Estimate the force required for punching a 25-mm diameter hole through a 3.2-mm thick

annealed titanium-alloy Ti-6Al-4V sheet at room temperature. Ultimate tensile strength of
the titanium-alloy Ti-6Al-4V is 100 MPa.

Solution:

The force is estimated from equation, where the UTS for this alloy is found to be 1000
MPa. Thus,

F = 0.7(3.2)(π)(25)(100 X 10 )
6
= ………………..

3.2.2 SHEARING OPERATIONS

1. The most common shearing operations are punching—where the sheared slug is
scrap or may be used for some other purpose—and blanking—where the slug is the

part to be used and the rest is scrap.

BPLK 66 DMV 20173