1. Shearing, Blanking and Punching
These are the three principal operations in press working that cut metal by shearing mechanism.
Shearing is used to cut large sheets into smaller pieces. It is performed on a machine called a power shears or squaring shears. The upper blade of the power shears is often inclined, to reduce the required cutting forces.

2. Blanking
Blanking : involves cutting of sheet metal along a closed outline in a single step to separate the piece from the surrounding stock.
The part that is cut out is the desired product in the operation is called as blank.

3. Punching & Blanking
a) Blanking b) Punching

4. Punching
Punching : similar to blanking except that the separated piece is scrap, called the slug.
The remaining stock is the desired product.

5. Analysis of sheet metal cutting
Important parameters in sheet metal cutting are :
Clearance between punch and die
Stock thickness
Type of metal & its strength
Length of cut

6. Clearance
The clearance “c” in shearing operation is the distance between the punch and die.
Typical clearance values are 4% to 8% of sheet thickness.
Special operations requiring very straight edges, such as shaving and fine blanking, clearance is 1% of sheet thickness.

7. Contd… c = Ac t c = clearance, mm Ac = clearance allowance
t = thickness of stock, mm

8. Contd…
These calculated “c” value can be applied to conventional blanking and punching operations to determine the proper punch and die sizes.
The die opening must always be larger than the punch size.

10. Contd…
Whether to add or subtract the clearance value to die size or punch size depends on whether the part being cut is a blank or slug.
Because of the geometry of the sheared edge, the outer dimension of the part cut out of the sheet will be larger than the hole size.

11. Contd…
Punch and die sizes for a round blank of diameter Db can be determined as :
For Blanking operation :
Blanking punch diameter = Db – 2c
Blanking die diameter = Db

12. Contd… For Punching operation : Hole punch diameter = Dh
Hole die diameter = Dh + 2c

13. Assignment
For roll numbers : 65, 94, 103, 101, 108, 120, 111, 122, 105, 123, 112, 118, 501, 144, 139, 146, 145, 143, 132, 213, 206, 207, 211, 227, 280, 283, 341, 380, 68
Submission date/day : 5th April, 2013 (Friday)
Timings : 12:00 to 13:00

14. Assignment Explain the following sheet metal operations with figures :
Ironing
Coining & Embossing
Lancing & Twisting