1. ENGR 241 – Introduction To Manufacturing
Chapter 16:
Sheet Metal Forming Processes

2. Shearing
Process that involves cutting sheet metal by subjecting it to shear stresses, usually between a punch and a die.
Process Variables
punch force
punch speed
lubrication
edge condition
Clearance
Material thickness

3. Punching Force Pmax = 0.7(UTS)(t)(L)
Approximate empirical formula for estimating maximum punch force:
Pmax = 0.7(UTS)(t)(L)
Pmax = maximum punch force
UTS = Ultimate Tensile Strength
t = thickness
L = total length of sheared edge

4. Shearing: Additional Concepts
Punching (piercing): sheared slug is discarded.
Blanking: slug is the part, the rest is discarded.
Fine Blanking (less distortion).
Burrs (clearance, ductility, dull tool edges).

5. Shearing
Discard
Punching
Blanking

6. Shearing Operations Die Cutting Perforating- punching holes.
Parting- shearing sheet into two or more pieces.
Notching-removing pieces or shapes from edge.
Lancing- leaving a tab without removing the material.

7. Shearing Operations
Parting
Perforating
Slitting
Notching
Lancing

8. Shearing Operations Slitting Driven Pull-through powered blades
idling blades

9. Shearing Operations Steel Rules Nibbling
thin strip of metal is bent and held on its edge on a flat base.
die is presses against the sheet to cut parts (similar to piercing, cookie cutting).
Nibbling
successive punched holes.

10. Shearing Operations (Nibbling)
successive holes

11. Shearing Dies Compound Progressive Dies Transfer Dies
several operations on same strip (punching).
Progressive Dies
different operations performed with each stroke.
Transfer Dies
sheet metal undergoes different operations at different stations.

12. Bending
One of the most common forming operations, used for producing parts and to impart stiffness.
After bending, outer fibers of the material are under tension while the inner fibers are in compression.

13. Bending Process Variables minimum bend radius bending allowance
Smallest radius allowed before cracks on outer surface (2T, 3T, etc.)
bending allowance
Springback: elastic recovery
Overbending

14. Bending Radius
The bending radius is measured to the inner surface of the bend and it is normally expressed in terms of material thickness. r t

15. Bending Radius
Theoretically, the strains at the outer and inner fibers are equal in magnitude.
where: eo = outer strain
ei = inner strain
r = bending radius
t = thickness

16. Bending Allowance t L d=0.4t  r where: L = bending allowance
r = bending radius
t = thickness

17. Bendability
A material may be reduced in thinkness to increase its bendability
A material may be heated to increase its bendability
Anisotropy is an important factor in bending operations (Fig )
The larger the minimum bend radius, the more resistance to bending

18. Bending Operations Press Brake Beading Flanging
machine utilizes long dies in a press
Beading
edge of sheet metal is bent into die cavity
Flanging
bending of edges

19. Bending Operations Hemming edge folded over itself Roll Forming
bent by series of rolls
Tube Bending and Forming
pack inside of tube with sand to prevent buckling and folding.
flexible mandrels

20. Other Methods Stretch, draw, compression bending Rubber Forming
one die is replaced with a rubber or polyurethane pad.
Spinning
axisymmetric parts over a mandrel with tools or rollers.

21. Other Methods Peen Forming Explosive Forming Magnetic-Pulse Forming
Superplastic Forming