ENGR 241 – Introduction To Manufacturing

Chapter 16:

Sheet Metal Forming Processes

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

Punching Force• Approximate empirical formula for estimating

maximum punch force:

Pmax = 0.7(UTS)(t)(L)

Pmax= maximum punch forceUTS = Ultimate Tensile Strength t = thickness L = total length of sheared edge

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).

Shearing

Punching Blanking

Discard

Shearing Operations

1. 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.

Shearing Operations

Perforating

Slitting

Lancing

Notching

Parting

Shearing Operations

2. Slitting• Driven

• powered blades

• Pull-through• idling blades

Shearing Operations

3. Steel Rules• 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).

4. Nibbling• successive punched holes.

Shearing Operations (Nibbling)

successive holes

Shearing Dies• Compound

• several operations on same strip (punching).

• Progressive Dies• different operations performed with each stroke.

• Transfer Dies• sheet metal undergoes different operations at

different stations.

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.

Bending• Process Variables

• minimum bend radius• Smallest radius allowed before cracks on outer

surface (2T, 3T, etc.)

• bending allowance

• Springback: elastic recovery

• Overbending

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

Bending Radius

• Theoretically, the strains at the outer and inner fibers are equal in magnitude.

1)/2(

1

tree io

where: eo = outer strain

r = bending radius

t = thickness

ei = inner strain

Bending Allowance

d=0.4t

L

t

r

360

πα2)4.0(

trL

3.57

α)4.0( tr

where: L = bending allowance

r = bending radius

t = thickness

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. 16.17)• The larger the minimum bend radius,

the more resistance to bending

Bending Operations• Press Brake

• machine utilizes long dies in a press

• Beading• edge of sheet metal is bent into die cavity

• Flanging• bending of edges

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

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.

Other Methods• Peen Forming

• Explosive Forming

• Magnetic-Pulse Forming

• Superplastic Forming