IE 337: Materials & Manufacturing

Processes
Lecture 13:
Metal Forming
Operations 2
Chapter 20
2
Issues To Address
Sheet Metal Processes
Cutting
Bending

Homework 4 (Due Tuesday)



3
Three Major Sheet Metal Processes
1. Cutting
Shearing to separate large sheets, or cut part
perimeters, or make holes in sheets
2. Bending
Straining sheet around a straight axis
3. Drawing
Forming of sheet into convex or concave shapes
4
Cutting: Blanking & Punching
(a) Blanking and (b) Punching
5
Extending the concept
End Cap
Header and Channel
Header and Fin
End Cap
Micrograph courtesy of PNNL
Microscale
heat exchangers
and reactors
Paul et al, OSU
6
Cutting Parameters
Clearance between punch
and die
Stock thickness
Type of metal & strength
Length of cut
7
Clearance in Sheet Metal Cutting
Distance between the punch
and die
Clearance allowance values
range between 4% and 8%
of stock thickness
If too small, fracture lines
pass each other, causing a
poor cut and larger force
If too large, metal is pinched
between cutting edges and
excessive burr results
8
Clearance in Sheet Metal Cutting
Recommended clearance can be calculated
by:
c = at
Where:
c = clearance;
a = allowance; and
t = stock thickness

Allowance a is determined according to type of
metal
9
Allowance a for Three Sheet Metals
Metal group a
1100S and 5052S aluminum alloys, all
tempers
0.045
2024ST and 6061ST aluminum alloys;
brass, soft cold rolled steel, soft
stainless steel
0.060

Cold rolled steel, half hard; stainless
steel, half hard and full hard
0.075
Table 20.1, Page 444
10
Die and Blank Size Relationships
Die size determines blank size D
b
;
Punch size determines hole size D
h
.; c = clearance
11
Punch & Die Sizes for Blanking/Punching
For a round blank of diameter D
b
:
Blanking punch diameter = D
b
- 2c
Blanking die diameter = D
b

where c = clearance

For a round hole of diameter D
h
:
Hole punch diameter = D
h

Hole die diameter = D
h
+ 2c
where c = clearance

12
Example
A compound die will be used to blank and
punch a large washer out of aluminum alloy
sheet stock 3.2 mm thick. The outside diameter
of the washer = 65 mm and the inside diameter
= 30 mm.
Determine:
(a) the punch and die sizes for the blanking
operation, and
(b) the punch and die sizes for the punching
operation.
13
Example: Solution
From Table 20.1, a = 0.045. Thus, c = at = 0.045(3.2) =
0.144 mm

(a) Blanking punch diameter = D
b
- 2c
= 65 - 2(0.144) = 64.71 mm
Blanking die diameter = D
b
= 65 mm

(b) Punching punch diameter = D
h
= 30 mm
Punching die diameter = D
h
+ 2c = 30 + 2(0.144) = 30.29
mm

14
Cutting
F = ?
15
Cutting Forces
Important for determining press size (tonnage)
F = S t L
Where:
S = shear strength of metal, MPa
t = stock thickness, mm, and
L = length of cut edge, mm

Since shear strength is not always available, it may be
estimated from the tensile strength (TS):
S = 0.7 TS

16
Example
The outside diameter of an Al washer = 65 mm
and the inside diameter = 30 mm. Determine
the minimum tonnage press to perform the
blanking and punching operation on the
washer if the aluminum sheet metal of
thickness 3.2 mm has a tensile strength = 290
MPa.

17
Example: Solution
F = 0.7(TS)tL
t =3.2 mm
L = 65p + 30p = 95p = 298.5 mm
F = 0.7(290)(3.2)(298.5)
= 193,874 N

18
Bending - 1
Straining sheet metal around a straight axis
to take a permanent bend
19
Bending - 2
Metal on inside of
neutral plane is
compressed,
while metal on
outside of neutral
plane is stretched
both compression and tensile elongation of
the metal occur in bending
20
Types of Sheetmetal Bending
V-bending - performed with a V-shaped die
Bottom-bending
Workpiece is in complete contact with punch on one side,
and the with the die on the other side
Angle is set by the form of the tooling (punch & die)

Air-bending
A form of three-point bending
Angle is continuously variable* - set by a stop

Edge bending - performed with a wiping die
21
V - Bending
For low production
rates

Performed on a
press brake

V-dies are simple
and inexpensive
22
Edge - Bending
For high
production
rates

Pressure pad
required

Dies are more
complicated
and costly
23
Stretching during Bending
If bend radius is small relative to stock
thickness, metal tends to stretch during
bending

Important to estimate amount of stretching, so
that final part length = specified dimension

24
Bend Allowance Formula
where:
BA = bend allowance;
A = bend angle; degrees
R= bend radius;
t = stock thickness; and
K
ba
is factor to estimate stretching
Rule of Thumb: Compare bend radius to thickness
R < 2t K
ba
= 0.33
R 2t K
ba
= 0.50
) ( t K R
A
BA
ba

360
2p
25
Springback in Bending
Springback in bending shows itself as a decrease in bend angle and
an increase in bend radius:
(1) during bending, the work is forced to take the radius R
b
and
included angle A
b
' of the bending tool (punch in V-bending),
(2) after punch is removed, the work springs back to radius R and
angle A'
26
Springback in Bending
Springback = increase in included angle of
bent part relative to included angle of forming
tool after tool is removed

Reason for springback: When bending
pressure is removed, elastic energy remains in
bent part, causing it to recover partially toward
its original shape
27
Bending Force
Maximum bending force estimated as follows:
Where:
F = bending force;
TS = tensile strength of sheet metal;
w = part width in direction of bend axis;
t = stock thickness; and
D = die opening dimension.
For V - bending, K
bf
= 1.33;
For edge bending, K
bf
= 0.33
D
TSwt K
F
bf
2

28
Die Opening Dimension
Die opening dimension D: (a) V-die, (b) wiping die
29
This Time
Shearing and bending operations
Punch and die calculations for blanking
Estimating cutting & bending forces
Estimating bend allowance
Springback

30
Assignment
HW 4 (Due Tuesday):
CH 18 & 20 Problems:
18.2
18.15
20.4
20.9
20.12
CH 6, 18 & 20 Questions
To be posted by the
end of the day
press brake
Glass and Ceramics Processing

31
Next Time
Chapter 12 & 17