1. IE 337: Materials & Manufacturing Processes
IE 337 Lecture 12: Forming 2
IE 337: Materials & Manufacturing Processes
Lecture 13:
Metal Forming Operations 2
Chapter 20
S.V. Atre

2. Issues To Address Sheet Metal Processes Homework 4 (Due Tuesday)
Cutting
Bending
Homework 4 (Due Tuesday)

3. Three Major Sheet Metal Processes
Cutting
Shearing to separate large sheets, or cut part perimeters, or make holes in sheets
Bending
Straining sheet around a straight axis
Drawing
Forming of sheet into convex or concave shapes

4. Cutting: Blanking & Punching
(a) Blanking and (b) Punching

5. heat exchangers and reactors
Extending the concept
Microscale
heat exchangers and reactors
Paul et al, OSU
Micrograph courtesy of PNNL

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 Db;
Punch size determines hole size Dh.; c = clearance

11. Punch & Die Sizes for Blanking/Punching
For a round blank of diameter Db:
Blanking punch diameter = Db ‑ 2c
Blanking die diameter = Db
where c = clearance
For a round hole of diameter Dh:
Hole punch diameter = Dh
Hole die diameter = Dh + 2c

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 = Thus, c = at = 0.045(3.2) = mm
(a) Blanking punch diameter = Db - 2c
= (0.144) = mm
Blanking die diameter = Db = 65 mm
(b) Punching punch diameter = Dh = 30 mm
Punching die diameter = Dh + 2c = (0.144) = 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 = 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. both compression and tensile elongation of the metal occur in bending
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
Kba is factor to estimate stretching
Rule of Thumb: Compare bend radius to thickness
R < 2t Kba = 0.33
R  2t Kba = 0.50

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 Rb and included angle Ab' 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, Kbf = 1.33;
For edge bending, Kbf = 0.33

28. 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: press brake 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

31. Next Time Glass and Ceramics Processing Chapter 12 & 17
IE 337 Lecture 12: Forming 2
Next Time
Glass and Ceramics Processing
Chapter 12 & 17
S.V. Atre