1. SHEET METALWORKING
Cutting Operations
Bending Operations
Drawing
Other Sheet Metal Forming Operations

2. Advantages of Sheet Metal Parts
High strength
Good dimensional accuracy
Good surface finish
Relatively low cost
Economical mass production for large quantities

3. Basic Types of Sheet Metal Processes
Cutting
Shearing to separate large sheets
Blanking to cut part perimeters out of sheet metal
Punching to make holes in sheet metal
Bending
Straining sheet around a straight axis
Drawing
Forming of sheet into convex or concave shapes

4. Sheet metal cutting examples: Shearing, Blanking, & Punching
(1) punch before contact, clearance ‘c’ between punch and die
(2) punch causes material to plastically deform
(3) smooth cut surface is formed
(4) fracture initiated at the opposing cutting edges which separates the sheet.

5. 1. Shearing
Sheet metal cutting operation along a straight line between two cutting edges
Typically used to cut large sheets
(a) side view of the shearing operation; (b) front view of power shears equipped with inclined upper cutting blade.

6. 2. Blanking and Punching
Blanking - sheet metal cutting to separate piece (called a blank) from surrounding stock
Punching - similar to blanking except cut piece is scrap, called a slug
(a) Blanking and (b) punching.

7. Clearance in Sheet Metal Cutting
Distance between punch cutting edge and die cutting edge (typically 4 - 8% of thickness)
If too small (a), fracture lines pass each other, causing double burnishing and larger force
If too large (b), metal is pinched between cutting edges and excessive burr results

8. Clearance in Sheet Metal Cutting
Recommended clearance is calculated by:
c = a∙t
where c = clearance; a = allowance; t = stock thickness
Allowance is determined according to type of metal:
Metal group a _
aluminum alloys (1100, 5052)
aluminum alloys (2024 and 6061); brass,
soft cold rolled steel, soft stainless steel
cold rolled steel, stainless steel, (hard & half-hard) 0.075

9. Punch and Die Sizes For a round blank of dia. Db and clearance c:
Blanking punch diameter =
Blanking die diameter =
For a round hole of dia. Dh and clearance c :
Hole punch diameter =
Hole die diameter =

10. Angular Clearance Purpose: allows slug or blank to drop through die
Typical values: 0.25 to 1.5 on each side
Cutting Force:
F = S t L
where S = shear strength;
t = stock thickness,
L = length of cut edge
(disregard ‘c’)
1 ton ≈ 8896 N

11. Progressive Die

12. Types of Sheet Metal Bending
V‑bending - performed with a V‑shaped die
Performed on a press brake
V-dies are simple and inexpensive
Edge bending - performed with a wiping die
Pressure pad required
Dies are more complicated and costly

13. Stretching during Bending
If bend radius is relative to stock thickness, metal tends to stretch during bending
Important to estimate amount of stretching, so final part length = specified dimension
Problem: to determine the length of neutral axis of the part before bending

14. Bend Allowance Formula
where Ab = bend allowance; = bend angle; R= bend radius; t = stock thickness; Kba is factor to estimate stretching
If R < 2t, Kba = 0.33
If R  2t, Kba = 0.50

15. 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, remains in bent part,
(1) during bending, the work is forced to take radius Rb and included angle b' of the bending tool, (2) after punch is removed, the work springs back to radius R and angle ‘.

16. Bending Force Maximum bending force estimated as follows:
where F = bending force;
TS = tensile strength;
w = part width;
t = stock thickness.
D = die opening
For V- bending, Kbf = 1.33;
For edge bending, Kbf = 0.33

17. Drawing
Sheet metal forming to make cup‑shaped, box‑shaped, or other hollow‑shaped parts
clearance c = 1.1 t
where t = stock thickness
In other words, clearance is about 10% greater than stock thickness

18. Tests of Drawing Feasibility
Drawing ratio
where Db = blank diameter; and Dp = punch diameter
Indicates severity of a given drawing operation
Upper limit: DR  2.0
Reduction
Value of r should be less than 0.50 for a cylinder
Thickness-to-diameter ratio
Thickness of starting blank divided by blank diameter
Desirable for t/Db ratio to be greater than 1%
As t/Db decreases, tendency for wrinkling increases

19. Blank Size Determination
For final dimensions of drawn shape to be correct, starting blank diameter Db must be right
Solve for Db by setting starting sheet metal blank volume = final product volume
To facilitate calculation, assume negligible thinning of part wall

20. Other Sheet Metal Forming
Ironing
Embossing
The Guerin process
Stretch forming
Roll bending
Roll forming
Spinning

21. Ironing Makes wall thickness of cylindrical cup more uniform
Ironing to achieve more uniform wall thickness in a drawn cup: (1) start of process; (2) during process. Note thinning and elongation of walls.

22. Embossing
Creates indentations in sheet, such as raised (or indented) lettering, or
Embossing: (a) cross‑section of punch and die configuration during pressing; (b) finished part with embossed ribs.

23. Guerin Process Advantages: Low tooling cost
Rubber pad can be used with different form blocks
Suited for prototype production

24. Stretch Forming Sheet metal is stretched and simultaneously bent:
F = stretching force; L = length in direction perpendicular to stretching (e.g ); t = thickness; Yf = flow stress
Initially assume ε = , calculate Yf =
As ‘length’ is increased ‘t’ is
Find tf by conservation of volume: wotoLo=
Die force Fdie can be determined by balancing vertical force components: Fdie =

25. Roll Bending
Large metal sheets and plates are formed into curved sections using rolls

26. Roll Forming
Continuous bending process in which opposing rolls produce long sections of formed shapes from coil or strip stock

27. Spinning
Metal forming process in which an axially symmetric part is gradually shaped over a rotating mandrel using a rounded tool or roller
Conventional spinning to = tf, Final diameter?
Shear spinning tf = to sin α Final diameter?

28. Quotes:
True friendship is like [good] health; the value of it is seldom known until it is lost. - Charles Colton
Humility is the solid foundation of all other virtues.- Confucius
No person was ever honored for what he received. Honor has been the reward for what he gave. - Calvin Coolidge
Money-giving is a good criterion of a person’s mental health. Generous people are rarely mentally ill people. - Dr. Karl Menninger