1. Chapter 7 Sheet Metalworking Cutting Operations Bending Operations
Drawing
Other Sheet Metal Forming Operations
Dies and Presses for Sheet Metal Processes
Sheet Metal Operations Not Performed on Presses
Bending of Tube Stock
Prepared by:
Dr. Tan Soo Jin

2. Sheet Metalworking Defined
Cutting and forming operations performed on relatively thin sheets of metal
Thickness of sheet metal = 0.4 mm to 6 mm
If thickness of plate > 6 mm, referred as a plate rather than a sheet
Sheet or plate is produced by rolling.
Operations usually performed as cold working (at room temperature)
Thickness= 0.4 to 6 mm

3. Sheet and Plate Metal Products
Sheet and plate metal parts for consumer and industrial products such as
Automobiles and trucks
Airplanes
Railway cars and locomotives
Farm and construction equipment
Small and large appliances
Office furniture
Computers and office equipment

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

5. Sheet Metalworking Terminology
Punch‑and‑die - tooling to perform cutting, bending, and drawing
Stamping press - machine tool that performs most sheet metal operations
The term stamping die sometimes used for high production dies
Stampings - sheet metal products

6. 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

7. Cutting Operations

8. Cutting Shearing between two sharp cutting edges
Figure 1 ‑ Shearing of sheet metal between two cutting edges:
(1) In which upper cutting edge (punch) sweeps down past a stationary lower cutting edge (die)

9. Figure 1 ‑ Shearing of sheet metal between two cutting edges:
(2) As punch begins to push into work, causing plastic deformation occurs in the surface of the sheet.

10. Figure 1 ‑ Shearing of sheet metal between two cutting edges:
(3) As punch moves downward, punch compresses and penetrates into work causing a smooth cut surface. Penetration zone is 1/3 the thickness of the sheet.

11. Figure 1 ‑ Shearing of sheet metal between two cutting edges:
(4) As punch continues to travel into the work, fracture is initiated in the work at the two cutting edge.

12. Shearing, Blanking, and Punching
There are hree principal operations in pressworking that cut sheet metal by the shearing mechanism just described:
Shearing
Blanking
Punching

13. Shearing
Sheet metal cutting operation along a straight line between two cutting edges
Typically used to cut large sheets
Figure 2 Shearing operation: (a) side view of the shearing operation; (b) front view of power shears equipped with inclined upper cutting blade (cutting starts from one edge).

14. Blanking
Blanking –Cutting out the sheet metal along a closed outline in a single step to separate the piece from the surrounding stock.
The separate piece is called a blank.

15. Punching
Punching - similar to blanking except cut piece is scrap, called a slug . The remaining stock is the desired part.

16. Clearance in Sheet Metal Cutting
Distance between the punch and die
Typical values range between 4% and 8% of stock thickness
If too small, fracture lines pass each other, causing double burnishing and larger force
If too large, metal is pinched between cutting edges and excessive burr results

17. 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

18. Figure 2 ‑ Die size determines blank size Db; punch size determines hole size Dh.; c = clearance

19. Angular Clearance Purpose: allows slug or blank to drop through die
Typical values: 0.25 to 1.5 on each side

20. Cutting Forces Important for determining press size (tonnage)
F = S t L
where
S = shear strength of metal;
t = stock thickness, and
L = length of cut edge
(F = Stress x Area = σ. A)

21. Length of cut edge F = S t L For a rectangular shape: L=2(a+b)
For a circular shape:
L=2πR

22. Bending Operations

23. Sheet Metal Bending
Straining sheetmetal around a straight axis to take a permanent bend.
Metal is plastically deformed so that bend takes a permanent set upon removal of the stresses that caused it.
Bending produces little or no change in thickness of the sheet metal.
During bending operation, metal on inside of neutral plane is compressed, while metal on outside of neutral plane is stretched.

24. Types of Sheet Metal Bending
V‑bending - performed with a V‑shaped die
Edge bending - performed with a wiping die

25. V-Bending Sheet metal is bent between a V-shaped punch and die.
Generally used for low production
Performed on a press brake
V-dies are simple and inexpensive

26. Edge Bending
Pressure pad is used to apply a force, Fh to hold the base of the part against the die, while punch forces the part to yield and bend over the edge of the die
For high production
Dies are more complicated and costly

27. 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 final part length = specified dimension
Problem to solve by the designer: to determine the length of neutral axis of the part before bending
Bend radius is small. Metal has been stretched.

28. Bend Allowance Formula
where Ab = bend allowance (mm);
A = bend angle (o);
R= bend radius (mm);
t = stock thickness (mm); and
Kba = factor to estimate stretching
The following design values are recommended for Kba
If R < 2t, Kba = 0.33
If R  2t, Kba = 0.50

29. 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
Original shape
Desired deformed shape
Springback

30. Springback
Figure 5 Springback in bending is seen as a decrease in bend angle and an increase in bend radius: (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 ‘.

31. Bending Force
The force required to perform bending depends on the geometry of the punch-and-die and the strength, thickness and length of the sheet metal.
Maximum bending force estimated as follows:
where F = bending force (N); TS = tensile strength of sheet metal (MPa); w = part width in direction of bend axis (mm); and t = stock thickness (mm), D = die opening dimension (mm). For V- bending, Kbf = 1.33; for edge bending, Kbf = 0.33

32. Figure 7 ‑ Die opening dimension D: (a) V‑die, (b) wiping die

33. Drawing
Used to make cup-shaped, box-shaped, complex curved, hollow-shaped parts.

34. Drawing
Sheet metal forming to make cup‑shaped, box‑shaped, or other complex‑curved, hollow‑shaped parts
Sheet metal blank is positioned over die cavity and then punch pushes metal into opening
Products: beverage cans, ammunition shells, automobile body panels
Also known as deep drawing (to distinguish it from wire and bar drawing)

35. Drawing
Figure 6 (a) Drawing of cup‑shaped part: (1) before punch contacts work, (2) near end of stroke;
(b) workpart: (1) starting blank, (2) drawn part.

36. Clearance in Drawing Clearance= 1.1*thickness=> c = 1.1 t
Sides of punch and die separated by a clearance c given by:
Clearance= 1.1*thickness=> c = 1.1 t
In other words, clearance c is about 10% greater than stock thickness

37. Drawing Ratio DR
One of the measures of the severity of a deep drawing operations is, called drawing ratio.
Most easily defined for cylindrical shape:
where Db = blank diameter; and Dp = punch diameter
Upper limit on the drawing ratio is a value of 2.0

38. Reduction r Another way to characterize a given operation.
Very closely related to drawing ratio.
Again, defined for cylindrical shape:
Value of r should be less than 0.50

39. Other Sheet Metal Forming on Presses
Other sheet metal forming operations performed on conventional presses
Operations performed with metal tooling (ironing, coining and embossing)
Operations performed with flexible rubber tooling (Guerin process)

40. Ironing (Metal Tooling)
In deep drawing the flange is compressed by the squeezing action of the blank perimeter seeking a smaller circumference as it is drawn toward the die opening. Because of this compression, the sheet metal near the outer edge of the blank becomes thicker as it moves inward. If the thickness of this stock is greater than clearance between the punch and die, it will be squeezed to the size of the clearance.
Makes wall thickness of cylindrical cup more uniform
Examples: beverage cans and artillery shells
Figure 9 ‑ Ironing to achieve a more uniform wall thickness in a drawn cup: (1) start of process; (2) during process
Note thinning and elongation of walls

41. Coining and Embossing (Metal Tooling)
Used to create indentations in sheet, such as raised (or indented) lettering or strengthening ribs
Figure 10 ‑ Embossing: (a) cross‑section of punch and die configuration during pressing; (b) finished part with embossed ribs

42. Guerin Process (Rubber Tooling)
Uses a thick rubber pad (or other flexible material) to form sheet metal over a positive form block.
-The rubber pad is confined in a steel container.
-As the ram descends, the rubber gradually surrounds the sheet, applying pressure to deform it to the shape of the form block.
Figure 10 ‑ Guerin process: (1) before and (2) after
Symbols v and F indicate motion and applied force respectively

43. Advantages of Guerin Process
Low tooling cost
Form block can be made of wood, plastic, or other materials that are easy to shape
Rubber pad can be used with different form blocks
Process attractive in small quantity production

44. Dies for Sheet Metal Processes
Most pressworking operations performed with conventional punch‑and‑die tooling
Custom‑designed for particular part
The term stamping die sometimes used for high production dies

45. Figure 11 ‑ Components of a punch and die for a blanking operation
Punch holder also called upper shoe while die holder called lower shoe.

46. Figure 12 ‑
Progressive die;
associated strip development

47. Power and Drive Systems
Hydraulic presses - use a large piston and cylinder to drive the ram
Longer ram stroke than mechanical types
Suited to deep drawing
Slower than mechanical drives
Mechanical presses – convert rotation of motor to linear motion of ram
High forces at bottom of stroke
Suited to blanking and punching

48. Sheet Metal Operations Not Performed on Presses
A number of sheet-metal operations are not performed on conventional stamping presses.
Stretch forming
Roll bending and forming
Spinning
High‑energy‑rate forming processes.

49. Stretch Forming
Sheet metal is stretched and simultaneously bent to achieve shape change
Figure 17 ‑ Stretch forming: (1) start of process; (2) form die is pressed into the work with force Fdie, causing it to be stretched and bent over the form. F = stretching force

50. Force Required in Stretch Forming
where F = stretching force (N); L = length of sheet in direction perpendicular to stretching (mm); t = instantaneous stock thickness(mm); and Yf = flow stress of work metal (MPa)
Die force Fdie can be determined by balancing vertical force components

51. Roll Bending
Large metal sheets and plates are formed into curved sections using rolls
As the sheet passes between the rolls, the rolls are brought toward each other to a configuration that achieves desired radius of curvature on the work.

52. Roll Forming or Contour Roll Forming
Continuous bending process in which opposing rolls produce long sections of formed shapes from coil or strip stock
Figure 17 ‑ Roll forming of a continuous channel section:
straight rolls
partial form
final form

53. Spinning
Metal forming process in which an axially symmetric part is gradually shaped over a rotating mandrel using a rounded tool or roller
Three types:
Conventional spinning
Shear spinning
Tube spinning

54. Figure 18 ‑ Conventional spinning: (1) setup at start of process; (2) during spinning; and (3) completion of process

55. High‑Energy‑Rate Forming (HERF)
Processes to form metals using large amounts of energy over a very short time
HERF processes include:
Explosive forming
Electrohydraulic forming
Electromagnetic forming

56. Explosive Forming
Use of explosive charge to form sheet (or plate) metal into a die cavity
Explosive charge causes a shock wave whose energy is transmitted to force part into cavity
Applications: large parts, typical of aerospace industry

57. Figure 19 ‑ Explosive forming:
(1) setup, (2) explosive is detonated, and
(3) shock wave forms part and plume escapes water surface

58. Electromagnetic Forming
Sheet metal is deformed by mechanical force of an electromagnetic field induced in workpart by an energized coil
Presently the most widely used HERF process
Applications: tubular parts

59. Figure 20 ‑ Electromagnetic forming: (1) setup in which coil is inserted into tubular workpart surrounded by die; (2) formed part