1. SHEET METAL WORKING PROCESSES
ME PRODUCTION PROCESSES II
CHAPTER 6
SHEET METAL WORKING PROCESSES
6.1 INTRODUCTION
Sheet metalworking includes cutting and forming operations performed on relatively thin sheets of metal (0.4-6 mm).
The tooling used to perform sheet metalwork is called punch and die. Most sheet metal operations are performed on machine tools called presses.
The term stamping press is used to distinguish these presses from forging and extrusion presses. The sheet metal products are called stampings.
The commercial importance of sheet metalworking is significant.
The number of consumer and industrial products that include sheet metal parts: automobile and truck bodies, airplanes, railway cars and locomotives, farm and construction equipment, small and large appliances, office furniture, computers and office equipment, and more. Sheet metal parts are generally characterized by high strength, good dimensional tolerances, good surface finish, and relatively low cost.
CHAPTER 6 SHEET METAL WORKING PROCESSES

2. These are usually cases of warm working rather than hot working.
ME PRODUCTION PROCESSES II
Sheet-metal processing is usually performed at room temperatures (cold working).
The exemptions are when the stock is thick, the metal is brittle, or the deformation is significant.
These are usually cases of warm working rather than hot working.
CHAPTER 6 SHEET METAL WORKING PROCESSES

3. The three major categories of sheet-metal processes:
ME PRODUCTION PROCESSES II
The three major categories of sheet-metal processes:
cutting (shearing, blanking, piercing)
bending
drawing.
Cutting is used to separate large sheets into smaller pieces, to cut out a part perimeter, or to make holes in a part.
Bending and drawing are used to form sheet metal parts into their required shapes.
Fig.6.1 Some cutting operations
CHAPTER 6 SHEET METAL WORKING PROCESSES

4. Classification of Sheet Metalworking Processes
ME PRODUCTION PROCESSES II
Classification of Sheet Metalworking Processes
Fig.6.2 Basic sheet metalworking operations:
(a) bending,
(b) drawing, and
(c) shearing;
(1) as punch first contacts sheet and
(2) after cutting.
Force and relative motion are indicated by F and v
CHAPTER 6 SHEET METAL WORKING PROCESSES

5. Classification of Sheet Metalworking Processes
ME PRODUCTION PROCESSES II
Classification of Sheet Metalworking Processes
Fig.6.2 Basic processes involved in forming sheet metal components. (a) Processes involving local deformation.
CHAPTER 6 SHEET METAL WORKING PROCESSES

6. ME 333 PRODUCTION PROCESSES II
6.2. PIERCING AND BLANKING
A commonly used piercing-blanking die set and related terms are shown in the following figure.
Fig.6.3 Components of a punch and die
CHAPTER 6 SHEET METAL WORKING PROCESSES

7. ME 333 PRODUCTION PROCESSES II
Blanking and punching
Blanking and punching are similar sheet metal cutting operations that involve cutting the sheet metal along a closed outline.
If the part that is cut out is the desired product, the operation is called blanking and the product is called blank.
If the remaining stock is the desired part, the operation is called punching. Both operations are illustrated on the example of producing a washer:
Starting stock produced by shearing operation from a big metal sheet
Fig.6.4 Steps in production of washer
CHAPTER 6 SHEET METAL WORKING PROCESSES

8. ME 333 PRODUCTION PROCESSES II
The cutting of metal between die components is a shearing process in which the metal is stressed in shear between two cutting edges to the point of fracture, or beyond its ultimate strength.
The metal is subjected to both tensile and compressive stresses; stretching beyond the elastic limit occurs; then plastic deformation, reduction in area, and, finally, fracturing starts and becomes complete.
Blanking punch diameter= Db-2c
Blanking die diameter= Db
Hole punch diameter= Dh
Hole die diameter= Dh+2c
Fig.6.5 Shearing of sheet metal between punch and die
CHAPTER 6 SHEET METAL WORKING PROCESSES

9. ME 333 PRODUCTION PROCESSES II
The cutting of metal between die components is a shearing process in which the metal is stressed in shear between two cutting edges to the point of fracture, or beyond its ultimate strength. The metal is subjected to both tensile and compressive stresses; stretching beyond the elastic limit occurs; then plastic deformation, reduction in area, and, finally, fracturing starts and becomes complete.
Fig.6.5 Shearing of sheet metal between punch and die
CHAPTER 6 SHEET METAL WORKING PROCESSES

10. Engineering analysis of metal cutting:
ME PRODUCTION PROCESSES II
Engineering analysis of metal cutting:
Cutting of sheet metal is accomplished by a shearing action between two sharp edges. The shearing action is illustrated in the figure:
Fig Shearing of sheet metal between two cutting edges:
just before the punch contacts work;
punch begins to push into work, causing plastic deformation;
punch compresses and penetrates into work, causing a smooth cut surface; and
fracture is initiated at the opposing cutting edges that separate the sheet.
Symbols v and F indicate motion and applied force, respectively.
Fig.6.3 Shearing
CHAPTER 6 SHEET METAL WORKING PROCESSES

11. ME 333 PRODUCTION PROCESSES II
At the top of the cut surface is a region called the rollover. This corresponds to the depression made by the punch in the work prior to cutting. It is where initial plastic deformation occured in the work.
Just below the rollover is a relatively small region called the burnish. This results from penetration of the punch into the work before fracture began.
Beneath the burnish is the fractured zone, a relatively rough surface of the cut edge where continued downward movement of the punch caused fracture of the metal.
Finally, at the bottom of the edge is a burr, a sharp corner on the edge caused by elongation of the metal during final seperation of the two pieces.
CHAPTER 6 SHEET METAL WORKING PROCESSES

12. 6.2.1. Engineering Analysis_CLEARANCE
ME PRODUCTION PROCESSES II
Engineering Analysis_CLEARANCE
Process parameters in sheet metal cutting are clearence between punch and die, stock thickness, type of metal and its strength and length of the cut
Clearance c in a shearing operation is the space between the mating members of a die set (e.g.punch and die).
For optimum finish of cut edge, proper clearance is necessary and is a function of the kind, thickness, and hardness of the work material.
In an ideal cutting operation the punch penetrates the material to a depth equal to about 1/3 of its thickness before fracture occurs, and forces an equal portion of the material into the die opening.
Common die clearances (linear clearance) are 2-5% of the material thickness.
Angular clearance is gradient given to the hole in the die such that cut material will easily be removed. Angular clearance is usually ground from 0.25⁰ to 1.5⁰ per side.
CHAPTER 6 SHEET METAL WORKING PROCESSES

13. The correct clearance depends on sheet-metal type and thickness t:
ME PRODUCTION PROCESSES II
The correct clearance depends on sheet-metal type and thickness t:
c = a*t
where a is the allowance (a = for steels and for aluminum alloys).
If the clearance is not set correctly, either an excessive force or an oversized burr can occur:
Fig.6.7 Effect of clearance: (Left) clearance too small causes less than optimal fracture and excessive forces, and (Right) clearance too large causes oversized burr.
CHAPTER 6 SHEET METAL WORKING PROCESSES

14. ME 333 PRODUCTION PROCESSES II
Figure (a) Effect of the clearance, c, between punch and die on the deformation zone in shearing.
As the clearance increases, the material tends to be pulled into the die rather than be sheared. In practice, clearances usually range between 2% and 10% of the thickness of the sheet. (b) Microhardness (HV) contours for a 6.4-mm (0.25-in) thick AISI 1020 hot-rolled steel in the sheared region.
CHAPTER 6 SHEET METAL WORKING PROCESSES

15. The calculated clearance value must be;
ME PRODUCTION PROCESSES II
The calculated clearance value must be;
substracted from the die punch diameter for blanking operations or
added to die hole diameter for punching:
Fig.6.8
Die diameter is enlarged with clearance c in punching.
In blanking, the punch diameter is decreased to account for clearance.
D is the nominal size of the final product.
CHAPTER 6 SHEET METAL WORKING PROCESSES

16. ME 333 PRODUCTION PROCESSES II
An angular clearance must be provided for the die hole to allow parts to drop through it:
Fig.6.9 Angular clearance for the die opening in punching and blanking.
CHAPTER 6 SHEET METAL WORKING PROCESSES

17. ME 333 PRODUCTION PROCESSES II
CUTTING FORCE
The pressure (or stress) required to cut (shear) work material is;
(for round holes)
(for any contours)
where;
S= shear strength of material, kg/mm2
D= hole diameter, mm
L= shear length, mm
t= material thickness, mm
For example to produce a hole of 20mmX20mm in a material 2mm in thickness with 40 kg/mm2 shear strength:
P= 40 kg/mm2x(2x20+2x20)mmx2mm
P= 40x160 kg= 6400 kg force is required.
CHAPTER 6 SHEET METAL WORKING PROCESSES

18. 6.2.3 TOOLS AND DIES FOR CUTTING OPERATIONS
ME PRODUCTION PROCESSES II
6.2.3 TOOLS AND DIES FOR CUTTING OPERATIONS
Simple dies
When the die is designed to perform a single operation (for example, cutting, blanking, or punching) with each stroke of the press, it is referred to as a simple die:
Fig.6.10 The basic components of the simple blanking and punching dies
CHAPTER 6 SHEET METAL WORKING PROCESSES

19. Multi-operational dies
ME PRODUCTION PROCESSES II
Multi-operational dies
More complicated pressworking dies include:
compound die to perform two or more operations at a single position of the metal strip
progressive die to perform two or more operations at two or more positions of the metal strip
Fig.6.11 Method of making a simple washer in a compound blanking and punching die
CHAPTER 6 SHEET METAL WORKING PROCESSES

20. Multi-operational dies
ME PRODUCTION PROCESSES II
Multi-operational dies
Schematic illustrations: (a) before and (b) after blanking a common washer in a compound die. Note the separate movements of the die (for blanking) and the punch (for punching the hole in the washer). (c) Schematic illustration of making a washer in a progressive die. (d) Forming of the top piece of an aerosol spray can in a progressive die. Note that the part is attached to the strip until the last operation is completed.
CHAPTER 6 SHEET METAL WORKING PROCESSES

21. ME 333 PRODUCTION PROCESSES II
6.2.4 CENTRE OF PRESSURE
Sheet metal part that to be blanked is of irregular shape the summation of shearing forces on one side of the center of the ram may greatly exceed the forces on the other side. This result in bending and undesirable deflections might happen. Center of pressure is a point, which the summation of shearing forces will be symmetrical. This point is the center of gravity of the line that is the perimeter of the blank. It is not the center of gravity of the area. α
Fig.6.14 Center of pressure for some shapes
CHAPTER 6 SHEET METAL WORKING PROCESSES

22. Procedure to find center of pressure:
ME PRODUCTION PROCESSES II
Procedure to find center of pressure:
Divide cutting edges into line elements, 1,2,3, ...
Find the lengths l1, l2, l3, ...
Find the center of gravity of each element as x1, x2, x3, ..., y1, y2, y3, ...
Calculate the center of pressure from:
CHAPTER 6 SHEET METAL WORKING PROCESSES

23. ME 333 PRODUCTION PROCESSES II
EXAMPLE
Find the center of pressure and the required cutting force of the following blank (S=40 kg/mm2 and t=2mm).
Element l X Y
(l)(x)
(l)(y) 1
4.00
0.00
6.25
25.00 2
4.71
1.50
9.20
7.05
43.33 3
3.20
7.00
12.80
22.40 4
2.50
5.00
10.00
12.50 5
3.00
4.25
4.50
12.75 6
1.57
1.00
TOTAL
18.98
35.92
115.98
and the cutting force is;
P= LtS 189.8mmx2mmx40kg/mm2 =15184 kg
CHAPTER 6 SHEET METAL WORKING PROCESSES

24. 6.2.5 REDUCING CUTTING FORCES
ME PRODUCTION PROCESSES II
6.2.5 REDUCING CUTTING FORCES
Since cutting operations are characterized by very high forces exerted over very short periods of time, it is some times desirable to reduce the force and spread it over a longer portion of the ram stroke.
Two methods are frequently used to reduce cutting forces and to smooth out the heavy loads.
Step the punch lengths; the load may thus be reduced approx. 50%.
Tapering the punch; grind the face of the punch or die at a small shear angle with the horizontal. This has the effect of reducing the area in shear at any time, and may reduce cutting force as much as 50%. The angle chosen should provide a change in punch length of about 1.5 times of material thickness. It is usually preferable to a double cut to prevent setup of lateral force components.
Fig.6.15 Different configurations for reducing the cutting force
CHAPTER 6 SHEET METAL WORKING PROCESSES

25. ME 333 PRODUCTION PROCESSES II
Fig.6.– Effect of different clearances when punching hard and soft alloys
CHAPTER 6 SHEET METAL WORKING PROCESSES

26. 6.3 SCRAP-STRIP LAYOUT FOR BLANKING
ME PRODUCTION PROCESSES II
6.3 SCRAP-STRIP LAYOUT FOR BLANKING
In designing parts to be blanked from strip material, economical strip utilization is of high importance. The goal should be at least 75% utilization.
where;
t : thickness of the stock,
W: width of the stock,
B: space between part and edge (1.5t),
C: lead of the die (L+B),
L&H: dimensions of the work piece.
CHAPTER 6 SHEET METAL WORKING PROCESSES

27. Locating the work piece for maximum economy is very important.
ME PRODUCTION PROCESSES II
Locating the work piece for maximum economy is very important.
CHAPTER 6 SHEET METAL WORKING PROCESSES

28. ME 333 PRODUCTION PROCESSES II
HOMEWORK:
If two strips (250 mm and 125 mm width) are available for the production of 100 mm blanks, which one have to be preferred for maximum material utilization?
CHAPTER 6 SHEET METAL WORKING PROCESSES

29. ME 333 PRODUCTION PROCESSES II
6.4 BENDING
Bending is defined as the straining of the sheet metal around a straight edge:
Fig Bending of sheet metal
CHAPTER 6 SHEET METAL WORKING PROCESSES

30. ME 333 PRODUCTION PROCESSES II
Bending operations involve the processes of V-bending and edge bending:
Fig.6.16 (Left) V-bending, and (Right) edge bending; (1) before and (2) after bending
V-bending—sheet metal is bent along a straight line between a V-shape punch and die.
Edge bending—bending of the cantilever part of the sheet around the die edge.
CHAPTER 6 SHEET METAL WORKING PROCESSES

31. ME 333 PRODUCTION PROCESSES II
Bending is the process by which a straight length is transformed into a curved length. It is a very common forming process for changing sheet and plate into channel, tanks, etc.
For a given bending operation the bend radius can not be made smaller than a certain value, otherwise the metal will crack on the outer tensile surface. Minimum bend radius is usually expressed in multiples of the sheets thickness. It varies considerable between different metal and always increases with cold working. Bend radius is not less than 1 mm and for high strength sheet alloys the minimum bend radius may be 5t or higher.
R - bend radius
BA - bend allowance
 - bend angle
L0 - original length
t - sheet thickness
Lf  L0
Lf=L1+L2+BA Rmin>5t practical
CHAPTER 6 SHEET METAL WORKING PROCESSES

32. where A is the bend angle; t is the sheet thickness;
ME PRODUCTION PROCESSES II
This is the stretching length that occurs during bending. It must be accounted to determine the length of the blank,
where Lb is the length of the blank, L are the lengths of the straight parts of the blank, BA is the bend allowance,
Fig.6.17 Calculation of bend allowance
where A is the bend angle; t is the sheet thickness;
R is the bend radius; Kba is a factor to estimate stretching, defined as follows:
Kba = 0.33 for R < 2t
Kba = for R ≥ 2t
CHAPTER 6 SHEET METAL WORKING PROCESSES

33. ME 333 PRODUCTION PROCESSES II
The minimum bend radius for a given thickness of sheet can be predicted fairly accurately from the reduction of area measured in tension test, Ar.
for Ar< 0.2,
for Ar> 0.2,
Another common problem is springback. It is the dimensional change of the formed part after pressure of the forming tool has been removed. It results from the change in strain produced by elastic recovery.
CHAPTER 6 SHEET METAL WORKING PROCESSES

34. To compensate for springback two methods are commonly used:
ME PRODUCTION PROCESSES II
The commonest method of compensating for springback is to bend the part to a smaller radius of curvature than is desired so that after springback the part has the proper radius.
Springback is the elastic recovery leading to the increase of the included angle when the bending pressure is removed.
To compensate for springback two methods are commonly used:
Overbending—the punch angle and radius are smaller than the final ones.
Bottoming—squeezing the part at the end of the stroke.
Fig.6.19 Compensation of springback by:
(a) and (b) overbending; (c) and (d) bottoming
Fig.6.18 Springback in bending
CHAPTER 6 SHEET METAL WORKING PROCESSES

35. The maximum bending force is estimated as
ME PRODUCTION PROCESSES II
The force required bending a length L about a radius R may be estimated from;
Bending forces
The maximum bending force is estimated as
where Kbf is the constant that depends on the process, Kbf = 1.33 for V-bending and Kbf = 0.33 for edge bending; w is the width of bending; D is the die opening dimension as shown in the figure:
Fig.6.20 Die opening dimension D, (a) V-bending, (b) edge bending
CHAPTER 6 SHEET METAL WORKING PROCESSES

36. Equipment for bending operations
ME PRODUCTION PROCESSES II
Equipment for bending operations
Fig.6.22 Dies and stages in the press brake forming of a roll bead
Fig.6.21 Press brake with CNC gauging system
CHAPTER 6 SHEET METAL WORKING PROCESSES

37. 6.5 DEEP DRAWING (Derin Çekme)
ME PRODUCTION PROCESSES II
6.5 DEEP DRAWING (Derin Çekme)
Deep drawing is the metal working process used for shaping flat sheets into cup-shaped articles such as bathtubs, shell cases, and automobile fenders. Generally a hold down or pressure pad is required to press the blank against the die to prevent wrinkling. Optional pressure pad from the bottom may also be used.
Fig.6.23 Drawing of a cup shaped part
CHAPTER 6 SHEET METAL WORKING PROCESSES

38. Deep drawing of a cup-shaped part
ME PRODUCTION PROCESSES II
Deep drawing of a cup-shaped part
Fig.6.24 Deep drawing of a cup-shaped part: (Left) start of the operation before punch contacts blank, and (Right) end of stroke
CHAPTER 6 SHEET METAL WORKING PROCESSES

39. ME 333 PRODUCTION PROCESSES II
In the deep drawing of a cup the metal is subjected to three different types of deformations. In the flange part, as it is drawn in, the outer circumference must continuously decrease from that of the original blank Do to that of the finish cup Dp. This means that it is subjected to a compressive strain in the hoop (tangential) direction and a tensile strain in the radial direction. As a result of these principal strains, there is a continual increase in the thickness as the metal moves inward. However, as the metal pass over the die radius, it is first bend and then straightened while at the same time being subjected to a tensile stress. This plastic bending under tension results in considerable thinning. Punch region is under very little stress.
Fig.6.23 Types of deformations in different region during deep drawing of a cup shaped part
CHAPTER 6 SHEET METAL WORKING PROCESSES

40. ME 333 PRODUCTION PROCESSES II
Clearance
Clearance c is the distance between the punch and die and is about 10% greater than the stock thickness:
c = 1.1t
Holding force
The improper application of the holding force can cause severe defects in the drawn parts such as (a) flange wrinkling or (b) wall wrinkling if the holding force is too small, and (c) tearing if the folding force is overestimated.
Fig.6.25 Defects in deep drawing of a cup-shaped part
CHAPTER 6 SHEET METAL WORKING PROCESSES

41. P = total punch load, o= average flow stress,
ME PRODUCTION PROCESSES II
The force on the punch required to produce a cup is the summation of the ideal force of deformation, the frictional forces, and the force required to produce ironing. Mathematical calculation of the drawing force is very complex. Following approximate equation is developed:
where;
P = total punch load, o= average flow stress,
d = punch diameter, D = blank diameter,
H = hold drawn force, B = force required to bend,
t = wall thickness,  = coefficient of friction,
 = efficiency
Drawing force may be calculated for practical purposes by:
when LDR  2 (Limiting Drawing Ratio)
CHAPTER 6 SHEET METAL WORKING PROCESSES

42. Some of the practical considerations which affect drawability: Rd 10t
ME PRODUCTION PROCESSES II
The drawability of a metal is measured by the ratio of the blank diameter to the diameter of the cup drawn from the blank (usually accepted as punch diameter). For a given material there is a Limiting Drawing Ratio (LDR), representing the largest blank that can be drawn through a die without tearing.
Where,  is an efficiency term to account for frictional losses. If =1, then LDR=2.7 while =0.7, LDR2 which is used in most practical applications.
Some of the practical considerations which affect drawability:
Rd 10t
Rp should be big enough to prevent tearing.
Clearance between punch and die; 20 to 40% greater than “t”.
Hold-down pressure; 2% of o and lubricate die walls
The diameter of blank required to draw a given cup may be obtained approximately by equating surface areas.
and
where; h is height of cup.
CHAPTER 6 SHEET METAL WORKING PROCESSES

43. ME 333 PRODUCTION PROCESSES II
6.5.1 REDRAWING
If the shape change required by the part design is too severe (limiting drawing ratio is too high, or LDR is not sufficient to form a desired cup), complete forming of the part require more than one drawing step. The second drawing step and any further drawing steps if needed, are referred to as redrawing. Throat angle is 10-15. Redrawing is generally done in decreasing ratios as given below:
(D/d)= 1.43, 1.33, 1.25, 1.19, 1.14 and 1.11.
If these redrawing steps are not enough to reach required cup diameter, annealing have to be performed and then redrawing can be performed.
Fig.6.26 Redrawing of a cup
CHAPTER 6 SHEET METAL WORKING PROCESSES

44. ME 333 PRODUCTION PROCESSES II
6.5.2 EXAMPLE:
A  200 mm blank is to be drawn to a 50 mm cup. Estimate the minimum number of draws required using the drawing ratios given below:
Draw
1st
2nd
3rd
4th
5th
6th
Ratio
1.43
1.33
1.25
1.19
1.14
1.10
Solution:
LDR=2 D/d = 200/50 = 4 > 2
So that redrawing is necessary. 1. 4. 2. 5. 3. 6.
56.38>50
CHAPTER 6 SHEET METAL WORKING PROCESSES

45. ME 333 PRODUCTION PROCESSES II
Therefore annealing should be applied. But it might be better to anneal the blank before 6th draw to reduce number of redraws. We know that LDR=2. So that if annealing is performed after 3rd draw where D3 = mm, than ratio to reach required cup diameter is:
< 2
Therefore, after 3rd draw, blank is annealed and then redraw with a ratio of 1.68 to obtain required cup diameter. The required number of drawing is then 4.
CHAPTER 6 SHEET METAL WORKING PROCESSES

46. 6.6 OTHER SHEET-METAL FORMING OPERATIONS
ME PRODUCTION PROCESSES II
6.6 OTHER SHEET-METAL FORMING OPERATIONS
The Guerin process
The Guerin process involves the use of a thick rubber pad to form sheet metal over a positive form block:
Fig.6.27 The Guerin process: (Left) start of the operation before rubber pad contacts sheet, and (Right) end of stroke
CHAPTER 6 SHEET METAL WORKING PROCESSES

47. Examples of equipment and products manufactured by the Guerin process:
ME PRODUCTION PROCESSES II
Examples of equipment and products manufactured by the Guerin process:
Fig.6.29 A large number of different components can be made simultaneously during one press cycle with rubber pad presses
Fig.6.28 Rubber pad press showing forming tools on the press table
Advantages: small cost of tooling
Limitations: for relatively shallow shapes
Area of application: small-quantity production
CHAPTER 6 SHEET METAL WORKING PROCESSES

48. Advantages: small cost of tooling Limitations: simple shapes
ME PRODUCTION PROCESSES II
Hydroforming
It is similar to Guerin process but instead of rubber pad a rubber diaphragm filled with fluid is used:
Fig Hydroform process:
(1) start-up, no fluid in the cavity;
(2) press closed, cavity pressurized with hydraulic fluid;
(3) punch pressed into work to form part.
Symbols:
v - velocity,
F – applied force, and
p - hydraulic pressure
Advantages: small cost of tooling
Limitations: simple shapes
Area of application: small-quantity production
CHAPTER 6 SHEET METAL WORKING PROCESSES

49. Advantages: small cost of tooling, large parts
ME PRODUCTION PROCESSES II
Stretch forming
In stretch forming the sheet metal is stretched and bent to achieve the desired shape:
Fig.6.31 Stretch forming: (1) start of the process; (2) form die is pressed into the work causing it to stretched and bent over the form. Symbols: v - velocity, Fdie - applied force
Advantages: small cost of tooling, large parts
Limitations: simple shapes
Area of application: small-quantity production
CHAPTER 6 SHEET METAL WORKING PROCESSES

50. Advantages: small cost of tooling, large parts (up to 5 m or more)
ME PRODUCTION PROCESSES II
Spinning
Spinning is a metal forming process in which an axially symmetric part is gradually shaped over a mandrel by means of a rounded tool or roller:
Fig.6.32 In spinning operation, flat circular blanks are often formed into hollow shapes such as photographic reflectors. In a lathe, tool is forced against a rotating disk, gradually forcing the metal over the chuck to conform to its shape. Chucks and follow blocks are usually made of wood for this operation
Advantages: small cost of tooling, large parts (up to 5 m or more)
Limitations: only axially symmetric parts
Area of application: small-quantity production
CHAPTER 6 SHEET METAL WORKING PROCESSES

51. HIGH-ENERGY-RATE FORMING (HERF)
ME PRODUCTION PROCESSES II
HIGH-ENERGY-RATE FORMING (HERF)
These are metal forming processes in which large amount of energy is applied in a very short time. Some of the most important HREF operations include:
Explosive forming
It involves the use of an explosive charge placed in water to form sheet into the die cavity.
Fig.6.33 Explosive forming: (1) set-up, (2) explosive is detonated, and (3) shock wave forms part
CHAPTER 6 SHEET METAL WORKING PROCESSES

52. Advantages: small cost of tooling, large parts
ME PRODUCTION PROCESSES II
Explosively formed elliptical dome 3-m in diameter being removed from the forming die
Fig.6.34 Explosively formed elliptical dome 3-m in diameter being removed from the forming die
Advantages: small cost of tooling, large parts
Limitations: skilled and experienced labor
Area of application: large parts typical of the aerospace industry
CHAPTER 6 SHEET METAL WORKING PROCESSES

53. Electrohydraulic forming
ME PRODUCTION PROCESSES II
Electrohydraulic forming
This is a HREF process in which a shock wave to deform the work into a die cavity is generated by the discharge of electrical energy between two electrodes submerged in water. Similar to explosive forming, but applied only to small part sizes.
Fig.6.35 Setup of electrohydraulic forming
CHAPTER 6 SHEET METAL WORKING PROCESSES

54. Electromagnetic forming
ME PRODUCTION PROCESSES II
Electromagnetic forming
The sheet metal is deformed by the mechanical force of an electromagnetic field induced in the workpiece by a coil:
Fig.6.36 Electromagnetic forming: (1) set-up in which coil is inserted into tubular workpiece surrounded by die, (2) formed part
Advantages: can produce shapes, which cannot be produced easily by the other processes
Limitations: suitable for magnetic materials
Area of application: most widely used HERF process to form tubular parts
CHAPTER 6 SHEET METAL WORKING PROCESSES

55. ME 333 PRODUCTION PROCESSES II
HOMEWORK:
If two strips (250 mm and 125 mm width) are available for the production of 100 mm blanks, which one have to be preferred for maximum material utilization?
CHAPTER 6 SHEET METAL WORKING PROCESSES

56. THE END 13.04.2017 CHAPTER 6 SHEET METAL WORKING PROCESSES
ME PRODUCTION PROCESSES II
THE END
CHAPTER 6 SHEET METAL WORKING PROCESSES

57. CHAPTER 6 SHEET METAL WORKING PROCESSES
ME PRODUCTION PROCESSES II
CHAPTER 6 SHEET METAL WORKING PROCESSES