1. Shaping operations are generally divided into three groups based upon how the parent metal flows or deforms during the shaping process ,namely
Bending
Forming
Drawing

2. Bending :
Bending is defined as shaping metal around a straight axis which extends completely across the material.
The result is a plane surface at an angle to the original plane of the work piece material.
Metal flow is uniform along the bend axis , with inner surface of the bend in compression and the outer surface in tension.

3. Forming:
Forming is defined as shaping metal around a curved axis instead of a straight one.
A part that is formed generally takes the shape of the punch or die.
Metal flow is not uniform .

4. Drawing:
Drawing is the name given to the production of cups , shells , boxes and similar articles from metal blank.
Drawing is the most complicated of the three sheet metal shaping process.
A drawing operations begins with a flat blank which is transformed into a cup or shell.
The parent metal is subjected to severe plastic deformation.

5. Bottoming Dies : Figure Please See Class Note ( Punch bottoms out ).
Bending Methods
V Bending
Air Bending Dies
Bottoming Dies
Wiping Bending
In Case Of,
Air Bending Dies : Figure Please See Class Note (Punch does not bottom out).
Bottoming Dies : Figure Please See Class Note ( Punch bottoms out ).

6. Air Bending : When the punch does not bottom out then it is called air bending.
Advantages Of Air Bending Dies :
One set of die & punch can be used to bend w/p with different thickness to various angles
Least pressure Is needed
Limitations Of Air Bending Dies :
Less Accuracy
Spring back is more.(tendency of material to be straight for elastic property )

7. Bottoming Dies :
The punch is allowed to bottom out at the V bend dies . The material is squeezed between the punch & die at the end of the punch stroke .
Advantages of Bottoming Dies:
Higher degree of accuracy
Sharper inside corners
Less spring back
Limitations of Bottoming Dies:
Only one type of bending can be done by one set of die and punch
3-10 times more pressure than air bending

8. Wiping Dies:
The work piece is clamped to a die block by a spring loaded or fluid cylinder pressure pad and the punch wipes the extended material over one edge of the die.

9. Spring Back:
When the force used to create the bend is removed, the recovery of the elastic region results in the occurrence of spring back.
Spring back is the partial recovery of the work from the bend to its geometry before the bending force was applied.
The magnitude of spring back depends largely on the modulus of elasticity and the yield strength of the material.
Typically the results of spring back will only act to increase the bend angle by a few degrees, however, all sheet metal bending processes must consider the factor of spring back.

11. Methods of Preventing Spring Back:
Over bending :
It is used for V Die air Bending . Some more bending is made than required , then due to spring back it results in desired bending.
Corner Setting :
Spring back is prevented by coining (squeezing) the metal slightly at the corner in order to relieve elastic stresses.

12. Methods Of Corner Setting :
Making the edge of the punch slightly flat.
Making the edge of the punch slightly rough.
Ironing:

13. Methods Of Ironing:
Fig: 10-6 (a)
Fig:10-6(b)
Fig:10-6(c)

14. Bend Allowance:
As a sheet metal is bent (Fig), its fibres experience a distortion such that those nearer its outside, convex surface are forced to stretch and come in tension, while the inner fibres come in compression. Somewhere, in the cross section, there is a plane which separates the tension and compression zones. This plane is parallel to the surface around which the sheet is bending, and is called neutral axis. The position of neutral axis depends on the radius and angle of bend.
       BEND ALLOWANCE
It is the length of the neutral axis in the bend,(Fig.) This determines the blank length needed for a bent part.

15. Fig: BEND ALLOWANCE

17. Spring back prevention in U dies:

18. Determination of Bend Allowance :
B = (A/360) * 2π (IR + kt)
Where,
B = Bending allowance (arc length of neutral axis in inch )
A = Bend angle ( in degree )
IR = Inside radius of bend ( in inch )
t = Metal thickness ( in inch )
k = constant for neutral axis location
Use , k = 0.33 for IR < 2t
k = 0.50 for IR > 2t

19. For Figure Please See Class Note
Determine the blank length of the following product :
For Figure Please See Class Note
Blank Length , L = L₁ + L₂ + L₃ + L₄ + L₅
L = 2L₁ + 2L₂ + L₃ ; [ L₁ = L₅ , L₂ = L₄ ]
L₁ = L₅ = 2 – (IR + t) = 2 – (3/ 6 + 1/8 ) = inch
L ₃ = 4 – 2(IR + t) = 4 – [ 2 (3/16 + 1/8) ] = 3.375inch

20. L₂ = L₄ = Bend Allowance
= (A/360) * 2π (IR + k *t)
= (90/360) * 2 * ( 3/ * 1/8 ) =
Here , IR = 3/16 , 2t = 2 * 1/8 = 4/16
So, IR <2t ; k = 0.33
L = 2L₁ + 2L₂ + L₃
= 2 * *
= “

21. Bending Pressure /Force:
F = ( kLst² ) / W
Where,
F = Bending force required ( in tons )
L = Length of the bend part ( in inch )
s = Ultimate tensile strength (in ton / in² )
t = Metal thickness
k = Die opening Factor
w =Width between contact points on die
Cont,d

22. k = 1.33 ; when Die opening = 8*Material thickness
For Air bending
k = 1.33 ; when Die opening = 8*Material thickness
k = 1.20 ; when Die opening =16*Material thickness
For U bending and channel bending
k = 0.67 ;
For Wiping bending
k = 0.333
W is determined as shown in fig:
For both U bending & wiping ,
W = R₁ + R₂ + C , C = t

23. Problem : Determine the bending force in tons for a 45degree bend in 24ST3 aluminium [with s = psi] ; in thickness ; 4ft long . The bend is air bending . The die opening is 8 times the material thickness .
Solution : Here , k = 1.33 ; ( as the die opening is 8 times the material thickness)
L = 4ft = 48inch
t = inch
s = psi
W = 8 * inch

24. Problem : Calculate the bending force in tons for the channel bend
Problem : Calculate the bending force in tons for the channel bend. The bend length is 36 inch and die radius is inch[ s = psi ](Fig please see class note)
Solution : Here , k = 0.67; ( For U bending )
L = 36inch
t = c = 1/8
s = psi
W = R₁ + R₂ + C
= /16 + 1/8 =

25. Forming dies – types :
Solid form dies
Pad type form dies
Curling dies
Embossing dies
Coining dies
Bulging dies
Assembly die

26. Drawing Dies : Types of Drawing Dies : Shallow Drawing when h/d ≤ 0.5
Deep Drawing when h/d > 0.5
Where ,
h = outside height
d = outside diameter
Percent Reduction and depth of draw :
Percent reduction , P = 100 ( 1 – d/D )
where P= Percentage reduction, d=ID of drawn shell, D= OD of blank
Maximum allowable reduction in
one stroke / one draw ,
Theoritical ≈ 50 %
Practical ≈ 40 %
Example: The drawing of a 3 inch dia cup from a 5 in dia blank results in a 40% reduction.
Percent reduction , P = 100 ( 1 – d/D ) = 100 ( 1 – 3/5 ) = 40 %

27. Table 10.4 : (Donaldson)
Possible number of reduction for a given ratio of shell height to diameter :
Ratio h/d
Numbers of reduction
Reduction
1st Draw
2nd Draw
3rd Draw
4th Draw
Upto 0.75 1 40
0.75 – 1.5 2 25
1.5 – 3.0 3 15
3.0 – 4.5 4 10

28. √[(d–2r)²+4d(h-r)+2πr(d-0.7r)]
Blank size determination :
The following equations can be used to calculate the blank size for cylindrical shells of relatively thin metal.
D = Blank Diameter ,
d = Cup/Shell diameter (outside)
h = Cup/Shell height (outside) ,
r = Corner radius of punch
Blank size determination : (contd.)
D =
√(d² + 4dh)
If d/r ≥ 20
√((d² + 4dh) – 0.5 r)
If d/r = 15~20
√((d² + 4dh) – r)
If d/r =10~15
√[(d–2r)²+4d(h-r)+2πr(d-0.7r)]
If d/r < 10

29. Blank size determination : (contd.)
Use 1st equation when d/r is 20 or more
Use 2nd equation when d/r is between
Use 3rd equation when d/r is between
Use 4th equation when d/r is less than 10
Trim allowance: The extra material added to the blank diameter is referred to as trim allowance
Final/ Actual Blank Size = D + Trim Allowance
General Rule for trim allowance : Add 1/8 inch for cup dia upto 1inch , then add 1/8inch per extra 1inch dia.
Example:0 .5 inch should be added to the blank diameter for a cup diameter of 4 inch.

30. Drawing Force :
The amount of force required to shape a symmetrical cup by drawing can be calculated from the following formula from the ASTME “Die design handbook”.
P = πdts ( D/d – c )
Where ,
P = Drawing force/pressure.
d = Cup/shell outside diameter.
t = Metal thickness.
s = Yield strength of the material.
D = Blank diameter.
C = Constant to cover friction and bending( 0.6 to 0.7 for ductile materials)