MT-284
MANUFACTURING PROCESSES
INSTRUCTOR:
SHAMRAIZ AHMAD
MS-Design and Manufacturing Engineering
Shamraiz_88@yahoo.com
Topic:
Sheet Metal Working (Part-2)
By: Engr. Shamraiz
Sheet Metal Bending (Last Lecture)
Straining sheetmetal around a straight axis to
take a permanent bend
Figure 20.11 (a) Bending of sheet metal
By: Engr. Shamraiz
This Lecture
• Sheet metal work – Bending (Forming)
– Engineering Analysis of Bending
– Other Forms of Bending
• Sheet metal work – Drawing (Forming)
– Introduction to drawing operation
– Engineering Analysis of drawing
– Other Forms of drawing
– Defects in drawing
• Some other Sheet metal forminng operations
By: Engr. Shamraiz
Engineering Analysis of Bending
• Bending radius R is normally specified on the inside of the part, rather than at the
neutral axis. The bending radius is determined by the radius on the tooling used for
bending.
• Bending Allowance: If the bend radius is small relative to sheet thickness, the
metal tends to stretch during bending. Knowledge about Stretching is important to
have final part length according to specified dimensions.
• It is about determining the length of neutral axis before bending, in accordance
with final bend length. The bend allowance (BA) is the length of the arc of the
neutral line between the tangent points of a bend in any material.
– BA = 2pA(R + Kbat)/360
– Where BA – bend allowance(mm);
– A - bend angle, degrees;
– R – bend radius, in. (mm);
– t – sheet thickness; and
– Kba - factor to estimate stretching.
if R < 2t, Kba = 0.33;
and if R>=2t, Kba =0.5.
By: Engr. Shamraiz
Engineering Analysis of Bending
Spring back: When the bending pressure is removed at the end of
deformation, elastic energy remains in the bend part, causing it to recover
partially toward its original shape.
SB = (A’ – Ab’)/Ab’
Where SB – springback; A’ – included angle of sheet-metal part; and Ab’ –
included angle of bending tool, degrees.
By: Engr. Shamraiz
Engineering Analysis of Bending
Bending Force: The force required to perform bending depends on the geometry
of the punch and die and the strength, thickness, and width of the sheet metal. The
maximum bending force can be estimated by means of the following equation based
on bending of a simple beam:
F = (KbfTSwt2)/D
Where F – bending force, lb (N),; TS – tensile strength of the sheet metal, lb/in2. (Mpa); t
– sheet thickness, in. (mm); w- width of sheet metal and D – die opening dimension.
Kbf – a constant that counts for differences in an actual bending processes. For Vbending Kbf =1.33, and for edge bending Kbf =0.33
By: Engr. Shamraiz
Bending Problem
Metal to be bent with a modulus of elasticity E = 30x106 lb/in2., yield strength Y =
40,000lb/in2 , and tensile strength TS = 65,000 lb/in2. Determine (a) starting blank
size, and (b) bending force if V-die will be used with a die opening dimension D =
1.0in.
(a)
W = 1.75’, and the length of the part is: 1.5 +1.00 + BA.
R/t = 0.187/0.125 = 1.5 < 2.0, so Kba = 0.33
For an included angle A’ = 1200, then A = 600
BA = 2pA(R + Kbat)/360 =2p60(0.187 + 0.33 x 0.125)/360 = 0.239”
Length of the bank is 1.5+1+0.239 = 2.739”
(b)
Force:
F = (KbfTSwt2)/D
= 1.33 (65,000)(1.75)(0.125)2/1.0
= 2,364 lb
By: Engr. Shamraiz
Other Bending Operations
Other bending operations include:( Different from basic bending
due to curved axis and other different features)
a. Hemming involves bending the edge of the sheet over onto itself in
more than one bending step. This process is used to eliminate sharp
edges, increase stiffness, and improve appearance, such as the edges
in car doors.
b. Seaming is a bending operation in which two sheet metal edges are
joined together.
c. Curling (or beading) forms the edges of the part into a roll. Curling
is also used for safety, strength, and aesthetics.
(a) Hemming
(b) Seaming
(c) Curling
8
By: Engr. Shamraiz
Other Bending Operations
d. Flanging is a bending operation in which the edge of a sheet
metal is bent at a 90° angle to form a rim or flange. It is often
used to strengthen or stiffen sheet metal. The flange can be
straight, or it can involve stretching or shrinking as shown in the
figure below:
(a) Straight flanging
(b) Stretch flanging
(c) Shrink flanging
9
By: Engr. Shamraiz
3. Drawing Operation
Definition: Drawing is a Sheet metal forming to
make cup-shaped, box-shaped, or other
complex-curved, hollow-shaped parts . The
Sheet metal blank is positioned over die cavity
and then punch pushes metal into opening.
• Products: beverage cans, ammunition shells,
Cooking parts, automobile body panels
By: Engr. Shamraiz
Drawing Operation
(a) Drawing of
cup-shaped part: (1)
before punch contacts
work, (2) near end of
stroke; (b) workpart: (1)
starting blank, (2)
drawn part.
By: Engr. Shamraiz
Drawing Parameters
Clearance in Drawing
• Sides of punch and die separated by a clearance c
given by: the clearance in drawing is about 10%
of the stock thickness.
c = 1.1 t
where t = stock thickness
• In drawing as the punch moves downward, the
stock goes under complex stresses and strains
and is formed into the shape defined by punch
and die.
By: Engr. Shamraiz
Drawing Parameters
Friction:
The force being applied by punch is opposed by
the metal in form of deformation and friction
in the operation.
• Lubricants are generally used to reduce the
friction
By: Engr. Shamraiz
Drawing Parameters
Blank holding force:
• If this force is too small, the wrinkling occurs.
• If it is too large, it prevents metal from flowing
properly towards die cavity, resulting in
stretching and possible tearing of sheet metal.
• Determining proper holding force involves a
delicate balance between these opposing
factor.
By: Engr. Shamraiz
Engineering Analysis of Drawing
Tests of Drawing Feasibility
These tests provides information about
feasibility of the drawing operation before
actually performing the operation.
1. Drawing ratio
2. Reduction
3. Thickness-to-diameter ratio
By: Engr. Shamraiz
Engineering Analysis of Drawing
1. Drawing Ratio DR
The DR measures the severity of drawing operation. The
greater the ratio, the more severe the operation.
Most easily defined for cylindrical shape:
DR 
Db
Dp
where Db = blank diameter; and Dp =
punch diameter
– Upper limit: DR  2.0 (operation feasible)
By: Engr. Shamraiz
Engineering Analysis of Drawing
2. Reduction R:
• Defined for cylindrical shape:r  Db  Dp
Value of r should be less than 0.50
Db
3. Thickness-to-Diameter Ratio t/Db
•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
By: Engr. Shamraiz
Drawing Problem # 01
By: Engr. Shamraiz
Engineering Analysis of Drawing
Drawing Forces
The force required to perform drawing operation.
• F= Drawing Force, Db= Blank diameter, Dp= Punch Diameter
• t= Original blank thickness, TS= Tensile thickness
• 0.7 is correction factor to account for friction during operation
Drawing force varies throughout the downward movement of the punch
and usually reaches maximum at about one-third the length of the punch
stroke.
By: Engr. Shamraiz
Drawing Problem # 02
By: Engr. Shamraiz
Other Drawing Operations
1. Redrawing
• When too severe shape change is requires ( high drawing ratio),
complete forming may require more than one drawing steps.
• The second drawing step, and any further steps required, are
referred to as redrawing.
2. Drawing without Blank holder
• The blank holder is used to prevent wrinkling of the flange while
cup is being drawn.
• Tendency of wrinkling reduces as t/Db ratio increases
• So, when this ratio is large, the drawing can be done without
blank holders
• Governed by [ 5t > Db-Dp ]
• Without blank holder cost of tooling decreases and simple press
can be used.
By: Engr. Shamraiz
Other Drawing Operations
3. Shapes other than Cylindrical Cups
• Many shapes other than cylinder are also drawn.
• Square or rectangular boxes
• Stepped cups
• Cones
• Cups with spherical rather than flat bases
• Irregular curved forms (as in automobile body
panels)
Each of these shapes presents its own unique
technical problems in drawing
By: Engr. Shamraiz
Defects in Drawing
1. Wrinkling in the flange
•
•
Wrinkling is a major defect in drawing operation.
The movement of the blank into the die cavity induces compressive stress in the
flange causing series of ridges radially on undrawn part of flange.
• This can be reduced by keeping a blank holder under the effect of a holding force
2. Wrinkling in the wall
• When the flange is drawn into cup, these ridges appears into the vertical wall.
– t/D ↑ or Fh ↑ Wrinkling ↓
3. Tearing
• It is an open crack in the vertical wall, usually near the base of drawn cup which is
caused due to thinning and failure of metal in this location.
• It may also occur due to pulling metal over sharp die corner.
By: Engr. Shamraiz
Defects in Drawing
4. Earing
• This is the formation of irregularities (called ears)
in the upper edge of the deep drawn, caused by
anisotropy in the sheet metal
• If material is perfectly isotropic the earing does
not occur.
5. Surface scratch
• Scratches on surface of drawn parts appear when
surface of die and punch are not smooth or
sufficient lubricant is not used.
By: Engr. Shamraiz
4. Other Sheet Metal Forming on
Presses
In addition to bending and drawing, several
other sheet metal forming operations
performed on conventional presses. This
includes;
1. Ironing (using metal tooling)
2. Embossing (using metal tooling)
3. Hydro forming (using rubber forming
approach)
By: Engr. Shamraiz
1. Ironing
• A process which makes wall thickness of cylindrical cup
more uniform and it is normally done after drawing
• Beverage canes and artillery shell are ironed to increase
the life of material usage
(1) start of process; (2) during process. Note thinning and
elongation of walls.
By: Engr. Shamraiz
2. Embossing
• Creates indentations in sheet, such as raised (or indented)
lettering while maintaining the stock thickness uniform
Embossing: (a) cross-section of punch and die configuration during
pressing; (b) finished part with embossed ribs.
By: Engr. Shamraiz
3. Hydro forming
• Hydroforming is a specialized type of die forming that uses
pressurized hydraulic fluid in a diaphragm to form typical
metallic sheets in to a desired shape with a die cavity.
Hydroforming is a cost-effective way of shaping malleable
metals such as aluminium or brass into lightweight,
structurally stiff and strong pieces.
By: Engr. Shamraiz
Assignment # 02
Individual 6-pages assignment on
“Group Technology and Flexible Manufacturing
System”
Last date of submission:
16-05-2014
By: Engr. Shamraiz
Thank you
By: Engr. Shamraiz