Transcript INTRODUCTION

PRESS WORKING OPERATIONS(SHEET METAL)
PRESS WORKING OPERATIONS
(SHEET METAL)
CUTTING
BENDING
FORMING
DRAWING
BLANKING
BENDING
STRETCH FLANGING
CYLINDRICAL
CUP
DRAWING
PIERCING
FLANGING
SHRINK FLANGING
RECTANGULAR
SHELL
DRAWING
NOTCHING
HEMMING
REVERSE FLANGING
IRREGULAR
SHAPE
DRAWING
TRIMMING
HOLE FLANGING
LANCING
Embossing/
Form Beads
JOGGLE
THEORY OF CUTTING
When cutting sheet metal in a die the forces applied to
the sheet by the punch and die are basically shear
forces, that is, equal and opposite forces spaced at a
small distance apart on the metal and produce the
cutting.
The cutting or separation of the sheet metal is effected
through the following stages of shearing
• Roll over
• Penetration
• Fracture
From both punch side and Die side simultaneously
The spacing or the small distance of the two shearing
planes is called clearance.It will be generally about
8 - 10% of metal thickness for MS Sheets.
SHEET METAL CUTTING TERMINOLOGY
VARIOUS CUTTING OPERATIONS
BLANKING
It is the operation of cutting
or shearing a piece out of
sheet to a predetermined
contour
PIERCING
It is the operation of
producing the holes of round
or contoured shape in the
blanks or in formed parts.
NOTCHING
In Notching operation a small
piece or pieces of metal are
removed from the edges/
corners of a Strip or a Blank
TRIMMING
It is the operation of cutting
off the excess metal after the
Drawing or Forming
operation
LANCING
It is the operation of slitting
on three sides and opening
out by bending the lug about
an axis on the uncut fourth
side with in the blank.
THEORY OF BENDING AND SPRING BACK
•
•
•
•
BENDING
Bending is the process of folding a sheet about a straight line axis
which lies in the neutral plane.
Bends are made in sheet metal to gain rigidity, to produce a part of
desired shape & perform a particular function etc. The cross
section of the bend inward from neutral axis is in compression,
outward from neutral axis is in tension as shown in the fig.
SPRING BACK
During bending the metal nearer to the neutral axis is stressed to the
values below the elastic limit.This phenomenon creates a narrow
elastic band on both sides of the neutral axis.The metal further
away from the neutral axis is stressed beyond the yield strength
and is plastically deformed and permanently set. When the bending
force on the metal is released the elastic band tries to return to the
original flat condition but cannot return fully due to the restrictions
of the plastically deformed zones . Some slight return does occur
as the elastic and plastic zones come to an equilibrium and this
return is known as spring back.
The amount of spring back mainly depends on the ratio of bending
radius to stock thickness.
Spring back can be reduced by over bending, Bottoming or stretch
bending.
VARIOUS BENDING OPERATIONS
SIMLPE
BENDING
The operation of Folding the sheet about a
straight-line axis is called simple bending
FLANGING
It is similar to above in which the height of
bend is shorter compared to the overall size
of the part.
 It strengthens the edges of sheet metal
parts
 It provides flanges required for
assembling parts by spot welding or any
other joining processes.
HEMMING
Hemming is an operation in which the edge
0
of a component gets folded by 180 .
 It improves the rigidity of the edge
 It facilitates joining of two parts as in the
case of Bonnet assy. /Door assy.
DESIGN CONSIDERATIONS FOR BENDING
Radius of bend  T
Minimum height of bend  3T
Relief Notches for bend
DESIGN CONSIDERATIONS FOR HEMMING
Where appropriate provide an offset
 To minimise area of fitting
 To give additional rigidity to inner
panel
 To reduce potential assembly distortion

Height of flange after
0
+/- 1.0
90 flanging = 9.0
( except at corners and feature lines)
 Maximum permissible radius of bend
0
during 90 flanging = 0.5T
 Preferred radius of bend during 900
flanging = 0 (or as minimum as
possible)
THEORY OF FORMING
•
•
•
•
Forming is the process in which the
shape of the punch and die is directly
reproduced in the metal with little or
no metal flow.
Forming, Bending or drawing actions
may be combined in a die and is
classified as Form die or Draw die
depending on the dominant action of
process
The decision to use a form die instead
of a Draw die will depend largely on
the complexity of the shape &
geometrical criteria.
The use of draw die may be indicated,
if the form die would cause the metal
to tear because of excessive tensile
strain(stretch) or form objectionable
wrinkles because of excess crowding
of metal etc defects.
VARIOUS TYPES OF FORMING OPERATIONS

STRETCH
FLANGING


SHRINK
FLANGING

REVERSE
FLANGING

In stretch flanging tensile strain increases from zero at
the flange break line (axis of bending) to a maximum
at the flange edge. For that reason any tearing will
start from the flange edge.
The amount of tensile strain increases with increasing
forming angles and increasing flanging heights
In shrink flanging, the tendency to wrinkle increases
from zero at the flange break line to a maximum at the
flange edge.
Reverse flanging is the combination of stretch
flanging and shrink flanging
It is the process of forming a flange around a hole
HOLE
FLANGING
BEADS

JOGGLE

Beads are long narrow depressions embossed in the
sheet metal for stiffening purposes
It is a stepped surface provided at lap joint.
VARIOUS TYPES OF BEADS
ANGULAR These are used to stiffen the flat
BEADS
areas in large automotive panels
V-BEADS
These are used to stiffen the flat
areas of small and medium size
automotive panels
FLAT
V-BEADS
This is similar to above v-bead but
flat at the tip
These are commonly used for
small parts
These are generally used in draw
dies for controlling flow of material.
ROUND
BEADS
BEAKS
These are used to provide rigidity
and maintain accuracy in angle of
bend.
DESIGN CONSIDERATIONS FOR BEADS
ANGULAR
BEADS
V-BEADS
BEAKS
THEORY OF DRAWING
•
•
Drawing is a process in which the punch causes a flat , precut metal
blank in to the die cavity to assume the shape of seamless hollow
vessel without excessive wrinkling, thinning, or fracturing.
METAL FLOW IN DRAWING PROCESS
when the punch of draw die forces a portion of metal blank through
the bore of draw ring, different forces such as radial tension,
circumfrential compression and bending & frictional forces come in
to action as shown in the fig.These forces cause a complicated
plastic flow of the material in the blank. The volume and the
thickness of the component remain constant and the final shape of
the cup will be similar to contour of the punch.
The progressive stages of cupping are schematically shown in fig.
After a small stroke of the punch, cupping stage A, the metal
elements 2, 3, 4, & 5 of the blank move radially toward the center
of the blank. The flow of these elements go on till the final stage C
of the cup is reached. Thus by the end of the draw of the cup area 1
is unchanged in shape and size in the bottom of the cup. The areas
2,3 & 4 change from the shape of angular segments to longer
parallel-sided shapes and area 5 also changed in it’s size and shape.
Due to this metal flow phenomena in Drawing operation, Blank
holding pressure is to be applied on the blank in such a way that the
metal flow will be a controlled one so as to avoid wrinkling, tearing,
thinning etc.
DESIGN CONSIDERATIONS FOR CYLINDRICAL CUP DRAWING
 Empirical Rule to decide required
no. of Draws for Cylindrical cup
(Where h = inside height of shell
d = Mean dia. Of shell)
 Drawing Force for Cylindrical Cup
(Fd)
If  =h/d
No. of Draws
Up to 0.75 --------- 1
0.75 -- 1.5 --------- 2
1.5 -- 3.0 --------3
3.0 -- 4.5 --------4
Fd =  d t (Su + Sy)/2
Kg.
Where d = Punch diameter (mm.)
2
Su = Ultimate Tensile strength (Kg/mm )
2
Sy = Yield strength (Kg/mm )
T = Thickness of sheet. (mm.)
 Blank Holder Force(Fb)
Fb = 1/3 of Drawing force(Approx.)
 Empirical Rule for Punch and Die
radii.
Punch Radius =
Die radius
=
5t
6t
DESIGN CONSIDERATIONS FOR RECTANGULAR SHELL DRAWING
 Empirical Formulae to decide
required no. of Draws for
Rectangular shell
(Where h = inside height shell
r = Corner radius Of shell)
 Drawing Force Rectangular Shell
(Fr)
If  =h/r
No. of Draws
Up to 7 --------1
7 -13 --------- 2
13 -- 18 --------- 3
18 -- 24 --------- 4
Fr = t Su (2r + C1+L C2 )
Kg.
Where
2
Su = Ultimate Tensile strength (Kg/mm )
T = Thickness of sheet. (mm.)
r = Corner radius Of shell. (mm.)
L = Total length of straight sides of
rectangular shell
C1 = 0.5 – 2(lower values for shallow draw
Higher values for h/r > 0
C2 = 0.2 – 0.3 ( For easy to sever Draw
Conditions)
DESIGN CONSIDERATIONS FOR IRREGULAR SHAPE DRAWING
 Empirical Formulae to If e% < 5 , Part can be produced
by Forming
analyze the formability
If 5 < e% < 30 , Part requires Draw
of Irregular shaped
If e% > 30 , Part cannot be formed
shells
If e% > 5 Wrinkles tend to be shown
 General guide lines
related to Drawing
Operation
Where e% = Percentage of elongation
= (L – L0) x 100
L0 = Blank length before forming
L = Blank length after forming
e% = Difference in percentage of
elongation between neighboring
sections
 Draft angle on draw panel wall = 20 Min.
 Maximum permissible material thinning = 10%
 Material utilization Target = 70% and above
 Drawn corners to be achieved in one Draw