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| 46 | s h e e t m e t a l W O R K I N G T e c h n o l o g y
Analysing sheet metal forming
Mar/Apr 2011|EM
Asheet has a large surface area and a
small thickness, so that one must
pay attention to restrict further
thinning. Processes involving thinning
require the use of a larger initial thicknessincreasing the material input. Non-uniform
thinning is worse, as this necessitates the
use of better quality of sheet and/or a
thicker sheet. A machined product like a
gear on the other hand, is manufacturable
using a number of alternate machining
processes. In sheet metal products also,
alternate processing routes for a given
product are feasible, like spinning and
deep drawing for instance, but this calls
for very different sheet metal properties
for the two processes. The very process
of dimensioning in sheet metal productsdiffer from that of machined parts. So does
the process of inspection. Unlike machined
parts, the feature (usually a hole) that is most
unlikely to get deformed/displaced from its
original position or unlikely to change its
form or dimension during processing is
taken as the reference. If no such hole is
available then a feature that gets formed in
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Prof P P DateDepartment of Mechanical EngineeringIIT Mumbai, Powai
Manufacturability of a sheet metal part strongly depends on the interplay
between the process and material properties, making sheet metal processselection a crucial step. Differing from machining, which is often held
synonymous to manufacturing, at every stage, it is clear that sheet metal
forming is a completely different world altogether. Prof P P Date
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Sample of how sheet metal parts are dimensioned. Note that the edges do not get distorted, butstill their position is controlled with reference to a hole
the very first stage and that remains with
the product unchanged throughout, may
be selected. All dimensions are specified in
relation to this feature, and also inspected
using the same as reference.
Sheet metal forming
The large diversity of shapes to which a
sheet is deformed is largely asymmetric,while a large volume of machined products
is axisymmetric. Process design (design of
the processing route) is therefore crucial
in sheet forming. One pays attention
to the distribution of strain so as to
monitor thinning of the sheet. For this, a
grid of circles is etched/printed onto the
sheet and the distortion of these circles
is monitored. Unlike machining, sheet
forming is a non-steady state process. The
zone of deformation continuously evolves
over time. For instance, initially the sheet
wraps around the punch and even later
the shape and size of the zone undergoingdeformation changes continuously. The
points of maximum and minimum strain
shift from one location to another with
deformation. Similarly, the cross section
encompassing these two zones can also
change as deformation progresses. The
purpose of testing a raw material that
can be machined for its properties is
largely for the in-service requirements, as
the machinability of a material is not as
sensitive to raw material properties as the
sheet metal formability. Hence, wherever
the properties to satisfy in-service
requirements are in conflict with those
needed for formability, special processing
techniques and long processing routes
are required. Excess material than what
is necessary can similarly be damaging tothe quality of the sheet formed parts while
excess material to be machined would only
need an extra cut and hence slow down a
machining process. There is no finishing
operation for sheet metal and the surface
finish can be improved to a very limited
extent. Hence, handling and storage of
sheet metal coils as well as formed parts
is important.
Machining vs sheet metal
Sheet metal parts with errors cannot be
salvaged. They have to be discarded. Unlikemachining wherein machining occurs only
on the surface being machined and bulk of
the raw material remains unaffected, the
entire sheet metal blank participates in
deformation. Selection of the machine tool
(stiffness, the drive, etc,) for a sheet metal
operation depends on the material and
process being performed. For instance, the
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A sheet metal inspection fixture with sheet
metal part clamped on it
Sr. no. Point of comparison Machined components Sheet metal parts
1 Rigidity of parts Non-compliant parts Compliant parts
2 Designing and
dimensioning
Orthogonal machined datum
planes for dimensional
reference
Features that will not shift/get deformed
required as reference. Holes usually serve as
reference edges / blank boundaries w hich
no longer remain straight go into trim scrap.
3 Dimensional
gradients / deviations
Unintended dimensional
gradients can be avoided.
Unintended dimensional gradients eg.,
thinning of sheet metal and those from
springback are a part and parcel of the process.
4 Tolerances achievab le Close tolerances (in microns or
nanometers) can be achieved.
Elastic after-effects, part compliance and
variables not under complete contro l prevent
close tolerances from being achieved.
5 Zone (volume) of
metal affected by the
process at a given
time
Only the surface or a thickness
of metal close to the surface
participates in the machining
process, the bulk of the
material is relatively unaffected
The entire sheet metal blank gets
deformed (eg., flows into the die)
6 Rework and ease of
rework
Out of tolerance parts can be
salvaged.Salvaging an out-of tolerance part is not
possible.
7 Diversity of shapes Any shape can be made with
appropriate tool path.
Large diversity of tool designs and control
variables.
8 Purpose of testing of
raw material
Machinability, in service
requirements.
Formability and in-service requirements.
9 Tool materials Machinability is strongly
associated with tool life, which
in turn strongly depends on
tool material.
Tool material change does not ensure
formability. Tool geometry and sheet
metal properties together determine
formability.
10 Standardisation of
tool geometry and
control parameters
Easy, standards on tool angles,
standard tool geometries of
single and multi-point cutting
tools are widely available.
Only guidelines are feasible.
Standardisation across components is
impractical.
11 Cost of tooling Much lower. Single tool works
to generate many shapes
irrespective of complexity of
the tool. Smaller lead time in
procuring tools.
Tools are expensive as they are not mass
produced. Change of tool is expensive and
difficult. Lead time in making a new tool
is large.
12 Machine tools Machine tool drives are mechanical
drives which generate the requisite
tool paths that can be very complex.
Accuracy of tool path is crucial.
Cutter compensation, collision checks
are important. Forces involved are
much lower than in forming.
Machine tool drives are primarily built for
delivering power (heavy duty drives). The
forces involved are large and machine tool
frame stiffness is a major consideration.
Avalability of drive force, speed and
energy must be ensured after tool contact.
13 Springback and
residual stresses
Extremely small. Machining
relieves surface residual
stresses.
Un-released elastic stresses (springback)
on account of product geometry appear as
residual stresses. It is shape dependent and
processing history dependent.
14 In-process
stability of shape
Stable shape throughout. Better
form accuracy of holes as well
as external surfaces.
In-process stability of shape is determined
by an interplay between the residual
stresses and separating operations like
trimming and punching. Holes punched
with a circular punch might not
necessarily be round.
15 Inspection Form and dimensional errors
of individual parts can be
inspected. Assembly occurs by
selective assembly.
Inspection is performed at well defined
critical points only. Non-ideal,
untoleranced parts are required to get the
assembly to within dimensional
tolerances.
16 Fixture design 1-2-3 principle used for
location in fixtures.
n-2-1 principle (where n > 3) is required
for sheet metal parts. Clamping sequence
is more critical due to compliance of sheet
metal parts.
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variation / variability of velocity over the
stroke is required for some processes (long
stroke processes like deep drawing to ensure
fast approach & return and slow working
stroke) and materials (the rate sensitive
ones in particular). Hence, what can be
successfully formed on a hydraulic pressmight fail on a mechanical press. Stiffness of
the machines used for separating processes
is required to be much higher than those
for long stroke operations. Comparatively,
the machining process is not so sensitive
to machine tool characteristics as the
sheet forming process. The machinability
does change with machining speed, but
is not as sensitive to the machine tool
characteristics as the formability is. Dies
designed for steel as work material seldom
work for aluminum as work material, other
parameters (product design, machine tool,etc,) remaining the same. Hence a change
in raw material might call for a complete
change in the tool design or at least
substantial rework of the existing dies. A
change in work material to be machined
may call for a change in tool geometry,
but once standardised, all it requires is a
change of tool. Elastic after-effects in the
form of springback and residual stresses are
prominent in sheet formed parts leading
to inconsistent dimensions. Tolerances on
the rolled sheet thickness and those on
the yield strength introduce unpredictable
variations in the springback, depending onthe magnitudes of these two variables.
The given table gives a complete
comparison between sheet metal
parts and the machined ones with
reference to a good number of
manufacturing variables.