Chapter 4:
Sheet Metal Forming
4.3 USING PATTERNS
AND CUTTING METALS
Shafizan Bt. Shariffuddin
School of Manufacturing Engineering
UniMAP
Using Patterns and Making
Drawings
•
•
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Sheet metal articles are made of flat pieces of
metal cut according to outlines that are drawn
or traced on the sheets of metal.
To obtain the current size and shapes, patterns
are used.
These patterns may be drawn on paper first,
then transferred to the metal, or they may be
laid out directly on the metal.
Definition
a) Templates or master pattern- patterns
that are used repeatedly and are made
of metal.
b) Stretchout – the distance across the flat
pattern or flat piece of metal before it is
formed into shape. The illustration in
Figure 1 shows the stretchouts for
square and cylindrical jobs.
Figure 1. Stretchouts for square and cylindrical objects.
c) Layout – refers to the methods of
developing the lines which form the
patterns. The common layout methods
are :i. Parallel line development
ii. Radial line development
iii. Triangulation
i) Parallel line development
• Parallel line development is based upon the fact
that a line that is parallel to another line is an
equal distance from that line at all points.
• Objects that have opposite lines parallel to each
other or that have the same cross-sectional
shape throughout their length are developed by
this method.
• To gain a clear understanding of the parallel line
method, we will develop, step by step, a layout
of a truncated cylinder (fig. 2(a)).
Figure 2(a): Truncated cylinder
Figure 2(b): Development of a truncated cylinder
This piece of sheet metal is developed in the
following procedure:
1. First, draw a front and bottom view by
orthographic projection (fig.2(b), view A).
2. Divide half the circumference of the circle (view
A) into a number of equal parts. The parts
should be small enough so that when straight
lines are drawn on the development or layout
between division points, they will approximate
the length of the arc. Project lines from these
points to the front view, as shown in view B.
These resulting parallel lines of the front view
are called ELEMENTS.
3. Lay off the baseline, called the STRETCH-OUT
LINE, of the development to the right of the front
view, as shown in view C.
4. Divide the stretch-outline into twice the number of
equal parts equal to each division of the
circumference on the half circle of the
orthographic view (view C).
5. Erect perpendicular lines at each point, as shown
in view C.
6. Using a T-square edge, project the lengths of the
elements on the front view to the development
(View D).
7. Using a curve (french or other type), join the
resulting points of intersection in a smooth curve.
When the development is finished, add necessary
allowances for warns and joints, then cut out your
patterns.
ii) Radial line development
• The radial line method of pattern
development is used to develop patterns
of objects that have a tapering form with
lines converging at a common center.
Figure 3: Example of radial line development
iii) Triangulation
• Triangulation is slower and more difficult than
parallel line or radial line development, but it is
more practical for many types of figures.
• Additionally, it is the only method by which the
developments of warped surfaces may be
estimated.
• In development by triangulation, the piece is
divided into a series of triangles as in Radial line
development.
Figure 4: Triangulation of a warped transition piece join ing a large, square duct
and a small, round duct
d)
Pictorial drawings – show the object as it actually
appears after formed into shaped. This is illustrated in
Figure 5. Such a drawing cannot serve as means of
giving accurate information for the fabrication of the
project because the true shape and size of the object
is not shown.
Figure 5: Pictorial drawings show the type of objects as they appear after forming.
e) Mechanical drawing – show the exact
size and shape of each size. See Figure
6. It is only necessary to make the
number of view required to show the size
and shape of the object.
Figure 6. The white areas in the drawing at the right are mechanical or working drawings.
The drawings at the left and top show the placement of various views in relation to each
other.
Pattern Information
•
The master pattern should contain all of
the allowances and details necessary to
fabricate the job. These includes :a) Allowances for edges
– Different types of edges are used to stiffen
the edges of the sheet metal articles, and to
eliminate the sharp edges. The amount of
metal allowed for the edge depends upon
the type of edge used.
b) Allowances for seaming
– Sheet metal parts are joined by seams of various
kinds. The addition of the seams makes it necessary
to add material to the pattern.
*Allowance?
In engineering and machining, an allowance is a
planned deviation between an actual dimension and a
nominal or theoretical dimension.
For example, an area of excess metal may be left
because it is needed to complete subsequent
machining.
c) Prick punching brake lines
– The place or line where the metal to be bend or
folded is called brake line. On metal patterns, the
brake line are prick punched and shown by ‘x’. See
figure 7.
Figure 7
d) Notching and clipping
– Notching and clipping are used to cut away portions
of the metal to prevent over – lapping and budging on
seams and edges. The operations are different as
illustrated by Figure 8.
Figure 8: Notching and clipping
Cutting Metals
Cutting straight and curved lines
• The sheet metal worker generally uses
four different snips for all his cutting.
1) The bulldog snips are used for general
cutting in heavy metal – 24 gage or thicker.
2) The combination snips are also for general
cutting from 24 gage and thinner.
3) Right-hand airplane snip.
4) Left-hand airplane snip
– Both right-hand and left-hand airplane snips
are used for both heavy and light steel metal.
They can cut to 16 gage, cut very small land
complex curves, also to cut inside circles and
inside square, as shown in Figure 9.
Figure 9. Airplane snips are used to make small or complex cuts.
√
X
√
√
X
Figure 10 : proper and improper ways of cutting
•
Care and use of snips
1. Keep the small piece of metal over the
bottom blade of the snips.
2. Trim off excess metal before making the cut
on the line.
3. Whenever possible, rest the blade and
handle of the snips on the workbench.
4. When notching keep the end of the snips
blade at the point where the notch will end.
5. Keep oil from the blades snips.
6. Cut only sheet metal with snips.
Bench Lever Shear
• A shear which is used in almost every sheet
metal shop is shown in Figure 11.
• This is a heavy duty shears able to cut to 10 mm
thick metal with ease. They have a compound
leverage system for greater power.
• When using this type of sheets, keep the good
piece of metal over the lower blade and run the
scrap piece under blade since the piece that goes
under the upper blade will be curled and
disturbed by the cutting action.
• The blades are designed to cut curves, and
circles as well as straight lines.
• It posses a fixed lower blade and a moving
upper blade.
• The sheet being cut is prevented from tilting by a
clamping device which can be adjusted to the
thickness of the sheet.
• The knife-edge cutter of the upper blade is
curved so that the opening angle at the point of
cut remains constant.
Figure 11: Bench Lever Shear
Guillotine Shear
• The guillotine shear below may be set for cutting steel or
non-ferrous metal sheet, as well as plastics, wire netting
etc. The extremely logical construction of the shear,
produces several special new features, of which the
following deserving of mention :– Rear stop bar can be operated from the front with fast movement
and visual display scale.
– No lubrication point
– Quick blade gap adjustment
– Unobstructed view from above of the material being cut – no
parallex errors.
– Variable cutting table adjustment
– Low noise etc.
Guillotine Shear With Pivoting
Top Beam
1.
2.
3.
4.
5.
6.
Figure 12
Back stop
adjusting and
clamping lever
Reading
Optics
Treadle Switch
Main Switch
Cutting Gap
Adjusting
Device
Hang-up Peg
Figure 13