Singh R. Introduction to Basic Manufacturing Processes and Workshop Technology
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358 Introduction to Basic Manufacturing Processes and Workshop Technology
5. Dovetail seam
It is used to join a flat plate to a cylindrical piece. To layout such a seam, draw a line
parallel to one edge of sheet metal component at a distance of 6 to 20 mm. depending upon
the size of the hole of sheet metal part. Then draw lines to indicate where the sheet metal
part is to be slit. The width of the piece between slits ranges from 6 to 26 mm.
6. Flanged or burred bottom seam
It is used to fasten the bottom of a container made of sheet metal to its body in which
upper part is the sheet metal body and lower bottom of a container. To layout such a seam
on sheet metal part, draw a circle, which represents the outline of the bottom on sheet metal
part. If it is square or rectangular component, draw the bottom shape on the sheet metal part.
Then draw a second line to show the width of the flange. The width of this flange may range
from 3 to 6 mm on sheet metal part.
18.4.3 Notches
In the process of laying out on sheet metal part, some provision is to be made for bent
sections when they have folded edges. The opening left at corners of seams and edges of sheet
metal part are called notches. Five common types of notches are: (1) Straight notch, (2) V-
notch, (3) Square notch and (4) Slant notch.
1. Straight notch. It is prepared by making a straight cut where the bend is to occur
in sheet metal part.
2. V-notch. It is used where the corners of a flange
should fit together in sheet metal part. It is used for
laying out 90° and other than 90° angle on the sheet
metal part
3. Square notch. It is applicable where square or
rectangular box (made up of sheet metal) is formed.
4. Slant notch. It is used where single hems are to
meet at right angles. In this process they should be
clipped at 45° angle on the sheet metal part.
18.4.4 Folded Sheet Metal Joints
As per requirements, various types of sheet metal joints
are used in sheet metal work. Such joints are commonly
used to fasten bottoms to cylinders, square or rectangular
containers. Some of the commonly used such joints are shown
in Fig. 18.11.
1. Lap joint. It is very frequently used in sheet metal
work and can be prepared by means of soldering or
riveting.
2. Seam joint. This joint is a very generally used in
sheet metal work. It is locked, as shown in the
diagram, so as to ensure a positive grip and also to
make the joint flush with the surface.
3. Locked seam joint. This joint is used for locking seam joints in sheet metal work.
Lap
Seam
Locked seam
Hem
Wired ed
g
e
Cup or
circular
Flan
g
ed
Cap
Fig. 18.11 Various sheet
metal joints
Sheet Metal Work 359
4. Hem (single and double), wired edge, cup and angular joints. These enable
the edges sheet metal part to join the pieces along them.
5. Flanged joint. It is commonly used in sheet metal work frequently in making pipe
connections.
6. Cap joint. It provides another useful form of locked seam joint on sheet metal
component with good appearance and strength. It is generally used for assembling
cross seam of ducts made up of in sheet metal part.
18.5 SHEET METAL OPERATIONS
The major sheet metal operations carried out in sheet metal work are as follows:
1. Cleaning 2. Measuring
3. Marking 4. Laying out
5. Hand cutting 6. Hand shearing
7. Hand forming 8. Edge forming
9. Wiring 10. Joint making
11. Bending 12. Drawing
13. Soldering 14. Circle cutting
15. Machine shearing 16. Nibbling
17. Piercing 18. Blanking
The measuring and marking operations carried over on sheet metal are discussed as
under.
Measuring and Marking
The standard sizes of metal sheets available in the market are quite large. But the
required sheet size for making a component may be smaller and hence a standard size sheet
may have to be therefore cut into several smaller pieces. Each piece must be sufficient for
making one such component as per the needed size. Smaller sizes of sheet metal part are first
decided and are then marked on the larger metal sheet to cut the latter into small pieces
along the marked lines. A little allowance for cutting is always incorporated to the required
overall sizes. The overall dimensions of the required smaller sizes are marked on the larger
sheet with the help of marking tools such as a steel rule, a straight edge, a steel square and
a scriber. The sheet surface may have to be coated with a coloring media so that the scribed
lines arc clearly visible. If circular pieces are needed, a divider or trammel may be used to
mark the circles.
18.6 DEVELOMENT OF PAT TERN LAYOUT
Projective geometry forms the basis of successful layout of patterns for making different
jobs in sheet metal work. Most of the patterns are obtained from development of surfaces of
some common geometrical solids like cylinder, prism, pyramid and cone. These shapes may
be required to be developed either in single or in combination. A sheet metal planner should
have a thorough idea of the procedures employed in developing the surfaces of the above
solids and their inter-penetrations.
360 Introduction to Basic Manufacturing Processes and Workshop Technology
Sheet metal work consists mainly of cutting, forming and assembling the object that has
been laid out on flat metal sheet. The outline of the object or pattern is first either scratched
on the sheet or drawn on a paper and then transferred to the sheet. Metal sheet is then cut
to various shapes and sizes as per the pattern or shape of object, which may be later,
transformed to various articles by using different sheet metal processes. In the process of
laying out a pattern, certain allowances are kept for making edges and seams. The development
of surface is the unfolding or unrolling sheet in one plane. The sheet is used to mark correctly
using scriber. Important attention is also invited regarding accounting for the allowance in
preparing edges and seams to be made for making joints. The notches of metal pieces are
then cut. Finally the edges are then turned to form box as shown through step in Fig. 18.12.
Square notch
Ed
g
e turned to form pan
Notched pattern
Fig. 18.12 Pattern development of sheet metal part
18.7 MACHINES USED IN SHEET METAL SHOP
The various machines, which are in use to perform different operations on metal sheets,
can be listed as follows:
1. Shearing machine
2. Bending machine
3. Folding machine
4. Grooving machine
5. Peining machine
6. Beading machine
7. Swaging machine
8. Burring machine
9. Double seaming machine
Apart from the above, many other special purpose machines are also manufactured to suit
a particular kind of work especially in mass production of identical parts. The above machines
vary in shape and also slightly in the procedure of operating according to the mechanism
adopted by different manufactures. Some important machines are discussed as under.
A shearing machine consists of a base or frame, which can be conveniently fixed on a
bench in any position as desired. Two shearing blades are provided in the machine. One called
the fixed blade is rigidly fixed with the frame whereas the other known as movable blade is
operated by means of the hand lever provided at the rear. In operation, the metal sheet is
placed between the shear blades in such a way that the markings of the layout come exactly
under the cutting edge of the upper blade. Out of the two blades one is fixed and the other
Sheet Metal Work 361
moveable. When the lever is pulled by the operator the lower blade rises and thus the metal
sheet is cut. This particular provision enables the metal sheet to be locked against the upper
blade during the operation on account of the circular motion of the hand lever. With the
result, the metal sheet is prevented from being distorted.
Folding machines are used for bending and folding the edges of metal plates to form
the joint at the seam. Bending machines or bending rollers, as they are better known, are
used for shaping metal sheets into cylindrical objects. The machine consists of three rollers.
These rollers have different adjustments indifferent types of machines. In some-machines
two rollers at the bottom have fixed position and the third one (top roller) can be adjusted
in vertical direction to adjust the pressure and give the required curvature to the sheet.
A still better control is obtained by having two rollers exactly one over the other, out of
which the bottom roller is fixed and the top roller can be adjusted vertically to suit the
thickness of the sheet and the pressure required for rolling. The third roller, called radius
roller can be moved up and down to provide the desired curvature. Improved designs of
these machines are available which contain the rollers, which have grooves of various
shapes and sizes to form corresponding shapes in metal sheets. Conical shaped rollers are
used for tapered cylindrical articles. Similarly other machines named above are used to
perform different operations after which they are named.
Burring machine is used to make a burr on the edges of the bottom and covers. Burr
is the starting of the seam. Double seaming machine is used for double seaming flat bottoms
on straight or flared cylindrical pieces.
Various types of press machines with different capacities are used for mass production
of sheet metal components. These are equipped with different kind of press tools or dies.
These are commonly employed for fast and accurate processing of sheet metal work.
18.8 TYPE OF PRESSES
Presses are classified based upon the method of operation method of power source
method of activation of slide and number of slides in action. The sub-classification of each is
also gien as under.
1. Method of Operation
(i) Arbor Press. It is a hand operated press. It combines the principle of lever with
pinions to a related rack build into the ram. They are used where a very limited
no. of production of parts is required.
(ii) Foot Press. It is also manual operated press. Most of the presses have foot action
in their operating mechanism.
(iii) Open back inclinable press. It is a power operated press. They are having bench
or floor models.
Presses are classified according to
2. Method of Power Sources
(i) Manual presses
(ii) Mechanical power presses
(iii) Hydraulic presses.
362 Introduction to Basic Manufacturing Processes and Workshop Technology
3. Method of Activation of Slide
(i) Crank shaft presses
(ii) Knuckle joint presses
(iii) Toggle presses
4. Number of Slides in Action
(i) Triple action single action presses
(ii) Double action presses
18.9 GENERAL PRESS INFORMATION
It is necessary for the sheet metal planner to know certain fundamentals specification
of press machine. Some of them are press tonnages, stroke of the press, die area, shut height
etc., and are explained below.
Press tonnage
It indicates the amount of pressure in tones that a press exerts or works safely and is
used for rating a press. The press may exist in various capacities of tones varying from 5,
10, 20, 32, 50, 75, 200, 500 tons to tackle the sheet metal operation using proper die in press.
The tonnage of a hydraulic press is the area multiplied by the oil pressure in the cylinder.
The tonnage of a mechanical press is equal to the shear strength of the crankshaft metal
multiplied by the area of the crankshaft bearings.
Press stroke
It is the reciprocating motion of a press slide. It is adjustable on a hydraulic press.
Die area
It is the available surface for mounting punches and die components
Shut height
It is the distance from the top of the bed to the bottom of the slide when stroke down
and the adjustment up. The shut height of the die must be equal to or less than the shut
height of the press.
Die shut height = Punch shoe thickness + Die show thickness + Die steel height + Punch
steel Height-bypass of steels. The by-pass of steels may be taken as from 3 mm to 6 mm.
18.10 GENERAL PRESS OPERATIONS
1. Shearing. It takes place in form a cut when punch strikes and enters in the sheet
placed on die. The quality of the cut surface is greatly influenced by the clearance
between the two shearing edges of the punch and dies.
2. Cutting. It means severing a piece from a strip or sheet with a cut along a single
line using suitable punch and die of press tool in press machine.
3. Parting. It signifies that scrap is removed between the two pieces to part them
using suitable punch and die of press tool in press machine.
Sheet Metal Work 363
4. Blanking. It is a operation in which the punch removes a portion of material called
blank from the strip of sheet metal of the necessary thickness and width using
suitable punch and die of press tool in press machine.
5. Punching. It is the operation of producing circular holes on a sheet metal by a
punch and die. The material punched out is removed as waste. Piercing, on the
other hand, is the process of producing holes of any desired shape in the part or
sheet using suitable punch and die of press tool in press machine.
6. Notching. It is a process to cut a specified shape of metal from the side or edge
of the stock using suitable punch and die.
7. Slitting. When shearing is conducted along a line, the process is referred to as
slitting. It cuts the metal sheet lengthwise using suitable punch and die of press tool
in press machine.
8. Lancing. It makes a cut part way across a sheet and creates a bend along the cut
using suitable punch and die.
9. Nibbling. It is an operation of cutting any shape from sheet metal without special
tools. It is done on a nibbling machine.
10. Trimming. It is the operation of cutting away excess metal in a flange or flash from
a sheet metal part using suitable punch and die of press tool in press machine.
11. Bending. Bending is the operation of deforming a sheet around a straight axis. The
neutral plane lies on this straight axis. In bending all sheet material are stressed
beyond the elastic limit in tension on the outside and in compression on the inside
of the bend. There is only one line, the natural line that retains its original length.
The neutral axis lies at a distance of 30 to 50% of thickness of the sheet from the
inside of the bend. Stretching of the sheet metal on the outside makes the stock
thinner. Bending is sometimes called as forming, which involves angle bending, roll
bending, roll forming, seaming and spinning.
18.11 QUESTIONS
1. Explain various hand tools used in sheet metal work?
2. Name the metals commonly used in sheet metal work.
3. Write short notes on:
(a) Punching (b) Piercing
(c) Blanking (d) Slitting
(e) Bending (f) Lancing
4. Discuss the different sheering operations performed on sheet metal.
5. Explain the difference between punching, blanking and piercing.
6. Discuss the various types of hammers used in sheet metal work.
7. Describe the process of bending.
8. Write short notes on:
(a) Drawing (c) Shearing
(d) Riveting (e) Hollowing
(f) Planishing
9. Name various tools, machine and equipments used in sheet metal shop.
364 Introduction to Basic Manufacturing Processes and Workshop Technology
364
FITTING
19.1 INTRODUCTION
These days small, medium and heavy industries are using automatic machines. But
bench and fitting work also plays a significant role for completing and finishing a job to the
desired accuracy. Most of semi-finished works can be accomplished with fairly good degree of
accuracy in a reasonable time through various kinds of quick machining operations. They still
require some minor operations to be performed to finish the job by hand. The term bench
work denotes the production of an article by hand on the bench. Where as fitting is the
assembling of parts together and removing metals to secure the necessary fit, and may or
may not be carried out at the bench. These two types of work require the use of a large
number of hand tools and other devices or equipments that involve a number of operations
for accomplishing the work to the desired shape and size. Some of the commonly used tools
are discussed as under.
19.2 TOOLS USED IN FITTING SHOP
Tools used in bench and fitting shop are classified as under.
1. Marking tools
2. Measuring devices
3. Measuring instruments
4. Supporting tools
5. Holding tools
6. Striking tools
7. Cutting tools
8. Tightening tools, and
9. Miscellaneous tools
The above mentioned tools are further classified and discussed as under.
1. Markeing Tools
These are sub classified as steel rule, circumference rule, straight edge, flat steel square,
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CHAPTER
Fitting 365
scriber, semi-circular protractor, divider, trammel, prick punch, centre punch, try square,
bevel square, vernier protractor, combination set and surface gauge.
2. Measuring Devices
Commonly used measuring devices and instruments used in bench and fitting shop are
fillet and radius gauge, screw pitch gauge, surface plate, try square, dial gauge, feeler gauge,
plate gauge and wire gauge.
3. Measuring Instruments
Line measuring and end measuring devices. While using line measuring device, the
ends of a dimension being measured are aligned with the graduations of the scale from which
the length is read directly such as scales or steel rules. Whereas, with end measuring device,
the measurement is taken between two ends as in a micrometer, vernier calipers and gauge
block, etc. End measuring devices are commonly used for measuring accurate and precision
dimensions of components. Some measuring instruments are employed for measuring linear
dimensions and others are suitable for determining angular or geometric dimensions. Few
measuring instruments are also kept for reference purposes as standards of comparison. The
main measuring instruments are listed as under.
(i) Linear measurements
(A) Non-precision instruments
1. Steel rule
2. Calipers
3. Dividers
4. Telescopic gauge
5. Depth gauge
(B) Precision instruments
1. Micrometers
2. Vernier calipers
3. Vernier depth gauges
4. Vernier height gauges
5. Slip gauges
(C) Comparators
(D) Coordinate measuring machines
(ii) Angular measurements
(A) Non-precision instruments
1. Protector
2. Engineers square
3. Adjustable bevel
4. Combination set
366 Introduction to Basic Manufacturing Processes and Workshop Technology
(B) Precision instruments
1. Bevel protector
5. Angle gauges
6. Sine bar
7. Clinometers
8. Autocollimators
9. Sprit level
(iii) Surface measurement
1. Straight edge
2. Surface gauge
3. Surface table
4. Optical flat
5. Profilo-meter
4. Supporting Tools
These are vee-block, marking table, surface plate, and angle plate.
5. Holding Tools
These are vices and clamps. Various types of vices are used for different purposes. They
include hand vice, bench vice, leg vice, pipe vice, and pin vice. The clamps are also of different
types such as c or g clamp, plane slot, goose neck, double end finger, u-clamp, parallel jaw,
and clamping block.
6. Strking Tools
These are various types of hammers such as ball peen hammer; straight peen hammer;
cross-peen hammer; double face hammer; soft face hammer.
7. Cutting Tools
These involve various types of files, scrapers, chisels, drills, reamers, taps, snip or shear
and hacksaws.
Files. There are different types of files such as flat, square, round, triangular, knife,
pillar, needle and mill.
Scrapers. These are flat, hook, triangular, half round types.
Chisels. There are different types of chisels used in fitting work such as flat chisel, cross
cut chisel, diamond point chisel, half round chisel, cow mouth chisel and side cutting chisel.
The other cutting tools are drills, reamers, taps, snips, hacksaws (hand hacksaw and
power hacksaw) etc.
8. Tightening Tools
These are pliers and wrenches, which are sub classified as under.
Pliers. These are namely ordinary, needle nose, and special type.
Wrench. These are open single ended, open double ended, closed ended adjustable, ring
spanner, offset socket, t- socket, box wrench, pipe wrench and allen wrench.
Fitting 367
9. Miscellaneous Tools
These are die, drifts, counter sink tools, counter boring tools, spot facing bit and drill press.
Some of above mentioned important tools are discussed as under.
19.2.1 Measuring Tools
19.2.1.1 Steel Rule
Steel rule is generally employed for purpose of measuring rough dimensions and laying
out them. It is always advisable to start measuring from 1 cm mark because the end of the
rule is generally worn out
19.2.1.2 Circumference Rule
It is commonly used for measuring or laying out or as a straight edge. The specialty in
this rule is that the circumference can be taken directly, below the diameter dimension.
19.2.1.3 Straight Edges
There are two types of straight edges namely four edge type (Fig. 19.1(a)) and bridge type
(Fig. 19.1(b)) which are made of carbon tool steel and alloy steel. They are generally flat
graduated bar of steel with one longitudinal edge beveled. Straight edges come in various
lengths commonly varying from 2.5 mm up to one meter and above. They are mostly used
for scribing long straight lines.
Four ed
g
es
t
y
pe
(a)
Brid
g
e t
y
pe
(b)
Fig. 19.1 Straight edges
19.2.1.4 Flat Steel Squrae
It is a piece of flat hardened steel with graduations on either end. It is commonly used
for marking lines in the perpendicular direction to any base line.
19.2.1.5 Scribers
Fig. 19.2 shows the various types of scribers, which are sometimes called the metal
worker’s pencil. These are made up of high carbon steel and are hardened from the front edge.
Scriber is used for scratching lines on the sheet metal during the process of laying out a job.
Fig. 19.2 Scribers