Operation Management-ICAI-Inter-Special purpose machines

1.3 Special Purpose Machines
Honing Machine: Honing, lapping and super-finishing equipment are used to improve surface finish or
geometry to tight tolerances. Honing machines use small, bonded abrasive stones or super abrasive sticks
mounted in a fixture that rotates and reciprocates when applied to the surface or bore being finished.
Honing is used to correct the geometry and alignment of holes and to produce the surface required for the
application. Honing is final finishing operation conducted on a surface, typically of an inside cylinder
(bore), for example automobile engine cylinder. Abrasive stones are used to remove minute amounts of
material in order to tighten the tolerance on cylindricity.
Honing is a surface finish operation, not a gross geometry-modifying operation. Honing Holders apply a
slight, uniform, pressure using a number of abrasive sticks (Honing Sticks) that wipe over the entire surface
to be honed thus removing material from the surface. Honing is the final process used in the machining
cylinder bores, either during manufacture or in re-sizing (re-boring). Honing is used as a process to both
remove the final amount of metal to get a cylinder bore to within the required size limits, and to put a
surface on the cylinder bore which will give good life span, and aid lubrication and oil consumption
characteristics in use.
Honing is not only carried out on cylinder bores for internal combustion engines but on
compressors, hydraulics components and probably dozens of other applications.
Other Special Purpose Machine Tools:
The following list includes some of the most important Special Purpose Machines Tools:
• Boring Machines - These machines are used mainly for boring operations. By using Special Tools they
may be adapted to facing the end of work and cutting internal thread.
• Broaching Machines - These machines are used for altering the size of finish of holes in metallic parts.
• Gear-Cutting Machines - These machines are used for cutting gears.
• Honing Machines - (already described above under separate head) : These machines are used for
eliminating local irregularities of surface and giving good finishing.
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• Lapping Machines - These machines are used for finishing cuts.
• Turret and Capstan Lathes - The operations performed on these machines by using appropriate tools
are Turning, Forming, Parting off, Boring and Screw-cutting, the operations can be automatically
performed in any sequence.
Machine Tools used in a Workshop:
A workshop should, in the first place, contain all the General Purpose Machines and some Special Purpose
Machines according to need. A special Purpose Machine should be installed if it can be engaged for a
reasonable percentage of its useful life. Otherwise General Purpose Machines should be employed, so that
idle capacities of the machines may be reduced to the minimum.
The following machines are most usually found in an up-to-date machine shop manufacturing light
machines and their interchangeable parts:
(1) Lathes – (a) Centre or Engine Lathe, (b) Capstan Lathe, and (c) Turret-Lathe.
Lathes are used mainly for Turning, Boring and Screw-cutting and can be adapted for the operations of
Milling, Drilling, Slotting, Grinding, Gear and Rack cutting, Lapping, Facing, Reaming, Polishing, and
Knurling, Tapping and Parting off.
The Lathe:
The following are the different kinds of Lathes : (1) Centre or Engine Lathe, (2) Capstan Lathe, (3) Turret
Lathe and (4) Automatic Lathe.
Of these the Centre or Engine Lathe has been classified as a General Purpose Machine and the others as
Special Purpose Machines. The Centre Lathes are extremely versatile and with proper equipment can do
an endless variety of work. Capstan, Turret and Automatic Lathes have limited scope of work but can do
specialized jobs with great speed and accuracy. With the modern tendency towards specialisation, Centre
Lathe has been to a large extent superseded by Capstan, Turret and Automatic Lathes and Grinders.
By the term “Engine Lathe” is meant the Ordinary Centre Lathe with at least one automatic feed, either for
the “traverse”, or for the “surfacing”.
When both the traverse and the surfacing motions are power operating the Lathe is called a “Self-acting,
Sliding and Surfacing Lathe”.
When a Self-acting, Sliding and Surfacing Lathe is provided with a Lead Screw for Screw-cutting, it is
called a “Self-acting, Sliding, Surfacing and Screw-cutting Lathe”.
The Principle of Operation of the Lathe:
In a Lathe a great deal of work is in the form of cylindrical bars. To product cylindrical surfaces the job is
supported on two parts known as the Headstock and the Tailstock and is uniformly rotated by means of a
mechanical device. It is clear that if a cutting tool is applied on the rotating job, cylindrical surfaces of
specific diameter can be produced. The cutting tool can be operated on the entire surface of the job by
moving the former parallel to the axis of rotation of the job. The rotation of the job and the motion of the
cutting tool are produced by power.
Principal Parts of a Lathe:
(1) The Bed, (2) The Headstock, (3) The Tailstock, which is also called the Loose Headstock,
(4) The Carriag, (5) The Gearbox, (6) The Feed Shaft and the Lead Screw.
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Figure of Lathe
Lathe Attachment :
Lathe Attachments are employed for increasing the adaptability of the Lathe for handling special kinds of
jobs. The attachments are either devices for holding jobs or are Miscellaneous Tools suitable for different
operations.
The following devices are meant to hold jobs:
(1) Special Centres and Shaft Supports.
• When the usual Headstock and Tailstock Centres are unsuitable for special
kind of work, Special Centres are used.
• When a long slender shaft is turned between centres if is supportedby a device
called Travelling Stay or Steady Rest attached to the Lathe Spindle.
(2) Mandrel,
(3) Chuck,
(4) Faceplate.
Miscellaneous Tools : For Drilling and Boring, Special Tools are attached to the Tailstock, so that when the
job rotates, a hole is made into the job. Suitable Tools are used for other purposes.
Operations done on a Centre Lathe:
The following are the operations usually done on a Center Lathe: –
• Boring : Boring does the operation of enlarging the diameter of a hole already exist in a job. Small
Boring operations are often done in the Lathe either as a single operation or preparatory to Threading,
Reamering or other operations.
• Screw Cutting: this is the operation of cutting screw threads on the external surface of a cylindrical
job.
• Turning: Turning means producing cylindrical surface on a job. In this operation material from the
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surface of the job is removed by rotating the job against a Tool point, so as to produce a cylindrical
surface. The diameter of the work may be ascertained by Callipers, Micrometers or fixed gauges.
In additional to these normal uses, a Centre Lathe can readily be adapted for the following operations: –
• Milling: Milling is the process of giving a specific shape or form to a job by cutting it by means of a
revolving multiple point tool.
• Drilling: Drilling is the operation of making a hole in a job. The Drill is inserted into the Tailstock
Spindle and the Tailstock is clamped to the job in the necessary position. The feed is affected by the
Tailstock hand wheel.
• Slotting: This is the operation of making a long narrow aperture for something to be inserted or
worked in.
• Grinding: The Grinding operation consists in applying a wheel of abrasive character rotating at high
speed to a metallic surface for causing it to wear.
• Gear and Rack cutting: A Gear is a toothed wheel which, having the teeth meshed with those of
another Gear, can cause one shaft to drive another. A gear having the teeth in a straight line, instead
of on a circle, is called a Rack.
• Polishing: Polishing processes on job surfaces may be undertaken on a Centre Lathe.
• Lapping: Lapping is the process of polishing a work by-means of abrasive materials, to give fine
finish. Lapping operation may be undertaken after grinding to give fine adjustments.
• Facing: this is an operation of producing a flat surface and removing material from the surface. The
operation consists in rotating the job on an axis of rotation perpendicular to the flat surface.
• Reaming: The Reaming operation consists in (1) enlarging existing drilled holes; (2) making, a parallel
hole into a tapered hole and bringing existing holes actually to size.
• Knurling: Knurling is the process of producing a series of; right-hand and left-hand fine grooves on
the surface of a job to facilitate handing or to secure a better hold. Knurling operation may be
accomplished by the use of either a single or double wheel Knurling Tool. The work must be mounted
rigidly; either well up against Chuck Jaws or, in case of a long, slender work, a suitable Steady must
be employed.
• Tapping: Tapping is the operation of forming threads on the interior surface of a job. For tapping
threads a Tap should be used and the job should be very slowly rotated by engaging the Back Gear
system.
• Parting Off: when work is machined from bar, the completed job can be parted off from the bar on the
Centre Lathe.
Special purpose Lapping Machines are also used for this purpose.
Capstan and Turret Lathes:
The Centre Lathe is a versatile machine and can be used for many different kinds of operations. It however
requires for its operation a skilled operator whose hands produce intricate and precise work. With the
demand for speed and accuracy accompanied with low cost the need arose for more rapid results from less
skilled personnel. As a result, Capstan and Turret Lathes were developed.
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Capstan Lathe
The Capstan Lathe has the following principal parts
(1) A Bed.
(2) An all-geared Head-stock.
(3) A Saddle, in which the Cross-slide carries a revolving square Turret that can hold four Tools at once.
The Turret is unlocked by means of a lever, and by turning it manually, any of the Tools in the Turret
can be brought into the working position.
(4) The main Turret which replaces the Tailstock of the Centre Lathe.
Operations performed on a Capstan Lathe:
The following are the important operations performed on the Capstan Lathe:—
1. Turning, that is producing cylindrical surfaces.
2. Forming, that is turning intricate shapes by one traverse of a Tool, instead of several movements.
3. Parting-off, that is cutting away of a completed job from a bar.
4. Boring, that is enlarging the diameter of a hole already existing in a job.
5. Screw cutting, which is cutting screw threads on the external surface of a cylindrical job.
Turret Lathes
Turret Lathe is almost similar in construction to Capstan Lathe. Its principal parts are the following:—
(1) A bed.
(2) An alt-geared Headstock.
(3) A Saddle in which a revolving square Turret is carried by the Cross-Slide.
(4) The main hexagonal Turret which carries six different Tools in special holders that are bolted up to
the faces.
Operations performed on a Turret Lathe:
The operations performed on a Turret Lathe and on a Capstan Lathe are the same. Generally, however,
Turret Lathes are suitable for machining bigger part than Capstan Lathes.
The following are the important operations performed on a Turret Lathe:
(1) Turning,
(2) Forming,
(3) Parting-off,
(4) Boring,
(5) Screw-cutting.
Plaining Machines: are used for machining flat surfaces and cutting key-ways on large jobs. The function
of this machine is same as the shaping machine but here the tool is held fixed and gives the required feed
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whereas the work piece is rigidly fitted on the work table with a reciprocating motion. The length of the
workpiece is more than 1 metre and are normally massive.
Figure of Plaining Machines
The main parts of the machine are bed, work table, cross slide pillars and tool holders as shown in figure.
Tool holders, (for at least two tools) are mounted on cross beam and have provision to slide up or down
along two vertical pillars fixed on two sides of the bed. After first setting, this cross beam is firmly clamped
and then during operation the clapper box containing tool holders slide to give the feed.
Slotting Machines: are used for machining vertical or inclined flat surfaces, making holes with sharp corners
and tapered holes and for cutting T-slots and V-ways.
For mass production, Single-purpose Machines may be used in addition to the machines mentioned above.
This is another type of a machine to produce machined fiat surface like shaper and planer. The difference
with shaper or planer is that slotter makes vertical surface machining whereas the others produce horizontal
machining. As such it is sometime called as “vertical shaper”, since the ram holding the tool moves up and
down. Base and pillar is a composite L-shaped cast structure, the pillar having a vertical machined way for
the reciprocating movement of the ram. The work piece is held firm on a round table with saddle which is
fitted on a horizontal bed. The round table is suitable for adjustment for the movements at right angles in
horizontal plane and can also rotate about the vertical axis. The movement of the job gives the desired feed
whereas movement of ram produces cutting.
The slotting machine is suitable for making keyways on inside surface of the bore of a pulley or gear or on
the outside surface of a round. A typical shaping machine is shown in figure.
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Figure of Slotting Machine
Shaping Machine:
Shaping, like Plaining, is the machining of flat metallic surfaces by single-point cutting tools. Shaping is
carried out on small areas of metal not usually exceeding one foot in length. It consists of a massive body
supporting the Ram which slides along ways provided on the upper surface.
There is a Saddle which supports the Work Table. The Saddle can slide up or down the face of the body
and adjusts the height of the Work Table to accommodate jobs of different heights. When shaping
commences, the Saddle is locked, so as to keep the job at a constant height.
Figure of Shaping Machine
Uses of a Shaping Machine:
A Shaping Machine has the following uses:
1. Machining flat surfaces: The areas to be machined are how ever small, such as small Bed Plates of
Machines.
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2. Cutting Key-ways: For this purpose an aperture in the body of the machine enables shafts to be run
parallel to the Ram for the provision of the Keyways.
3. Cutting T-slats: T-steps are cut in three operations. Firstly, a plain slot is cut to the required depth.
Then each side is undercut by means of suitable undercutting tool to cut the side slots so that finally
a slot is produced resembling the letter ‘T’.
Drilling Machines:
Drilling Machines may be broadly grouped into the following four classes, such as
1. Sensitive Drilling Machines,
2. Pillar Drilling Machines,
3. Radial Drilling Machines,
4. Multiple Spindle Drilling Machines.
Sensitive Drilling Machines: Sensitive Drilling Machines are designed to take Drills up to about 8/l6 inch in
diameter. They are provided with hand feed for driving the Drill forward.
The Sensitive Drills are used for making small holes only. The hand feed is provided by hand by moving
the drill spindle by means of a rack and pinion arrangement. In operating these drills, pressure applied on
the drill bit can be felt by the operator by his senses. These Drills are therefore called Sensitive Drills.
Pillar Drilling Machine:
A Pillar Drilling Machine is an upright Drilling Machine as distinct from a Radial Drilling Machine It
consists of a Work Table on which the job to be drilled is placed.
The Work Table can be adjusted sideways and vertically to accommodate jobs of different heights.
Figure of Pillar Drilling Machine
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The arm carries the Spindle in a Saddle which accommodates the Spindle driving motor and the Gearboxes
for providing different Spindle speeds and feeds.
Radial Drilling Machine:
Radial Drilling Machine which has a horizontal arm which is attached to a sleeve capable of moving up
and down a vertical pillar and can be clamped at any position. The sleeve can freely rotate about the pillar
which is fixed to the base.
Figure of Milling Machine
Figure of Radial Drilling Machine
The arm carries the spindle in a saddle which accommodates the spindle driving motor and the gearboxes
for providing different spindle speeds and feeds.
Milling Machine:
Milling is the operation of removing metal by means of a rotating cutter against which the work is fed. The
milling cutters are generally discs of cylindrical Tools-usually made of high-speed steel and have serrations
or teeth about their edges.
The face of the tooth is given a definite cutting rake or angle appropriate for the material of the job to be
machined.
A Milling Machine is a very important machine tool in a modern workshop. The general purpose Milling
Machine may be broadly classified under (a) the horizontal milling machine and (b) the vertical milling machine.
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In a Vertical Milling Machine the working spindle is vertical, whereas in a Horizontal Milling Machine it is
horizontal.
A vertical Milling Machine is used in milling inside a recess or cavity and in performing operations which
cannot be done on a Horizontal Milling Machine. There are many operations which can foe performed on
the horizontal as well as on the Vertical Milling Machines.
Universal Milling Machine:
In a Universal Milling Machine the work-table can be swivelled about a vertical axis, so that its direction of
travel may be at right angles or at any other angles to the spindle axis. This enables helical grooves to be cut
on the outside of a cylindrical work, for instance cutting of helical teeth on gear blanks, milling of helical
groove on a twist drill, etc. These operations cannot be performed on a plain milling machine in which the
travel of the work-table is set permanently at right angles to the spindle axis.
Uses of a Milling Machine:
The following are the uses of a Milling Machine:—
1. Forming plain and irregular surfaces.
2. Cutting slots by using suitable Tools.
3. Cutting threads on dies, worms, screw, etc. internally or externally. Cutting threads on the Milling
Machine is known as Thread Milling.
4. Gear cutting by the uses of Special Gear Cutters.
Grinding Machines are used for machining cylindrical and tapered surfaces, for grinding internal and flat
surfaces and slender bars and delicate parts.
The Special Grinding:
The Grinding Process consists in machining jobs by means of abrasive wheels. In an abrasive wheel a large
number of abrasive particles are held together at the periphery by means of a, bonding material. The
abrasive particles act like minute cutters. When the abrasive wheel is rotated against a job, fine surface is
produced by the wearing away of the materials on the job. The chips produced are usually so minute that
they cannot be seen with the naked eye. The abrasive particles may be Aluminium Oxide, Silicon Carbide
or Diamonds in the form of “dust”.
Main Types of Grinders and their Uses:
There are four main types of Grinding Machines in common use:
1. The Cylindrical Grinders,
2. The Internal Grinding Machines,
3. The Flat Surface Grinding Machines,
4. The Centreless Grinding Machines.
The Cylindrical Grinding Machines are subdivided into. (1) Plan Grinders, (2) Universal Grinders.
The Cylindrical Grinders are used for machining parts like shafts, spindles, rollers, etc., having cylindrical
surfaces and also for machining tapered parts, cams, eccentrics, soulders of shafts, etc. The Internal Grinding
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Machines grind the internal surfaces of cylinders and other parts. Flat Surface Grinders are designed to
machine every type of flat surface. A Centreless Grinder produces more accurately a cylindrical surface.
Of the Cylindrical Grinders the Plain Grinder is used for machine parts either cylindrical or tapered in
from The Universal Grinder has a wider range of application and can be used for grinding work at any
desired angle. It can be adapted to Internal Grinding and is useful in the production of part used in small
machines and special tools.
Figure of Grinding Machine
The Hobbing Machine:
The Hobbing Machine is a Single Purpose Machine Tool designed for cutting gears. Hobbing operation is
the most accurate way of cutting Gears.
The cutting tool used in the Hobbing Machine is called the Hob which is in the form of a worm. A Hob is
nothing more than a screw provided with “gashes” or “flutes” forming cutting edges. The cutting action is
provided by the rotation of the hob, in conjunction with the gashing that provides the cutting edges.
The Hobbing Machine may be used to generate Spur Gears or Helical Gears. The Spur Gears have their
teeth parallel to the axis of rotation of the Gear, while in Helical Gears the teeth are not parallel to the axis.
Figure of Hobbing Machine
Problem 1 :
A shaft 1,000 mm. in length is being machined on a lathe. If the spindle executes 500 r.p.m. and the feed is
0.20 mm. per revolution, how long will it take the cutter to pass down the entire length of the shaft?
Solution:
Since the feed is 0.20 mm. per revolution, the number of revolutions in passing
1,000 m.m.=
1,000
0.20 m.m.
=5,000. Since the spindle executes 500 r.p.m.,
the Time required =
5,000
500
m.m. = 10 min.
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Problem: 2
Find the machining cost of a M. S. bar on a lathe from the following data: R. P. M. of the Job = 500. Feed of
tool per revolution of job = 0.5 mm. Depth of cut = 2 mm. Diameter of raw material = 60 mm. Diameter of
finished job = 40 mm. Length of Job = 1000 mm. Machining cost = Rs. 3 per hour.
Solution :
Number of revolutions in one traverse of 1000 mm.=
1,000 mm.
0.5 mm.
=
10,000
5
= 2000
Total depth of cut = ½ (60-40) mm. = 10mm. Cross-feed = 2 mm.
The number of transverse traverse over the job from end to end = 10/2 times = 5 times.
Thus the total number of revolutions of the job = 2,000 × 5 = 10,000 R. P. M. of the job = 500
Machine time =
10,000
500
min. = 20 min. = 1/3hr.
Cost of machining = Rs. 3×1/3 = Re.1.
Problem: 3
A shaft 500 mm. in diameter and 1 metre long is to be turned at a speed of 280 r.p.m. If the feed is 0.25 mm.
per revolution, calculate the time taken for one pass of the cutter.
Solution:
Length of the shaft = l metre = 1,000 mm.
Number of revolutions =
1,000 mm.
0.25 mm.
=
1,00,000
25
= 4,000
Time required for one pass of the cutter =
4,000
280
min. =
100
7
min. = 14.29 min.
Jigs and Fixtures:
A Jig is an appliance to which a work to be machined can be, fastened and which contains a device for
guiding the tool, so that the tool and the work are accurately located with respect to each other. A Fixture
is an appliance which holds the work when it is machined.
Uses of Jigs and Fixtures:
(1) Jigs quickly and accurately guide the tools. They render difficult operations easier, speedier and yet
more accurate.
(2) Jigs are suitable is mass production for producing accurately machined interchangeable parts.
(3) Fixtures are essential almost all machine work, because the work must be firmly held when the tools
work.
(4) Fixtures used in conjunction with jigs increase speed and accuracy of work.