wrkshop lathe

8/2/2019 Wrkshop Lathe

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Manufacturing Science & Engineering (Mechanical Engg. Department)

Workshop (Lathe)

Lathe:

A lathe is a machine tool which rotates the workpiece on its axis to

perform various operations such as cutting, sanding,knurling, drilling, or deformation with tools

that are applied to the workpiece to create an object which has symmetry about an axis of

rotation.

Fig (1): Lathe Machine

Lathe is the oldest, most basic, most versatile, widely used machine tool and lathe is known as

the Mother of all the Machines.

Why lathe are also known as turning machines?

Because the job to be machined is rotated or turned and the cutting tool

is moved relative to the job.


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Function of the lathe:

1. For producing cylindrical jobs.

2. Flat surfaces and holes.

3. With special attachments, it can be used for producing different types of

surfaces, cutting threads, cutting grooves.

4. As a whole, a skilled worker can produce any type of job with a lathe.

Lathe parts:

1. Bed: Base of the lathe supports all major components of the lathe

machine. Large mass and made from gray cast iron. Three main parts of

the lathe headstock, tailstock and carriage are mounted on the bed of

the lathe.Top of the bed has two guide ways or slide ways to provide

support and sliding surfaces for the carriage and for the tailstock.

2. Headstock: Secured permanently at the left hand end of the lathe bed.It

supports the spindle and is equipped with the power driving mechanism

for the spindle. The spindle speed can be set through speed selector

knobs. The spindle is hollow to facilitate holding of long work pieces.The

work holding devices such as chucks, centres and collets are attached to

the spindle.

3. Tailstock: Located at the right hand end of the bed Can be moved along

the guide ways and can be clamped in any position on the bed.Also

called loose headstock.Main purpose is to hold the dead centre and to

support the long work pieces during machining.It has a quill, into which

the deadcentre,drills,reamers can be fixed. The quill can move in and out

with the help of hand wheel.

4. Carriage: The carriage slides along the guide ways between headstock

and consists of an assembly of the cross-slide, tool post, the compound

rest and the apron.Main function is to hold the cutting tool and move it to


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give longitudinal and cross feed to it. The cross-slide moves radially in

and out, thus controlling the radial position of the cutting tool.

5. Feed rod:The feed rod is powered by a set of gears from the head stock.It rotates during the operation during the operation of the lathe and

provides mechanized movement to the carriage or the cross-slide by

means of gears, a friction clutch, and a keyways along the length of the

feed rod.

6. Lead screw: The lead screw is also powered by the gears from theheadstock and is used for providing specific accurate mechanizedmovement to the carriage for cutting threads on the work piece. The lead

screw has a definite pitch.

7. Split nut: A split nut in the apron is used to engage the lead screw with

the carriage. In some lathes, the lead screw performs the functions of

feed rod and there is no separate feed rod. Similarly a lathe not meant

for thread cutting will not have a lead screw.

8. Chucks: Chucks are used for mounting the difficult workpieces.3-jaw

chuck (Self Centering) and 4-jaw chuck (Independent) are most probably

being used while machining.

9. Gearbox: Gearbox is inside the headstock, providing multiple speeds with

a geometric ratio by moving levers.


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Fig (2): Figure showing different lathe parts

10. Apron: Attached to the front of the carriage, it has the mechanism and

controls the movement of the carriage and cross slide.

11. Compound rest: Mounted on the cross slide and pivots around the tool

post.

Machining operations done in lathe:

1. Straight turning

2. Taper turning

3. Chamfering

4. Drilling

5. Reaming

6. Boring


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7. Counter boring

8. Taper boring

9. Internal thread cutting

10. Tapping

11. Parting off

12. Thread cutting

13. Facing

14. Knurling

15. Filing

16. Polishing

17. Grooving

18. Spinning

19. Forming

Turning:

Fig (3): Turning Operation


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Fig (4): Operations related to turning

Turning operation is one of the most basic machining processes. That is, the part

is rotated while a single point cutting tool is moved parallel to the axis of rotation.Turning

can be done on the external surface of the part as well as internally (boring). The starting

material is generally a workpiece generated by other processes such as casting ,

forging, extrusion, or drawing. We can simply say that turning is nothing but reducing the

diameter of the workpiece.


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Different operations on Lathe:

Fig (5): Different operations on lathe

Facing:

Facing is part of the turning process. It involves moving the cutting

tool at right angles to the axis of rotation of the rotating workpiece.This can be

performed by the operation of the cross-slide, if one is fitted, as distinct from the

longitudinal feed (turning). It is frequently the first operation performed in the


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production of the workpiece, and often the last- hence the phrase "ending up".

And facing is nothing but reducing the length of the workpiece.

Fig (6) : Facing Operation

Cutting Parameters:

In turning, the speed and motion of the cutting tool is specified through several

parameters. These parameters are selected for each operation based upon theworkpiece material, tool material, tool size, and more.

1. Cutting feed: The distance that the cutting tool or workpiece advances during one

revolution of the spindle, measured in inches per revolution (IPR). In some operations

the tool feeds into the workpiece and in others the workpiece feeds into the tool. For a

multi-point tool, the cutting feed is also equal to the feed per tooth, measured in inches

per tooth (IPT), multiplied by the number of teeth on the cutting tool.

2. Cutting speed: The speed of the workpiece surface relative to the edge of the cutting

tool during a cut, measured in surface feet per minute (SFM).

3. Spindle speed: The rotational speed of the spindle and the workpiece in revolutions

per minute (RPM). The spindle speed is equal to the cutting speed divided by the

circumference of the workpiece where the cut is being made. In order to maintain a


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constant cutting speed, the spindle speed must vary based on the diameter of the cut.

If the spindle speed is held constant, then the cutting speed will vary.

4. Feed rate: The speed of the cutting tool's movement relative to the workpiece as the

tool makes a cut. The feed rate is measured in inches per minute (IPM) and is the

product of the cutting feed (IPR) and the spindle speed (RPM).

5. Axial depth of cut: The depth of the tool along the axis of the workpiece as it makes a

cut, as in a facing operation. A large axial depth of cut will require a low feed rate, or

else it will result in a high load on the tool and reduce the tool life. Therefore, a feature

is typically machined in several passes as the tool moves to the specified axial depth

of cut for each pass.

Tapers and Taper Turning:

A taper may be defined as a uniform increase or decrease in

diameter of a piece of work measured along its length. In a lathe taper turningmeans to produce a conical surface by gradual reduction in diameter from a

cylindrical work piece.

D= larger diameter of taper in mm

d= smaller diameter of tapered in mm

L= length of tapered part in mm

2= full taper angle

=angle of taper or half taper angle

Taper turning methods:

1. Using a form tool.

2. Swiveling the compound rest.

3. Using a taper turning attachment.

4. By combining longitudinal and cross feed in a special lathe.

5. Tail stock offset method.


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1. Taper turning using a form tool:

The form tool has a straight cutting edge set at the desired taper angle.

Form tool is a replica of the taper to be produced. That is the angle

between the straight cutting edge and the rotational axis of the job equals

taper angle or one half the included angle of the taper.Shape of the tool

is reproduced on the work piece.

Limitation:

This method is limited only for short length taper. Because the

metal is removed by the entire cutting edge, and only increase in the length of

the taper will necessitate the use of a wider cutting edge. This will require

excessive cutting pressure, which may distort the work due to vibration and spoil

the work piece.

2. Swiveling the compound rest.

The compound rest has a circular base graduated in degrees,

which can be swiveled at any angle.While turning a taper, the base of compound

rest is swiveled through an angle equal to the taper angle. The tool is then fed by

hand. Once the compound rest is set at the desired half taper angle, rotation of


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the compound slide screw will cause the tool to be fed at that angle and generate

a corresponding taper.

Limitations:

This method is limited to turn a short but steep taper owing to the

limited movement of the cross slide.But a small taper may also be turned. The compound rest

may be swiveled at 45 degree on turn a steep taper.

3. Taper turning by a taper attachment:The principle of turning taper by a taper attachment is toguide the tool in a straight path set at an angle to the axis of rotation of the work

piece, while the work is being revolved between centers or by a chuck aligned to

the lathe axis.A taper turning attachment consists essentially of a bracket orframe which is attached to the rear end of the lathe bed and supports a guide bar

pivoted at the centers. The bar having graduations in degrees may be swiveledon either side of the zero graduation and is set at the desired angle with the lathe

axis.

When the taper turning attachment is used, the cross slide is first made

free from the lead screw by removing the binder screw. The rear end the cross slide is

then tightened with the guide block by means of a bolt.When the longitudinal feed isengaged, the tool mounted on the cross slide will follow the angular path, as the guide


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block will slide on the gear bar at an angle to the lathe axis. The required depth of cut is

given by the compound slide which is placed at right angles to the lathe axis.

The guide bar must be set at half taper angle and the taper on the work

must be converted in degrees. The maximum angle through which the guide bar may be

swiveled is 10 degree to 12 degree on either side of the centre line.

If the diameters D,d and the length L of the taper are specified, the angle

of swiveling the guide bar can be determined from equation-

Tan= (D-d)/2L

The advantage of using a taper turning attachment are:

1. The alignment of live and dead centers being not disturbed, both straight and taper

turning may be performed on a work piece in one setting without much loss of time.


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2. Once the taper is set, any length of a piece of work may be turned within its limit.

3. Eery steep taper on a long work piece may be turned, which cannot be done by any

other method.

4. Accurate taper on a large number of work pieces may be turned.

5. Internal tapers can be turned with ease.

4. By combining longitudinal and cross feed in a special lathe:

Taper turning by combining feeds is a more specialized method of

turning taper.In certain lathes both longitudinal and cross feeds may be engaged

simultaneously causing the tool to follow a diagonal path which is the resultant of the

magnitude of the two feeds. The direction of the resultant may be changed by varying

the rate of feeds by change gears provided inside the apron.

5. Tail stock off-set:


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Thread cutting:

There are a large number of thread forms that can be machined in lathe

such as Whitworth, Acme, ISO metric, etc.Thread cutting can be considered as turningonly since the path to be travelled by the cutting tool is helical. Classification of threads

are done on the basis of different criteria.

1. Internal threads

2. External threads

And

3. Left hand threads

4. Right hand threads


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Fig (7) : Threads

Diameters:

There are several relevant diameters for screw threads: major diameter, minor diameter,

and pitch diameter.

1. Major diameter:

Major diameter is the largest diameter of the thread. For a male thread,

this means "outside diameter", but in careful usage the better term is "major diameter",

since the underlying physical property being referred to is independent of the

male/female context. On a female thread, the major diameter is not on the "outside". The

terms "inside" and "outside" invite confusion, whereas the terms "major" and "minor" are

always unambiguous.

2. Minor diameter:

Minor diameter is the smallest diameter of the thread.

3. Pitch diameter:

Pitch diameter, also known as mean diameter, is a diameter in between

major and minor. It is the diameter at which each pitch is equally divided between the

mating male and female threads. It is important to the fit between male and female

threads, because a thread can be cut to various depths in between the major and minor

diameters, with the roots and crests of the thread form being variously truncated, but

male and female threads will only mate properly if their sloping sides are in contact, and

that contact can only happen if the pitch diameters of male and female threads match

closely. Another way to think of pitch diameter is "the diameter on which male and

female should meet".


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Lead, pitch, and starts:

Fig(8): Figure showing Lead, pitch, and starts

Lead: Leadand pitchare closely related concepts. They can be confused because

they are the same for most screws. Leadis the distance along the screw's axis that is

covered by one complete rotation of the screw (360).

Pitch:Pitch is the distance from the crest of one thread to the next. Because the

vast majority of screw thread forms are single-startthread forms, their lead and

pitch are the same. Single-start means that there is only one "ridge" wrapped

around the cylinder of the screw's body. Each time that the screw's body rotates

one turn (360), it has advanced axially by the width of one ridge. "Double-start"

means that there are two "ridges" wrapped around the cylinder of the screw's

body.Each time that the screw's body rotates one turn (360), it has advanced

axially by the width of two ridges. Another way to express this is that lead and

pitch are parametrically related, and the parameter that relates them, the number

of starts, very often has a value of 1, in which case their relationship becomes

equality. In general, lead is equal to Stimes pitch, in which Sis the number of

starts.While specifying the pitch of a metric thread form is common, inch-based

standards usually use threads per inch(TPI), which is how many threads occur

per inch of axial screw length. Pitch and TPI describe the same underlying

physical propertymerely in different terms. When the inch is used as the unit of

measurement for pitch, TPI is the reciprocal of pitch and vice versa. For example,

a 14-20 thread has 20 TPI, which means that its pitch is120 inch (0.050 in or

1.27 mm)


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Fig (8) : Threading operation

Chucks:

A chuck is a specialized type of clamp used to hold an

object, usually an object with radial symmetry, especially a cylindrical object. It is

most commonly used to hold a rotating tool (such as the drill bit in a power tool)

or a rotating workpiece (such as the bar or blank in the headstock spindle of

a lathe). Some chucks can also hold irregularly shaped objects (ones that lackradial symmetry). In some applications, the tool or workpiece being held by the

chuck remains stationary while another tool or workpiece rotates (for example, a

drill bit in the tailstock spindle of a lathe, or a round workpiece being milled by

a milling cutter).

Different Chucks:

1. 3-Jaw chuck (Self-centering)

2. 4-Jaw chuck (Independent)3. Magnetic chucks

4. Vacuum chucks

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