Conventional Lathe Machine

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Conventional Lathe Machine

Mohd Zaid B. Akop15 January 2008

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CONTENTS

1. Introduction

2. Machine Function

3. Safety Precautions

4. Cutting Tools

5. Toolholders

6. Lathe Operations

7. Speeds & Feeds

8. Tools Geometry

FAKULTI KEJURUTERAAN MEKANIKALUNIVERSITI TEKNIKAL MALAYSIA MELAKA

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1. INTRODUCTION

Description: The purpose of a lathe is to rotate a part against a cutting

tool, thereby removing metal. It is used for fabricating parts and/or features that have a circular cross section. The spindle is the part of the lathe that rotates. Various work holding attachments such as three jaw chucks, collets, and centers can be held in the spindle. The spindle is driven by an electric motor through a system of belt drives and/or gear trains.


Lathe cutting action(Source: Kelmar Associates)

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1. INTRODUCTION-(cont’d)

Spindle speed is controlled by varying the geometry of the drive train. The tailstock can be used to support the end of the workpiece with a center, or to hold tools for drilling, reaming, threading, or cutting tapers. It can be adjusted in position along the ways to accomodate different length workpieces. The ram can be fed along the axis of rotation with the tailstock handwheel. The carriage controls and supports the cutting tool. It consists of:

1. A saddle that mates with and slides along the ways.

2. An apron that controls the feed mechanisms.

3. A cross slide that controls transverse motion of the tool (toward or away from the operator).

4. A tool compound that adjusts to permit angular tool movement.

5. A toolpost T-slot that holds the toolpost.


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1. INTRODUCTION-(cont’d)FAKULTI KEJURUTERAAN MEKANIKAL

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

Lathe Machine

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1. INTRODUCTION-(cont’d)

Types of Lathes:The engine lathe has kept pace with technological changes in

order to increase productivity and improve part quality. The most common engine lathes are:

1.The Engine Lathe – the accuracy of work produced is controlled by the operator. An experienced operator will be able to produce work to within 0.02 mm of accuracy.

2.The Engine Lathe with Digital Readout – it helps to improve the accuracy and performance of a lathe machine. Accuracy is up to 0.002 mm.

3.The Conventional/Programmable Lathe – it is equipped with digital readout (DR) and limited programming features. DR shows the location of cutting tool and X and Z workpiece dimensions.

4.The Computer-Numerically Controlled Lathe (CNC Lathe) – all movements are CNC controlled. Specially designed for production work that requires great precision and high productivity.


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2. MACHINE FUNCTION

Main Functions of a Lathe Machine:The main function of a lathe machine is to provide a means of rotating a workpiece against a cutting tool, thereby removing metal. All lathes have the same 3 basic functions:

1. A support for the lathe accessories or the workpiece.

2. A way of holding and revolving the workpiece.

3. A means of holding and moving the cutting tools.

Headstock - headstock supports spindle which rotates on ‘Zero Precision’ tapered roller bearings. Work holders are mounted on spindle nose.

Chuck - chuck is mounted on spindle nose. Adjustable jaws permit holding of larger diameter workpieces.


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2. MACHINE FUNCTION-(cont’d)FAKULTI KEJURUTERAAN MEKANIKAL


Tailstock - tailstock center supports right end of work held ‘between centers’. It can be offset to cut tapers, lock in any position along lathe bed, and has hand wheel feed for tailstock tools.

Apron - apron controls are centrally grouped with selector lever for power longitudinal and cross feeds, friction clutch for engaging feeds, half nut lever for thread cutting, and hand wheel for hand traverse of carriage.

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3. SAFETY PRECAUTIONSFAKULTI KEJURUTERAAN MEKANIKAL


General Turning:1. Wear approved safety goggles.

2. Confirm that all guards are in place.

3. Before starting the lathe, turn the spindle by hand to insure that it turns freely. If the spindle is locked in a stationary position with the back-gear lever as well as the bull-gear pin, release one of these devices for the desired kind of drive and speed.

4. Stop the machine to make adjustment.

5. Stop the machine if you need to make measurement.

6. Stop the machine to remove chips. Do not remove them with the hands - always use a brush or stick.

7. Stop the machine for oiling.

8. Always stop the machine when adjusting the tool in the tool post.

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3. SAFETY PRECAUTIONS-(cont’d)FAKULTI KEJURUTERAAN MEKANIKAL


General Turning:9. Keep the machine clear of long chips, rags, and

unnecessary hand tools.

10. Use the right type of cutting tool for the job.

11. Adjust the feed, speed, and depth of cut according to the size and type of metal.

Turning Between Centers:12. Be sure that the tailstock and the tailstock ladle are

locked securely.

13. When available, use a safety dog to drive the work piece. Select the smallest dog which will do the job and clamp it tightly.

14. Lubricate the tailstock dead center properly and frequently.

15. Always cut toward the headstock whenever possible.

16. Before starting the power feed, make certain at the carriage has sufficient free travel to complete the cut without running into the lathe dog.

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Turning Work in Chuck:17. Place a board under the chuck when mount ing it or

removing it from the spindle. Keep the fingers clear.

18. Be sure that the chuck is mounted tightly to the spindle.

19. Be sure that the work is mounted tightly in the chuck.

20. Always remove the chuck wrench or key from the chuck immediately after using it.

21. Turn the chuck one complete revolution by hand after the work is mounted to see that it clears the carriage and the ways.

22. Never allow the cutting tool or tool holder to come into contact with the revolving chuck jaws.

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Turning Work in Faceplate: 23. Be sure the faceplate is secured tightly to the lathe

spindle.

24. Use the shortest bolts and clamps possible for clamping work to the faceplate, and clamp the work securely. The clamps should be supported at the outer ends and should be parallel to the faceplate.

25. Use a counterweight, if necessary, to balance an off-center work piece.

26. Before starting the machine, turn the work one complete revolution by hand to see that it clears the carriage and the ways.

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4. CUTTING TOOLSFAKULTI KEJURUTERAAN MEKANIKAL


Lathe Cutting Tools: Tool bits - types of tool bits include

right and left hand turning tools, facing

tools, cutoff tools and threading tools.

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4. CUTTING TOOLS-(cont’d)FAKULTI KEJURUTERAAN MEKANIKAL


Common cuts made by various cutting tools:

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Carbide tipped:

These tool are resharpened as needed, using special silicon carbide or diamond grinding wheel.

Indexable throwaway inserts: Made of carbide, ceramic and diamond When the initial cutting edge became dulled, rotate to

the next cutting edge, reclamp and continue cutting without any change of tool holder position.

Regrinding – generally more costly than replacing them. Discarded after all cutting edges become dulled

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Standard shapes for indexable throwaway insert cutter

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Cutting Tool Materials: Cutting tools for metalworking are made of high speed

steel, cast alloys and cemented carbide. Also made of ceramic and diamonds for special purposes. Cutting tool must be made of material with suitable

properties:

1. Must have sufficient hardness to cut other materials

2. Must be capable of retaining hardness at high

temperatures which are produced at the cutting edge.

3. Must have good wear resistance

4. Must posses sufficient toughness to prevent chipping or

fracturing.

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5. TOOLHOLDERSFAKULTI KEJURUTERAAN MEKANIKAL


Types of toolholder

Straight shank toolholder-hold tool bit parallel to the base of the toolholder shank-Intended for holding carbide tipped tool bit also cast alloy bit

Throwaway insert toolholder-Used to hold a carbide/ceramiccutting tool and chip breaker-Many types throwaway inserttoolholder are made to hold different insert.

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5. TOOLHOLDERS-(cont’d)FAKULTI KEJURUTERAAN MEKANIKAL


Types of toolholder

Boring toolholders-Used for boring operations using lathe machine

Cutoff toolholders-Used for holding cutoff tool.-Used for cutting grooves, Cutting off stock.

Threading toolholders-Used for holding threading tool.

Knurling tool-Used for performing knurlingoperations

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6. LATHE OPERATIONSFAKULTI KEJURUTERAAN MEKANIKAL


INSTALLING A CUTTING TOOL

1. Lathe cutting tools are held by tool holders. To install a

tool, first clean the holder, then tighten the bolts. [Video1]

2. The tool post is secured to the compound with a T-bolt. The

tool holder is secured to the tool post using a quick release

lever. [Video2]

POSITIONING THE TOOL

3. First, loosen the bolts securing the compound to the

saddle. Then rotate the compound to the desired angle

referencing the dial indicator at the base of the compound.

Retighten the bolts. Now the tool can be hand fed along

the desired angle. No power feed is available for the

compound. If a fine finish is required, use both hands to

achieve a smoother feed rate.

4. The cross slide and compound have a micrometer dial to

allow accurate positioning, but the saddle doesn't. To

position the saddle accurately, you may use a dial indicator

mounted to the saddle.

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6. LATHE OPERATIONS-(cont’d)FAKULTI KEJURUTERAAN MEKANIKAL


TURNING

The lathe can be used to reduce the diameter of a part to a

desired dimension. First, clamp the part securely in a lathe

chuck. The part should not extend more that three times

its diameter. Then install a roughing or finishing tool

(whichever is appropriate). If you're feeding the saddle

toward the headstock (as in the clip below) use a right-

hand turning tool. Move the tool off the part by backing the

carriage up with the carriage handwheel, then use the

cross feed to set the desired depth of cut. In the clip below,

a finish cut is made using the power feed for a smoother

finish. Remember that for each thousandth depth of cut,

the work diameter is reduced by two thousandths. [Video3]

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FACING

A lathe can be used to create a smooth, flat, face very

accurately perpendicular to the axis of a cylindrical part.

First, clamp the part securely in a lathe chuck [Video4].

Then, install a facing tool. Bring the tool approximately

into position, but slightly off of the part. Always turn the

spindle by hand before turning it on. This ensures that no

parts interfere with the rotation of the spindle. [Video5]

Move the tool outside the part and adjust the saddle to

take the desired depth of cut. Then, feed the tool across

the face with the cross slide. The following clip shows a

roughing cut being made; about 50 thousandths are being

removed in one pass [Video6]. If a finer finish is required,

take just a few thousandths on the final cut and use the

power feed. Be careful clearing the ribbon-like chips; They

are very sharp. Do not clear the chips while the spindle is

turning. After facing, there is a very sharp edge on the

part. Break the edge with a file. [Video7]

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PARTING

1. A parting tool is deeper and narrower than a turning tool. It

is designed for making narrow grooves and for cutting off

parts. When a parting tool is installed, ensure that it hangs

over the tool holder enough that the holder will clear the

w/piece.

2. Ensure that the parting tool is perpendicular to the axis of

rotation and that the tip is the same height as the center of

the part. A good way to do this is to hold the tool against

the face of the part. Set the height of the tool, lay it flat

against the face of the part, then lock the tool in place [

Video8]. When the cut is deep, the side of the part can rub

against sides of the groove, so it’s especially important to

apply cutting fluid. In this clip, a part is cut off from a piece

of stock. [Video9]

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DRILLING

1. A lathe also be used to drill hole accurately concentric with

the centerline of a cylindrical part. First, install a drill chuck

into the tail stock. Make certain that the back of the drill

chuck seats properly in the tail stock. Draw the jaws of the

chuck and tap the chuck in place with a soft hammer. [

Video10]

2. Move the saddle forward to make room for the tailstock.

Move the tailstock into position and lock the it in place.

Before starting the machine turn the spindle by hand.

Always use a centerdrill to start the hole. You should use

cutting fluid with the center drill [Video11]. Always drill

past the beginning of the taper to create a funnel to guide

the bit in. In this clip, a hole is drilled with a drill bit. [

Video12]

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Boring on a Lathe

(www.eng.mu.edu)

BORINGIs an operation in which a hole is enlarged with a single point cutting tool. A boring bar is used to support the cutting tool as it extends into the hole. Because of the extension of the boring bar, the tool is supported less rigidity and is more likely to chatter. This can be corrected by using slower spindle speeds or grinding a smaller radius on the nose of the tool.

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Threading on a Lathe(www.phantasmechanics

.com)

SINGLE POINT THREAD TURNINGExternal threads can be cut with a die and internal thread can be cut with a tap. But for some diameter, no die or tap available. In these cases, threads can be cut on a lathe. A special cutting tool should be used, typically with 60 degree nose angle. To form threads with a specified number of thread per inch/mm, the spindle is mechanically coupled to the carriage lead screw.

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7. SPEEDS & FEEDSFAKULTI KEJURUTERAAN MEKANIKAL


CUTTING SPEED FOR LATHE WORK Correct cutting speeds is important for good tool life and

efficient machining. For lathe work, cutting speed refers to the rate in meter

per minute at which the surface of the workpiece moves

past the cutting tool. Condition that affect cutting speed:

1. Kind of material being cut

2. Kind of material the cutting tool is made

3. Shape of the cutting tool being used

4. Rigidity of the workpiece

5. Rigidity of the machine

6. Kind of cutting fluid being used

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7. SPEEDS & FEEDS-(cont’d)FAKULTI KEJURUTERAAN MEKANIKAL


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Cutting speeds are determined using the formula:

rpm = v / (D x )

where

rpm = revolutions per minutev = cutting speed, in meter per minute (mpm)

D = diameter of workpiece

= 3.14

Example 1:

If the cutting speed is 12 mpm for a certain alloy steel and

workpiece is 5cm in diameter, find the rpm.

rpm = v / (D x )

rpm = 12 / (5x10-2 x 3.14)

rpm = 76

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Example 2:

What rpm should be used for a heavy cut of 30.5 mpm on

a piece of low carbon steel of 50.8 mm diameter.

rpm = v / (D x )

rpm = 30.5 / (50.8x10-3 x 3.14)

rpm = 191

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CUTTING FEEDS FOR LATHE WORK Feeds is the distance a cutting tool advances per

revolution. Feeds are expressed in milimeters per revolution of the

spindle. Feeds are determined by the formula:

T = L / (f x N)

where

T = time, in minutes

L = length of cut, in milimeters

f = feed, in milimeters per revolution (mmpr)

N = lathe spindle speed, in rpm

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Example:

A shaft is being turned at 130 rpm and feed 0.5 mmpr. If

the length of cut is 100 mm, calculate the time required to

make the cut.

T = L / (f x N)

T = 100 mm / (0.5 mmpr x 130 rpm)

T = 1.54 min

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TURNING OPERATION

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The rotational speed in turning is related, to the desired cutting speed at

the surface of the cylindrical work piece by the equation

N (rpm) = v / (Do x )

where N = rotational speed , rev/min

v = cutting speed, (m/min)

Do = original diameter of the part, (m)

The turning operation reduces the diameter of the work

from Do to final diameter Df . The change in diameter is

determined by the depth of cut d:

Do – Df = 2d

The feed in turning is generally expressed in mm/rev. This

feed can be converted to a linear travel in mm/min by the

formula

fr = Nf

where fr = feed rate in mm/min and f = mm/rev and

N = rotational speed, rev/min.

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The time to machine from one end of a cylindrical workpart

to the other is given by

Tm = L/ fr

where Tm = time of actual machining, minute

L= length of the cylindrical work part in mm The volumetric rate of material removal can be most

conveniently determined by the following equation:

MRR = v f d

where MRR = material removal rate (mm3/min)

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Example:

A 60 mm long, 50 mm diameter aluminum rod being

reduced in diameter to 49 mm by turning on a lathe

machine. Find material removal rate (MRR) if cutting

speed 70 m/min and time to machine 2 min.

N (rpm) = v / (Do x )

= 70 (m/min) / [50x10-3 (m) x 3.14]

= 445.9 rev/min

Tm = L / fr

fr = L / Tm

= 60 mm / 2 min

= 30 mm/min

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fr = Nf

f = fr / N (f = feed in mm/rev)

= (30 mm/min) / (445.6 rev/min)

= 0.0673 mm/rev

determined by the depth of cut d:

Do – Df = 2d

d = (Do – Df ) / 2

= (50 – 49) / 2

= 0.5 mm

MRR = v f d

= 70 m/min x 0.0673 mm/rev x 0.5 mm x

1000

= 2355.5 mm3 /min

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8. TOOL GEOMETRYFAKULTI KEJURUTERAAN MEKANIKAL


TOOL GEOMETRY A cutting tool must posses a shape that is suited to the

machining operation. Important way to classify cutting

tools is according to the machining process. We have

turning tools, cutoff tools, drill bit, reamers, tap and many

other cutting tool that are name for operation. Cutting tools can be divided into two categories: Single

point tools and multiple cutting edge tools.

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8. TOOL GEOMETRY-(cont’d)FAKULTI KEJURUTERAAN MEKANIKAL


Cutting Tool Terminology:

(Source: R. Kibbe, 2006)

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Cutting Tool Terminology: The tool shank is that part held by the toolholder. Back rake is important to smooth chip flow, needed for

uniform chip and good finish. The side rake directs the chip flow away from the point

of cut. The end relief angle prevents the front edge of the tool

from rubbing on the work. The side relief angle provides for cutting action by

allowing the tool to feed into the work material. The side cutting edge angle (SCEA) directs the

cutting forces. Also helps to direct the chip flow away from

workpiece. The nose radius is important in controlling surface

finish. It will vary according to the finish required.

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MECHANICS OF BASIC MACHINING OPERATION

The important parameter involve

1. The thickness of the uncut layer (t1)

2. The thickness of the chip produced (t2)

3. The inclination of the chip-tool interface with respect to the

cutting velocity (the face of the tool in contact with the

chip known as rake face) i.e the rake angle(α)

4. The relative velocity of the

workpiece and the tool (v)

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EFFECT OF RAKE ANGLE

The performance of a cutting tool is markedly affected by

its rake angle as follows:

1. Strength of tool and heat conduction

a. A tool with large rake angle is weak so that the point

may break off. Also the heat will not be conducted

away from the point of tool effectively.

b. The shape with negative rake angle results is stronger

tool with better heat conductivity.

2. Effect on cutting force. As the rake angle is decreased, the

shear plane angle Ф will be decreased and for the same

depth of cut, the extent of the shear plane will be

increased so that the cutting force will be increased.

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TWO METHOD OF DETERMINING THE SHEAR ANGLE

1. Mathematical Analysis

The early study in determining the shear angle Ф

mathematically was done by Ernst and Merchant based on

the model of orthogonal cutting.2. Experiment technique

Using quick stop device to

freeze the tool on the

workpiece and the cutting

zone is captured by

photomicrography

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CHIP BREAKER Chip disposal is a problem that is often encountered in

turning and other continuous operations. Long, stringy

chips are often generated, especially when turning ductile

materials at high speeds. These chips cause a hazard to

the machine operator and to the workpart finish and they

interfere with automatic operation of the turning process. Chip breakers are frequently used with a single point tools

to force the chip to curl more tightly than they would

naturally be inclined to do, thus causing them to fracture.

Two principal forms of chip breaker design are commonly

used on single point turning tools.

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2 Types of Chip Breakers:

1. Groove type chipbreaker designed into the cutting tool

itself.

2. Obstruction type chip breaker designed as an additional

device on the rake face of tool. The chip breaker distance

can be adjusted in the obstruction type device for different

cutting conditions.

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THE END


THANK YOU

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HUreyyy…..QUIZZ time…..


A 6-in. long, ½-in. diameter 304 stainless steel rod is being reduced in diameter to 0.480 in. by turning on a lathe. The spindle rotates at N = 400 rpm, and the tool is travelling at an axial speed of 8 in./min. Calculate the :

1) cutting speed, 2) Material removal rate, 3) time to cut, 4) power dissipated and 5) cutting forces.

GOOD LUCK!!!!!!

Conventional Lathe Machine

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