Mechanical Engineering

Teacher: Mr. Tulloch

Centre Lathe

objectives

• Main purpose of the centre lathe• Safety precautions on the centre lathe• Types of Lathe• Identify main parts of the centre lathe• State the purpose of each main parts• Method of Holding work• Method of cutting tapers• Cutting speed on the Lathe

Purpose of centre Lathe

• The lathe is used for producing cylindrical work. The workpiece is rotated while the cutting tool movement is controlled by the machine or an operator.

Centre Lathe

• lathes may be operated directly by people (manual lathes) or computer controlled lathes (CNC machines) that have been programmed to carry out a particular task.

Safety Precautions• Always wear approved safety glasses• Rollup sleeves, remove tie and tuck in loose

clothing• Never operate machine if safety guards

removed• Stop lathe before measure work or clean, oil

or adjust machine• Always remove chuck key after use• Always remove chips with brush

Types of Lathe

Engine Lathe The most common

form of lathe, motor driven and comes in large variety of sizes and shapes. Not production lathe, found in school shops.

.

Types of Lathe

Bench Lathe A bench top

model usually of low power used to make precision machine small work pieces.

Types of Lathe

Tracer Lathe a lathe that has

the ability to follow a template to copy a shape or contour

Types of Lathe

Automatic LatheA lathe in which the

work piece is automatically fed and removed without use of an operator.

Types of Lathe–Turret Lathe lathe which have

multiple tools mounted on turret either attached to the tailstock or the cross-slide, which allows for quick changes in tooling and cutting operations.

Types of lathe

Computer Controlled Lathe

– A highly automated lathe, where both cutting, loading, tool changing, and part unloading are automatically controlled by computer coding.

Lathe Parts

Lathe Parts

Headstock

• Found on the left side of the bed.

• Head stock spindle

Headstock• Clamped on left-hand end of bed• Headstock spindle

– Hollow cylindrical shaft supported by bearings• Provides drive through gears to work-

holding devices– Live center, faceplate, or chuck fitted to

spindle nose to hold and drive work• Driven by stepped pulley or transmission

gears

Lathe Bed

Lathe Bed

• Heavy, rugged casting• Made to support working parts of lathe• On top section are machined ways

– Guide and align major parts of lathe

Carriage

Carriage• Used to move cutting tool along lathe bed• Consists of three main parts

– Saddle• H-shaped casting mounted on top of

lathe ways, provides means of mounting cross-slide and apron

– Cross-slide– Apron

Cross-slide

Cross-slide• Mounted on top of saddle• Provides manual or automatic cross

movement for cutting tool• Compound rest (fitted on top of cross-

slide)– Used to support cutting tool– Swiveled to any angle for taper-turning– Has graduated collar that ensure

accurate cutting-tool settings (.001 in.) (also cross-slide)

Apron

Apron

• Fastened to saddle• Houses gears and mechanism required to

move carriage or cross-slide automatically• Locking-off lever inside apron prevents

engaging split-nut lever and automatic feed lever at same time

• Apron handwheel turned manually to move carriage along lathe bed

Tailstock

Tailstock

• Upper and lower tailstock castings• Adjusted for taper or parallel turning by

two screws set in base• Tailstock clamp locks tailstock in any

position along bed of lathe• Tailstock spindle has internal taper to

receive dead center– Provides support for right-hand end of

work

Tool post

• There are three main types of tool post used for holding lathe cutting tools:

Ring and rockerFour-way turretQuick -change

Ring and Rocker

• With this type, tools are quickly adjusted at centre height by moving the rocker, which beds on the loose ring around the main post.

Four-way tool post

• It has four ways, or sides, which allow four cutting tools to be held at the same time.

Quick-change

• No need for packing. The adjustment is done by means of screw. There are separate tool holders that fit the four faces of the main block and are easily removed and returned to exactly the same setting.

Compound slide/ Top slide

• It is fixed on top of the cross-slide, and can be turned (set) to any desired angle. The compound slide is useful for turning and boring short tapers and chamfers.

Quick-Change Gearbox

Quick-Change Gearbox• Contains number of different-size gears• Provides feed rod and lead-screw with

various speeds for turning and thread-cutting operations– Feed rod advances carriage when automatic

feed lever engaged– Lead screw advances the carriage for thread-

cutting operations when split-nut lever engaged

Quiz points• Produce cylindrical work• Type of lathe Programme to

carry out task• Use to tighten the chuck• Types of lathe use in school

workshops• Use to make precision work

and small work pieces.• lathe which have multiple

tools• Tool post that doesn’t need

packing• Use to produce short tapers

• Made to support working parts of lathe

• Found at the left side of the bed

• Found at the right side of the bed

• Hold four tools at the same time

• Used to move cutting tool along lathe bed

• Parts of the machine use when facing off

Lathe Size and Capacity

• Designated by largest work diameter that can be swung over lathe ways and generally the maximum distance between centers

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Indicated by the swing and the length of the bed

Lathe Size

38

Lathe Size

39

Lathe Size

40

Lathe Size

Cutting speed

• Cutting speed for turning is the speed at which the work rotates. This is also known as surface speed

47-42

Cutting Speed

• Rate at which point on work circumference travels past cutting tool

• expressed in meters per minute (m/min)• Important to use correct speed for material

– Too high: cutting-tool breaks down rapidly

– Too low: time lost, low production rates

Cutting speed selection

• The softer the work material, the faster the recommended cutting speed

• The hardness of the cutting tool material has a great deal to do with the recommended cutting speed. The harder the cutting tool material, the faster the cutting speed. The softer the cutting tool material, the slower the recommended cutting speed

• The depth of the cut and the feed rate will also affect the cutting speed, but not to as great an extent as the work hardness

Factors that Determine cutting speed

• The material being cut• The rigidity and condition of the machine• The material of which the tool is made from• The depth of cut and the feed rate• Availability of coolant (cutting fluid)

Setting Speeds on a Lathe

• Speeds measured in revolutions per minute– Changed by stepped pulleys or gear levers

• Belt-driven lathe– Various speeds obtained by changing flat belt

and back gear drive• Geared-head lathe

– Speeds changed by moving speed levers into proper positions according to r/min chart fastened to headstock

Safety Note!! NEVER change speeds when lathe is running.

48

Setting Speeds on a Lathe• Speeds measured in revolutions per

minute– Changed by stepped pulleys or gear

levers• Belt-driven lathe

– Various speeds obtained by changing flat belt and back gear drive

• Geared-head lathe– Speeds changed by moving speed levers

into proper positions according to r/min chart fastened to headstock

Lathe Cutting SpeedsMaterial being cut Cutting speed (metres/minute

Mild steel 20 to 28

Cast iron 18 to 25

High carbon steel 12 to 18

Brass 45 to 90

Bronze 15 to 21

Aluminium Upto 30047-49

47-50

Calculating Lathe cutting speed

• Given in metres per minute• spindle speed of machine (N) and diameter

of work must be known

• S (m/min)= πDN/1000• Where π= 22/7 or 3.142• D= diameter of material• N= Spindle speed(rev/min)

Calculating Lathe Spindle speed (rev/min)

D xS1000 rev/min

S (m/min)= cutting speedWhere π= 22/7 or 3.142 D= diameter of material N= Spindle speed(rev/min

Examples

• Calculate rev/min required to rough-turn 150mm diameter piece of machine steel (CS 90)

• Find the cutting speed of a 50mm diameter bar being turned with a spindle of 178 rev/min.

Exercise• Calculate the spindle speed required to turn

200mm diameter piece of high speed steel, if the cutting speed is 28 m/min.

• Find the cutting speed of a 15mm diameter bar being turned with a spindle of 955 rev/min.

• Determine the lathe speed to cut a 40mm diameter at 30 m/min

47-53

Class work1. What is cutting speed for turning?2. State the unit for cutting speed.3. Which of the following materials have the

highest cutting speed. 1. Brass 2. Bronze 3. mild steel. Give reasons for your choice.4. List four factors that determine the cutting

speed. Explain any two factors. Continue on next slide.

5. Find the surface cutting speed for: Diameter 60mm. Lathe speed 180rev/min Diameter 225mm. Lathe speed 20rev/min Diameter 50mm. Lathe speed 160rev/min• Lathe speed 120 rev/min. Diameter 40mm.

6. Find the lathe speed for: Diameter 75mm. Surface speed 20 m/min Diameter 20mm. Surface speed 30 m/min Diameter 150mm. Surface speed 15 m/min.

Objectives

• Explain what is lathe accessories• Identify and explain lathe accessories

used in the metal work shop.

Lathe Accessories

• What is Lathe accessories?

• Equipment or tools used on the lathe to assist in performing an operation.

Lathe Accessories• Divided into two categories

– Work-holding, -supporting, and –driving devices• Lathe centers, chucks, faceplates• Mandrels, steady and follower rests• Lathe dogs, drive plates

– Cutting-tool-holding devices• Straight and offset toolholders• Threading toolholders, boring bars• Turret-type toolposts

Lathe Centers

• Work to be turned between centers must have center hole drilled in each end

• Support during cutting• Most common have

solid Morse taper shank60º centers, steel with carbide tips

• Care to adjust and lubricate occasionally

Type of Centres

Type of Lathe Centres

• Lathe centres is used as a support at the end of a work. It is usually made from carbon tool steel. There are three (3) main types of Lathe centres:

• Live centre (Revolving centre)• Dead centre• Half centre

Live centre (Revolving Centre)• It is constructed so that the 60° center runs in its own bearings .• The live centre fits in the spindle (headstock)and rotates with

the spindle. • Used when turning between centres

Centre in Headstock Spindle

Dead centre

• Fits in the tailstock spindle, remains stationary while the work rotates on its point

Half Centre

• A centre that is cut away almost to its point. It is often used in the tailstock for facing up to or for turning close to the end of the work.

Chucks

Chucks

• Used extensively for holding work for machining operations– Work large or unusual shape

• Most commonly used lathe chucks– Three-jaw universal– Four-jaw independent– Collet chuck

Three-jaw Universal Chuck

• Holds round and hexagonal work• Grasps work quickly and accurate • Three jaws move

simultaneously whenadjusted by chuck Key

Four-Jaw Independent Chuck• Used to hold round, square, hexagonal, and

irregularly shaped workpieces• Has four jaws

– Each can be adjusted independently by chuck Key• Jaws can be reversed to hold work by inside

diameter

Headstock Spindles

Universal and independent chuck fitted to three types of headstock spindles

1. Threaded spindle nose– Screws on in a

clockwise direction

2. Tapered spindle noseHeld by lock nut

that tightens on chuck

3. Cam-lock spindle nose• Held by tightening cam-locks

using T-wrench• Chuck aligned by taper

on spindle nose

Collet Chuck• Collet chuck is used to hold small Collet chuck is used to hold small

workpieces.workpieces.• Used for high-precision work• Spring collets available to hold round, square,

or hexagon-shaped workpieces• Each collet has range of only few thousandths

of an inch over or under size stamped on collet

|

Collet Chuck

Special adapter fitted into taper of headstock spindle, and hollow draw bar having internal thread inserted in opposite end of headstock spindle. It draws collet into tapered adapter causing collet to tighten on workpiece.

Lathe Dogs /Carrier

• A lathe dog ( lathe carrier) is a device that clamps around the workpiece and allows the rotary motion of the machine's spindle to be transmitted to the workpiece.

• A carrier is most often used when turning between centers on a lathe.

Types of Lathe Dogs

• Standard bent-tail lathe dog– Most commonly used for round workpieces– Available with square-head setscrews of headless

setscrews• Straight-tail lathe dog

– Driven by stud in driveplate– Used in precision turning

Types of Lathe Dogs

• Safety clamp lathe dog– Used to hold variety of work– Wide range of adjustment

• Clamp lathe dog– Wider range

than others– Used on all shapes

Work Held Between Centre

Work Held Between Centres

Face plate• A faceplate is the basic workholding accessory

for a lathe. It is a circular metal plate which fixes to the end of the lathe spindle. The workpiece is then clamped to the faceplate, typically using t-nuts in slots in the faceplate, or less commonly threaded holes in the faceplate itself.

Faceplate• Used to hold work too large or shaped so

it cannot be held in chuck or between centers

• Usually equipped with several slots to permit use of bolts to secure work

Drive plate

• Provide the drive when turning between centres

Steady Rest / Fixed Steady

• Used to support long work held in chuck or between lathe centers– Prevent springing

• Located on and aligned by ways of the lathe

• Positioned at any point along lathe bed• Three jaws tipped with plastic, bronze or

rollers may be adjusted to support any work diameter with steadyrest capacity

Steady Rest

Steady Rest

Travelling Steady/Follower Rest

• Mounted on saddle• Travels with carriage to prevent work from

springing up and away from cutting tool– Cutting tool generally positioned just ahead of

follower rest– Provide smooth bearing surface for two jaws

of follower rest

Travelling Steady

Travelling Steady

Mandrel

• Holds internally machined workpiece between centers so further machining operations are concentric with bore

• Several types, but most common– Plain mandrel– Expanding mandrel– Gang mandrel– Stub mandrel

Plain Mandrel

Expanding Mandrel

Gang Mandrel

Stub Mandrel

Taper Turning

• TapersUniform change in diameter of workpiece

measured along its axisProvides rapid and accurate method of

aligning machine parts and easy method of holding tools

Types of Tapers• Machine Tapers

–Self-holding tapers–Steep or self-releasing tapers

• Self-Holding Tapers• Remain in position because of the wedging

action of the small taper angle• Steep Tapers• Used mainly for alignment of milling

machine arbors and accessories

Types of Tapers

• Standard Tapers• Used for drills, reamers, and lathe center

shanks• Used for positioning and holding parts

together

Method of Cutting Tapers

• Using the tool angle• Setting the angle on the compound slide• Off set the tailstock• Using the taper turning attachment

Assignment 1.

Do a presentation on any of the following method using any of the following in group of three:

a. Videob. Picturec. Actingd. PowerPoint (picture and detail)e. movie

Assignment 2

• State and explain the process involve in knurling

• Three method of holding work on the lathe• The procedure involve in facing and parallel

turning• Four method of holding work on the drill press• Using the same presentation format

104

Metric Tailstock Offset Calculations

• If taper turned by offsetting tailstock, amount of offset O is calculated as follows:

where D = large diameter d = small diameter l = length of taper L = length of work

L x x 2

d - D Offset l

105

Metric Taper Calculations

d = small diameterk = unit length of taper l = total length of taper D = large diameter

If know d, k, and l, D may be calculated. D equal to small dia + amount of taper.Amount of taper is equal to 1k, so total taper equals lk.

D = d + total taperD = d + l/k

Using the compound Slide

• Taper:

LDD

2tan 21

Taper Turning Calculation

• Calculate tailstock offset required to turn a 1:30 taper X 60 mm long on a workpiece 300 mm long. The small diameter of tapered section is 20 mm.

question• Determine the angle at which the

compound rest would be swiveled for cutting a taper on a workpiece having a length of 150 mm and outside diameter 80 mm. The smallest diameter on the tapered end of the rod should be 50 mm and the required length of the tapered portion is 80 mm.

Using the compound slide• Calculate the taper angle for the following:

50105

2040

lOLdD

40100

2050

lOLdD

60120

3060

lOLdD

10:140

10020?

KlOLdD