lathe-types, parts, feed mechanisms, specifications,lathe accessories and attachments,lathe...

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LATHE MACHINE

MACHINE TOOLS

Machine Tools 2

The Lathe

Lathe is a machine tool which holds the work between two rigid and strong supports, called centers, or in a chuck or face plate.

The cutting tool is rigidly held and supported in a tool post and is fed against the revolving work.

While the work revolves about its own axis, the tool is made to move either parallel to (Cylindrical surface) or at an angle with the axis (Tapered surface) to cut the desired material.

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Lathe Basics In

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Parts of a Lathe

• Bed

• Head stock

• Tailstock

• Carriage

• Feed mechanism

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Lathe Basics

Headstock

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Lathe Basics

Bed

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Lathe Basics

Carriage/Apron

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Lathe Basics

Tailstock

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Figure: Diagram of an engine lathe, showing its principal components

Engine Lathe In

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Lathe Basics

Cross Slide

APRON Half Nut Lever Bed

TAILSTOCK

Carriage Hand

Wheel

Tailstock Spindle Clamp

Feed Change Lever

Tool Post Compound

Rest Saddle

BED WAYS

Motor Drive

Gear Box

Feed Reverse Lever

HEADSTOCK

Spindle Nose

Lead Screw

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Machine Tools 11

Types of Lathe 1. Speed lathe

Simplest of all types of lathe Contains Bed, Headstock, Tailstock, Tool post on adjustable slide No feed box, Lead screw, Carriage Headstock spindle speed:1200 to 3600 rpm Applications: Wood working, spinning, polishing etc.

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Types of Lathe … 2. Engine or Center lathe Most widely used Early lathes were driven by steam engines and so the name engine lathe Heavy duty Head stock is much more robust in construction Contains additional mechanism for multiple speeds Carriage, Feed rod and Lead screw are available and hence, longitudinal and

cross feeds are possible

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Machine Tools 13

Types of Lathe … 3. Bench lathe

Small lathe, mounted on a bench Just a scaled version of engine

lathe Used for small and precision work

4. Tool room lathe Greater accuracy More versatility Wide range of speeds and feeds Applications: Precision work on tools, dies,

gauges

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Capstan and Turret Lathes Tailstock of an engine lathe is

replaced by a hexagonal turret Several tools fitted on the turret

are fed into work in proper sequence to carryout different operations in one setting

Hence used for production work Capstan lathe Turret with its saddle can move

for a short distance over the sub bed.

Limitation: Length of a job to be machined is limited

Turret lathe: Turret with its saddle slides

directly on bed-ways enabling longer work to be machined

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Machine Tools 15

Types of Lathe … 6. Special purpose lathes

Used for special purposes Wheel lathe: For finishing journals, turning locomotive wheels Gap-bed lathe: A section of bed adjacent to tailstock is

removable thereby allowing very large diameter work to swing Duplicating lathe: To duplicate the shape of a template on to a

workpiece

Wheel lathe Duplicating lathe

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Types of Lathe … 7. Automatic lathe

Automatic control: Once the tools are set and the machine starts, all the operations will be performed automatically

Change of tools, speeds and feeds are done automatically

The jobs are made continuously one after the other without the intervention of operation

High speed, Heavy duty and Mass production lathe

Single or multi spindle Capable of simultaneous cuts

Operator’s role: Loading the bar stock Setting the tools Machine maintenance

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Machine Tools 17

Types of Lathe … 7. CNC lathe

Computer Numerical Control

Wide variety of process capability

Multiple axes

Indexing and contouring head

On-line and off-line programming available

Equipped with one or more turrets

Each turret is equipped with a variety of tools

Machine Tools 18

The Bed

• Rigid to withstand the cutting forces

• Massive, sufficient to absorb vibration

• Resist the torsional stresses set up.

• Should be seasoned naturally to avoid distortion.

Diagonal ribs

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The Headstock

• Headstock spindle

• Cone Pulley

• Backgears and backgear lever

• Gear Box

• Live center

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The Headstock Spindle

1. Threaded end 2. Taper sleeve 3. Live center 4. Threaded nose 5. Spindle hole

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The Headstock

• Need for speed change

– Workpiece material

– Cutting tool material

– Type of operation

– Workpiece size

– Surface finish

– Cutting fluid

– Rigidity of machine tool

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Methods to vary the speeds

1. By belt drive on cone pulley fitted on the headstock spindle with or without a back gear arrangement.

2. By all gear drive using sliding gears or clutches.

3. By variable speed motor.

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Machine Tools 23

Belt Driven Backgeared Headstock

In a belt driven back geared headstock, multiple speeds can be obtained 1. Direct speed or back gear out 2. Indirect speed or back gear in

Spindle Cone Pulley

Countershaft Cone Pulley

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Speed Ratios

• Direct speeds: Spindle speed =

• Indirect speeds:

Spindle speed =

Back gear ratio =

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Geared Headstock

15 – splined headstock spindle.

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Speed distribution, r (GP Ratio)

1 – Fast Pulley 2 – Loose pulley

3 – Spindle shaft

4 to 13 – gears with Z4 to Z13 teeth

10 – Intermediate shaft

14 – Gear box casting

Preferred values for ‘r’, = 1.12, 1.25, 1.4, 1.6 and 2

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Tailstock

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Carriage L

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Single Screw Tool post

5 – concave ring

Fig.: Single screw tool post

1 – screw

2 – body

3 – tool

4 – convex rocker

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Four way Tool post

Figure: Four way Tool post

1 – clamping handle

2 – central locking bolt

3 – set screws

4 – tool

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Feed Mechanism • Feed: The distance advanced by the tool into or along

the workpiece for each revolution of the work.

Units: mm/rev. or mm/min.

• Types – Longitudinal feed (cylindrical turning, thread

cutting, etc.)

– Cross feed (facing, parting, etc.)

– Angular feed (taper turning, chamfering, etc.)

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Po

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End of bed gearing

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Tumbler gear

feed reverse mechanism

Bevel gear

feed reverse mechanism

Feed gear box

1. Sliding gear mechanism

2. Sliding clutch

mechanism

3. Gear cone and

Norton feed gear

4. Sliding key

mechanism

5. Combination of two or more

of 1, 2, 3, 4

Feed Mechanism L

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Clutch handle

Figure: Layout diagram of feed drive

Tumbler gear

Change Gears

Norton Gears

Sliding Key Mechanism

Feed Mechanisms … Headstock spindle

Sliding key operating handle

Lead screw

Feed rod

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Feed Reverse Mechanism using Tumbler Gears

Tumbler gears are used to give the desired direction of movement to the lathe carriage, via lead screw or feed rod.

Tumbler Gear Mechanism Tumbler Gears

Forward motion: A-B-D: Carriage moves towards headstock

Reverse motion: A-C-B-D: Carriage moves away from headstock

Drawback: Non-rigid construction, hence not suitable for heavy duty

Two pinions B and C are mounted on a bracket.

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Feed Reverse Mechanism using Bevel Gear Mechanism

Constructional details 1 – headstock spindle 2 – spindle gear 3 – driver gear keyed to stud shaft (7) 5 – gear on the lead screw (6) 8, 10, 12 – bevel gears with equal no. of teeth 11 – clutch (keyed to stud 7 by a feather key) 9 – lever (to engage clutch to gear 8 or 10 or

keep in neutral position) (Gear 13-sleeve-bevel gear 8) rotate freely on

stud 7 as one unit bevel gear 10 also rotates freely on stud 7

The clutch operated bevel gear feed reversing mechanism incorporated below the headstock or in the apron provides sufficient rigidity in construction.

Operation: When clutch is engaged to bevel gear 8: Motion flow: gear 2 – Idler gear – (gear 13, sleeve, bevel gear 8) – (stud 7, gear 3) Result: Gear 3 rotates in the same direction as that of gear 2. Hence, Forward motion, Carriage moves towards headstock. When clutch is engage to gear 10: Motion flow: gear 2 – Idler – (gear 13, sleeve, bevel gear 8) – bevel gear 12 – bevel gear 10 – (stud 7, gear 3) Result: Gear 3 rotates in opposite direction to that of gear 2. Hence, Reverse motion, Carriage moves away from headstock.

Feed Reverse Mechanism …

using Bevel Gear Mechanism

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Feed Mechanisms …


Change Gears

Change gears are used in thread cutting to give proper relative motion between the work and tool

Change Gear Mechanism

Figure: Change gears

Change Gears

E G

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Feed Mechanisms …


Norton Feed Gears

Norton Feed Gears

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Feed Mechanisms …




I – driving shaft; II – driven shaft 1 – sliding key operating handle 2 – sliding rod 3 – lever spring

3

I

II

It consists of a cone of gears mounted on a hollow shaft.

Gears rotate independently freely on the shaft.

Steel washers are placed between the gears to prevent simultaneous engagement of adjacent gears.

With the help of sliding key (that slides within the hollow shaft), any one of the gears may be engaged to the cone gears on the driven shaft.

Motion is then transmitted finally to the lead screw or feed rod.

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Apron Mechanism 1 – lead screw; 2- feed rod 3 – half-nut; 4 – cross feed screw 5, 6, 7 – Gear train for cross feed 9 – cone clutch for longitudinal feed 10, 11 – gear train for long. feed. 12 – gear for hand wheel 13 – pinion; 14 – apron 15 – rack; 16 – hand wheel 17 – knob for longitudinal feed 18 – cone clutch for cross feed 19 – knob for cross feed 20 – lever; 21 – cam 22 – sliding gear 23 – gear on worm shaft 24 – worm; 25 – cam plate 26 – half nut operating handle

Apron Mechanism

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Feed Rod & Lead Screw

lead screw feed rod

Feed rod Used to engage automatic tool feed during turning, boring, facing etc.

Automatic tool feed is obtained by engaging feed rod so as to transmit power from the lathe spindle to the apron gears

Lead screw: Long threaded master screw

Engaged during thread cutting operation

In lathes without feed rod, lead screw may be used for turning, boring etc.

Split Half Nut Mechanism

1 – cam slot; 2 – frame; 3 – lead screw; 4 – hand lever; 5 – pin; 6 – circular plate; 7 – half nuts

Fig. b) Sectional views of Half Nut Mechanism

Half Nut is engaged to the lead screw during threading.

The nut is made in two halves.

Each of these halves carries a pin at the back (P1 and P2 as shown in Fig. a) which engages into the cam slots provided in a plate at their back. When this plate is rotated by means of the lever L, it engages or disengages the split half nut with the lead screw depending upon the direction of rotation of the lever.

Fig. a) Half nut

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Kinematic Structure of a Lathe L

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Specifications of a Lathe

A = Length between centers B = Swing over gap (Gap bed lathes) C = Height of centers D = Swing over carriage E = Swing over bed

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Machine Tools 49

Turning Operation

FIGURE : Schematic illustration of a turning operation showing depth of cut, d, and feed, f. cutting speed is the surface speed of the workpiece at the tool tip.

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Lathe Operations

Fig: Various cutting operations that can be performed on a lathe.

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Machine Tools 51

Lathe Operations

Fig: Various cutting operations that can be performed on a lathe.

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Lathe – Accessories & Attachments

Accessories Devices employed for holding and supporting the work and the tool.

Attachments Devices to increase production and efficiency and widen its scope of use.

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Machine Tools 53

Lathe – Accessories & Attachments

Accessories Centers

Face & Angle Plates

Dogs or Carriers

Chucks

Mandrels

Rests

Attachments Bar Stops

Thread Chasing Dials

Taper turning

Milling

Grinding

Gear Cutting

Hydro-copying

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Centers

A – Ordinary Center E – Tipped (brazed) Center

B – Ball Center F – Insert type Center

D – Half Center H – Use of half center 1 – Half center 2 – Facing tool

G - Pipe Center

C – Frictionless Center 1 – Insert type center 2 – Nut 3 – Roller bearing 4 – Thrust bearing 5 - Housing

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Machine Tools 55

Driving Plates and Dogs

Fig: Eccentric Turning

Fig. Straight dog

Boss Threaded Hole

Projected Pin

Fig: Driving Plate

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Holding the work between Centers

Fig.: mounting the work between centers using a "dog” Acc

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Chuck

Fig: Four Jaw Chuck

1- concentric circle; 2 – chuck body; 3 – Jaw screw; 4 – jaw; 5 – gripping surface; 6 – recess for back plate.

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Chuck

Fig: Three Jaw Chuck Acc

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Chuck

Fig: Magnetic Chuck

1-work; 2-magnetic material; 3-keepers; 4-face plates, 5-magnet

Fig: Hydraulic Chuck

1-cylinder; 2-air passage; 3-head stock spindle; 4-piston; 5-valves;; 6-piston rod; 7-link; 8-jaw; 9-guide; 10-sliding unit

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Collet

1-Bearing; 2-Spindle; 3-Nut; 4-Key; 5-Chuck; 6-Collet

Fig: Collet Chuck

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Collet

Fig.: collet

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Face Plate & Angle Plate

Clamping Plate

Balance Weight

Packing Work

Fig: Face Plate

Fig: Angle Plate

1-face plate; 2-balance weight; 3-elbow pipe; 4-clamping nut; 5-angle plate; 6-clamping nut

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Face Plate

Fig.: face plate for non-cylindrical workparts Acc

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Mandrels

Plain Mandrel

Step Mandrel Threaded Mandrel

Collar Mandrel

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Mandrels

Fig: Cone Mandrel

1-solid cone; 2-sliding cone; 3-nut

Fig: Gang Mandrel

1-fixed collar; 2-hollow workpieces; 3-movable collar; 4-nut

Fig: Expansion Mandrel

1-sleeve; 2-tapered pin

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Steady Rest

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Follower Rest

Fig.: Follower rest

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Thread Cutting - Terminology

Fig. Elements of external threads

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Thread Cutting - Terminology

Fig. Single Start Fig. Double Start

Fig. Triple Start

.

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Thread Cutting

Fig: General set-up of lathe for screw cutting

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Thread Cutting - Change Gear Ratio

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Cutting metric thread on British standard lead screws or Vice Versa

To cut metric threads on lathes having British standard lead screw:

To cut British standard threads to be cut on a lathe having lead screw of metric pitch:

5127

1275

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Machine Tools 73

Cutting R.H. and L.H. Threads Right Hand threads: Threads slope up to the right when

screw is held vertically.

The spindle and the lead screw will rotate in the same direction.

Left Hand threads: Threads slope up to the left when

screw is held vertically.

The spindle and the lead screw will rotate in opposite direction.

This change of direction of rotation on lathe is effected by tumbler gear or by using proper number of idlers.

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Thread Cutting - Gear Trains

Simple gear train Compound gear train

Often lathes are equipped with a set of gears ranging from 20T to 120T in steps of 5T.

In addition, a gear of 127 teeth is also provided which is known as translating gear.

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Machine Tools 75

Left Hand thread - Applications Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten, e.g. on a left-hand bicycle pedal.

In combination with right-handed threads in turnbuckles.

In Gold ornaments, e.g. ear studs.

In some gas supply connections to prevent dangerous misconnections, for example in gas welding the flammable gas supply uses left-handed threads.

RH ThreaLH Thread

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Examples:

26

13

13

2020

2060

Simple train with one idler Simple train with one idler

13

2525

2575


13

3030

3090


Calculate change gears for cutting R.H. threads of 2 mm pitch on a lathe having lead screw of 6 mm pitch.

Pb.:

Soln:

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Machine Tools 77

Examples: Calculate change gears for cutting L.H. threads of 1.5 mm pitch on a lathe having lead screw of 6 mm pitch.

Pb.:

Soln:

1.56

14

14

2020

2080


14

2525

20100

Simple train with one idler Simple train with one idler Acc

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Examples: Calculate change gears for cutting R.H.B.S.W. “V” threads of 12 TPI on a lathe having lead screw of 8 TPI.

1 12⁄1 8⁄

23

2020

2060


23

2525

2575


812

23

Pb.:

Soln:

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Machine Tools 79

Examples: Calculate change gears for cutting R.H. threads of 1 mm pitch on a lathe having lead screw of 8 mm pitch.

18

18

2020

20160

Compound Gear train with no idler Gear A = 20 Gear B = 80 Gear C=30 Gear D = 60

Compound Gear train with no idler Gear A = 20 Gear B = 80 Gear C=30 Gear D = 60

18

14 2

14

12

14

2020

12

3030

2080

3060

Pb.:

Soln:

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Examples: Calculate change gears to cut R.H. threads of 25 TPI on a lathe having lead screw of 6 TPI.

Pb.:

Soln:

625

625

2 35 5

25

2020

35

1515

40100

4575

Compound Gear train with no idler Gear A = 40 Gear B = 100

Gear C = 45 Gear D = 75

Compound Gear train with no idler Gear A = 40 Gear B = 100


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Machine Tools 81

Examples: Calculate change gears to cut L.H. single start threads of 0.25” pitch on a lathe having lead screw of 8 mm lead.

Compound Gear train with one idler Gear A = 127 Gear B = 80


Compound Gear train with one idler Gear A = 127 Gear B = 80


0.25"14

1275

1 4⁄8

1275 32

1275 16 2

⇒12780

1 502 50

127 5080 100

Pb.:

Soln:

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Setting of Tools for Threading

Fig. Setting threading tool for external threading

Fig. Setting threading tool for internal threading

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Feeding the Tool in Threading

Fig: Straight Thread Fig: Inclined feed

Fig: Inclined feed using a form tool

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Fig.: Cutting a screw thread on a lathe Inset shows face of threading dial

Thread Cutting

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Providing undercut

Fig.: Under Cut Acc

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Cutting Square and Acme Threads

Fig. Tool setting for square threads

nceCircumfere

threadofLead ..tan

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Fig.: Threading cycle to produce threads

Thread Cutting Cycle A

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Att

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Thread Chasing Dial

Fig: Thread Chasing Dial Acc

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Machine Tools 89

Taper Turning

1100

1 100

tan 2⁄

2

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Tail Stock Set-over Method Let, H = set-over required D = larger diameter d = smaller diameter L = total length of work l = length of taper (all dimensions in ‘mm’)

sin

α is very small and hence, sin tan

tan 2

But, ∴2

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Machine Tools 91

Swiveling the Compound Rest

Fig. Turning taper by swiveling the compound rest Acc

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Taper turning attachment

Fig. Use of taper turning attachment

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Form or Broad Nose Tool

Fig. Turning short taper by a form (broad nose) tool

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Machining Time Calculations in Turning Cutting speed:

where d = diameter of the work in mm; N = speed of the work in rpm.

Feed: mm / rev. or mm / min.

f (mm / min) = f (mm / rev) X N (rev / min)

Machining time:

where l = length of the job, mm

f = feed in mm/rev.;

N = speed in rpm.

1000

/

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Machine Tools 95

References Te

xt B

oo

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• Materials and Processes in Manufacturing, by E. Paul DeGarmo, et al, Wiley

• Manufacturing Engineering and Technology, by S Kalpakjian and Steven R Schmid, Pearson

• Machining Technology – Machine Tools and Operations, by Helmi A Youssef and Hassan El-Hofy, CRC Press

• Manufacturing Science, by A Ghosh and AK Mallik, EWP

• Machining & Machine Tools, by AB Chattopadhyay, Wiley