LATHE MACHINE PROCESSES & OPERATIONS

Introduction

The lathe operates on the principle of the

work being rotated against the edge of a

cutting tool.

It is one of the oldest and most important

machine tools.

The cutting tool is controllable and can be

moved lengthwise on the lathe bed and into

any desired angle across the revolving work.

Types of Lathes

Speed

Simple construction of a head stock and tail stock

with a tool post. Used for wood turning, metal

polishing, or metal spinning.

Engine

Most frequently used lathe

Heavy duty

power drive for most tool movements

Size range 12”x24” to 24”x48” - can be larger

Types of Lathes (contd.)

CNC

Computer controlled

Wide variety of process capability

multiple axis

Indexing and contouring head

On- line and off- line programming available

CNC Lathe & Turret

Types of Lathes (contd.)

Toolroom

Greater accuracy

More versatility

Wider range of speeds and feeds

Capstan & Turret

Hex turret replaces tailstock

Multiple tools set to machine part

High production rates

Still may require some operator skill

Turret Lathe

Capable of performing multiple cutting operations on the same

workpiece

Turning

Boring

Drilling

Thread cutting

Facing

Turret lathes are very versatile

Types of turret lathes

Ram-type: ram slides in a separate base on the saddle

Saddle type:

more heavily constructed

Used to machine large workpeiceces

Turret Lathe

Difference between Capstan & Turret Lathe

Turret Lathe

Capstan Lathe

Types of Lathes (contd.)

Tracer Lathes

Machine tools with attachments

Capable of turning parts with various contours

A tracer finger follows the template and guides the cutting tool

Automatic Lathes

Automatic cutting off lathe

Single spindle automatic lathe

Swiss type automatic lathe, screw machines multiple spindle automatic lathes

Types of Lathes (contd.)

Automatic Bar Machines

Formerly called automatic screw machines

Designed for high-production-rate machining of screws and other

threaded parts

All operations are preformed automatically

Equipped with single or multiple spindles

Types of Lathes (contd.)

1

2

3

Lathe Size

Lathe size is determined by the SWING and LENGTH OF THE BED.

The swing indicates the largest diameter that can be turned over the

ways (flat or V-shaped bearing surface that aligns and guides movable

part of machine). Bed length is entire length of the ways.

Bed length must not be mistaken for the maximum length of the work

that can be turned between centers. The longest piece that can be

turned is equal to the length of the bed MINUS the distance taken up

by the headstock and tailstock.

As an example, consider the capacity and clearance of a modern 13 in

by 6 ft (325mm by 1800mm) lathe:

Swing over bed 13 in (325mm)

Swing over cross slide 8 3/4 in. (218mm)

Bed length 72 in. (1800mm)

Distance between centers 50 in. (1240mm)

Various Cutting Operations Turing – produces straight, conical, curved, or grooved

workpieces

Facing – produces a flat surface at the end of the part

Boring – to enlarge a hole

Drilling - to produce a hole

Cutting off – to cut off a workpeiece

Threading – to produce threads

Knurling – produces a regularly shaped roughness

Lathe Operations

Turning Operations

Schematic illustration of the basic turning operation, showing depth-of-cut, d; feed, f; and

spindle rotational speed, N in rev/min. Cutting speed is the surface speed of the

workpiece at the tool tip.

Summary of

Turning

Parameters

and

Formulas

Forces Acting on a Cutting Tool in Turning

Forces acting on a cutting tool in turning, Fc is the cutting force, Ft is the thrust of feed force (in the

direction of feed), and Fr is the radial force that tends to push the tool away from the workpiece

being machined.

Basic Turning Operations

Knurling

Standard Engine Lathe

Schematic of a standard engine lathe performing a turning operation, with the cutting tool shown in inset.

Engine Lathe Parts

Engine Lathe Parts

Lathe Design and Terminology

Lathe Engine essential

components:

Bed

Gray cast for vibration

dampening

Headstock assembly

Spindle

Transmission

Drive motor

Tailstock assembly

Longitudinal way clamp

Transverse way clamp

Quill for cutting tools,

live centers, or dead

centers

Schematic diagram of an engine lathe, showing basic components.

Lathe Design and Terminology

Lathe Engine essential components:

Quick-change gearbox

Powers Carriage Assembly movement with lead screw

Carriage Assembly

Fixed to cross slide

Holds tool post at variable orientations

Provides longitudinal and transverse movement of tooling

Ways

Provides precise guidance to carriage assembly and tailstock

Schematic diagram of an engine lathe, showing basic components.

Lathe Design and Terminology

Lathe Design and Terminology

Lathe Design and Terminology

Lathe Components

Lathe Components

Lathe Components

Lathe Components

BED

The bed of the lathe provides the foundation for the whole machine and holds

the headstock, tailstock and carriage in alignment. The surfaces of the bed

that are finely machined - and upon which the carriage and tailstock slide -

are known as "ways".

Some beds have a gap near the headstock to allow extra-large diameters to

be turned. Sometimes the gap is formed by the machined ways stopping

short of the headstock, sometimes by a piece of bed that can be unbolted,

removed and lost.

Some very large lathes have a "sliding bed" where the upper part, on which

the carriage and tailstock sit, can be slid along a lower separate part - and so

make the gap correspondingly larger or smaller.

SADDLE

The casting that fits onto the top of the bed and slides along it is known,

almost universally, as the "Saddle" - a self-explanatory and very suitable

term.

Lathe Components

APRON

The vertical, often flat and rectangular "plate" fastened to the front of the

"Saddle" is known as the "Apron" and carries a selection of gears and

controls that allow the carriage to be power driven up and down the bed -

and also engage the screwcutting feed and various powered tool feeds,

should they be fitted. The leadscrew, and sometimes a power shaft, usually

pass through the apron and provide it with a drive for the various functions it

has to perform.

Apron design can be roughly divided into "single-wall" and "double-wall"

types. The "single-wall" apron has just one thickness of metal and,

protruding from it (and unsupported on their outer ends) are studs that carry

gears. The "double-wall" apron is a much more robust structure, rather like a

narrow, open-topped box with the gear-carrying studs fitted between the two

walls - and hence rigidly supported at both ends

Lathe Components CROSS SLIDE and TOP SLIDE

Sitting on top of the "Saddle" is the "Cross Slide" - that, as its name implies,

moves across the bed - and on top of that there is often a "Top Slide" or

"Tool Slide" that is invariably arranged so that it can be swiveled and locked

into a new position.

Very early lathes had a simple T-shaped piece of metal against which the

turner "rested" his tool (all turning being done by hand) but when it became

possible to move this "Rest" across the bed by a screw feed it became

known, appropriately enough, as a "Slide-rest".

When two slides are provided (or sometimes, on watchmaker's lathes, three)

the complete assembly is known as a "Compound" or "Compound Slide" or

even "Compound Slide-rest". Some makers label the "Top Slide" as the

"Compound Rest" or even the "Compound Slide" - but as "to compound"

means the 'joining of two or more' - not 'one' - so this is not a correct use of

the term.

Lathe Components

CARRIAGE

The whole assembly of Saddle, Apron, Top and Cross Slide is known as the

"Carriage". Some American publications (even makers' handbooks) have

been known to casually refer to the "Saddle" as the "Carriage" - but this

incorrect.

HEADSTOCK.

The lathe Headstock used, at one time, to be called the "Fixed Headstock" or

"Fixed Head", and the rotating shaft within it the "Mandrel". Today the mandrel

is usually called the "Spindle", but this can cause confusion with the tailstock,

where the sliding bar is known variously as the "ram", "barrel" - and "spindle".

The headstock is normally mounted rigidly to the bed (exceptions exist in

some production and CNC lathes) and holds all the mechanisms, including

various kinds and combinations of pulleys or gears, so that the spindle can be

made to turn at different speeds.

Lathe Components

HEADSTOCK SPINDLE

The end of the headstock spindle is usually machined so that it can carry a

faceplate, chuck, drive-plate, internal or external collets - or even special

attachments designed for particular jobs. In turn, these attachments hold the

workpiece that is going to be machined.

BACKGEAR

As its name implies, "backgear" is a gear mounted at the back of the

headstock (although in practice it is often located in other positions) that

allows the chuck to rotate slowly with greatly-increased torque (turning

power).

Lathe Components LEADSCREW

Originally termed a "master thread", or described as the "leading screw", but now

always referred to as the "leadscrew", this is a long threaded rod normally found

running along the front of the bed or, on some early examples running between

the bed ways down the bed's centre line. By using a train of gears to connect the

lathe spindle to the leadscrew - and the leadscrew to the lathe carriage - the

latter, together with its cutting tool, could be forced to move a set distance for

every revolution of the spindle.

TAILSTOCK

The Tailstock was once known as the "Loose headstock", " Poppet head" or

"Loose head“. The unit is arranged to slide along the bed and can be locked to it

at any convenient point; the upper portion of the unit is fitted with what is variously

called a "barrel", "spindle" "ram" or "shoot" that can be moved in and out of the

main casting by hand, lever or screw feed and carries a "Dead Centre" that

supports the other end of work held (by various means) in the headstock.

Special centres, which rotate with the work, can be used in the tailstock ; these

are known as "Rotating Centres" and should not be referred to as "live centres" -

that term being reserved for the centre carried in the headstock spindle.

Cutting Tools for Lathes

Tools consists of cutting surface and support

Cutting surfaces can be of same material as

support or a separate insert

Supports materials must be rigid and strong

enough to prevent tool deflection during cutting

Cutting materials are typically carbides, carbide

coatings, ceramics, or high carbon steels

Inserts are used to decrease cost in that the insert

is disposed of, and the support reused.

Cutting Tools for Lathes

External

Right hand turning

Left hand turning

Round nose turning

Cut-off

Left hand facing

Cutting Tools for Lathes

External (con’t)

Broad nose finishing

Right hand facing

Threading

Form

Internal

Boring

Threading

Grooving

Form

Typical Tool Holders

FIGURE 22-16 Common

types of forged tool holders:

(a) right-hand turning,

(b) facing, (c) grooving cutoff,

(d) boring, (e) threading.

(Courtesy of Armstrong Brothers

Tool Company.)

Quick Change Tool Holders

Tool changing can take

over 50% of manual lathe

operations

Quick Change holders

are used to reduce

manual tool change time

and increase production

Workholding Devices for Lathes

Work pieces can be

held by various

methods

Work piece mounted

between centers

Work piece mounted

within a single chuck

Work piece mounted

within a collet

Work piece mounted on a

faceplate

Lathe Centers

A lathe center hold the end of the work piece, providing

support to preventing the work piece from deflecting

during machining

Lather centers can be mounted in the spindle hole, or in

the tailstock quill

Lathe centers fall into two categories

Dead Center: solid steel tip that work piece spins against

Live Center: centers contact point is mounted on bearings and

allowed to spin with work piece

Lathe Centers

FIGURE 22-21 Work being

turned between centers in a

lathe, showing the use of a dog

and dog

plate. (Courtesy of South Bend

Lathe.)

FIGURE 22-22 Live lathe

center can rotate with the part.

Lathe Chucks

Lathe Chucks are adjustable mechanical vises that hold

the work piece and transfer rotation motion from the

drive motor to the work piece

Lathe Chucks come in two basic types

Three-jaw self-centering chucks

Used to center round or hexagonal stock

Four-jaw independent chucks

Each jaw moves independently to accommodate various work piece

shapes

Lathe Chucks

The jaws on

chucks for lathes (four-jaw

independent or three-jaw selfcentering)

can be removed and

reversed.

Hydraulically

actuated through-hole three-jaw

power chuck shown in section

view to left and in the spindle of

the lathe above connected to

the actuator.

Face Plates

Mandrels

Lathe Collets

Collets are used to hold round stock of

standard sizes

Most accurate holding method for round

stock

Run out less than 0.0005 inch

Stock should be no more than 0.002 inch larger or

0.005 smaller than the collet

Typically used for drill-rod, cold-rolled, extruded,

or previously machined stock

Lathe Collets

FIGURE 22-26 Several types

of lathe collets. (Courtesy of

South Bend Lathe.)

Summary Lathes are used for turning, boring, drilling

and facing

Lathe typically holds the work piece in a

rotating chuck, with the opposite end

supported by a center held in the tailstock

A wide variety of lathe types, and tool types

are available depending upon the application

and the rate of production

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