Lathe Machine

A lathe is a powered mechanical device in which the work is held and rotated against a suitable cutting tool for producing cylindrical forms in the metal, wood or any other suitable machinable material.

Principle of working

The principle of lathe machine is that it holds the work between two strong supports called as centers  or in a check or face plate which revolves. The chuck or the center is mounted on the main spindle of the machine. The cutting tool is rigidly supported in a tool port and it is fed against the rotating job. The job rotates in its axis and the tool is moved either parallel or to an inclination with its axis as such a cylindrical or tapered surface is produced.

Types of lathe

There are various types of lathe. They are as follows:
a. Precision lathe
b. Tool room lathe
c. Capstan and turret lathe
d. Automatic lathe
e. Speed lathe
f. Engine lathe
g. Bench lathe
h. Special purpose lathe

The principle parts of lathe are discussed:

Bed: 

The bed is the function of the lathe. It is supported on board box columns and is made of cast iron. It consist of two heavy metal slides running lengthwise with ways. Units which are mounted on the bed are the head-stock, the tail-stock and the carriage. Lathe bed is made of high grade special cast iron having high vibration damping qualities. It is secured rigidly over cabinet leg and leg. In its use care should be taken to avoid formation of scratches, nicks and dents by falling tools. It should be lubricated regularly.

Gap beds: 
Some beds have a gap just adjacent to the front of the head stock. It is gap beds. This type of bed can accommodate jobs which are bigger in diameter, but it is true for jobs of shorter length only.

Head stock: 

It supports the main spindle in the bearings and aligns it properly. It also houses necessary transmission mechanism with speed changing levers to obtain different speeds. Cone pulleys or gears could be used to change the speed of spindle. Holding devices which are mounted on head-stock spindle:

a. Three jaw chuck
b. Four jaw chuck
c. Face plate
d. Collet chuck
e. Magnetic chuck
f. Lathe dog

Main spindle: 

It is a hollow cylindrical shaft and long cylindrical jobs can pass through the main spindle. The spin end facing the tail-stock is called the spindle nose. The spindle nose has a mores taper hole (self locking taper) and threads on outside. The mores taper is used to accommodate center or collet chuck or threaded portion of chuck.

Tail-stock: 

It is a movable part located opposite to head-stock on the ways of the bed. It is capable of sliding along the bed maintaining its alignment with head-stock. On medium size and smaller lathe it is moved along the bed manually, whereas in heavier lathes it is moved by hand wheel through a pinion that meshes with rack provided in front of lathe bed. It is used for four purposes:
a. To support free end during machining.
b. To hold tool for performing operations like drilling, reaming, tapping etc.
c. To center the job and the tool before any lathe operations.
d. To help in the operation of taper turning by tail stock set over method.

Carriage: 

It is located between head-stock and tail-stock. It can slide along bed guide ways and be locked at any position by tightening the carriage lock screws. It consist of following five main parts

a. APRON: 

It is fastened to saddle. It contains gears and clutches for transmitting motions from feed rod and hand wheel to the carriage. Also split nut which engages with the lead screws during threading. The clutch mechanism is used for transmitting motion from feed rod whereas the split nut along with the lead screw moves the carriage during thread cutting.

b. SADDLE: 

It is made up of H-shaped casting. It aids saddle to slide on bed guide ways by operating hand wheel. 

c. COMPOUND REST: 

It supports the tool post and cutting tool in its various positions. It may be swiveled on the cross-slide to any angle in the horizontal plane.

d. CROSS-SLIDE: 

It is provided with a female dovetail one side and assembled on top of saddle having a male dovetail.

e. TOOL POST: 

It is used to hold various tool holders and tools. Three types of tool post commonly used are:
• Ring and rocker tool post
• Square head tool post
• Quick change tool post

Legs: 

They are supports which carry entire load of the machine. Legs are casted and it is placed on the floor of the shop of foundation by grouting. The left leg acts as a housing for the motor, the pulleys and the counter shaft at the same time the right leg acts as a housing or the coolant tank, pump and the connecting pipes.

SPECIFICATION OF LATHE

The size of the lathe is specified by one of the following ways:
a. Length of the bed (Bed length will include head-stock)
b. Distance between centers (Maximum job length that can be held between centers)
c. Diameter of the work which can be turned between the ways (Maximum diameter that can be rotated)
d. Heights of centers
e. Swing over carriage

OPERATIONS PERFORMED ON LATHE MACHINE

1. Facing

2. Rough Turning

3. Shouldering

4. Radius Turning

5. Necking

6. Taper turning

7. Thread cutting

8. Forming

9. Chamfering

10. Knurling

11. Drilling

12. Boring

13. Counter sinking

14. Counter boring

DIFFERENT METHODS OF TAPER TURNING

Compound rest method: 

Compound rest method is used for turning steeps and short tapers. It is having a circular base graduated in degree. When taper of required degrees is turned, the base of compound rest is swiveled through an angle equal to half of the taper angle.

Tapper turning attachment: 

It is bolted on the back of the lathe and has a guide bar which may be set at the desired angle of taper. As the carriage moves along the bed length, a slide over the bar causes the tool to move in and out according to the setting of the bar. Lathe center remains aligned and same taper can be given on various jobs.

Form tool method: 

When the length of the taper is very short it can be turned by a bored nosed tool consisting of cutting edge half the taper angle. While using this type of tool lot of chattering is there, as such this type of tool is used specially when the machine have a very rigid base.