What is lathe machine ?

What is lathe machine ?


Introduction

The lathe is one of the oldest machine tools. and come into existence form the early tree lathe  which was then a novel device for rotating and machining a piece of work held between two adjacent trees. A rope wound round the work with its one end attached to a flexible branch of a tree and the other end being pulled by a man caused the job to rotate intermittently .Hand tools were then used .With its further development a strip of wood called "lathe" was used to support the rope and that is how the machine came to be known as "lathe".

Function of the lathe

The main function of a lathe is to remove metal from a piece of work to give it the required shape and size.This is accomplished by holding the work securely and rigidly on the machine and then turning it against cutting tool which will remove metal from the work in the from of chips .To cut the material properly the tool should be harder than the material of the workpiece ,should be rigidly held on the machine and should be fed or progressed in a definite way relative to the work.


Types of lathe

Lathe of various designs and constructions have been developed to suit the various conditions of metal machining .But all of them employ the same fundamental principle of operation and perform the same function.

The types generally used are:

1. Speed lathe                                    3. Bench lathe .
(a) Wood working                             4. Tool room lathe.
(b) Centering                                     5. Capstan and Turrret lathe.
(c) Polishing                                      6. Special purpose
(d) Spinning                                           (a) Wheel lathe.
                                                               (b) Gap bed lathe 
2. Engine lathe.                                      (c) T-lathe
    (a) Belt drive                                      (d) Duplicating lathe.
    (b) Individual motor drive            7.  Automatic lathe.
    (c) Gear head lathe.


The size of lathe


The size of  a lathe is expressed or specified by the following items and illustrated in
1. The height of the centres measured from the lathe bed.
2. The swing diameter over bed. This is the largest diameter of work that will revolve without                  touching  the bed and is twice the height of the center measured from the bed of the lathe.
3. The length between centres. This is the maximum length of work that can be mounted between the       lathe  centres.
4. The length of bed .This indicates the approximate floor space occupied by the lathe.



DESCRIPTIOINS AND FUCTIONS OF LATHE PARTS 


Illustrates the basic parts of a geared head lathe. Following are the principal parts:

1. Bed                     4.Carriage
2. Headstock          5. Feed mechanism
3. Tailstock             6. Screw cutting mechanism


1. The Bed

    The lathe bed forms the base of the machine .The headstock and the tailstock  are located at either      end of the bed and the carriage rests over the lathe bed and slides on it. the lathe bed being the           main guiding member of the tool,for accurate machining work, must satisfy the following                   conditions:

(a) It should be sufficiently rigid to prevent deflection under tremendous cutting pressure transmitted        through the tool-post and carriage to the lathe bed.
(b) it must be massive with sufficient depth and width to absorb vibration.
(c) The bed should be seasoned naturally to avoid distortion or warp that may develop when it is              cooled after the bed is cast.


2. The Headstock

    The headstock is secured permanently on the innerways at the left hand end of the lathe bed and it      provides mechanical means of rotating the work at multiple speeds. It comprises essentially a              hollow  spindle and mechanism for driving and altering the spindle speed. All the parts are housed      within the headstock casting.
     The spindle of the headstock illustrated in is made of carbon or nickel-chrome steel.This is                   usually of a large diameter to resist bending and it should be perfectly aligned with the lathe axis        and  accurately machined for producing true work surface.A hole extends through the spindle so
    that  a long bar may be passed  through the bore.The front end of the hole is appeared for holding
     centres and other tools having a standard Morse taper shank.A taper sleeve fits into the taper hole,
     and a live centre which supports the work and revolves with the work fits into the sleeve that acts
     as a bush.There are common types of spindle noses: the threaded design which carries the chuck,
     driving plate and face plate and the flanged nose which enable them to be directly attached.The
     lathe most commonly used has a threaded spindle nose.
    The spindle revolves on two bearings housed on the headstock casting. The clearance between            the spindle and the bearing should be minimum to prevent vibration .The bearing may be either
    bush,ball or roller  type  depending on whether it is a   high speed , heavy duty or precision
    machine .Thrust bearings are provided to take up the end load owing to the feeding action of  the
   tool. Provision is made for expansion of the spindle when it gets heated under high speed metal
   cutting .


3. TAILSTOCK

    The tailstock is located on the innerways at the right hand end of  the bed, this has two main uses :
    (1) it supports the other end of the work when it is being machined between centres and
    (2) it holds a tool for performing operation such as drilling ,reaming,tapping etc



4. CARRIAGE

    The carriage of a lathe has several parts that serve to support move and control the cutting tool ,It
     consists of the following parts:
     (1) saddle       (2) cross-slide    (3) compound slide or compound rest
     (4) tool post, and   (5) apron. A sectional view of the carriage


5  FEED MECHANISM

    The movement of  the relative to the work is termed as "feed" .A lathe tool may have three types
     of feed-longitudinal ,cross, and angular.When the tool moves parallel to the lathe axis the
    movement is  termed as the longitudinal feed and is effected by the movement of the carriage .
    When the tool moves at the right angel to the lathe axis with the help of the cross slide the
     movement is termed as cross feed,While the movement of the tool by compound slide when it is
     swivelled at an angle to the lathe axis is termed as angular feed. Cross and longitudinal feed ate
     both hand and power operated ,but angular feed is only hand operated .
     The feed mechanism has different units through which motion is transmitted from the headstock
     spindle to the carriage .Following are the untis
   
     1. End of bed gearing
     2. Feed gear box
     3  Feed rod and lead screw
     4  Apron mechanism

6.  Screw cutting mechanism





LATHE OPERATION


In order to perform different machining operation in a lathe,the workpiece may be supported and driven by any one of the following

1. CUTTING SPEED

The cutting speed (v) of a tool is the speed at which the metal is removed by the tool from the workpiece .In a lathe it is the peripheral speed of the work past the cutting tool expressed in meters per minute.
   
                                          Pdn
         cutting speed  =   -------------      m / min
                                         1000        



2. DEPTH OF CUT

The depth of cut (t) is the perpendicular distance measured from the machined surface to the uncut surface of the workpiece .In  a lathe the depth of cut is expressed as follows :
           
                                         D    __     D
                                           1              2
       Depth of cut  =       ---------------------
                                                2

Where, D     = diameter of the work surface before machining
                1
            D      = diameter of the machined surface
               2


3.  MACHINING TIME


The machining  time in lathe work can be calculated for a particular operation if the speed of the job feed length of the job is known :
     If a is the feed of the job per revolution expressed in mm per revolution and l the length of the job in mm then number of revolution of the job required for a complete cut will be :
     If the r.p.m. of the work is n , time taken to revolve the job through  l/s number revolution for a complete cut will be :
                                        l
                                  ---------
                                   S  ×  N                                          l
     Therefore the time taken  for a complete cut =    ------------        min.
                                                                                   S   ×   N 

                                         
                                                                 



This article was taken by
S.K. Hajra choudhury
A.K. Hajra choudhury




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