Lathe Machine and it's Operation

                    Lathes were developed as early as the 15th century and were known as "bow" lathes. The operator rotated the workpiece by drawing a bow back and forth, either by hand or with the use of a foot treadle. Next came Bessons lathe in 1568, which was driven by a cord passing over a pulley above the machine. This in turn drove two other pulleys on the same shaft which rotated the workpiece and a crude, wooden lead screw, which in turn allowed the operator to remove metal from the piece being machined. The screw cutting lathe originates in the 17th century. Development and advancements have continued and today we have sophisticated computerized controlled lathes. Lathes have allowed man to reshape, machine and manufacture many precision cylindrical components made of various types of metal, wood, plastics, and other materials. Without the lathe, man would still be trying to produce cylindrical components in some crude fashion or another. However, because of advanced technology, the lathe has allowed man to become an important asset in developing and machining many precision components needed to operate and function in many areas of our industrial complex.

Classification of Lathes:
Lathes are very versatile of wide use and are classified according to several aspects:
(a) According to configuration
          • Horizontal
                     - Most common for ergonomic conveniences
          • Vertical
                     - Occupies less floor space, only some large lathes are of this type.
(b) According to purpose of use
          • General purpose
                     - Very versatile where almost all possible types of operations are carried out on wide ranges of size, shape and materials of jobs; example : centre lathes
          • Single purpose
                     - Only one (occasionally two) type of operation is done on limited ranges of size and material of jobs; example – facing lathe, roll turning lathe etc.
          • Special purpose
                     - Where a definite number and type of operations are done repeatedly over long time on a specific type of blank; example: gear blank machining lathe etc.

(c) According to size or capacity
         • Small (low duty)
                     - In such light duty lathes (upto 1.1 kW), only small and medium size jobs of generally soft and easily machinable materials are machined
         • Medium (medium duty)
                     - These lathes of power nearly upto 11 kW are most versatile and commonly used
         • Large (heavy duty)
         • Mini or micro lathe
                     - These are tiny table-top lathes used for extremely small size jobs and precision work; example : swiss type automatic lathe

(d) According to degree of automation
         • Non-automatic
                    - Almost all the handling operations are done manually; example: centre lathes
         • Semi-automatic
                    - Nearly half of the handling operations, irrespective of the processing operations, are done automatically and rest manually; example : capstan lathe, turret lathe, copying lathe relieving lathe etc.
         • Automatic
                    - Almost all the handling operations (and obviously all the processing operations) are done automatically; example – single spindle automat (automatic lathe), swiss type automatic lathe, etc.

(e) According to type of automation
         • Fixed automation
                   - Conventional; example – single spindle automat, swiss type automatic lathe etc.
         • Flexible automation
                   - Modern; example CNC lathe, turning centre etc.

(f) According to configuration of the jobs being handled
         • Bar type
                   - Slender rod like jobs being held in collets
         • Chucking type
                   - Disc type jobs being held in chucks
         • Housing type
                   - Odd shape jobs, being held in face plate

(g) According to precision
         • Ordinary
         • Precision (lathes)
                   - These sophisticated lathes meant for high accuracy and finish and are relatively more expensive.

(h) According to number of spindles
        • Single spindle
                  - Common
        • Multispindle (2, 4, 6 or 8 spindles)
                  - Such uncommon lathes are suitably used for fast and mass production of small size and simple shaped jobs.

Kinematics Systems and Working System of Lathes:
              Amongst the various types of lathes, centre lathes are the most versatile and commanly used.

For machining in machine tools the job and the cutting tool need to be moved relative to each other.
The tool-work motions are :
• Formative motions : - cutting motion
       - feed motion
• Auxiliary motions : - indexing motion
       - relieving motion etc
In lathes
o Cutting motion is attained by rotating the job
o Feed motion by linear travel of the tool
       - either axially for longitudinal feed
       - or radially for cross feed
• The job gets rotation (and power) from the motor through the belt-pulley, clutch and then the speed gear box which splits the input speed into a number (here 12) of speeds by operating the cluster gears.
• The cutting tool derives its automatic feed motion(s) from the rotation of the spindle via the gear quadrant, feed gear box and then the appron mechanism where the rotation of the feed rod is transmitted - either to the pinion which being rolled along the rack provides the longitudinal feed
       - or to the screw of the cross slide for cross or transverse feed.
• While cutting screw threads the half nuts are engaged with the rotating leadscrew to positively cause travel of the carriage and hence the tool parallel to the lathe bed i.e., job axis.
• The feed-rate for both turning and threading is varied as needed by operating the Norton gear and the Meander drive systems existing in the feed gear box (FGR). The range of feeds can be augmented by changing the gear ratio in the gear quadrant connecting the FGB with the spindle
• As and when required, the tailstock is shifted along the lathe bed by operating the clamping bolt and the tailstock quil is moved forward or backward or is kept locked in the desired location.
• The versatility or working range of the centre lathes is augmented by using several attachments like
             - Taper turning attachment
             - Thread milling attachment
             - Copying attachment

Construction:
 The main parts of the lathe are the bed, headstock, quick changing gear box, carriage and tailstock.


1. Bed: The bed is a heavy, rugged casting in which are mounted the working parts of the lathe. It carries the headstock and tail stock for supporting the workpiece and provides a base for the movement of carriage assembly which carries the tool.

2. Legs: The legs carry the entire load of machine and are firmly secured to floor by foundation bolts.

3. Headstock: The headstock is clamped on the left hand side of the bed and it serves as housing for the driving pulleys, back gears, headstock spindle, live centre and the feed reverse gear. The headstock spindle is a hollow cylindrical shaft that provides a drive from the motor to work holding devices.

4. Gear Box: The quick-change gear-box is placed below the headstock and contains a number of different sized gears.

5. Carriage: The carriage is located between the headstock and tailstock and serves the purpose of supporting, guiding and feeding the tool against the job during operation. The main parts of carriage are:

a). The saddle is an H-shaped casting mounted on the top of lathe ways. It provides support to cross-slide, compound rest and tool post.

b). The cross slide is mounted on the top of saddle, and it provides a mounted or automatic cross movement for the cutting tool.

c). The compound rest is fitted on the top of cross slide and is used to support the tool post and the cutting tool.

d). The tool post is mounted on the compound rest, and it rigidly clamps the cutting tool or tool holder at the proper height relative to the work centre line.

e). The apron is fastened to the saddle and it houses the gears, clutches and levers required to move the carriage or cross slide. The engagement of split nut lever and the automatic feed lever at the same time is prevented she carriage along the lathe bed.

6. Tailstock: The tailstock is a movable casting located opposite the headstock on the ways of the bed. The tailstock can slide along the bed to accommodate different lengths of workpiece between the centers. A tailstock clamp is provided to lock the tailstock at any desired position. The tailstock spindle has an internal taper to hold the dead centre and the tapered shank tools such as reamers and drills.

Machining Operations usually done in Centre Lathes:
The machining operations generally carried out in centre lathes are :
• Facing
• Centering
• Rough and finish turning
• Chamfering, shouldering, grooving, recessing etc
• Axial drilling and reaming by holding the cutting tool in the tailstock barrel
• Taper turning by
 -offsetting the tailstoc
-swivelling the compound slide
- using form tool with taper over short length
 -using taper turning attachment if available
 -combining longitudinal feed and cross feed, if feasible.
• Boring (internal turning); straight and taper
• Forming; external and internal
• Cutting helical threads; external and internal
• Parting off
• Knurling
In addition to the aforesaid regular machining operations, some more operations are also occasionally done, if desired, in centre lathes by mounting suitable attachments available in the market, such as,

• Grinding, both external and internal by mounting a grinding attachment on the saddle
• Copying (profiles) by using hydraulic copying attachment
• Machining long and large threads for leadscrews, power-screws, worms etc. by using thread milling attachment.


                                Published by Ravindra.K (Mechanical Engineering)