LATHE MACHINE
- INTRODUCTION : In a machine shop, metals are cut to shape on different machine tools. A lathe is used to cut and shape the metal by revolving the work against a cutting tool. The work is clamped either in a chuck, fitted onto the lathe spindle or in-between the centres. The cutting tool can be fed into the work, either lengthwise or cross-wise. While turning, the chuck rotates in counter- clockwise direction, when viewed from the tail-stock end.
- FUNCTION OF THE LATHE: The main function of a lathe is to remove metal from a piece of a piece of work to give it required shape and size.
- TYPES OF LATHE: The types of lathes are follows-
- Speed Lathe (Wood working, Centering, Polishing, Spinning)
- Engine Lathe (Belt drive, Individual motor drive, Gear Head)
- Bench Lathe
- Tool room Lathe
- Capstan and Turret Lathe
- Special Purpose Lathe ( Wheel Lathe, Gap bed Lathe, T-lathe, Duplicate lathe)
- Automatic Lathe
- THE SIZE OF A LATHE: The size of a Lathe specified by-
- The height of the centres measured from the lathe bed.
- The swing diameter over bed.
- The length between centres.
- The swing diameter over carriage.
- The maximum bar diameter.
- The length of bed
- PRINCIPAL PARTS OF A LATHE : The image shows a centre Lathe, indicating the main parts. The name is due to the fact that work pieces are held by the centres.
- BED : It is an essential part of a lathe, which must be strong and rigid. It carries all parts of the machine and resists the cutting forces. The carriage and the tail-stock move along the guide ways provided on the bed.
- HEAD-STOCK : It contains either a cone pully or gearing to provide the necessary range of speeds and feeds. It contains the main spindle, to which the work is held and rotated.
- TAIL-STOCK : It is used to support the right hand end of a long work piece. It may be clamped in any position along with the bed. The tail-stock spindle has an internal morse taper to receive the dead centre that supports the work. Drills, reamers, taps may also be fitted into the spindle, for performing operations such as drilling, reaming and tapping.
- CARRIAGE OR SADDLE : It is used to control the movement of the cutting tool. The carriage assembly consist of the longitudinal slide, cross slide and the compound slide and apron. The cross slide moves across the length of the bed and perpendicular to the axis of the spindle. This movement is used for facing and to provide the necessary depth of cut while turning. The apron, which is bolted to the saddle, is on the front of the lathe and contains the longitudinal and cross slide controls.
- COMPOUND REST : It supports the tool post. By swiveling the compound rest on the cross slide, short tapers may be turned to any desired angles.
- TOOL POST : The tool post, holds the tool holder or the tool, which may be adjusted to any working position.
- LEAD SCREW : It is a long threaded shaft, located in front of the carriage, running from the head-stock to the tail stock. It is geared to the spindle and control the movement of the tool, either for automatic feeding or for cutting threads.
- CENTRES : There are two centres known as dead centre and live centre. The dead centre is positioned in the tail-stock spindle and the live centre, in the head stock spindle. While turning between centres, the dead centre does not revolve with the work while the live centre revolves with the work.
- WORK-HOLDING DEVICES
- THREE JAW CHUCK : It is a work holding device having three jaw(self-centering) which will close or open with respect to the chuck centre or the spindle centre. It is used for holding regular objects like round bars, hexagonal rods, etc.
2. FOUR JAW CHUCK : In a four jaw(independent) chuck, all the jaws have independent movement. It is used for holding square, octagonal or irregular shaped works.
3. FACE PLATE : It is a plate of large diameter, used for turning operations. Certain types of work that cannot be held in chucks, are held on the face plate with the help of various accessories.
4. LATHE DOGS AND DRIVING PLATE : These are used to drive a work piece that is held between centers. These are provided with an opening to receive and clamp the work piece and dog tail. The tail of the dog is carried by the pin provided in the driving plate for driving plate for driving the work piece.
- LATHE CENTRES: The most common method of holding the work in lathe is between the two centres- live center and dead center. These two centres take up the thrust due to metal cutting and the entire load of the workpiece on small bearing surface.
- MANDRELS: A mandrel is a device for holding and rotating a hollow piece of work that has been previously drilled or bored. The work revolves with the mandrel which is mounted between two centres.
- STEADY REST : Steady rest consists of a cast iron base which may be made to slide on the lathe bed ways and clamped at any desired position where is support is necessary.
- F0LLOWER REST: A follower raised consists of a c like casting having two adjustable jobs which support the workpiece the rest is bolted to the back end of the carriage and moves with it.
- LATHE OPERATIONS :
- TAPER TURNING METHODS: A taper may be turned in by feeding the tool at an angle to the axis of rotation of the workpiece. The angle formed by the part of the tool with the axis of the workpiece should correspond to the half taper angle. While taper turning it is essential that the tool cutting age should be set accurately on the centre line of the workpiece otherwise correct taper will not be obtained it may be done by any one of the following methods
- By a board nose form tool
- By setting over the tailstock centre
- By swiveling the compound rest
- By a taper turning attachment
- By combining longitudinal and cross feed in a special lathe
- TOOL GEOMETRY: Single point cutting tool used on lathe may be considered as a simple wedge. The common turning tools used for different operation, the basic angles of a simple turning tool shows the lid tool holder with HSS tool mounted and tool holders used with carbide tipped tools
