Pdf Centre Lathe Notes-1 PDF

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notes on center lathe machines...

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LATHE MACHINE. A centre lathe is one of the oldest and perhaps most important machine tool ever developed. The job to be machined is rotated (turned) and the tool is moved relative to the job. That is why, the lathe are also called “Turning machines”. If a tool moves parallel to the axis of the rotation of the workpiece, cylindrical surface is produced, while if it moves perpendicular to the axis, it produces a flat surface. Working principle. In a lathe machine the workpiece is held in a chuck or between the centres and rotated about the axis at a uniform speed. The cutting tool held in tool post is fed into the workpiece for desired depth and desired direction i.e. in linear transverse or lateral direction. Since there exist a relative motion between the workpiece and the cutting tool, therefore the material is removed in the form of chip and the desired shape is obtained.

Parts of Lathe. The main parts of the lathe machine are; 1. 2. 3. 4. 5.

Bed Headstock Tailstock Carriage Feed mechanism.

The Bed. It is the base or foundation of the lathe. It is a heavy rigid casting made in one piece. In majority of cases the beds are made of grey cast iron-nodular cast iron, or high strength wear resistant cast iron. The cast iron offers the following advantages over the other materials,

a) b) c) d) e)

It is self-lubricant; it can be hardened by induction hardening process. It has better compressive strength. It has excellent shock absorbing capacity. It can easily be cast and machined. It can easily be cast and machined.

It holds or supports all other parts of the lathe. The top pf the bed is planned to form guide ways for the carriage and the tailstock. The guide ways are of two types; a) Flat guideways or inverted Vee guideways. b) Flat and inverted Vee guideways.

Headstock: It is permanently fastened to the inner ways at the left hand end of the bed. It serves to support the simple and driving arrangements. All the lathe machines receives their power through the headstock, which may be equipped with a step-cone pulley or a gear head drive (the modern lathe are provided with all geared type head stock to get larger variation of spindle speeds). In order to allow the long bar or work holding devices to pass through, the headstock spindle is made hollow. A tapered sleeve fits into the tapered spindle hole. Tailstock: It is situated at the right end of the bed. It is used for supporting the right end of the work. It is also used for holding and feeding the tools such as drills, reamers, taps etc.

Carriage:

The carriage controls and supports the cutting tool. The carriage has the following five major parts; a) Saddle. It is a H-shaped casting fitted over the bed. It moves along the guideways of the lathe. b) Cross-slide. It carries the compound slide and tool post. It can move by power or by hand. c) Compound rest. It sis marked in degrees. It is used during taper turning to set the tool for angular cuts. d) Tool post. The tool is clamped on the tool post. e) Apron. It is attached to the saddle and hangs in front of the bed. It has gears, levers and clutches for moving the carriage with the lead screw for thread cutting. Feed mechanism: It is employed for imparting various feeds (longitudinal, cross and angular) to the cutting tool. It consists of feed reverse lever, tumbler reversing mechanism, change gears, feed gear box, quick change gear box, lead screw, feed rod, apron mechanism and half nut mechanism

Size and Specifications of the lathe machine.

The size of the lathe machine is specified in any of the following ways,

1. The height of the centres measured over the lathe bed. 2. Swing or maximum diameter that can be rotated over the bed ways. 3. Swing or diameter over carriage. This is the largest diameter of work that will revolve over the lathe saddle. 4. Maximum job length in mm that may be held between the centres (headstock and tailstock centres). 5. Bed length in metres which may include the headstock length also. 6. Diameter of the hole through lathe spindle for turning bar material. In addition to the above, the following specifications are necessary while ordering a lathe machine. a) b) c) d) e) f) g) h) i) j)

The length, width and depth of the bed. The depth and width of the gap, if it is a gap lathe. The swing over gap The number and range of spindle speeds. The lead screw diameter. The number and range of metric threads that can be cut. The tailstock spindle travel. The tailstock spindle set over. The back gear ratio. The power rating of electric motor.

Types of Lathe. a) Speed lathe.

In this lathe spindle can rotate at a very high speed with the help of a variable speed motor built inside the headstock of the lathe. It is used mainly for wood working, centering, metal spinning, polishing etc. b) Engine/ Centre lathe. It is the most common type of the lathe and is widely used in workshops. The speed of the spindle can be widely varied as desired which is not possible with a speed lathe. The cutting tool may be fed both in cross and longitudinal directions with reference to the lathe axis with the help of the carriage. c) Bench lathe. It is usually mounted on a bench. It is very similar to the centre lathe, the only difference being it is smaller in size which enables it to handle small work (usually requiring considerable accuracy as in the production of gauges, punches and beds for press tools). d) Tool room lathe. It is similar to an engine lathe, designed for obtaining accuracy. It is used for manufacturing precision components, dies, tools, jigs, etc. and hence it is called tool room lathe. e) Turret and capstan lathes. These lathe have provision to hold a number of tools and can be used for performing wider range of operations. They are particularly suitable for mass production of identical parts at minimum time. f) Automatic lathes: These lathes are also designed that the tools are automatically fed to work and withdrawn after all operations, to finish the work are complete. They require little attention of the operator, since the entire operation is automatic. These lathes are used for production of identical parts. g) Special purpose lathe: These lathes are primarily designed for carrying out a particular operation with utmost efficiency. The lathes included in this category are: a) Gap lathe. It is used for machining extra larger diameter pieces. b) Special purpose engine lathe. These lathes are designed for machining special types of workpieces, e.g. wheel turning lathes for turning the tread on rail-car and locomotives and so on. c) Instrument lathe. These lathes are smaller size than Bench lathe and are used by instrument makers. d) Facing lathe. These lathes are used to machine workpieces of larger diameter but short in length in single piece production and in repair shops. Now these lathes have been replaced by more advanced boring and turning machines. e) Flow turning lathe. A flow turning is used for roll flowing, as a method of coldflowing metal. As heavy pressure is applied spirally with two hardened rollers against a metal block.

f) Heavy –duty lathe. A lathe that has a swing of 500mm or more and is used for roughing and finishing cuts, is often referred to as heavy-duty lathe. Lathe Tools. In a lathe, for a general purpose work, the tool used is a single point tool (a tool having one cutting edge), but for special operations multi-point tools maybe used. The commonly used materials are high carbon steel, high speed steel, cemented carbide, diamond tips and ceramics. Depending on the nature of operation done by the tool, the lathe tools are classified as; a) b) c) d) e) f) g) h)

Turning tool (left hand or right hand) Facing tool (left hand or right hand) Chamfering tool (left hand or right hand) Form or profile tool External threading tool Internal threading tool Boring tool Knurling tool.

Work holding devices.

Centres. Lathe centres are hardened steel devices used for holding the work to be turned. The center that is fitted to the stock spindle is called the live centre and the one on the tailstock spindle is referred to as the dead centre. The various forms of lathe centres are; Ordinary center, Ball centre, Tipped centre, Half centre, Revolving centre, Pipe centre, Insert type centre. a) Ordinary centre. It is used for general purpose work.

b) Ball centre. This has a ball shape at the end to minimize the wear and strain. This is particularly suitable for taper turning.

c) Tipped centre. This is a centre that has a hard alloy tip in to the steel shank. The hard tip is wear resistant.

d) Half centre. The half centre is similar to an ordinary centre only that its half to facilitate facing of the bar without removal of the centre.

e) Revolving centre. The ball and roller bearings are fitted into the housing to reduce friction and to take up end thrust. It is mostly used at the tailstock forr supporting heavy work revolving at high speed.

f) Pipe centre. This is used for supporting pipe shells and hollow end jobs.

g) Insert type centre. This insert type centre is used for the reason that steel “inserts” can be replaced instead of replacing the whole component.

Between centres.

Face plate.

Face plate is a circular plate. It is screwed to the lathe spindle. There are many holes and slots on the face of the face plate. The workpiece is mounted on the face of the face plate by use of Tbolts, clamps and nuts. When the spindle of the machine rotates, the face plate rotates and the work too rotates. Large irregular and heavy jobs which cannot be held between centres or by use of chucks can be held on the face plate.

Mandrel. Mandrel is used for holding hollow jobs. The workpiece is mounted over the mandrel and the mandrel is rotated between centres. The outside diameter of the mandrel should be equal to the

inside diameter of the work. If tapered the taper should also be equal. Different types of mandrel are used for different types of jobs.

The catch plate.

Types of mandrel; a) Solid mandrel. This is also known as plain mandrel. It is most commonly used in the shops. The body of the mandrel has a slight taper which aids in gripping the workpiece. The difference in diameter is about 1 to 2 per 100mm length. This type of mandrel is available in various sizes. This type of mandrel is used for holding workpieces with the same bore diameters. For workpieces with different bore diameters different mandrels are used.

b) Gang mandrel. This type of mandrel has a fixed collar at one end. The other end of the mandrel has a threaded portion. At this threaded end there is a movable collar. This movable collar is adjusted to the required position by a nut. A set of hollow workpieces can be held between the two collars.

c) Expanding mandrel.

This mandrel has a tapered arbor. Over this arbor a bush is mounted. This bush has a tapered hole and a cylindrical outside surface. Three longitudinal slots are cut on the bush. Two slots are cut through. Third slot is cut of full length. By this construction it is possible to expand the split bushing with a limit. Therefore various workpieces of hole diameters ranging from 0.5 to 2mm. using bushes of different sizes this range can be increased. d) Cone mandrel. This has two conical ends. One end has a solid cone. The other end has threads. At the end of the threads is a sliding cone. This sliding cone can be adjusted by a nut. This cone mandrel is used for holding workpieces of different diameters. The workpieces are held between two cones.

e) Screwed mandrel. This is threaded at one end. It has a collar. The screwed mandrel is used to hold workpieces having threaded holes. The workpiece is screwed on the mandrel against the collar.

f) Step mandrel. This type of mandrel has steps of different diameters and therefore used to hold workpieces having different size of holes.

g) Collar mandrel. This mandrel has two collars. The collars fit firmly into the hole of workpieces. The collar mandrel is lighter in weight. It is used for holding workpieces having larger diameter holes (above 100mm)

Turning fixture.

When the workpiece cannot be held by the convectional work holding devices i.e. the chucks, mandrel, face plate among others, the turning fixtures are used. The fixture will hold the work and locate it in position for machining. In preparing the turning fixture various work holding devices like face plate, Vee blocks, step blocks, angle plates, bolts and clamps etc. are used. The main functions of the turning fixtures are; 1. To accurately locate the work. 2. To grip the workpiece properly. 3. To load and unload the workpiece quickly; clamping time is reduced. 4. To avoid marking on each workpiece. Used in mass production. The fixture body is connected to the lathe spindle by three studs. The fixture body has a circular recess. The workpiece is located in this circular recess and clamped in position.

Chucks. a) Collet chuck. Collet chucks are used for setting quickly and for holding accurately the small cylindrical work during mass production of parts. The collet chuck is attached to the spindle of the lathe. The collet are split at the end so that as the taper is pulled into the lathe spindle, it tightens on the workpiece.

Collet chuck. b) Three jaw chuck. It is a self- centering chuck. It is used for holding symmetrical workpieces. It has three jaws and all the three jaws moves towards the centre on a scroll arrangement. The centering and gripping of the workpiece is done quickly since all the three jaws move simultaneously by turning the key.

c) Four jaw independent chuck. It is used for holding complex and asymmetrical shaped workpieces. It has four jaws. Each jaw can be moved independently. When the chuck key is tightened, that particular jaw will move. Therefore irregular or eccentric jobs can be held in this chuck by moving each jaw to the required amount.

Lathe dog. It’s also known as carrier dog. It is used to transfer motion from the driving plate to the workpiece held between centres. The workpiece is inserted into the hole of the dog and firmly secured by the means of set screw. The different types of carriers are; •

Bent tail carrier.

•

Straight tail heavy duty carrier

Steady. For machining long piece of work, steadies are used to support long pieces of work. They are of two type; the 2 point travelling steady (follower rest) and the 3 point fixed steady. The 3-point fixed steady supports the workpiece at convenient point between the headstock and the tailstock. This gives support to the work and thus minimizes vibrations and bending of workpiece when being worked on. Its three jaws are adjustable. If one end of the work is held by a live centre,

drilling and boring may be done on either end when the 3-point fixed steady (steady rest) is used as a support.

The 2-point movable steady (follower rest) performs the same functions only that it is mounted on the saddle and moves together with the tool. It has two jaws that supports the work opposite the tool. When the entire length of the work is turned in one setting without any disturbance a two point steady is used.

Lathe Operations. Setting the cutting tool. In case a cutting tool is fixed to a table, thin metal plates are put between the tool and table, and the height of the edge is adjusted to the centre of material. In the case of using the general cutting tool, when the edge is higher than the centre of material, and it cannot cut all. Conversely, if the edge is

low, it becomes impossible to cut the centre of the material. Moreover, the scale of a handle does not have correct value, then accurate processing becomes impossible.

•

Through the height of cutting tool is adjusted in carefully, we cannot unite with the centre of material completely. Therefore, we have to set the tool to the direction, that the edge is easy to touch the material. The general cutting tool and the parting tool have to be set a few low position. The boring bar has to set a few high position.

The common lathe operations which can be carried out on a lathe machine are; Turning, taper turning, boring, knurling, drilling, undercutting or grooving, forming, reaming and threading. 1. Facing. This is an operation of machining the ends of a workpiece to produce a flat surface square with the axis. It is also used to cut the workpiece to the required length. The operation involves feeding the tool perpendicular to the axis of rotation of the workpiece. A properly ground facing tool is mounted in the tool post. A regular turning tool may also be used for facing a large workpiece. The cutting edge should be set at the same height as the centre of the workpiece.

The facing operations is usually performed in two steps. In the first step a rough facing operation is done by using a heavy cross feed of the order of 0.5 to 0.7 mm and deeper cut up to 5mm (maximum). It is followed by a finer cross feed of 0.1 to 0.3 mm and a smaller depth of cut of about 0.5mm. 2. Plain turning. It is an operation, of removing excess material from the surface of the cylindrical workpiece. In this operation, the work is held either in the chuck or between the centres and the longitudinal feed is given to the tool either by hand or power.

3. Step turning. In this type of lathe operation various steps of different diameters in the workpiece are produced. It is carried out in the similar way as plain turning.

4. Taper turning. A taper maybe defined as a uniform increase or decrease in diameter of a piece of work measured along its length. The taper angle α can be found by using the following relationship; tan α = D-d 2L Or, α = tan-1 (D-d/2L) Where, D = Large diameter of the taper in mm, d =Small diameter of the taper in mm, L = Length of the tapered part in mm, and α = Half of the taper angle.

Taper turning – taper turning means to produce a conical surface by gradual reduction in diameter from a cylindrical workpiece. The tapering of the part has a wide applications in the construction of machines. Almost all machines spindles have taper holes which receive taper shanks of various tools and work holding devices. Taper turning methods. Taper turning can be carried out on lathes by the following methods; a) By setting over the tailstock centre. b) By swiveling the compound slide rest. c) By using a taper turning attachment. d) By manipulating the transverse and longitudinal feeds of the slide tool simultaneously. e) By using a board nose from tool. Cutting Speed, Feed and Depth of cut.

Thread cutting.

Simple gear train.

Compound gear train....