Various Operations Performed on Workshop Lathe Machine

The lathe is a machine tool, which removes the metal from a piece of work to give 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 form of chips. To cut the material properly, the tool should be harder than the material of the work piece. It should be rigidly held on the machine and should be fed or progressed in a definite way relative to the work.

THESE ARE THE TYPES OF OPERATIONS CARRIED OUT ON A LATHE MACHINE

There are various types of operations carried out on lathe machine which are described below:

(a) Plain Turning or Straight Turning
(i) Rough Turning
(ii) Finish Turning
(b) Facing
(c) Knurling
(d) Recessing
(e) Chamfering
(f) Parting off
(g) Taper Turning
(h) Shoulder Turning
(j) Thread cutting
(k) Boring

PLAIN TURNING OR STRAIGHT TURNING

[Full Operation Procedure Explained Here]

The work is turned straight when it is made to rotate about the lathe axis, and the tool is fed parallel to the lathe axis. The straight turning produces a cylindrical surface by removing excess metal from the work piece. After facing the ends and drilling the center, the job is carefully mounted between the centers using a lathe dog attached to the work piece. A properly ground right hand turning tool is used for this purpose. Tool is clamped on the tool post with the minimum overhang and is set with its cutting edge approximately at the lathe axis or slightly above it. For light cuts the tool may be inclined towards the headstock, but for heavy cuts the tool must be inclined towards the tailstock. The automatic feed is engaged to move the carriage to the desired length, then the feed is disengaged and the carriage is brought back to the starting position. The process is repeated until the job is finally finished. There are two kinds of cuts that can be given onto a work piece:

[Read detailed operation procedure about Straight Turning OperationHERE]

(a) Rough Turning

The rough turning is the process of removal of excess material from the work piece in a minimum time by applying high rate of feed and heavy depth of cut. The roughing cut should be so made that the machine, the tool, and the work piece can bear the load. The depth of cut for roughing operations in average machine shop work is from 2 to 5mm and the rate of feed is from 0.3 to 1.5 mm per revolution of the work. Rough turning operations are carried out by rough turning tool.

(b) Finish Turning

The finish turning operation requires high cutting speed, small feed, and a very small depth of cut to generate a smooth surface. A finish turning tool having sharp cutting edge is held securely on the tool post for this purpose. In finish turning operation, the depth of cut ranges from 0.5 to 1 mm and feed from 0.1 to 0.3 mm per revolution of the work piece. The cross-feed dial is used to set an accurate depth of cut. After measuring the diameter of rough turned surface, the depth of cut to be given is determined by subtracting the finished diameter from the measured value. The tool is then made to advance by half of the above value by rotating the cross-slide hand-wheel through required number of divisions on the dial. The machine is started and a trial cut is made from the end of the work to 5 or 6 mm by applying hand feed and the finished diameter is checked by a micrometer. Once the correct diameter is made, the rest is finished by the automatic feed.

FACING

[Full Operation Procedure Explained Here]

Facing is the operation of machining the ends of a piece of work to produce a flat surface square with the axis. This is also used to cut the work to the required length. The operation involves feeding the tool perpendicular to the axis of rotation of the work piece. A properly ground facing tool is mounted in a tool holder in the tool post. A regular turning tool may also be used for facing a large work piece. The cutting edge should be set at the same height as the center of the work piece. The selection of hand-feed or power feed depends upon the length of the cut. The surface is finished to the size by giving usual roughing and finishing cuts. For roughing, the average value of the cross feed is from 0.3 to 0.7 mm per revolution and the depth of cut is from 2 to 5 mm. For finishing, the feed ranges from 0.1 to 0.3 mm per rev. and the depth of cut is from 0.7 to 1 mm.

KNURLING

[Full Operation Procedure Explained Here]

Knurling is the process of embossing a diamond shape pattern on the surface of a work piece. The purpose of knurling is to provide an effective gripping surface on a work piece, to prevent it from slipping when operated by hand. In some press fit work knurling is done to increase the diameter of a shaft. The operation is performed by a special knurling tool which consists of a set of hardened steel rollers in a holder with the teeth cut on their surface in a definite pattern. Knurls are available in coarse, medium and fine pitches.

RECESSING

[Full Operation Procedure Explained Here]

Recessing can also be called as grooving or necking. So, the recessing tools are sometimes called necking tools. Recessing tools may be either straight or bent shank types. As the recess is usually narrow, the cutting edge is kept narrow. It is relieved by 1o to 2o on each side towards the shank. The sides are relieved to make the tool free cutting. The rake angle should be decreased or the face should be made hollow to the radius. The tool should be set exactly in center. If tool is set below or above the center, it will break.

CHAMFERING

[Full Operation Procedure Explained Here]

Chamfering is the operation of beveling the extreme end of a workpiece. This is done to remove the burrs, to protect the end of the workpiece from being damaged and to have a better look. The operation may be performed after knurling, rough turning, boring, drilling. Chamfering is an essential operation before thread cutting so that the nut may pass freely on the threaded workpiece.

PARTING OFF

[Full Operation Procedure Explained Here]

Parting-off is the operation of cutting a workpiece after it has been machined to the desired size and shape. The process involves rotating the workpiece on a chuck or faceplate at half the speed to that of turning and feeding by a narrow parting off tool perpendicular to the lathe axis by rotating the cross-slide screw by hand. Before the operation starts, the carriage is locked in position on the lathe bed and the cutting tool is held rigidly on the tool post with the compound slide set parallel to the lathe axis. The tool should be fed very slowly to prevent chatter. The feed varies from 0.07 to 0.15 mm per revolution and the depth of cut is equal to the width of the tool. In parting off, a work of very large diameter, cuts are made in stages. The parting off tool is first fed through a certain depth, then withdrawn and two more cuts are made at the two sides of the central groove. The tool is next fed into the central groove until the work is cut off in two parts.

TAPER TURNING

[Full Operation Procedure Explained Here]

Taper turning means, to produce a conical surface by gradual reduction or increase in diameter from a cylindrical work piece. This tapering operation has wide range of use in construction of machines. Almost all machine spindles have taper holes which receive taper shank of various tools and work holding devices.

Taper Turning Methods  

A taper may be turned by any one of the following methods:

(a) By a broad nose form tool.

(b) By setting over the tailstock center.

(c) By Swivelling the compound rest.

(d) By a taper turning attachment.

(e) By combining longitudinal and cross feed in a special lathe.

Taper Turning by a Form Tool

A broad nose tool having straight cutting edge (form tool) is set on to the work at half taper angle, and is fed straight into the work to generate a tapered surface. In this method, the tool angle should be properly checked before use. This method is limited to turn short length of taper only. Tool will require excessive cutting pressure, which may distort the work due to vibration and spoil the work surface.

Taper Turning by Setting over the Tailstock

The principle of turning taper by this method is to shift the axis of rotation of the workpiece, at an angle to the lathe axis, and feeding the tool parallel to the lathe axis. The angle at which the axis of rotation of the workpiece is shifted is equal to half the angle of the taper. The body of the tailstock is made to slide on its base towards or away from the operator by a set over screw. The amount of set over being limited, this method is suitable for turning small taper on long jobs. The main disadvantage of this method is that the live and dead centers are not equally stressed and the wear is not uniform. Moreover, the lathe carrier being set at an angle, the angular velocity of the work is not constant.

Taper Turning by Swivelling the Compound Rest                                 

This method employs the principle of turning taper by rotating the work piece on the lathe axis and feeding the tool at an angle to the axis of rotation of the work piece. The tool mounted on the compound rest is attached to a circular base, graduated in degree, which may be swiveled and clamped at any desired angle. Once the compound rest is set at the desired half taper angle, rotation of the compound slide screw will cause the tool to be fed at that angle and generate a corresponding taper. This method is limited to turning a short taper owing to the limited movement of the cross slide. But a small taper may also be turned. The compound rest may be swiveled at 45°on either side of the lathe axis enabling it to turn a steep taper. The movement of the tool in this method is being purely controlled by hand, thus giving a low production capacity and poor surface finish. The setting of the compound rest is done by Swivelling the rest at half taper angle, if this is already known. If the diameter of the small and large end and Length of taper are known, the half taper angle can be calculated from the equation {Tan α = (D-d) / 2L}

Taper Turning by a Taper Attachment   

The principle of turning taper by a taper attachment is to guide the tool in a straight path set at an angle to the axis of rotation of the work piece. A taper turning attachment consists essentially of a bracket or frame which is attached to the rear end of the lathe bed and supports a guide plate pivoted at the center. The plate having graduations in degrees may be swiveled on either side of the zero graduation and is set at the desired angle with the lathe axis. When the taper turning attachment is used, the cross slide is first made free from the lead screw by removing the binder screw. The rear end of the cross slide is then tightened with the guide block by means of a bolt. When the longitudinal feed is engaged, the tool mounted on the cross slide will follow the angular path, as the guide block will slide on the guide plate set at an angle to the lathe axis. The required depth of cut is given by the compound slide which is placed at right angles to the lathe axis. The guide plate must be set at half taper angle and the taper on the work must be converted in degrees. The maximum angle through which the guide plate may be swiveled is100 to 12°on either side of the center line.  If the Large diameter (D), Small diameter (d), and the taper length (L) are specified, the angle of Swivelling the guide plate can be determined from equation Tan ά = (D-d) / 2L

Advantages of Taper Turning Attachment

The advantages of using a taper turning attachment are:

(a) The alignment of live and dead centers being not disturbed, both straight and taper turning may be performed on a work piece in one setting without losing much time.

(b) Once the taper is set, any length of a piece of work may be taper turned within its limit.

(c) Accurate taper on many work pieces may be turned.

(d) Internal tapers can be turned with ease.

Taper Turning by Combining Feeds     

Taper turning by combining feeds is a more specialized method of turning taper. In certain lathes both longitudinal and cross feeds may be engaged simultaneously causing the tool to follow a diagonal path. This is the resultant of the magnitudes of the two feeds. The direction of the resultant may be changed by varying the rate of feeds by change gears provided inside the apron.

SHOULDER TURNING

[Full Operation Procedure Explained Here]

When a workpiece having different diameters is turned, the surface forming the step from one diameter to the other is called the shoulder, and machining this part of the workpiece is called shoulder turning.

THREAD CUTTING

[Full Operation Procedure Explained Here]

Thread cutting is one of the most important operations performed in a lathe. The principle of thread cutting is to produce a helical groove on a cylindrical or conical surface by feeding the tool longitudinally when the job is revolved between centers or by a chuck. The longitudinal feed should be equal to the pitch of the thread to be cut per revolution of the workpiece. The lead screw of the lathe, through which the saddle receives its traversing motion, has a definite pitch. A definite ratio between the longitudinal feed and rotation of the headstock spindle should therefore be found so that the relative speeds of rotation of the work and the lead screw will result in cutting of a screw of the desired pitch. This is affected by Change gears arranged between the spindle and the lead screw or by the Change Gear Mechanism or feed box used in a modern lathe where it provides a wider range of feed and the speed ratio can be easily and quickly changed.

Thread Cutting Operation

(a) External Thread Cutting

In a thread cutting operation the first step is to remove the excess material from the workpiece to make its diameter equal to the major diameter of the screw thread, Change gears of correct size are then fitted to the end of the bed between the spindle and the lead screw. In case of all geared machines, the change gears are already fitted, only the levers are to be shifted to the required positions. The shape or form of the thread depends on the shape of the cutting tool to be used. In a metric thread, the included angle of the cutting edge should be ground exactly 60°. The top of the tool nose should be set at the center of the workpiece. A thread tool gauge or angle gauge is usually used against the turned surface to check the cutting tool so that each face of the tool may be equally inclined to the center line of the workpiece.  The speed of the spindle is reduced by one half to one-fourth of the speed required for turning according to the type of the material being machined, and the half-nut is then engaged. The depth of cut, which usually varies from 0.05 to 0.2 mm, is applied by advancing the tool perpendicular to the axis of the work. After the tool has produced a helical groove upto the end of the work, it is quickly withdrawn using the cross slide, the half nut disengaged, and the tool is brought back to the starting position to give a fresh cut. Before re-engaging the half nut, it is necessary to ensure that the tool will follow the same path it has traversed in the previous cut, otherwise the job will be spoiled. Several cuts are necessary before the full depth of thread is reached.

(b) Internal Thread Cutting

The principle of cutting internal threads is like that of an external thread, the only difference being in the tool used. The tool is like a boring tool with cutting edges ground to the shape conforming to the type of thread to be cut. The hole is first bored to the root diameter of the thread. The tool is fixed on the tool post or on the boring bar after setting it at right angles to the lathe axis, using a thread gauge/angle gauge. The depth of cut is given by the compound slide and the thread is finished in the usual manner.

BORING

Boring is the operation of enlarging and truing a hole produced by drilling, punching, casting or forging. Boring cannot originate a hole. Boring is like the external turning operation and can be performed in a lathe.

(a) Counter Boring

Counter Boring is the operation of enlarging a hole through a certain distance from one end instead of enlarging the whole drilled surface. It is like a shoulder turning operation in external turning.

(b) Taper Boring

The principle of turning a taper hole is like the external taper turning operation and is accomplished by rotating the work on a chuck and feeding the tool at an angle to the axis of rotation of the work piece.

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