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TURNING OPERATIONLathe is the oldestMost basic Most versatileWidely used machine toollathe is known as the MOTHER OF MACHINE TOOLS.Why lathe are also known as TURNING MACHINES.Because the job to be machined is rotated or turned and the cutting tool is moved relative to the job Function of the latheFor producing cylindrical jobs.Flat surfaces and holes.With special attachments, it can be used for producing different types of surfaces, cutting threads, cutting grooves.As a whole, a skilled worker can produce any type of job with a lathe.

Machining operation done in latheStraight turningTaper turningChamferingDrillingReamingBoringCounter boringTaper boringInternal thread cuttingTappingParting offThread cuttingFacingKnurlingFilingPolishingGroovingSpinningForming

Tail stockSpeed gearboxHead stockFeed rodLead screwHalf nutApron boxLathe bedrackFeed gear boxDead centreramcarriageclampMain driving pulleyTool post6

Lathe parts BedHeadstockTailstockCarriageFeed mechanismScrew cutting mechanismAssignment for next class is to read all the parts of a lathe machineLathe parts contd.Bed-Base of the latheSupports all major components of the lathe machine.Large mass and made from gray cast ironThree main parts of the lathe headstock, tailstock and carriage are mounted on the bed of the lathe.Top of the bed has two guide ways or slide ways to provide support and sliding surfaces for the carriage and for the tailstock.Contd.HeadstockSecured permanently at the left hand end of the lathe bed.It supports the spindle and is equipped with the power driving mechanism for the spindle. The spindle speed can be set through speed selector knobs. The spindle is hollow to facilitate holding of long work pieces.The work holding devices such as chucks,centres and collets are attached to the spindle.

Contd.Tail stockLocated at the right hand end of the bed Can be moved along the guide ways and can be clamped in any position on the bed.Also called loose headstockMain purpose is to hold the dead centre and to support the long work pieces during machining.It has a quill, into which the deadcentre,drills,reamers can be fixed. The quill can move in and out with the help of hand wheel.

Contd.Carriage

The carriage slides along the guide ways between headstock and consists of an assembly of the cross-slide ,tool post, the compound rest and the apron.Main function is to hold the cutting tool and move it to give longitudinal and cross feed to it. The cross-slide moves radially in and out, thus controlling the radial position of the cutting tool.The compound rest, also called compound slide is mounted on the top of the cross slide and has circular base graduated in degrees. It is used for obtaining angular cuts and short tapers. Compound rest swivels the tool for positioning and adjustment. The tool post is located at the top of the compound rest to hold the tool and to enable it to be adjusted to a convenient working position. The apron is equipped with mechanisms for both manual and mechanized movements of the carriage and the cross-slide , by means of a lead screw and feed rod. Contd.Feed rod and lead screwThe feed rod is powered by a set of gears from the head stock .It rotates during the operation during the operation of the lathe and provides mechanized movement to the carriage or the cross-slide by means of gears, a friction clutch, and a keyways along the length of the feed rod.The lead screw is also powered by the gears from the headstock and is used for providing specific accurate mechanized movement to the carriage for cutting threads on the work piece. The lead screw has a definite pitch.A split nut in the apron is used to engage the lead screw with the carriage. In some lathes, the lead screw performs the functions of feed rod and there is no separate feed rod. Similarly a lathe not meant for thread cutting will not have a lead screw.

Size of a latheThe size of a lathe is expressed or specified by the following items.The height of the centers measured from the lathe bed.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 centre measured from the bed of the lathe.)The length between centers.(This is the maximum length of work that can be mounted between the lathe centers.)

Contd.(4)The swing diameter over carriage.(This is the largest diameter of work that will revolve over the lathe saddle and is always less than the swing diameter over bed.)(5)The maximum bar diameter(This is the maximum diameter of bar stock that will pass through the hole of the head stock spindle)(6)The length of bed.(This indicates the approximate floor space occupied by the lathe)Lathe accessories and attachmentsOperating condition in a latheCutting speedFeedDepth of cut1-Cutting speedIn a lathe, for the turning operation, cutting speed is the peripheral speed of the work piece past the cutting tool.Expressed in meters/minute.

Cutting speed= m/min

Where D= diameter of the work piece in mm. N=rpm of the work To get the speed in m/min conversion factor of 1000mm/m is used. The

Contd.The material passes the stationary cutting tool with this peripheral speed as shown in figure.Hence V=cutting speed=peripheral speed=STherefore,V=

2-Feed

The feed of a cutting tool in a lathe work is the distance the tool advances for each revolution of the work.Feed is expressed as mm/revolution.Increased feed reduces cutting time. But increased feed greatly reduces the tool life.The feed depends on factors such as size, shape, strength and method of holding the component, the tool shape and its setting as regards overhang, the rigidity of the machine, depth of cut, power available etc.Coarser feeds are used for roughing and finer feeds for finishing cuts.

Depth of cut The depth of cut(d) is the perpendicular distance measured.From the machined surface to the uncut surface of the work piece.For turning operation, the depth of cut is expressed as-

d = mm

Where D1=original diameter of the work piece in mmD2=final diameter of the work piece in mm In a turning operation, if the depth of cut is 1mm then the diameter will be reduced by 2mm.

Machining timeThe time required to machine a component is called MACHINING TIME.Machining time depends upon Size of the work pieceAmount of material removed and The operating condition(speed,feed,depth of cut)Consider the feed and speed(fN) the feed rate in mm/min.It gives the distance that the tool moves (fN) in mm is one minute.Hence , for a distance L , the time required for one complete cut, t in minute is given by t= minuteAlternativeIf f is the feed of the job per revolution expressed in mm/revolution and L is the length of the job in mm , then number of revolutions of the job required for a complete cut will be

If the rpm of the work is N , time taken to revolve the job through number of revolution for a complete cut will be:There fore time taken for a complete cutt= minute

Manufacturing timeTotal manufacturing time = machining time + set up time + moving and waiting time+ inspection time Time required for setting the tool , work piece and machine is known as set up time.Movement time is the time when component has to move from machine to machine and may wait before the machines getting processed, that time is called waiting timeDuring and after machining , components are inspected that time is called inspection time

Tapers and taper turningA taper may be defined as a uniform increase or decrease in diameter of a piece of work measured along its length. In a lathe taper turning means to produce a conical surface by gradual reduction in diameter from a cylindrical work piece.D= larger diameter of taper in mmd= smaller diameter of tapered in mmL= length of tapered part in mm2= full taper angle=angle of taper or half taper angle

A tapered job showing taper angle

BACFrom the geometry tan=The amount of taper in a work piece is usually specified by the ratio of the difference in diameters of the taper to its length.This is termed as the CONICITY and it is designated by the letter K.K=

TAPER TURNING METHODS1- using a form tool.2- swiveling the compound rest.3- using a taper turning attachment.4- by combining longitudinal and cross feed in a special lathe.

Taper turning using a form tool

The form tool has a straight cutting edge set at the desired taper angle.Form tool is a replica of the taper to be produced. That is the angle between the straight cutting edge and the rotational axis of the job equals taper angle or one half the included angle of the taper.Shape of the tool is reproduced on the work piece.

Limitation-This method is limited only for short length taper. Because the metal is removed by the entire cutting edge, and only increase in the length of the taper will necessitate the use of a wider cutting edge. This will require excessive cutting pressure, which may distort the work due to vibration and spoil the work piece.

Taper turning by swiveling the compound rest

The compound rest has a circular base graduated in degrees, which can be swiveled at any angle.While turning a taper, the base of compound rest is swiveled through an angle equal to the taper angle. The tool is then fed by hand.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 turn a short but steep 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 degree on turn a steep taper.The movement of the tool in this method being purely controlled by hand, this gives a low production capacity and poorer surface finish. The setting of the compound rest is done by swiveling the rest at the 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.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, while the work is being revolved between centers or by a chuck aligned to the lathe axis.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 bar pivoted at the centers. The bar 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 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 gear bar 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 bar 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 bar may be swiveled is 10 degree to 12 degree on either side of the centre line.If the diameters D,d and the length L of the taper are specified, the angle of swiveling the guide bar can be determined from equationtan=

The advantage of using a taper turning attachment are-1-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 much loss of time.2-once the taper is set, any length of a piece of work may be turned with in its limit.3-very steep taper on a long work piece may be turned, which cannot be done by any other method.4-accurate taper on a large number of work pieces may be turned.5-internal tapers can be turned with ease.Taper turning by combining feedsTaper 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 which is the resultant of the magnitude 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.Fig for combining feedExtra- Tail stock off-set

By offsetting the tailstock, the axis of rotation of the job is inclined by the half angle of taper S=AB sin =L sin Thread cuttingThere are a large number of thread forms that can be machined in lathe such as Whitworth, Acme, ISO metric, etc.

Thread cutting can be considered as turning only since the path to be travelled by the cutting tool is helical

summaryLathe is the most important and common machine tool found in practically all machine shops.

A large variety of lathes have been developed to cater for different processing requirements.

A lathe consists of a bed, headstock, tailstock and a carriage as major components along with a few other items that provide the necessary support and motions.

summaryA variety of chucks such as universal 3-jaw, independent 4-jaw, and faceplate, are used to locate and support work pieces in a lathe for common machining applications.

There are a variety of tools available depending upon the type of surface that needs to be generated.

There are a large variety of operations such as turning, facing, knurling, contouring, etc. that can be carried out in a lathe. In fact practically all types of surfaces can be generated in a lathe.

summaryTaper turning is a special type of operation that requires the tool to be moved in two different direction simultaneously to generate the surface.

For this purpose, a variety of methods are used in a lathe such as compound slide, tailstock offset or a special attachment.

Precision threads can be cut in a lathe using the lead screw and special methods

There are various special attachments such as milling attachment, grinding attachment, etc. have been developed that enhance the range of surfaces that can be generated in a lathe.

Machining time for different operations can be estimated using the cutting process parameters and the geometry of the part

summary

3 jaw chuck

Four jaw chuck

Holding jobs in centre lathe by three jaw chuck

Holding jobs in centre lathe by four jaw chuck