22MET42 MACHINING AND MEASUREMENT

Types of Lathe

• Speed lathe - is a power driven simplest lathe often used for wood turning.
• Centre lathe or engine lathe - Stepped cone pulley arrangement with motor is used for varying the speed of lathe spindle.
• Gap bed lathe
• Geared head lathe
• Bench lathe
• Turret and capstan lathes
• Tool room lathes
• Automatic lathes

Operations Performed on Lathe

• Taper turning - tan a = (D – d)/2I.
• Facing
• Boring
• Threading
• Drilling
• Knurling
• Grooving or undercutting
• Parting off

Lathe Accessories and Attachments

• Accessories are the devices used for holding or supporting job on a lathe during machining. These include lathe centres, face plate, dog carrier, chucks, angle plate, mandrel, steady rest, follower rest.
• Lathe centres

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• Face plate- It carries a number of slots for bolting jobs directly on the face plate or through the angle plate on which the job is mounted.

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• Angle plate for holding typical right angled bend jobs for boring,

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• The dog carrier at its one end is clamped with the job while its other end is engaged in the open slot of the face plate screwed on the lathe spindle.
• Mandrels are hardened steel pieces of round bar and are used for holding the bored jobs (jobs having drilled or bored holes) for the purpose of turning them at outside.

• Steady rest is used when a long job is machined or drilled at its end by holding the job in a chuck. The use of steady rest avoids the deflection of job under its own weight or cutting forces of the tool.
• Follower rest is used for turning a long and thin job which may be held between centres and may thus deflect under cutting forces of the tool.

TURRET AND CAPSTAN LATHES

• Turret and capstan lathes are the natural developments of a centre lathe and are built to machine workpieces that are large in number and on repetitive basis.
• The main distinguishing feature of turret and capstan lathes is the multiple tool holders which enables presetting of all the tools for a job.

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Capstan Lathe

Turret Lathe

Capstan Lathe

Turret Lathe

Thread Cutting on Lathe

• Lead is the axial movement of the screw travelled in its one revolution. In case of single start threads, lead is equal to pitch which is the distance from one point on one thread to the corresponding point on the adjacent thread.

Lead = No. of start × Pitch

• For thread cutting, the ratio of gears between the stud gear (driver) to the lead screw gear (driven) is found from the following relation.

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• Find the charge gears to cut RH single-start threads of 2 mm pitch on a lathe having lead screw of 8 mm pitch.

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Taper and Taper Turning

• Taper turning is a type of turning operation in which the diameter of the job is gradually reduced as the turning proceeds along the job length.
• Common methods of taper turning on lathe are
• Compound rest method

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• Tail stock set over method

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• Case I For giving taper 1 in 10 on a job 80 mm long, find taper on 80 mm length = 80/10 = 8 mm. Then, tail stock set over = 8/2 = 4 mm.
• Case II For a taper of 12° on a job 80 mm long, here sine of half of taper angle
• Case III When the given major dia is D, minor dia is d and tapered length is l and the total length of the job (including tapered portion) is L, all in mm, then,

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• Taper turning with a form tool

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• Taper turning with taper turning attachment
• These attachments are available for turning tapers on lathe. Longer tapers are easily turned with these.

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Problem: Find the angle at which compound rest should be swivelled for cutting a taper on job 150 mm long and having diameter 80 mm. The smallest diameter on the tapered end should be 60 mm and required length of tapered portion 100 mm.

Given: D = 80 mm; d = 60 mm; l = 100 mm

Let a be the angle at which compound rest will be swivelled,

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Problem: A mild steel rod has a length of 80 mm and a tapered portion of length 50 mm. Large diameter of taper is 90 mm and small diameter 80 mm. Find:

(i) Taper in mm/metre and in degrees

(ii) Angle to which compound rest should be set

(iii) Tail stock set over

Given: L = 80 mm; l = 50 mm; D = 90 mm; d = 80 mm

Work Holding Devices

Collet chucks

Jaw chucks

Tooling Layout

• Proper planning for systematic operations to be carried out on turret and capstan lathes is done in advance before setting the work on the lathe.
• The following procedures should be adopted for proper working on the machine.
• go through the capacity chart of the machine
• Get a copy of the drawings and specifications of the product to be made
• Selection of proper tools and tooling arrangement
• Proper spindle speeds, feeds and depth of cut are decided for various operations
• Work and the tools are then set at proper locations as also the trips and bar stop, etc

To make a hexagonal bolt (15 mm dia) from a hexagonal bar stock on a capstan lathe

AUTOMATIC LATHES

• Automatic lathe machines are those in which both the workpiece handling (during metal cutting) and the metal cutting operations are performed automatically.

Classification of Automatic Lathes

(a) According to the type of stock (or workpiece) material used

• Bar automatics
• Chucking machines

(b) According to the spindle position

• Horizontal machines
• Vertical machines

(c) According to the number of spindles

• Single-spindle automatics
• Multiple-spindle automatics