10eme14 - unit 5

Nagarjuna College of Engg and Technology 10EME14/24 Notes

UNIT-5

LATHE AND DRILLING MACHINES

LATHE MACHINE

Working principle of Lathe:

The lathe machine works on the principle that, a cutting tool can remove material when it is moved against a rotating workpiece. This is accomplished by rotating the workpiece between two rigid and strong supports, while the cutting tool is fed against it. The principle of operation is shown in figure (a).

When the tool is moved against the rotating workpiece, the excess material is removed from the workpiece in the form of fine chips. The type of surface produced depends on the movement of the tool with respect to the axis of rotation of the workpiece.

• When the tool moves parallel to the axis of rotation of the workpiece, a cylindrical surface is produced.

• When the tool moves perpendicular to the axis of rotation of the workpiece, a flat surface is produced.

• When the tool moves at an angle to the axis of rotation of the workpiece, a tapered surface can be produced.

Working principle of lathe


Parts of a Lathe:

1. Bed: The bed is a heavy, rugged casting in which are mounted the working parts of the lathe. It carries the headstock and tail stock for supporting the workpiece and provides a base for the movement of carriage assembly which carries the tool. 2. Legs: The legs carry the entire load of machine and are firmly secured to floor by foundation bolts. 3. Headstock: The headstock is clamped on the left hand side of the bed and it serves as housing for the driving pulleys, back gears, headstock spindle, live centre and the feed reverse gear. The headstock spindle is a hollow cylindrical shaft that provides a drive from the motor to work holding devices. 4. Gear Box: The quick-change gear-box is placed below the headstock and contains a number of different sized gears. 5. Carriage: The carriage is located between the headstock and tailstock and serves the purpose of supporting, guiding and feeding the tool against the job during operation. The main parts of carriage are: a). The saddle is an H-shaped casting mounted on the top of lathe ways. It provides support to cross-slide, compound rest and tool post. b). The cross slide is mounted on the top of saddle, and it provides a mounted or automatic cross movement for the cutting tool. c). The compound rest is fitted on the top of cross slide and is used to support the tool post and the cutting tool. d). The tool post is mounted on the compound rest, and it rigidly clamps the cutting tool or tool holder at the proper height relative to the work centre line. e). The apron is fastened to the saddle and it houses the gears, clutches and levers required to move the carriage or cross slide. The engagement of split nut lever and the automatic feed lever at the same time is prevented she carriage along the lathe bed. 6. Tailstock: The tailstock is a movable casting located opposite the headstock on the ways of the bed. The tailstock can slide along the bed to accommodate different lengths of workpiece


between the centers. A tailstock clamp is provided to lock the tailstock at any desired position. The tailstock spindle has an internal taper to hold the dead centre and the tapered shank tools such as reamers and drills. Classification of Lathe:

Based on the construction and functions, lathes are classified as follows 1. Engine lathe or center lathe 2. Speed lathe 3. Automatic lathe 4. Bench lathe 5. Tool room lathe 6. Capstan lathe 7. Turret lathe 8. CNC lathe (Computer Numerical Control)

Lathe operations

The operations performed on a Lathe machine are: 1. Plain Turning or cylindrical Turning. 2. Facing 3. Knurling 4. Drilling 5. Threading 6. Taper turning 7. Parting

(1) Plain Turning:

It is the operation of removing excess amount of material from the workpiece to produce

a cylinder work piece. In this operation, shown in fig., the work is held either in the chuck or between centers, the cutting tool is fed against the revolving workpiece and is then moved parallel to the lathe axis so as to produce a cylindrical surface.

(2) Facing:


It is the operation for generating a flat surface at the end of the workpiece. In this operation, as shown in fig., the work piece is held in the chuck and the facing tool is fed from the center of the work piece towards the outer surface or from the outer surface to the center, with the help of a cross-slide. Facing is also carried out to reduce or cut the workpiece to the required length.

(3) Knurling:

knurling

It is the process carried out on a lathe, where a visually-attractive diamond shaped pattern is cut or rolled on the surface of metallic parts. In this operation, as shown in fig., the workpiece is held rigidly between two centers, the knurling tool is pressed against the rotating workpiece and pressure is slowly increased until the tool produces a pattern on the workpiece. The surface on the workpiece formed by knurling is used for applicatiions where grip is required to hold the part.

(4) Drilling :

drilling

It is the operation of producing a cylindrical hole in a work piece with the help of a drill. In this operation, as shown in fig., the workpiece is held In the tapered hole of the tail stock sleeve and is fed into the rotating work piece, by rotating the tail stock hand wheel.


(5) Thread cutting:

It is a operation for cutting screw threads on metallic parts. In this operation, as shown in fig., the workpiece is held In between the two centers, the cutting tool is mounted on the tool post and the carriage is connected to the lead screw with the help of a split nut. The rotation of the lead screw gives the required motion to the carriage relative to the rotation of the workpiece. The depth of cut is selected and the tool is made to move parallel to the axis of rotation of the workpiece by means of automatic arrangement. By disengaging split nut or half nut, the carriageis brought back to its initial position to start another cut.

(6) Taper turning:

It is the operation of producing a conical surfaces on the workpieces. A taper can be produced by any one of the following methods.

i. By swivelling the compound rest ii. By off-setting the tailstock

iii. By using a taper turning attachment iv. By form tool method

i) Taper turning by swivelling the compound rest.

chuck workpiece compound rest


In this method, the compound rest is swivelled to the desired angle at which thje taper is to be produced. The compound rest has a circular base gratuated in degrees. The angle at which the compound rest to be swivelled is calculated by using the equation,

tan α = (D-d)/2Lt α = tan-1{(D-d)/2Lt}

where D = Larger diameter of taper in mm d = Smaller diameter of taper in mm Lt = Length of taper in mm and α = Half taper angle in degrees This method is more suitable for producing large tapers on short length workpieces.

ii) Taper turning by off-setting the tailstock.

In this method, the taper is produced by shifting the axis of rotation of the workpiece at an angle to the lathe axis, and then the cutting tool is moved parallel to the lathe axis. This method is used for producing small tapers on long length workpieces. The angle at which the axis of rotation of the workpiece is shifted is equal to the half angle of the taper. This is done by offsetting the body of the tailstock with respect to its base by an amount equal to,

Offset = (D-d)Lw/2Lt

Where D = Larger diameter of taper in mm d = Smaller diameter of taper in mm Lt = Length of taper in mm and Lw= Length of entire workpiece in mm Specification of lathe: The size of a lathe is specified by the following, as shown in figure.

1. Maximum diameter of the workpiece that can be revolved over the lathe bed. 2. The maximum diameter and the width of the workpiece that can swing when the lathe has

a gap bed.


3. The maximum length of the workpiece that can be mounted between the centres. 4. Overall length of the bed. 5. Range of spindle speeds.

DRILLING MACHINE Drilling: Drilling is a machining operation of producing a cylindrical hole in a solid workpiece by means of a revolving tool called twist drill.

Twist drill

Classification of drilling machines: Based on the construction and functions, drilling machines are classified as follows

1. Portable drilling machine 2. Bench or sensitive drilling machine 3. Radial drilling machine 4. Upright drilling machine 5. Gang drilling machine 6. Automatic drilling machine 7. Deep hole drilling machine 8. Multi-spindle drilling machine 9. CNC drilling machine (Computer Numerical Control)


Bench or sensitive drilling machine: Bench drilling machines are used for drilling small holes at high speeds in small sized workpieces. The diameter of the hole usually ranges from 1.5mm to 15mm.

Bench drilling machine The machine consists of the following parts.

1. Base: The base of the machine is made from cast iron material and supports the vertical column.

2. Vertical column: The column is a hollow steel pipe mounted rigidly on the base. It supports the drill head and the worktable.

3. Worktable: The worktable supports the workpiece to be drilled. The table can be raised or lowered, and can be clamped to the vertical column at any desired position. The table can also be swiveled around the vertical column to any desired position.

4. Drill head: A fixed drill head located at the top end of the vertical column carries an electric motor and a mechanism through which the spindle can be made to rotate, as well as slide up and down.

Working: In operation, the workpiece is clamped rigidly on the worktable. With the help of a center punch,an indentation mark is made on the workpiece at the location where the hole is to be drilled. The drill bit is made to rotate at a suitable speed by sdjusting the V-belt on the stepped cone pulley. With the help of hand feed lever, the drill bit is moved downwards till the point of the drill bit touches the indentation mark. Gradual feed to the drill bit is given by operating the hand feed lever till the desired depth of the hole is achieved. The drill bit is withdrawn slowly from the workpiece by operating the hand feed lever in the reverse direction.


Radial drilling machine: Radial drilling machines are used for drilling medium or large diameter holes of upto 50mm in heavy workpieces.

Radial drilling machine

The machine consists of the following parts. 1. Base: The base of the machine is made from cast iron material, which is provided with T-

slots, which help the workpiece to be clamped rigidly to the base of the machine. 2. Vertical column: The column is a hollow steel pipe mounted rigidly on the base. The

column carries a radial arm that can be raised or lowered by means of an electric motor and can be clamped to any desired position. The radial arm can also be rotated in a complete circle around the column.

3. Drill head: The drill head is mounted on the radial arm and carries a driving motor and a mechanism for revolving and feeding the drill bit in to the workpiece. The drill head can be moved horizontally on the guideways provided in the radial arm, and can be clamped to any desired position.

With the combination of the movements of radial arm and the drill head, it is possible to move the drill bit and hence generate a hole at any desired position without moving the workpiece.

Specification of a radial drilling machine: Following are some of the important specifications of a radial drilling machine.

1. Drill capacity- It is the depth of the hole that can be drilled. Example 20mm 2. Power required. Example 1HP for spindle drive motor and 0.5HP for elevating motor. 3. Length of arm. Example 600mm 4. Vertical movement of arm. Example 500mm 5. Angular swing of the arm. Example 3600 6. Number of spindle speeds. Example 8 (gear drive) 7. Range of spindle speeds. Example 80-1800 rpm


Drilling machine operations: A variety of operations performed on a drilling machine are

1. Drilling 2. Reaming 3. Boring 4. Tapping 5. Counter boring 6. Counter sinking 7. Spot facing.

1. Drilling:

Drilling of a hole

Drilling is a machining operation of producing a cylindrical hole in a solid workpiece by means of a revolving tool called drill bit. In operation, the drill bit is held rigidly in the chuck of the machine and rotated by the spindle at high speeds. With the help of hand wheel, a drill bit is forced to move against the rigidly clamped workpiece. A hole is generated by the sharp cutting edges of the rotating drill bit and meanwhile, the excess material removed gets curled and escapes through the helical grooves provided in the drill bit.

2. Reaming:

It is the operation of finishing a previously drilled hole to bring it to a more exact size and to improve the surface finish of the hole. This operation is carried out using a multi-tooth revolving tool called reamer. A reamer is similar to the twist drill, but has straight flutes. Reaming should be used only to remove small amounts of material.


reaming 3. Boring:

boring

It is the operation of enlarging a previously drilled hole by means of an adjustable cutting tool having only one cutting edge. This operation is performed when a drill bit of the required dimension is not available. In such cases, a hole is first drilled to the nearest dimension, and then a single point cutting tool is fastened and adjudted to a boring bar to enlarge the size of the existing hole to the required dimension.

4. Tapping:

It is the operation of producing internal threads in a previously drilled hole by means of a

tool called tap. The tap has threads cut on its periphery and is hardened to improve its properties. Taps are available in standard sizes. Hence, to generate a specific size thread in the workpiece, a hole with diameter smaller than the size of the tap is first drilled using a twist drill and then using a standard size tap, threads are cut in the workpiece. 5. Counter boring:


It is the operation of enlarging one end of a previously drilled hole to the required depth. This operation is performed in order to allow the head of bolts, studs etc., to sit with or below the surface of the surrounding material. The tool used for this operation is called counter bore, which is provided with a pilot at its bottom as shown in figure, for maintaining alignment and true concentricity of the counter bored hole with the previously drilled hole. 6. Counter sinking:

It is the operation of enlarging one end of a previously drilled hole into a conical shapefor a short distance. This operation is performed in order to allow the conical head of screws and rivits to sit with or below the surface of the surrounding material. The tool used for this operation is called counter sink and are made in angles of 600 for centering, 820 for counter sinking, 900 for deburring and 1200 for chamfering operations.

7. Spot facing: Spot facing is a finishing operation to produce a flat surface at the opening of a drilled

hole to provide a seat for the underside of a bolt head, washer or nut. Spot facing is done using a tool similar to the counter bore, but has cutting teeth on one end only.

Spot facing

10eme14 - unit 5

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