CONTOUR ATTACHMENT FOR LATHE

SYNOPSIS

A concave attachment is a versatile one to cut concave-convex profiles. A set of

rotating plates have T-slots for the T-bolts that hold the boring bar. These plates are

rotated by a train of gears. The bar is held roughly yet free enough to rotate. When the

tool is built, the boring bar is first clamped with both pivots at lead centre. So, the tool

describes a zero radius when the crank is turned.

At this time, one set up pin hole is drilled in each of the rotating plates, as

indicated so that the pin touches the boring bar. Then to set the tool, for a desired radius,

it is merely necessary to place a piece of flat stock the same thickness as the desired

radius between the bar and the set up pins while securing the bar in position. At this

point, data the flat stock and set up pin can be removed.

INTRODUCTION

The concave attachment is fixed to the carriage of the lathe. The compound rest

that is locked solid with its base is removed and the attachment is fixed on the cross lied

of the carriage with its axis parallel to the lathe bed.

The work is secured rigidly in a chuck. By the concave-convex movement of the

boring bar, the material is being removed. This attachment is used to produce contour

profiles and round balls.

NEED FOR ATTACHMENT

In recent years, new fabrication techniques have been developed to satisfy the

technological demands. Moreover, emphasis is stressed on attachments.

Attachments are used in various fields and machines depending upon the needs to

be fulfilled and mode of operation. An attachment eliminates the purchasing of a new

unit which serves the same purpose. For example, a lathe occupies a place opposite to

that of a milling machine, the ten machines mainly used to produce cylindrical and plain

surfaces respectively. By implementing an attachment to a unit, the capacity of the unit

can be increased which is very economical.

WORKING PRINCIPLE

The worm wheel is rotated with the help of a worm shaft. Rotating plates with T-

slots that hold the boring are screwed to the worm wheel. The worm wheel in turn is

screwed to its base with a simple gear train. The rotary motion is imported to the blank

through two spur gears and an intermediate gear namely “Idler Gear”.

The idler gear serves to keep the rotation of rotating plates in the same direction.

The worm shaft is rotated in clockwise direction, the boring bar machines a contour

profile in the clockwise direction and vice versa. Thus a concave-convex profile is

machined by boring bar by simultaneous engagement of the two plates.

DESCRIPTION OF EQUIPMENTS

Base Plate:

The base plate is in ‘I’ shape with drilled holes to suit the gear stems. The

compound reset that is locked solid with its base of the crosslide of the carriage is

removed and in its place, the base plate is so designed to seat. The base plate’s axis is

parallel to the lathe bed.

Rotating Plate:

The rotating plate is a primary part of the unit. It is circular in shape. It has ‘T’

slot along its diameter for the ‘T’ bolts that hold the boring bar. It is screwed with the

help of Allen screws to the worm wheel, so that the direction of the rotation of the worm

wheel and the rotating plate is the same. A set up in hold is drilled in the rotating plate to

set the tool for desired radius.

Worm and Worm Wheel:

The worm wheel is screwed between the spur gear and rotating plate with its axial

to the lathe bed. The worm shaft is provided with a handle for easy rotation. The height

of the worm shaft from base plate is the height of the worm from the base when the

handle is rotated; the worm shaft rotates the proportionate worm wheel. A seating is

provided on the worm wheel to seat the rotating plate.

Spur Gear:

The spur gear is screwed between the worm wheel and the base plate. A step

down bush is used to fit the worm wheel and the spur gear songly yet free enough to

rotate.

Idler Gear:

The idler gear is screwed to the base plate by press fitting the idler gear and its

stem to the hold provided in the base plate. The rotary motion between 2 spur gears is

directed through the idler gear.

Seating Connecting Blanks:

This forms the base of the unit. The setting blank is screwed to the connecting

blank by the help of Allen screws. The blanks are circular in shape. The connecting

blank connects the setting blank and the base plate. These blanks are so designed that the

component reset is removed and the blanks are bolted to cross slide of the carriage. The

whole unit resets on these blanks.

Boring Bar:

The boring bar is held songly in the T-slots provided in the rotating plates by the

T-bolts. The tool that cuts the radial is screwed to the boring bar. The boring bar moves

over a certain radius when the crank is turned or rotated.

WORKING OF THE UNIT

When the worm shaft is rotated in a clockwise direction, the worm wheel rotates in

the clockwise direction and vice versa.

Idler gears are used to rotate the rotating plates in the same direction. When the

boring bar is bolted to only one of the ‘I’ slots in the rotating plate and when the crank is

turned, the material is removed at a certain radius depending on the extension of the

boring bar. When the boring bar is bolted to the T-slots provided in the rotating plates

and when the worm is rotated, the boring bar described a concave-convex profile. This

movement of the bar is used to produce spherical balls also.

DESCRIPTION OF EQUIPMENTS

Base Plate:

The base plate is in ‘I’ shape with drilled holes to suit the gear stems. The

compound reset that is locked solid with its base of the crosslide of the carriage is

removed and in its place, the base plate is so designed to seat. The base plate’s axis is

parallel to the lathe bed.

Rotating Plate:

The rotating plate is a primary part of the unit. It is circular in shape. It has ‘T’

slot along its diameter for the ‘T’ bolts that hold the boring bar. It is screwed with the

help of Allen screws to the worm wheel, so that the direction of the rotation of the worm

wheel and the rotating plate is the same. A set up in hold is drilled in the rotating plate to

set the tool for desired radius.

Worm and Worm Wheel:

The worm wheel is screwed between the spur gear and rotating plate with its axial

to the lathe bed. The worm shaft is provided with a handle for easy rotation. The height

of the worm shaft from base plate is the height of the worm from the base when the

handle is rotated; the worm shaft rotates the proportionate worm wheel. A seating is

provided on the worm wheel to seat the rotating plate.

Spur Gear:

The spur gear is screwed between the worm wheel and the base plate. A step

down bush is used to fit the worm wheel and the spur gear songly yet free enough to

rotate.

Idler Gear:

The idler gear is screwed to the base plate by press fitting the idler gear and its

stem to the hold provided in the base plate. The rotary motion between 2 spur gears is

directed through the idler gear.

Seating Connecting Blanks:

This forms the base of the unit. The setting blank is screwed to the connecting

blank by the help of Allen screws. The blanks are circular in shape. The connecting

blank connects the setting blank and the base plate. These blanks are so designed that the

component reset is removed and the blanks are bolted to cross slide of the carriage. The

whole unit resets on these blanks.

Boring Bar:

The boring bar is held songly in the T-slots provided in the rotating plates by the

T-bolts. The tool that cuts the radial is screwed to the boring bar. The boring bar moves

over a certain radius when the crank is turned or rotated.

ADVANTAGES

The unit is compact in size.

Less maintenance is essential

The unit gives long life with proper alignment of gears.

Jobs can be easily handled in this unit.

APPLICATIONS

Concave and convex profiles of desired radius can be easily turned.

Round balls can be produced.

DESIGN OF SPUR GEAR

SPEEDS IN GEAR BOX:

Measured Specifications:

N1/N2 = D2/D1

Where,

N1 = Motor speed in RPM---40 RPM

N2 = Output speed

D2 = Diameter of the roller gear wheel

= 88 mm

D1 = Diameter of the motor gear wheel

= 35 mm

∴ N2 = (D1/D2) x N1

= (35 / 88) x40 = 16 rpm

SPUR GEAR:

The spur gears, which are designed to transmit motion and power between parallel

shafts, are the most economical gears in the power transmission industry.

APPLICATION:

Material handling

Feed drives

Machine tools

Conveyors

Marine hoists

INTERNAL SPUR GEAR:

The internal gears are spur gears turned "inside out." In other words, the teeth are

cut into the inside diameter while the outside diameter is kept smooth. This design allows

for the driving pinion to rotate internal to the gear, which, in turn, allows for clean

operation. Intended for light duty applications, these gears are available only in brass.

When choosing a mating spur gear, always remember that the difference in the number of

teeth between the internal gear and pinion should not be less than 15 or 12.

APPLICATIONS:

Light duty applications

Timing

Positioning

Rollers

Indexing

EXTERNAL SPUR GEAR:

Perhaps the most often used and simplest gear system, external spur gears are

cylindrical gears with straight teeth parallel to the axis. They are used to transmit rotary

motion between parallel shafts and the shafts rotate in opposite directions.

They tend to be noisy at high speed as the two gear surfaces come into contact at

once. Internal spur gears: The internal spur gear works similarly to the external spur gears

except that the pinion is inside the spur gear. They are used to transmit rotary motion

between parallel shafts but the shafts rotate in the same direction with this arrangement.

BASIC SHAFT DESIGN FORMULA

The drive shaft with multiple pulleys experience two kinds of stresses, bending stress and shear stress. The maximum bending stress generated at the outer most fiber of the shaft. And on the other hand, the shear stress is generated at the inner most fiber. Also, the value of maximum bending stress is much more than the shear stress. So, the design of the shaft will be based on the maximum bending stress and will be driven by the following formula:

Maximum bending stress Tb = (M * r) / I……………….Eqn.1.1

Where,

M is maximum bending moment on the shaft.

r is the radius of the shaft.

I is area moment of inertia of the shaft.

Design Procedure

Draw the bending moment diagram to find out the maximum bending moment (M) on the shaft.

Calculate the area moment of inertia (I) for the shaft. Replace the maximum bending stress (Tb) with the given allowable stress for the

shaft material. Calculate the radius of the shaft.

Shaft Design Problem

Refer the above picture, where a steel shaft is supported by two bearings and a pulley is placed in between the bearings. You have to design the shaft. Weight of the pulley is 1000 N.

Input data:

Maximum allowable shear stress for the shaft material= 40 N/mm2

Solution:

From the bending moment diagram, the maximum bending moment (M) is calculated as 66666.67 N/mm2.

Area moment of inertia (I)of the circular shaft is:

I = pi * r^4 *0.25

= 0.785*r^4………………..Eqn. 1.2

From Eqn1.1 we can write:

40 = (66666.67 *r)/ (0.785*r^4)

r= 12.85 mm

So, the minimum radius of the shaft should be 12.85 or 13 mm.

DESIGN OF IDLER GEAR:

Pressure Angle = 14 ½°

No. of Teeth (N) = 25

Diameteral Pitch (DP) = 18

PCD = ( N / DP ) x 25.4

= ( 25 / 18 ) x 25.4

= 35.28 mm

Outer Diameter (OD) = { ( N+2 ) / DP } x 25.4

= { ( 25 + 2) / 18} x 25.4

= 38.1 mm

Depth of Cut (or) Tool depth = ( 2.157 /DP ) x 25.4

= ( 2.157 / 18 ) x 25.4

= 3.044 mm

Dedendum = ( 1.157 / DP ) x 25.4

= ( 1.157 / 18 ) x 25.4

= 1.633 mm

DESIGN OF WORM SHAFT:

Outer Diameter = 20 mm

Modulo = 2

OD = 20 mm = 0.7874015

OD = PCD + Working Depth

∴PCD = OD – Working Depth

= OD – ( Pitch x 0.636 )

= 0.7874015 – 0.159

= 0.628” = 15.95 mm

TO FIND ANGLE AND LEAD:

{ Lead / ( PCD x T) }

Lead = pitch x No. of Start

0.250 x 1

Lead = 0.250

Angle = { 0.250 / ( 0.628 x II ) }

Where, ( 0.628 x II = 1.9741818 )

Angle = 0.250 / 1.9741818

= 7° R.H

To milling machine held is tilted at an angle of 7¼° R.H

To find No. of thread = Inch / pitch

= 1 / 0.25

= 4

( NB : The angle of the form tool is 30° )

DESIGN OF WORM WHEEL:

Bore Diameter = 25.4 mm

No. of teeth = 42

Pitch circle diameter = No. of teeth x module

= ( No. of teeth + 2 ) x 2

= ( 42 +2 ) x 2

= 88

Throat diameter = PCD + 2 module

= 88 + ( 2 x 2 )

= 92 mm

Tip diameter = Throat diameter + (1.5 module)

= 92 + ( 1.5 x 2 )

= 95 mm

Centre Distance = ( ½ x PCD of worm ) +

( ½ x PCD of worm )

= ( ½ x 23 ) + ( ½ x 88 )

= 11.50 + 44

= 55.50 mm

ASSEMBLY

A train of simple gears is press fitted to the base plate by bushes. These gears are

hence held singly. Yet free enough to rotate. A worm wheel is screwed to one of the

spur gear a circular blank is screwed with the help of Allen screws. A worm shaft is

fitted to the base plate with the help ‘L’ clams.

This worm shaft in engagement rotates the worm wheel. Rotating circular plates

are screwed to the worm wheel and the circular plates are Allen screws. Seating is also

provided on the worm wheel and the circular blank to hold the rotating plates. The

rotating plates have T-slots for T-bolts that secure the boring bar. Setting end connecting

blanks are provided to the bottom of the base plate to fasten entire unit to the crosslide of

the carriage.

4. Step down bushes:

For songly holding and to enable free rotation of the train of gears the step down

bushes are machines to suit the design values M.S. material of 35 x 601 is turned on a

lathe as per the drawing specification. A press fit is obtained on the town portion of the

bush by choosing a shaft hole combination of H7 P6.

5. Idle Gear Bush:

Similar to the other bush the same tolerance range is observed on 30 x 351 M.S.

rods and the bush is turned lather. A press fit of P6 tolerance is closely maintained to

enable the location of the idle gear.

6. Worm Gear:

As it is already said, the hand feed eliminates the need for materials like EWS to

be used as the gears. Hence C.I. of roughly 90.50 x 15f is turned on a chuck. To enable

the seating of the rotating plate on the gear a boss of 45 x 3mm is turned on the face of

the gear.

Directly opposite of the boss a recess is cut an undercutting rod is used to recess

35.1.25c on this recessed portion rests the primarily spur gear.

For a pitch of 0.250” and 90.50 OD the lead, depths of cut are calculated. For

these design values the worm gear is cut on a milling machine using to form relived

cutter.

The marking of the step down bushes with the worm gear is checked beforehand.

For a 35mm shaft combination a bore of slightly less in size is made on the worm wheel

blank to obtain a press fit. Once the seating is aer, four tapes are done on the outer face

of the worm wheel blank to grip the rotating slot firmly (by means of Allen screw)

7. Spur Gear:

a) Primary secondary spur gear:

The gears are cut similar to that of the worm. For an OD of 73.37mm and 50 T

the DP is calculated. The gears are cut on a milling machine using form relived cutter.

Here too C.I. is preferred to minimize the material cost. Prior to cutting the gear a boss 1

x 44 is turned and a 1” bore is done to a close tolerance of H7.

The seating of the other rotating plate is carried out similar to that of the previous

blank.

8. Rotating Plates:

C.I. of 1 x 44.02 mm is turned on a lathe to suit the boss of the making parts. For

holding the boring tool T slot of 3/8” is done on a slotting machine using a key way

slotter.

9. Bearing bar:

The boring bar is fabricated in mild steel. It has a T slot to suit the T bolt. The T

slot is machined in a slotting machine with the help of a T slot cutter. It has screws

provided at the side of the slots to tighten on loosen the tool that is placed in the slot.0

10. Maintenance and Lubrication:

Lubrication is an important performance factor in power transmission because of

its influence on operating efficiency. The contact of a spur gear pair involved maximum

sliding action and minimum roll between engaging teeth. Such combinations aid film

formation in a favorable environment, being most effectively early and late in a typical

‘contact’ and less efficient as pitch line is approached. A necessary characteristic of a

gear lubricant is its resistance to thermal degradation and chemical change. “Grease” is

used as the chief lubricant since it’s a nonconductor of heat thus achieving temperature

limitations.

Maintenance aids to good precision, longer life and higher efficiency. The

following precautions should be followed:

1. Proper handling is necessary to avoid the teeth damage in the gears.

2. Periodic lubrication is necessary to avoid film formation.

3. On any account there should not be any novelty in gear blanks

4. While machining, proper dead centre should be maintained.

5. The axis of the unit should be always paralleled to lathe bed.

DESIGN OF WARM AND WARM WHEEL

WORM AND WORM-WHEEL DIMENSIONS:

No. of teeths in worm wheel = 24 teeths = Z

Out side diameter = 55 mm

Start = Double start

1. WORM-WHEEL DIMENSIONS:

Outside diameter = d’ + 2ha

= m (Z+2)

55 = m (24 + 2)

m = 55/(24+2)

Modulus (m) = 2