STANDARDISATION AND INTERCHAGEABILITY copy

Component selected randomly should assemble

correctly with any other mating component.

This is interchangeability. Possible when certain

standards are strictly followed‐‐‐international

standards

Reqd fit in an assembly can be obtained by:

• Universal or full interchangeability

• Selective assembly

Full interchangeability :‐

Any component will match with any other mating part

without classifying manufactured components into

subgroups or carrying out minor alterations.

Selective assembly:

In this parts are graded according to size and only matched

grades of mating parts are assembled e.g.

Mating of piston in car cylinder – The bore size is 63.5mm

and the skirt clearance of piston is 0.13mm on the

diameter, The tolerance on bore and on piston skirt each is

0.04mm.

HL of bore – 63.5+0.02 = 63.52mm

LL of bore ‐ 63.5‐0.02 = 63.48mm

Piston bore is 63.5‐ 0.13 = 63.37 mm

HL of piston – 63.37+0.02=63.39

LL of piston ‐ 63.37 – 0.02=63.35

Max clearance = HL of bore – LL of piston =

63.52‐63.35=0.17mm

Min clearance= LL of bore‐ HL of piston =

63.48‐ 63.39=0.09mm

By grading bore and piston, selective assembly will give :

Cylinder bore 63.48 63.5 63.52

Piston 63.35 63.37 63.39

Comparators:

The general principle of comparator is to indicate the

differences in size between the standard and the work

being measured by means of some pointer on a scale

with sufficient magnification.

It thus does not measure the actual dimension but

indicates how much it differs from the basic dimension

All comparators consist of three basic features

1)A sensing device

• which faithfully senses the input signal

2) A Magnifying or amplifying system

• to increase the signal to suitable magnitude.

Mechanical, Optical, Pneumatic,hydraulic and electronic

methods are used for this purpose.

3) A display system (usually a scale and pointer)

Which utilizes the amplified signal to provide a suitable

readout.

Comparators can be classified as

1. Mechanical

2. Optical

3. Electrical &Electronic

4. Pneumatic

5. Fluid displacement comparators

6. Mechanical optical comparator

7. Electro-mechanical comparator

8. Multi-check comparator.

Mechanical Comparator:

• In Mechanical type, the required magnification is obtained by

using mechanical linkages, levers, gears and other

mechanical devices.

• Magnification (M): It is the ratio between the movement of

the plunger and the resultant movement of the pointer

M= P(pointer movement)/p Plunger movement

SIGMA COMPARATOR( Mechanical Comparator):

It has magnifications in the range of 300 to 5000.

It has a plunger attached to a rectangular bar which is

supported at its upper and lower ends by flexure plates.

A knife edge is fixed to the side of rectangular bar which

bears on a moving block.

The moving block and the fixed block are connected

by flexible strips at right angles to each other.

Scale

Drum

Bronze band

Forked arm

Plunger

Flexure plate

Fixed block

Flexible strips

Flexure plate

Pointer

Y X

Knife edge

Moving block

If an external force is applied to the moving block, it would pivot

about the hinge .

A forked arm or Y-arm attached to the moving block transmits the

rotary motion to the indicator driving drum through a bronze band

wrapped around the drum.

Magnification: If Y is the length of forked arm and X is the distance

from the knife edge to hinge, then first stage magnification is Y/X

If the pointer length is R and the radius of the drum is r the second

stage magnification is R/r such that the total magnification is

(Y/X)* (R/r)

Advantages of Mechanical Comparators

• They are cheaper compared to other amplifying

devices

• Do not require electricity or air and such the

variations in the outside sources do not affect the

accuracy.

• They have a linear scale, robust and easy to handle.

Disadvantages of Mechanical Comparators

• They have more linkages due to which friction is more and

accuracy is less.

• Mechanisms used have more inertia and hence they become

sensitive to vibrations.

• The range of the instruments is limited as the pointer moves

over a fixed scale.

• It is also difficult to incorporate arrangement for adjusting

magnification.

O

d

C

A

ScreenLamp

Pivot

Mirror

Plunger

NormalO

C

A

Screen

Lamp

Mirror

Normal

d

2

h angleof tilt

PRINCIPLE OF OPTICAL LEVER

OPTICAL COMPARATORS

OPTICAL COMPARATORS

• An optical comparator works on one of the following two main principles:

1)Use of optical lever 2) Use of enlarged image

– If a beam of light AC is directed on to a mirror as shown in fig, it will

be reflected onto the screen at O as a dot.

The angle of incidence = angle of reflection

– When the plunger moves up, the mirror tilts by . Then the reflected

beam moves through 2 . i.e. the illuminated dot moves to B.

– Also the increase in distance CO of the screen from the tilting mirror

will increase the magnification.

In this comparator, small displacements of the

measuring plunger are first amplified by a mechanical

lever. It is equal to (L2/L1).

The amplified mechanical movement is further

amplified by optical system due to tilting of mirror by .

The reflected ray D (image of index line) will be turned

through an angle 2, and hence optical amplification

=2(L4/L3)

The overall magnification = (L2/L1)* 2(L4/L3)

L1 L2 L3

L4

PivotMeasuringPlunger

Mecahnicallever

Screen with scale

Light source

Mirror pivot

Mirror

Projection lens

Glass plate carrying indexline

Collimating lens

D

Mechanical-Optical Comparator

Advantages & Disadvantages of Optical comparators

• Advantages:

1) Few moving linkages and hence no friction & wear.

2) High range of measurements and no parallax error

3) Magnification is usually high.

• Disadvantages:

1) Heat from the source of light, transformers etc., may cause the setting to drift.

2) An electric supply is required to operate these comparators.

3) The size is large and costly.

4) Use of eyepiece to view is inconvenient for prolonged use. Also a dark room is essential to take readings.

Electrical& Electronic Comparators

R1 R2

R3R4

Battery

Coils

Arm

Measuring

PlungerCoils

IronArmature

Electrical& Electronic Comparators

• These comparators depend on the principle of balancing the

Wheatstone bridge,

(R1/R2) = (R3/R4) applicable for only to direct current

obtained from a battery.

• In actual instruments, one pair of inductances takes the form

of a pair of coils in the measuring head of the instrument.

• An iron armature inside these coils moves along with the

measuring plunger, and upsets the balance of the circuit and

causes the deflection of the pointer meter which is calibrated

directly in linear units.

• Magnifications of the order of 30,000 are possible.

Pneumatic Comparators (Solex Gauge):

Principle:

It works on the principle of pressure difference generated by the

air flow.

Air is supplied at constant pressure through the orifice and the air

escapes in the form of jets through a restricted space which

exerts a back pressure.

The variation in the back pressure is then used to find the

dimensions of a component.

Working:

As shown in Figure (a) the air is compressed in the compressor at

high pressure which is equal to Water head H.

The excess air escapes in the form of bubbles. Then the metric

amount of air is passed through the orifice at the constant

pressure.

Due to restricted area, at A1 position, the back pressure is

generated by the head of water displaced in the manometer

tube.

To determine the roundness of the job, the job is rotated along the

jet axis, if no variation in the pressure reading is obtained then we

can say that the job is perfectly circular at position A1.

Then the same procedure is repeated at various positions A2, A3,

A4, position and variation in the pressure reading is found out.

Also the diameter is measured at position A1 corresponding to the

portion against two jets and diameter is also measured at various

position along the length of the bore.

Fluid Displacement Comparators

In this comparator, a fine bore capillary tube is arranged so that its

lower end is placed in a chamber of relatively large cross-sectional

area and containing a fluid of low viscosity.

At the bottom of the chamber, a diaphragm is fitted, which gets

deflected by pressure transmitted to it by the measuring plunger.

Due to deflection of diaphragm, a small quantity of liquid rises up

in the capillary tube.

A scale is arranged at the side of tube and limit pointers are set

relative to this to indicate the high and low limits of the dimension

being measured. These limits are obtained by master setting.

Instrument body comprises of a cone shape with a thin base

containing a small button centrally. Any movement of the

base causes a displacement of liquid up the tube to indicate

actual dimension.

The magnification of this type of comparator is usually of the

order of 1500. Various disadvantages of the instrument are :

(1) It is too much dependent upon the temperature as the fluid

changes its volume with change in temperature.

(2) The deflection of the diaphragm per unit measuring force is

not constant.

(3) There is certain passivity in the instrument due to the

characteristics of diaphragm and the viscosity of the fluid.

Automatic Gauging Machines.

These machines incorporate comparator amplifying

methods and are similar to multicheck devices.

They eliminate manual inspection and parts are

inspected for all the dimensions simultaneously and

segregated and classified automatically.

Coordinate Measuring Machine (CMM) is a 3-dimensional measuring

device that uses a contact probe to detect the surface of the object.

The probe is generally a highly sensitive pressure sensing device that

is triggered by any contact with a surface.

The linear distances moved along the 3 axes are recorded, thus

providing the x, y and z coordinates of the point.

CMMs are classified as either vertical or horizontal, according to the

orientation of the probe with respect to the measuring table.

Advantages and Disadvantages of CMM

Advantages:

1) High precision and accuracy.

2) Requires less labor.

3) Accurate dimensions can be obtained just by knowing

the coordinates and distance between the two reference

points.

Disadvantages:

1) The Coordinate measuring machines are very costly.

2) The CMMs are less portable.

3) If the operating software cracks down it is difficult to

restart the entire system.

1) It needs to construct some feature on its own as some

parts of the work piece are unreachable by the probe.