Comparators

Definition: “A comparator is a precision instrument employed to

compare the dimension of a given component with a working

standard( usually slip gauges)”.

It works on relative measurement i.e. it does not measure the

actual dimension but indicates how much it differs form the basic

dimension.

It has a sensing device, magnifying or amplifying systems and

or display system ( usually scale & pointer) to provide suitable

read out.

Eliminates human element in taking measurement & gives

accurate result consistently.

Basic Principle of Operation of Comparator:

The comparator is first adjusted to zero on its dial or recording

device with a gauge block in position. The dimensions of gauge

block and work piece are same.

Then gauge block is replaced by work piece to be checked.

The comparator gives the dimensional difference using

suitable magnification.

Uses of Comparators:

1. Mass production: where components are to be checked at a

very fast rate.

2. Laboratory standards: from which working or inspection

gauges are set and correlated.

3.Receiving inspection gauges: Checking parts received from

outside sources.

4.Working gauges: Attached with some machines, comparators

can be used as to prevent work spoilage.

5. In selective assembly of parts: where parts are graded in three

or more groups depending upon their tolerances.

Characteristics of Good Comparators:

1. Robust design and construction

2. High degree of accuracy and precision

3. The scale should be linear and must have straight line characteristic

4. High amplification and good resolution

5. Versatility

6. Compensation for temperature effects

7. Minimum wear of contact point

8. Quick in results

9. Quick insertion of work piece

10. Free from oscillation

Classification of Comparators:According to the method used for amplifying & recording the

variations

1. Mechanical Comparator: It works by gears pinions, linkages,

levers, springs etc.

2. Pneumatic Comparator: Pneumatic comparator works by using

high pressure air, valves, back pressure etc.

3. Optical Comparator: Optical comparator works by using lens,

mirrors, light source etc.

4. Electrical Comparator: Works by using step up, step down

transformers.

5. Electronic Comparator: It works by using amplifier, digital

signal etc.

6. Combined Comparator: The combination of any two of the

above types can give the best result.

Mechanical Comparators:

1. Dial Indicator

2. The Johansson ‘Mikrokator’

3. Reed Type Mechanical Comparator

4. Sigma comparator

In these comparators, magnification is obtained by mechanical

linkages and other mechanical devices. The usual magnification

ranges from about X250 to X1000.

Systems of Displacement Amplification used in

Mechanical Comparators:

Rack and Pinion

Cam and gear train

Lever with toothed sector

Twisted Strip

Lever combined with band wound around drum

Tilting mirror projecting light spots

1.Dial Indicator:

Dial Gauge Set up:

Working of Dial Gauge:

Measurement of Dimensional Difference:

Dimensional Difference:

2. The Johansson ‘Mikrokator’

Principle: It works on the principle of a Button spinning on a loop

of string like in the case of Children’s toys.

Construction & Working:

Magnification up to 5000X can be obtained by this comparator

Magnification:

Magnification of the instrument dQ / dL α L/ w2 n

Where,

Q = twist of mid point of strip w.r.t. the end

L = length of twisted strip measured along its neutral axis

w = width of twisted strip

n = no of turns

For high magnification thin rectangular strip must be used.

w

L

Components:1. Measuring plunger2. Slit diaphragm3. Knife edge4. Cross strip hinge5. Light metal Y- forked arms6. Thin phosphor bronze ribbon7. Driving drum8. Pointer and scaleFig. Cross strip liver

Magnification:First stage magnification: The ratio of effective length (L) of arm and

the distance (a) of the knife edge from the pivot i.e. L/a

Second stage magnification: The ratio of the pointer length (l) and

the radius (r) of the driving drum i.e. l/r

Total magnification: ( L/a x l/r)

Sigma Comparator:Advantages:1. Safe operation2. Dead beat readings are obtained by mounting a non-

ferrous disc on the pointer spindle and making it move in the field of a permanent magnet.

3. Avoid parallax error4. Constant pressure over the range of the instrument5. Fine adjustments are possible6. Provides magnification of X 300 to X 5000 and sensitivity

0.0001 mmDisadvantages:1. Wear due to motion of parts2. Not sensible as optical comparator due to friction of moving

parts

Graticule

Mechanical –Optical Comparators:Principle: In mechanical optical comparator, small variation in the plunger

movement is magnified: first by mechanical system and then by optical system.

Magnification:

Mechanical amplification: L2/L1 by lever principle

Optical amplification: If the movement of the plunger causes

the mirror to tilt by an angle α, then the image will be tilted by

2α

Therefore optical amplification = 2 x L4/L3

Total magnification: (2 L2/L1 x L4/L3)

Advantages:

1. Weightless and have less number of moving parts, due to this

there is less wear and hence less friction.

2. Higher range even at high magnification is possible.

3. No parallax errors.

4. They are used to magnify parts of very small size and of

complex configuration such as intricate grooves, radii.

Disadvantages:

1. The accuracy of measurement is limited to 0.001 mm

2. Electrical supply is required.

3. Eyepiece type instrument may cause strain on the operator.

4. Occupy large space and they are expensive.

5. Dark room is required as the scale is projected on a screen.

Mechanical –Optical Comparators:

x

Magnification:

x- distance from the plunger centre line to the first mirror pivot

f- focal length

h- plunger moves a height

Then, the angular movement of the mirror δθ = h/x

Movement of the scale = 2f.δθ = 2f h/x

Therefore, magnification = 2fh/xh = 2f/x

Overall magnification: (2f/x) x (Eyepiece magnification)

Zeiss Ultra Optimeter:

Electrical Comparator:

Pneumatic Comparators:Working 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.

Solex Pneumatic Gauges:

Angular Measurement

Sine Bars:

Sine bar is a precision measuring instrument used along with

slip gauges for the measurement of angles.

It consists of steel bar and rollers.

Sine bar is made up of high carbon, high chromium corrosion

resistant steel, suitably hardened, precision ground and stabilized.

It should be noted here that the holes are drilled in the body of

sine bar to

(i) make it lighter

(ii) facilitate handling

Most common types of Sine bar:

Characteristics of Sine Bar:1. The axes of the rollers must be parallel to each other and the

center distance L must be known. The size of the sine bar is

specified by this distance.

2. The top surface of the bar must have a high degree of flatness.

It should be parallel to the plane connecting the axes of rollers.

3. The rollers must have identical diameters and round within a

close tolerance,

Sine Bar is used:1. To measure the angle very accurately

2. To locate the work piece to a given angle within a very close

limits

Principle of Working:It works on trigonometric principle. The angle to be measured (or

to be set) is determined as a function of sine for this reason the

device is called a sin bar

Use of Sine Bar.(1) Measuring known angles or locating any work to a given angle.

(2) Checking or Measuring unknown angles.

If ‘dx’ is the variation in parallelism over a distance ‘x’

Then, the corresponding variation in angle,

Approximate or Nominal angle =

(3) Checking of unknown angles of heavy component.

Advantages of sine bar:

1. It is used for accurate and precise angular measurement.

2. It is available easily.

3. It is cheap.

Disadvantages:

1. The application is limited for a fixed center distance between

two plugs or rollers.

2. It is difficult to handle and position the slip gauges.

3. If the angle exceeds 45°, sine bars are impracticable and

inaccurate.

4. Large angular error may results due to slight error in sine bar.

Sine Centers:

Conical

Auto-collimator

• Auto-collimator is an optical instrument used for the

measurement of small angular differences.

• For small angular measurements it provides a very

sensitive and accurate approach.

• It is essentially an infinity telescope and acollimator

combined into one instrument.

Applications:-

1. Checking of an internal right angle.

2. Comparative measurement using master

angles.

3. Measuring straightness and flatness of the

surfaces.

4. Assessment of squareness and parallelism of

component.

5. For machine tool adjustment setting.

• Angle Dekkor:-

• This is also a type of auto-collimator.

• There is an illuminated scale in the focal plane of the collimating lens.

• This illuminated scale is projected as a parallel beam by the

collimating lens which after striking a reflector below the instrument

is refocused by the lens in the filed of view of the eyepiece.

• In the field of view of microscope, there is another datum scale fixed

across the center of screen.

• The reflected image of the illuminated scale is received at right angle

to the fixed scale.

• Thus the changes in angular position of the reflector in two planes are

indicated by changes in the point of intersection of the two scales.

One division on the scale is calibrated to read 1 minute.

Applications:

• Measuring the angle of a component.

• Checking the sloping angle of a vee-block.

• Measuring the angle of a cone or taper

gauges.

• Precise angular settings for machining

operations.

Slip Gauges:Slip gauges or gauge blocks are universally accepted end

standard of length in industry.

These were introduced by Johnson, a Swedish engineer, and

are also called as Johnson Gauges

Slip gauges are rectangular blocks of high grade steel with

exceptionally close tolerances. These blocks are suitably

hardened through out to ensure maximum

resistance to wear.

They are then stabilized by heating and cooling successively

in stages so that hardening

stresses are removed, After being hardened they are

carefully finished by high grade

Lapping to a high degree of finish, flatness and accuracy.

For successful use of slip gauges their workingfaces are made truly flat and parallel. Slip gaugesare also made from tungsten carbide which isextremely hard and wear resistance.

The cross sections of these gauges are 9 mm x 30mm for sizes up to 10 mm and 9 mm x 35 mm forlarger sizes. Any two slips when perfectly cleanmay be wrung together. The

dimensions are permanently marked on one ofthe measuring faces of gauge blocks

Uses of Slip Gauges:1. Direct precise measurement, where the accuracy of the

work piece demands it. 2. For checking accuracy of vernier calipers, micrometers, and

such other measuring instruments. 3. Setting up a comparator to a specific dimension. 4. For measuring angle of work piece and also for angular

setting in conjunction with a sine bar. 5. The distances of plugs, spigots, etc. on fixture are often best

measured with the slip gauges or end bars for large dimensions.

6. To check gap between parallel locations such as in gap gauges or between two mating parts