comparators

Contents by: CB.EN.P2MFG15006 Ilyas Hussain

introduction The general principle of the comparator is to indicate

the differences in size between the standard and the

work being measured by the 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.

Classification of comparators Mechanical comparators.

Mechanical-optical comparators.

Electrical and Electronic comparators.

Pneumatic comparators.

Fluid displacement comparators.

Projection comparators.

Multi-check comparators.

Automatic gauging machines.

characteristics Instrument must be of robust design and

construction. Readings should be obtained in least possible time. Provision must be made for temperature effect. Scale must be linear and must have straight line

characteristics. Indicator should be constant in its return to zero. Instrument must withstand a reasonable ill usage

without permanent harm.

uses Where components are to be checked at a very fast

rate.

Newly purchased gauges can be inspected.

Attached with some machines, to maintain required

tolerance at all stages of manufacturing.

Used in selective assembly.

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.

Advantages

Accurate dimensioning of simple parts.

Easy construction.

It is economic.

Do not require external supply of energy.

Portable and easily issued from store.

Disadvantages of the instrument

It is too much dependent upon the temperature as the

fluid changes its volume with change in temperature.

The deflection of the diaphragm per unit measuring force

is not constant.

There is certain passivity in the instrument due to the

characteristics of diaphragm and the viscosity of the fluid.

Projection comparator Fundamentally projection comparator (projector) is a

measuring instrument which projects an enlarged

shadow of the part being measured on a screen,

where it is compared to a master drawing.

In any projection system, there are four essential

elements, source of light, collimating or condensing

lens, projection lens and screen.

The illumination required for any projector is a

parallel beam of light, which can be achieved if the

source of light is a point source and is placed at the

principal focus of the collimating lens.

The purpose of collimating lens is to render the beam

of light from point source parallel.

By this device, complicated shaped parts can be

easily checked.

advantages It has small number of moving parts and hence a

higher accuracy.

In the optical comparators, the scale can be made to

move past a datum line and thus have high range and

no parallax errors.

It has very high magnification.

Optical lever is weightless.

disadvantages As the instrument has high magnification, heat from

the lamp, transformer etc. may cause the setting to drift.

An electrical supply is necessary. The apparatus is usually large and expensive. When the scale is projected on a screen, then it is

essential to use the instrument to a dark room in order to take the readings easily.

The instruments in which the scale is viewed through the eyepiece of a microscope are not convenient for continuous use.

Multi check comparators Now-a-days modern trend in comparison is towards

the inspection of all dimensions at the same time, as it is economical procedure.

It is particularly desired where various dimensions have some relationship with each other, e.g., diameter and concentricity measurement.

This job is done by multi check comparators, which are of the following types and incorporate the following systems of amplification.

(a) Electrical (b) Air (c) Combination ‘air-electric’.

A. Electric Multi checks: This is combination gauge incorporating about

twenty or more electrical-check heads to measure simultaneously a number of dimensions of one part.

For each dimension, there is one measuring head and signal lights for each dimension indicate whether it is within tolerance, undersize or oversize.

The instrument is used for high production checking and chances of any error in any dimension are less.

As it will not be desirable to see all the lights for

various dimensions, a master signal light is used to

integrate all the individual lights, and inspector has to

watch only one light.

On seeing the master light warning individual signal

lights are examined.

This instrument is well suited for all sizes of works.

B. Air Multi checks: In this instrument, group of air comparators are set

up to check a number of dimensions. In this type of instrument, initially some difficulty is experienced in setting the various comparators to check various dimensions and to arrange them in compact form.

In one setting all the dimensions can be checked and the time of inspection is reduced considerably.

Mass inspection becomes possible and high quality can be ensured.

C. An air Electric Multi check:  This makes use of both types of comparators.

It is very easy to use air comparator for checking diameter and concentricity, so for this purpose air comparator is used, while for other measurements electric-checks are used.

Automatic gauging machines

Automatic inspection machines are very economical for final inspection in high production manufacturing facilities.

Arrangements for automatic transport of parts from one measuring station to other by electro-mechanical, hydraulic, or pneumatic mechanisms are provided.

Part transportation system is designed for efficient and safe movement through machine.

Information received from the measuring station(s) is processed by the gauging amplifier and compared to a desired and predetermined standard.

Good or bad components are identified automatically and bad components are discharged in a reject chute for collection of scrap parts.

Automatic gauging machines are used because these provide output information that can be easily interfaced with statistical analyzers, computers, CRTs and printers.

The following points should be considered while designing automatic gauging systems:

Gauging probes should be designed to be actuated by an auxiliary probe, rather than the gauging probe connecting the part directly.

This way the service life of gauging probe is enhanced considerably because now only auxiliary probe rams into slides against the parts.

The probes should be made retractable type to prevent possible wear or damage to the probes. The probe heads are thus held back till the part is in gauging position.

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