Comparator 431q17

COMPARATORS A comparator is an indirect type of precision instrument employed to compare the dimension of a given component with a working standard (Usually Slip gauges). In general the comparators compares the objects with reference to a standard master piece. Comparators finds their application is all type of production works as these require very less skill and reduces the time of inspection as it requires just a comparison of the values or the dimensions of the product produced. Types: 1) Mechanical comparator 2) Mechanical-optical comparator 3) Electrical and electronic comparator 4) Pneumatic comparator

CHARACTERISTICS OF COMPARATORS 1) A comparator should posses a robust design and construction so as it should give an accurate value even at the worst possible conditions at available at the level of ordinary usage. 2) When a magnification system is used for obtaining deadbeat readings care should be taken to eliminate the backlash, oscillations, wear resistance in the mechanical components and make the inertia gets reduced to the minimum possible extent. 3) A large range of temperature should be taken in to consideration. The comparators should sustain all the temperatures. 4) A scale with linear and having a straight line characteristic should be employed 5) The indicator should be constant when it retains its position back to zero 6) High magnification to detect smallest deviation in size. 7) The comparator should be prepared in such a way that it can be employed for a wide range of applications . 8) The measuring pressure should be always constant and should be low.

USES OF COMPARATORS Comparators find their usage in many areas of production irrespective of the job being produced. 1) They are used as Working Gauges in mass production where the components are required to be checked at a faster rate. 2) These are also used as laboratory standards and are used for making the working or inspection gauges correlated and set. 3) Comparators are used for inspecting newly purchased gauges 4) These can also be used as some special attachments to the production machines such that the work being done can be regularly checked so as to reduce the wastage of the work due to errors 5) These finds usage in assembly sections where more than three parts are to be assembled

MECHANICAL COMPARATORS In these comparators, magnification is obtained by mechanical linkages and other mechanical devices (lever, gear trains, rack and pinion or a combination).

Lever with Toothed gear. Rack and pinion. In it the measuring spindle integral with a rack, engages a pinion which amplifies the movement of plunger through a gear train.

Cam and gear train. The measuring spindle acts on a cam which transmits the motion to the amplifying gear train.

A lever with a toothed sector at its end engages a pinion in the hub of a crown gear sector which further meshes with a final pinion to produce indication.

Compound levers. Levers forming a couple with compound action are connected through segments and pinion to produce final pointer movement.

Twisted taut strip. The movement of measuring spindle tilts the knee causing straining which further causes the twisted taut band to rotate proportionally. The motion of strip is displayed by the attached pointer.

DIAL INDICATOR. One of the most commonly used mechanical comparators is essentially of the same type as a dial indicator. It consists of a robust base whose surface is perfectly flat and a pillar carrying a bracket in which is incorporated a spindle and indicator. The linear movement of the spindle is magnified by means of a gear and pinion train into sizable rotation of the pointer on the dial scale. The indicator is set to zero by the use of slip gauges representing the basic size of the part. This is generally used for inspection of small precision-machined parts. This type of comparator can be used with various attachments so that it may be suitable for large number of works. With a V-block attachment it can be used for checking out-ofroundness of a cylindrical component.

Johansson Mikrokator.

Disadvantages

Sigma Comparator The plunger is mounted on a pair of slit diaphragms (Flat steel strings) in order to have frictionless linear movement. A knife edge is mounted on it and bears upon the face of the moving member of a cross strip hinge. The cross strip hinge consists of the moving component and a fixed member which are connected by thin flexible strips alternately at right angle to each other. Thus if an external force is applied to the moving member ; it will pivot, as would a hinge, about the line of intersection of the strips. To the moving member an arm of Y shape and having effective length “I” is attached. If the distance of the hinge from the knife edge be “a” L then the magnification of the first stage will be a and the ratio of pointer length “l” and the radius “r” of the driving drum gives the second stage Magnification. So, the total magnification of the instrument is L x l . In order to adjust the magnification, distance “a” a r must be changed by slackening and tightening the two screws attaching the knife edge to the plunger.

Diagrammatic sketch showing the movement of Sigma Comparator

Some of the interesting features of the instrument are : (1)Safety: As the knife edge moves away from the moving member of the hinge and is followed by it, therefore, if too robust movement of the plunger is made due to shock load, that will not be transmitted through the movement. (2) Dead Beat Reading: By mounting a non-ferrous disc on the pointer spindle and making it move in field of a permanent magnet, dead beat readings can be obtained. (3) Parallax: The error due to parallax is avoided by having a reflective strip on the scale. (4) The constant measuring pressure over the range of the instrument is obtained by the use of a magnet plunger on the frame and a keeper bar on the top of the plunger. As the plunger is raised the force required increases but the keeper bar approaches the magnet and the magnetic attraction between the two increases. Thus as the deflecting force increases, the assistance by the magnet increases and total force remains constant. Disadvantages: 1. Due to motion of the parts there is a wear in the moving parts. 2. It is not sensible as optical comparator due to friction of the moving parts.

Mechanical comparators Advantages 1) Cheaper than all the other type of comparators 2) Does not require any external source of power or air supply 3) These comparators use a linear scale that can be easily understood. 4) Usually these comparators are robust and compact but are very easy to handle 5) These are small in size and can are portable from one place to other very easily without much difficulty Disadvantages 1) Contains more number of moving parts so there develops friction which in turn reduces the accuracy. 2) Slackness in the moving parts reduces accuracy very drastically. 3) These have more inertia so the instrument is prone to vibrational effects 4) Limited range of the instrument is another drawback as the pointer moves over a fixed scale 5) Parallax error may also arise as the pointer moves over a fixed scale.

OPTICAL COMPARATORS

Advantages 1) High accuracy: Less number of moving parts hence more accuracy 2) No parallax error: High range and no parallax error 3) High magnification possible 4) Weight less optical lever

Disadvantages 1) Due to high magnification the heat produced from the lamp, transformers etc may cause drift. 2) Electrical supply is necessary 3) Large in size and expensive 4) Dark room is required to take the readings 5) These cannot be used continuously as the scale is viewed through an eye piece.

PNEUMATIC COMPARATORS

Principle of Working:

Advantages: 1) No wear 2) High accuracy 3) Less friction and less inertia 4) Indicating instrument can be remote from the measuring instrument 5) High magnification is possible (30,000:1) 6) Very small diameter holes can be easily measured even when the length is very large 7) Best instrument for determining the ovality and taperness of the circular bores Disadvantages: 1) many instruments are used in addition to the normal set up 2) scale is not uniform 3) when the indicating device is kept in a glass tube a high level of magnification is required so as to minimize meniscus errors 4) machine is not easily portable as it has large equipment 5) different gauging heads are required