INTRODUCTIONComparators are the instruments calibrated by means of end standards to measure unknown dimensions. The purpose of a comparator is to detect and display the small differences between the unknown linear dimensions and the length of the standard. The difference in lengths is detected as a displacement of a sensing probe. The important and essential function of the instruments is to magnify or amplify the small input displacement so that it is displayed on an analog scale. Comparators are classified on the basis of type of the amplification method used. Accordingly comparators are of following types or hybrid thereof.(a) Mechanical comparators, (b) Optical Comparators.(c) Pneumatic comparators, (d) Electrical comparators.

Essential Parts of a Comparator1. Fixed surface from which all measurements are taken

2. A very sensitive indicator which will show the movement of a sliding piece usually terminating in an anvil with curved surface

3. Some means of setting the curved face

Arrangement of above parts is used to measure the difference between the length or diameter of a component & a standard of length, usually made up of slip gauges

Desirable Features…1. Recording scale should be linear

2. Maximum rigidity

3. Maximum compensation of temperature effect

4. Neither lag nor backlash in the movement of the plunger & recording mechanism

5. Indicator should be constant in its return to zero

6. Measurement pressure should be low & constant

7. Instrument should be capable of a wide measuring range

8. Should be able to record variations of 0.0025mm.

Uses of Comparators…1. To inspect newly purchased gauges

2. In mass production, where components are to be checked at a very fast rate

3. As laboratory standards from which working or inspection gauges are set & correlated

4. As working, to prevent work spoilage & to maintain required tolerance at all important stages of manufacture

MECHANICAL COMPARATORSConventional mechanical methods to obtain magnification are not suitable in construction of mechanical comparators as it causes backlash and friction. Also they require a large input force. Let us understand the mechanical comparators by studying a reed comparator which is strictly a mechanical comparator. The diagram represents a REED type mechanical comparator.

REED COMPARATOR

In mechanical comparator, the gauging head is usually a sensitive, high quality, dial indicator mounted on a base supported by a sturdy column.

The reed mechanism is frictionless device for magnifying small motions of spindle.

It consists of a fixed block A which is rigidly fastened to the gauge head case.

A floating block B, which carries the gauging spindle and is connected horizontally to the fixed block by reedsC.

A vertical reed is attached to each block with upper ends joined together. These verticalreeds are shown in the figure by letter D.

Beyond this joint extends a pointer or target.

REED COMPARATOR

A linear motion of the spindle moves the free block vertically causing the vertical reed on the floating block to slide past the vertical reed on the fixed block. However, as these vertical reeds are joined at the upper end, instead of slipping, the movement causes both reeds swing through an arc and as the target is merely an extension of the vertical reeds, it swings through a much wider arc. The amount of target swing is proportional to the distance the floating block has moved but of course very much magnified. The scale may be calibrated by means of gauge block (slip gauges) to indicate any deviation from an initial setting.Comparators using this type of linkage have sensitivities of the order of 0.25 micron per scale division.The mechanical amplification is usually less than 100, but it is multiplied by the optical lens system.

REED COMPARATOR

SIGMA COMPARATOR

The vertical beam is mounted on flat steel springs A connected to fixed members, which in turn are screwed to a back plate.The assembly provides a frictionless movement with a restraint from the springs.The shank B at the base of the vertical beam is arranged to take a measuring contact, selecting from the available range.The stop C is provided to restrict movement at the lower extremity of the scale.Mounted on the fixed members, is the hinged assembly D carrying the forked arms E.

SIGMA COMPARATOR

This assembly incorporates a hardened fulcrum (provided with means for adjustment of controlling the ratio of trans-mitted motion) operative on the face of a jeweled insert on the flexible portion of the assembly.The metal ribbon, attached to the forked arms, passes around the spindle G causing it to rotate in specially designed miniature ball bearings. Damping action to the movement is affected by a metal disc, mounted on the spindle, rotating in a magnetic field between a permanent magnet and a steel plate.The indicating pointer H is secured to a boss on the disc.

SIGMA COMPARATOR

SIGMA COMPARATORThe instrument is available with vertical capacities of 150 mm, 300 mm and 600 mm and magnifications of 500,1000,1500,3000 and 5000. The scales are graduated in both English and Metric systems.The least count which one division represents is of the order of 0.25 microns.Advantage :

It has got a bold scale and larger indicating pointer.

Disadvantages : Due to motion of the parts, there is wear in the moving parts. It is not as sensitive as optical or other types of comparators due to friction beingpresent in the moving parts.

…