Subject:- Metrology and Quality Engineering

Topic:- Comparators

Difficulty:- High

Mechanical Comparators:-

Working principle of a Mechanical Comparator :- A mechanical comparator employees mechanical means for magnifying the small movement of the stylus the magnification of the small stylus movement is obtained by means of levers, gear trains, rack and pinion or a combination The usual magnification obtained by these comparators ranges from about 250 to 1000

*Types of Mechanical Comparators

1. Dial indicator (Dial Gauge)
2. Read Type
3. Johansson Mikrokator
4. Sigma Comparator

1. Dial indicator/ Dial Gauge

The simplest type of mechanical comparator. It consists of a base with a rigid column rising from its rear. An arm mounted on this column and it carries a dial gauge at its outer end. The indicator is set at zero by the use of slip gauges. The part to be checked is placed below the plunger. The linear movement of the plunger is magnified by means of mechanical means to a sizable rotation of the pointer.This type is generally used for inspection of small precision machined parts The dial gauge with various attachments are commonly used This comparator is ideal for the checking of components with a tolerance of + 0.005 mm

Johansson Mikrokator:-

Working of the comparator:-
A very thin metal strip at the center carries a light pointer made up of glass. One end of the strip is connected to the adjustable cantilever strip and the other end is to the spring elbow, in turn connected to the plunger. The slight movements of the plunger will make the bell crank lever to rotate. This rotation will create tension in the strip and causes the strip to rotate thereby the strip start to untwist resulting in the movement of the point. The spring ensures that the plunger returns when the contact is removed The length of the cantilever can be varied to adjust the magnification The mechanical magnification is obtained by “Abramson Movement”

Reed Type 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 friction less device for magnifying small motions of spindle. It consists of a fixed block A which is rigidly fastened to the gauge head case, and floating block B, which carries the gauging spindle and is connected horizontally to the fixed block by reeds C Vertical reed is attached to each block with upper ends joined together. These vertical reeds are shown in the figure by letter D. Beyond this joint extends a pointer or target. 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. How- ever, 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. 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. It is available in amplifications ranging from x 500 to x 1000.

Sigma Comparator:- The plunger is mounted on a pair of slit diaphragms in order to have friction less 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.

Advantages of mechanical comparators

1. Cheaper than all the other type of comparators 2.Does not require any external source of power or air supply
2. These comparators use a linear scale that can be easily understood.
3. Usually these comparators are robust and compact but are very easy to handle
4. 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 when proper scale is used