Mechanical Comparators

By :Bhavik Pandya

COMPARATORS

The general principle of comparator is to indicate the differences in size between the standard and the work being measured by 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

All comparators consist of three basic features

1)A sensing device• which faithfully senses the input signal

2) A Magnifying or amplifying system• to increase the signal to suitable magnitude.

Mechanical, Optical, Pneumatic,hydraulic and electronic methods are used for this purpose.

3) A display system (usually a scale and pointer) Which utilizes the amplified signal to provide a suitable

readout.

Need for a comparator A comparator is used in mass production to inspect the

components to close tolerances with high degree of precision and speed. Ex: Piston

• Use of line standards such as vernier and micrometer calipers require considerable skill

• Many dimensions can be checked in a very short timeComparators can be classified as1. Mechanical 2. Optical 3. Electrical &Electronic 4. Pneumatic  5. Fluid displacement comparators  6. Mechanical optical comparator 7. Electro-mechanical comparator 8. Multi-check comparator.

Design Consideration

1.Ability to record variation of at least 0.0025 mm2.Linear recording scale3.Variance free (Instrument readings should not vary

when repeated measurements of given quantity are made

4.Scale indicators clear and free from oscillations5.Free from backlash, unnecessary friction &clearances

inertia should be low.6.Measuring pressure should be low and constant7.Supporting frame to be rigid and withstand reasonable

ill usage without permanent damage8.Capable of measuring wide range.

Basic operation1. The comparator is first adjusted to zero on its dial

with a gauge block in position2. The work piece to be checked is then placed in

position 3. The comparator gives the difference in

dimension in relation to the gauge block.

Uses of comparator

1. Laboratory Standards: to set the working or inspection gauges2.Working GaugesTo prevent the work spoilage and to maintain required

tolerances3. Final Inspection Gauges4. Receiving Inspection Gauges5. For checking newly purchased gauges

Essential characteristics of a comparator1. Robust design and construction2. Linear characteristics of scale3. High magnification4. Quick in Results5. Versatility6. Minimum Wear of contact Point7. Free from Oscillations8. Free from backlash9. Quick insertion of work piece10. Adjustable table11. Compensation from temperature effects12. Means to prevent damage

Mechanical comparator

Types of Mechanical Comparators

1. Dial indicator (Dial Gauge)2. Read Type 3. Johansson Mikrokator4. Sigma Comparator

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 centre 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 frictionless 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 scaledivision.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 frictionless linear movement. A knife edge is mounted on it and bears upon the face of themoving 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 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 when proper scale is used