Sigma comparator - construction and working | The Mechanical post

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Definition of Sigma comparator

The sigma comparator is a mechanical comparator which is used to compare the dimensions of a workpiece with a standard or an another workpiece.

Construction of Sigma Comparator:

A Sigma Comparator consists of the following components:

  • Plunger
  • Blade Edge or Knife edge
  • Slit Diaphragm
  • Sapphire Bearing Block
  • Moving Block
  • Fixed Block
  • Cross Strip Hinge
  • Y-arm
  • Phosphor Bronze Driving Band
  • Driving Drum
  • Pointer
  • Scale


It is the detecting unit which detects or senses any irregularity of the surface. This segment of the comparator is straightforwardly in contact with the surface. The plunger is able to slide vertically (up & down) when pressure is applied at its tip.

Slit diaphragm

For the most part, a diaphragm is made of elastic type material (polymer), in Sigma comparator there are 2 cuts that the diaphragm is given to have frictionless movement of the plunger.

Blade Edge or Knife edge

It is a knife edged metallic strip, which is fitted with the plunger with the help of screws as shown in figure.

Sapphire Bearing Block

The bearing block is placed on the moving block as shown in the figure. The knife edge rests on the sapphire bearing block and is in continuous contact with it.

Moving Block

Moving square is a moving arm or part which is straight forwardly associated with blade edge part, and it is a base piece of Cross Strip.The moving block is capable of rotating or turning around its upper left hand corner.

Fixed Block

As the name suggests it is a fixed member attached to the cross strip.

Cross Strip Hinge

The cross strip hinge is the pivoted portion which helps the moving block to rotate.


It is a basic Y shaped arm intended to connect the driving drum via the phosphor bronze strip and the moving block as shown in the figure below.

Phosphor Bronze Driving Band

This arm is made of Phosphor Bronze alloy as it has desirable properties like corrosion resistance, spring qualities, resistance to fatigue etc.

Driving Drum

It is a small cylindrical shaped member which rotates clockwise as well as anticlockwise. The phosphor bronze strip movement drives the drum. The pointer is mounted upon the same spindle as that of the driving drum. So as the drum rotates the pointer also rotates.


A pointer is used to display the reading of the measurement.


The scale and the pointer display the output of the measurement. The scale is marked accordingly with proper units. The scale may be circular or linear.

design or chart of sigma comparator

Working of Sigma Comparator

The working of a Sigma Comparator is expressed underneath:

  • The plunger is fitted through the slit diaphragm to give a frictionless surface and allow vertical movement of the plunger.      
  • At the point when the plunger is pushed upward because of the irregularity in the surface of the component.              
  • The knife edge which is fitted with the plunger moves upward with it.   
  • Because of the upward movement of the knife edge, the moving block also rotates accordingly in the anticlockwise direction as it is pivoted. 
  • As the moving block moves the Y arm also rotates as it is connected to the moving block.                                                              
  • The phosphor bronze strip also moves with the movement of the Y arm. This movement of the strip rotates the driving drum which in turn causes the pointer to move. This gives the magnified reading on the scale.  
  • The main amplification of this instrument relies upon the Y-arm and the length of the Y-arm, and the subsequent amplification relies upon the proportion of the span of the pointer and the radius of the spindle.

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Sigma comparator is a kind of mechanical comparator invented by the Sigma Instrument Company, based in USA.

Uses of Sigma Comparator

Sigma Comparator is utilized for different purposes like:

  • It is utilized in selective assembly.
  • In large scale manufacturing, we can see this sort of instrument moreover.
  • In medium to large workshops for inspection purposes.

Advantages of Sigma Comparator

Here are a few advantages of Sigma Comparator:
  • It is easy to use.
  • Less expensive in price.
  • It doesn't need any external source of energy to operate.
  • It is portable.
  • It is powerful, minimal and simple to deal with the instrument.                                  

Disadvantages of Sigma Comparator:

Here are a few limitations or disadvantages of Sigma Comparator:

  • Complex design.
  • Because of numerous mechanical segments, there is wear and tear of components.
  • Parallax error can happen in this sort of instrument.
  • It is not capable of sensing small variations as the optical comparators can.

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