Sigma comparator - construction, working and uses with PDF

The Mechanical post welcomes you back! Today let's check out the construction and working of the sigma comparator, its advantages, disadvantages, and applications starting off by defining sigma comparator.

You can download the PDF on sigma comparator by clicking on the button at the end.

Sigma comparator - construction and working | The Mechanical post

Definition of Sigma comparator

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


Construction of Sigma Comparator:

A Sigma Comparator consists of the following components: (refer diagram given below)

  • 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 that 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 the 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 straightforwardly associated with the 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 that 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 that 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.

diagram of sigma comparator
Diagram 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.

You might also like to read

Magnification of sigma comparator

There are 2 ways in which magnification can be obtained in the sigma comparator.

  • The main magnification is calculated as the effective length of the Y arm (L) to the distance between the hinge pivot and the knife edge (x). 
i.e. Magnification = L / x
  • The second stage of magnification is achieved by the length of the pointer (R) to the radius of the driving drum (r).
 i.e. Magnification = R / r

Uses of Sigma Comparator

Sigma Comparator is utilized for different purposes like:

  • It is utilized in the selective assembly.
  • In large scale manufacturing, we can see this sort of instrument moreover.
  • In the 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.


From which category sigma comparator belongs?

The sigma comparator belongs to the category of "mechanical comparator".

What is sigma comparator used for?

Sigma comparator is used to compare the flatness or roughness of a standard component with the specimen.


If you like this post or have any suggestions do let us know in the comments we would love to hear from you. Do follow The Mechanical post on Instagram, Facebook, and Twitter. 

Also, share this post with friends and family with the social links provided below. 

Till then keep learning!


Post a Comment

Post a Comment (0)

Previous Post Next Post