Sigma Comparator – Parts, Working, Advantages and Disadvantages

Comparator is a device that is used to measure the roughness of the surface. It is a precision measuring instrument in the metrological field.
Sigma comparator was developed by a company named Sigma Instrument Company, USA. It is mechanical comparator in which magnification ranges from 300 to 5000.
This comparator does not require any power supply.

Sigma Comparator
Sigma Comparator

Parts of Sigma Comparator:-

1 Plunger:-
It is the sensing unit of the sigma comparator. This part of the comparator is in contact with the surface and senses any unevenness in the surface that is analyzed. The reading of this plunger is magnified using the linking inside the comparator.

2 Knife Edge:-
The plunger has a groove to which knife edge is connected. The knife edge is in point contact with the groove of the plunger. This knife is attached to the plunger with the help of screws.
The corners of knife are sharp.

3. Adjustable Screw:-
Adjustable screws are used to adjust the height or keeping the contact of the knife edge with the movable block.

4. Bearing Block:-
The bearing block is directly in contact with the moving block and knife edge. This block will bear complete external force.

5. Movable Block:-
Moving block is a part of the sigma comparator which is directly in contact with the knife-edge. It is present at the bottom part of the cross strip.

6. Fixed Block:-
Fixed block is present at the upper part of the cross strip and cannot move i.e it is stationary.

7. Cross Strip:-
Cross Strip is a pivoted portion of the fixed block and moving block. This strip is made up of elastic material so that when the force is applied this strip will get deformed and when the forces are removed it will return back to its original position.

8. Y-arm:-
Y-arm is a Y shaped armed which is connected to the movable block. This Y-arm is in contact with driving drum or driving pulley. This Y-arm is made of Phosphorus Bronze alloy.
The length of this Y-arm is generally denoted by L.

9. Driving Drum:-
The driving drum is used to connect the Y-arm to the pointer with the help of pointer spindle.

10. Pointer Spindle:-
Pointer spindle is used to connect the pointer to the driving drum.

11. Pointer:-
A pointer is used for pointing the measured value on the scale.

12. Scale:-
The scale of the sigma comparator can be linear or circular in shape and contains the reading of the comparator. The pointer points at the measured reading on this scale.

Working of Sigma Comparator:-

In the sigma comparator, the reading of the plunger is magnified through the linkage.
The plunger is mounted on the pair of slit diaphragm which gives it a frictionless linear motion.
The plunger reads a slight change in the evenness of the surface that is being read using sigma comparator. Due to unevenness of the surface, the plunger moves upward or downward.
This up or down movement of the plunger is magnified so much through a linkage that we can easily measure or read the movement of the plunger.
When the plunger moves upward or downward due to unevenness of the surface being examined, the knife-edge either moves upward or downward.
The moving block is connected to the knife-edge using a Saphire bearing block.
As the knife-edge move upward or downward, the moving block also moves accordingly as it is hinged with the knife edge.
Due to upward or downward movement of the moving block, the cross-strip rotates and this rotation of cross-strip is transferred to the driving drum with the help of Y-arm which is ditectly connected to the cross-strip.
The driving drum which receives the rotation of the cross-strip has a spindle that connects the driving drum to the pointer.
Now the rotation of the driving drum is transferred to the pointer with the help of spindle and pointer moves and points at the measured value on the scale.
If the length of Y-arm is l and the distance from hing pivot to the knife-edge is a. Then the first stage of magnification is l/a.

If the radius of driving drum is and R is the total length of the pointer. Then second stage of magnification is R/r.

Total Magnification = Stage 1 Magnification x Stage 2 maginfication

i.e M = l/a x R/r

Advantages of Sigma Comparator:-

1) It is cheaper in price as compared to other comparators.
2) It doesn’t require any external energy source to work.
3) It has bold scale and large pointer, hence the measured value can be noted easily.
4) It is small in size and easily portable.
5) It is very easy to operate.

Disadvantages of Sigma Comparator:-

1) Friction developed is more in it which affects accuracy.
2) As there are many mechanical components, chance of wear and tear is high.
3) The chance of parallax error is high in this instrument.
4) Sigma comparator also develops vibration.

Applications of Sigma Comparator:-

1 Sigma comparator is used in selective assembly.
2 It can also be used in mass production.

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