The name ‘vernier’as applied to several precision tools is derived from the name of a French Scientist and mathematician, ‘Pierre vernier’.
A vernier is a graduated short scale that is mounted on the measuring instrument that its graduations subdivided the divisions on the main scale of the instrument.
Avernier calliper is used to make both inside and outside measurement with the help of its specially designed jaws.
Parts of a vernier caliper:
1. Outside jaws: used to measure external diameter or width of an object.
2. Inside jaws: used to measure internal diameter of an object.
3. Depth probe: used to measure depths of an object or a hole.
4. Main scale: scale marked every mm.
5. Main scale: scale marked in inches and fractions.
6. Vernier scale: gives interpolated measurements to 0.1 mm or better.
7. Vernier scale: gives interpolated measurements in fractions of an inch.
8. Retainer: used to block movable part to allow the easy transferring of a measurement.
The vernier, dial, and digital calipers give a direct reading of the distance measured to high accuracy. They are functionally identical, with different ways of reading the result. These calipers comprise a calibrated scale with a fixed jaw, and another jaw, with a pointer, that slides along the scale. The distance between the jaws is then read in different ways for the three types.
The simplest method is to read the position of the pointer directly on the scale. When the pointer is between two markings, the user can mentally interpolate to improve the precision of the reading. This would be a simple calibrated caliper; but the addition of a vernier scale allows more accurate interpolation, and is the universal practice; this is the vernier caliper.
Vernier, dial, and digital calipers can measure internal dimensions (using the uppermost jaws in the picture at right), external dimensions using the pictured lower jaws, and in many cases depth by the use of a probe that is attached to the movable head and slides along the centre of the body. This probe is slender and can get into deep grooves that may prove difficult for other measuring tools. The vernier scales may include metric measurements on the lower part of the scale and inch measurements on the upper, or vice versa, in countries that use inches. Vernier calipers commonly used in industry provide a precision to 0.01 mm (10 micrometres), or one thousandth of an inch. They are available in sizes that can measure up to 1,829 mm (72 inches).
Procedure of taking measurement with a vernier calliper
If we want to measure an outside diameter of cylinder:
1. The sliding jaw is moved along the beam until the sliding jaw almost contact the cylinder kept against the fixed jaw. In this way the cylinder is held between the fixed jaw and sliding jaw.
2. Then the sliding jaw assembly that carries the fine adjustment screw should be clamped to the beam with the help of fine adjustment clamp.
3. The two jaws are now brought into contact with the work piece by moving the sliding jaw with the help of fine adjustment screw.
4. The jaws should make definite contact with the surface of the cylinder but should not be tight.
5. The main slide assembly is then locked to the beam with the help of clamp.
6. The calliper is carefully removed from the work piece to prevent springing of the jaws.
The reading is then take as the procedure described below :
Least count is the minimum possible measurement which can be measured by the measuring instruments.
Least count = Main scale reading/Vernier scale reading.
(1) Take that least main scale reading just ahead of which zero of the vernier scale takes place. Assume that the zero of vernier scale is just ahead of 32mm reading of main scale. So take note of 32 mm. (A) Main scale reading = 32mm
(2) Now note the least count (L.C) of the vernier calliper. Least count is the minimum possible measurement which can be measured by the measuring instruments. Assume in this case L.C is.02 mm (B) Least count = 0.02 mm
(3) Now look for the graduation mark of the vernier scale that coincides with any graduation mark of the main scale.
