Vernier caliper – Precision linear Measuring Instrument
A vernier caliper is a measuring device used to precisely measure linear dimensions.
It is a very useful tool to use when measuring the diameter of around objects like cylinders because the measuring jaws can be secured on either side of the circumference.
The Vernier Caliper is a precision instrument that can be used to measure internal and external distances extremely accurately. The example shown here is a manual caliper.
Vernier caliper can be used for three types of measurements, outside distance (such as the length of an object),2nd one is inside distance such as the width of a groove or the diameter of a large hole, 3rd one is depth, such as the depth of a hole or the height of a step.
Vernier Caliper Parts :
1. Vernier caliper upper jaws
The upper jaws of a caliper are used for taking inside measurements such as the diameter of a hole or slot.
2. Vernier caliper lower jaws
The lower jaws are used for measuring outside dimensions such as width, length, and diameter.
3. Vernier caliper depth rod
The depth rod is used for measuring the depth of holes. It protrudes when the thumbscrew is turned.
4. Vernier caliper main scale
The main scale runs along the beam of the caliper. On a metric vernier caliper, the main scale is graduated in centimeters (cm) and millimeters (mm). On an imperial vernier caliper, the main scale is graduated in inches, with each inch divided into increments of a tenth of an inch (0.1 inch).
5. Vernier scale
The vernier scale adds an extra degree of accuracy to measurements. On the vernier scale, the smallest increment on the main scale is further divided. The metric vernier scale is divided into 50 equal parts. The measuring range of the vernier scale is usually 1 mm, with each increment representing 0.02 mm. On an imperial caliper with a main scale of 6 inches, the vernier scale is 0.6 inches long and is divided into 25 parts. Each increment has a length of 0.001 inch.
6. Vernier caliper thumb screw
The thumbscrew is used to precisely adjust the measuring faces of the caliper (both sets of jaws and the depth rod). It helps the user to get a tight grip on the material they are measuring.
7. Vernier caliper lock screw
The lock screw secures the jaws into place, so the object you are measuring can be removed, and readings are taken.
Vernier scale and Main scale :
A vernier scale has a main scale and a vernier scale. The main scale has the normal resolution with a 1 division value is 1 mm. The vernier scale is attached to the main scale which can slide on it and has 50 divisions on it.
The VERNIER is a small movable graduated scale for obtaining fractional parts of subdivisions on a fixed main scale of any measuring instrument. Usually vernier calipers have both imperial (inches) and metric (mm) scales.
Other parts of vernier calipers are external jaws – these jaws are used for measuring external dimensions .job is placed in between these two jaws. internal jaws are used for internal dimensions measurement like washer ID, grove, slot, cutout pocket dimension, the stem is used for depth measurement or height measurement. lock screw is used for locking a vernier scale moving body.
Principle of Vernier Caliper
The vernier scale works on the principle of using the alignment of line segments displaced by a small amount to make fine measurements. The human eye can easily detect this alignment of lines which is the main fact that drives a vernier.
Least Count Of Vernier Caliper :
Least count is nothing but the smallest possible value that can be measured using instruments. this can be calculated by the smallest division value on the main scale in mm divided by the total number of divisions on the main scale .here in this case, 1 mm is one division value divided by 50 numbers which is the total count of vernier scale line = 0.02 mm. so least count is 0.02 mm
Least count = ( smallest division on main scale) / ( total no. of divisions on vernier scale )
= 1/50 = 0.02 cm
Vernier Caliper How to Use and Application
How to measure vernier caliper
Each division on the main scale is 1mm. The metric Vernier scale is 49 mm long and divided into 50 equal parts. Each division is 49/50, which is equal to 0.98 mm. The difference between one division on the main scale and one division on the metric vernier scale is 1/50 or 0.02 mm which is the Least Count.
How to read Vernier Caliper :
- Note the main scale reading, immediately preceding the zero line on the vernier scale.
- Here, zero of the vernier scale immediately preceding 28mm.
- This (28mm) must be added with decimal reading on the vernier scale.
- Note the line on the vernier scale, which is coinciding, with a line on the main scale.
- 31ST line coincides with a line on the main scale.
- So, the reading is 28mm plus 31 divisions of 0.02mm TOTAL: 28mm + 31 * 0.02 = 28.62mm
Types of Vernier Calipers :
IS:3651 recommends three types of vernier calipers A, B and C to meet the various needs of internal and external measurements upto 2000 mm with a vernier accuracy of 0.02, 0.05 and 0.1 mm and the recommended measuring ranges are 0-125, 0-200, 0-250, 0-300, 0-500,
0-750, 0-1000, 750-1500 and 750-2000 mm.
This type of vernier caliper has jaws on both sides of the beam for external and internal measurements. Also, a blade is provided, which is connected to the movable jaw, for depth measurements.
This is provided with jaws only on one side. The jaws are so designed to enable both internal and external measurements. The width of the measuring jaws is added to the reading on the scale for internal measurements. Usually, the width is marked on the jaws of the vernier caliper.
This is provided with jaws on both sides. The lower jaws are used for measurements and upper jaws are used for marking purposes.
Disadvantages of Vernier Calipers :
Limitations of Vernier Calipers are as follows,
- It is difficult to obtain the correct feel of the part. As both the part and caliper are always to some extent elastic, the amount of force used by the operator affects the indication.
- The scales are difficult to read.
- The accuracy is about 0.02 mm
- The caliper must be properly used in order to take the desired measurement.
Errors in Measurement with Vernier Caliper
- Errors occur due to manipulation of vernier caliper and its jaws on the workpiece i.e Care should be taken to properly align the workpiece between the measuring faces of the jaws.
- Vernier caliper is tilted during use especially when measuring depth.
- Dirty measuring surfaces.
- The improper conditions of the jaws of the caliper, like natural wear and warping, so the caliper jaws must be tested frequently by setting the 0.0 point of the main and vernier scales. The caliper is held against a light source in this position.
Precautions in using a Vernier Caliper
- The fixed jaw of the vernier caliper should be used as the reference point and the measured point is obtained by advancing or with drawing the sliding jaw. For this purpose, the sliding jaw is provided with a fine adjustment nut in all vernier calipers.
- If play exists between the movable jaw and the frame, the accuracy of the vernier will be lost. So, any play should be removed by adjusting the gib at the back of the jaw assembly.
- On repeated use, the tips of the measuring jaws get worn out and that must be taken into account. Before lifting the caliper from the workpiece, release the slide otherwise, the measuring surfaces will get worn.
- Always ensure that the vernier caliper is not tilted or twisted.
- The vernier caliper should be gripped near or opposite the jaws, one hand for the stationary jaw, and the other hand for supporting the movable jaw.
- The sense of touch is an important factor in measurement. When measuring, gentle pressure must be applied.
- The measuring surfaces should be clean.
- When taking a reading, our viewpoint should be vertical to the scale.
Application Of Vernier Caliper :
- By using Outer jaws Of vernier we can measure the length of a rod, the diameter of a sphere, the external diameter of a hollow cylinder.
- by using Inner jaw we can measure internal dimensions such as the inner diameter of pipe and seals, washers.
- By using stem we can measure depth or length of bottle or beaker.
- Measuring workpiece thickness.
- Measuring shaft diameter, keyway dimensions, shaft length.
- Measuring Slots, undercut groves.
Care in use of Vernier Calipers
- The vernier caliper should not be dropped or tossed aside after measurement but should be set down gently or placed in the box.
- Vernier calipers are not a rugged instrument, hence they should not be treated or used as wrench or hammer.
- Grit, Chips, and oil must be wiped off from the instrument, before and after measurements.
- The component should not be clamped in the vernier caliper jaws and waived in the air.
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