A Vernier calliper consists of two main parts: the main scale engraved on a solid L-shaped frame and the vernier scale that can slide along the main scale. The sliding nature of the vernier has given it another name sliding calliper. The main scale is graduated in millimetres, up to a least count of 1 mm. The vernier also has engraved graduations, which is either a forward vernier or a backward vernier. The vernier calliper is made of either stainless steel or tool steel, depending on the nature and severity of application.
Fig.1 Main parts of a Vernier calliper
Fig.2 Vernier Calliper
Figure.1 illustrates the main parts of a vernier calliper. The L-shaped main frame also serves as the fixed jaw at its end. The movable jaw, which also has a vernier scale plate, can slide over the entire length of the main scale, which is engraved on the main frame or the beam. A clamping screw enables clamping of the movable jaw in a particular position after the jaws have been set accurately over the job being measured. This arrests further motion of the movable jaw, so that the operator can note down the reading in a convenient position. In order to capture a dimension, the operator has to open out the two jaws, hold the instrument over the job, and slide the movable jaw inwards, until the two jaws are in firm contact with the job.
Fig.3 Measurement of dimensions (a) Outside dimension (b) Inside dimension
A fine adjustment screw enables the operator to accurately enclose the portion of the job where measurement is required by applying optimum clamping pressure. In the absence of the fine adjustment screw, the operator has to rely on his careful judgement to apply the minimum force that is required to close the two jaws firmly over the job. This is easier said than done, since any excessive application of pressure increases wear and tear of the instrument and may also cause damage to delicate or fragile jobs. The two jaws are shaped in such a manner that they can be used to measure both inside and outside dimensions. Notice the nibs in Fig.1, which can be used to measure inside dimension. Figure.3 illustrates the method of measuring inside and outside dimensions using a vernier calliper.
Whenever the vernier slides over the main frame, a depth-measuring blade also slides in and out of the beam of the calliper. This is a useful attachment for measuring depths to a high degree of accuracy. Divider setting holes are provided, which enable the use of a divider to aid the measurement process.
Measuring a diameter is easier than measuring between flat surfaces, because the diameter is the greatest distance separating the reference and the measured points. Compared to the measurement between flat surfaces, the area of contact between the calliper and the job is much lesser in diameter measurement. Therefore, the resultant force acting either on the job or on the jaws of the calliper is lesser, with the result that there is no deformation or buckling of the jaws.
This not only improves the accuracy of measurement, but also reduces the wear and tear of the instrument. Whether the measurement is done for the inside diameter or outside diameter, the operator has to rely on his/her feel to judge if proper contact is made between the measured surfaces and also that excessive force is not exerted on the instrument or the job. Continued closing of the calliper will increase the springing. High gauging pressure causes rapid wear of the jaws, burnishes the part (localized hardening of metal), and may cause damage to the calliper.
The following guidelines are useful for the proper use of a vernier calliper:
1. Clean the vernier calliper and the job being measured thoroughly. Ensure that there are no burrs attached to the job, which could have resulted from a previous machining operation.
2. When a calliper’s jaws are fully closed, it should indicate zero. If it does not, it must be recalibrated or repaired.
3. Loosen the clamping screw and slide the movable jaw until the opening between the jaws is slightly more than the feature to be measured.
4. Place the fixed jaw in contact with the reference point of the feature being measured and align the beam of the calliper approximately with the line of measurement.
5. Slide the movable jaw closer to the feature and operate the fine adjustment screw to establish a light contact between the jaws and the job.
6. Tighten the clamp screw on the movable jaw without disturbing the light contact between the calliper and the job.
7. Remove the calliper and note down the reading in a comfortable position, holding the graduations on the scale perpendicular to the line of sight.
8. Repeat the measurement a couple of times to ensure an accurate measurement.
9. After completing the reading, loosen the clamping screw, open out the jaws, and clean and lubricate them.
10. Always store the calliper in the instrument box provided by the supplier. Avoid keeping the vernier calliper in the open for long durations, since it may get damaged by other objects or contaminants.
11. Strictly adhere to the schedule of periodic calibration of the vernier calliper.
According to IS: 3651-1974, vernier callipers are of three types: type A, type B, and type C. While all the three types have the scale on the front of the beam, type A vernier scale has jaws on both sides for external and internal measurements, along with a blade for depth measurement.
Type B, shown in Fig. 1, is provided with jaws on one side only for both external and internal measurements. Type C has jaws on both sides for making the measurements. However, the jaws have knife edge faces for marking purpose. The recommended measuring ranges for vernier callipers are 0–125, 0–200, 0–250, 0–300, 0–500, 0–750, 0–1000, 750–1500, and 750– 2000 mm.
A vernier calliper is useful for accurate linear measurements. However, it demands basic mathematical skill on the part of the user. One should be able to do simple calculations involving MSD, vernier coinciding division, and least count, in order to compute the measured value of a dimension. In addition, considerable care should be exercised in identifying the coinciding vernier division. These problems can be offset by using a dial calliper (Fig. 4.).
Fig.4 Dial Calliper
In a dial calliper, the reading can be directly taken from a dial gauge that is attached to the calliper. The dial gauge has its own least count, which is clearly indicated on the face of the dial. By multiplying the value of the reading indicated by the least count, one can calculate the measured value easily. A small but precise pair of rack and pinion drives a pointer on a circular scale. This facilitates direct reading without the need to read a vernier scale. Typically, the pointer undergoes one complete rotation per centimetre or per millimetre of linear measurement. This measurement should be added to the main scale reading to get the actual reading. A dial calliper also eliminates parallax error, which is associated with a conventional vernier calliper.
A dial calliper is more expensive than the vernier calliper. In addition, the accuracy of the reading mechanism of the dial caliper is a function of length of travel, unlike the vernier calliper that has the same accuracy throughout its length. A dial calliper is also subject to malfunctioning because of the delicate nature of the dial mechanism.
Electronic Digital Calliper
An electronic digital calliper is a battery-operated instrument that displays the reading on a liquid crystal display (LCD) screen. The digital display eliminates the need for calculations and provides an easier way of taking readings. Figure 5 illustrates the main parts of an electronic digital calliper.
Fig.5 Electronic digital calliper
Fig.6 Electronic digital calliper
The LCD display is turned on or off with a button. In order to initialize the instrument, the external jaws are brought together until they touch and the ‘zero button’ is pressed to set the reading to zero. The digital calliper can now be used to measure a linear dimension. Some digital callipers can be switched between centimetres or millimetres, and inches. Digital callipers are made of stainless steel and are generally available in three sizes: 150, 200, and 300 mm.
The two greatest advantages of an electronic digital calliper are its electronic calculator functions and capability to be interfaced with a computer. It can be set to either metric or British system of units. The ‘floating zero’ option allows any place within the scale range to be set to zero. The digital display will then exhibit either plus or minus deviations of the jaw from a reference value. This enables the instrument to be also used as a limit gauge.
More importantly, a digital calliper can be interfaced with a dedicated recorder or personal computer through a serial data cable. The digital interface provides secured storage for a series of readings, thereby improving the reliability of the records. It can be connected to a printer to provide a printed record or can be directly interfaced with a computer of a statistical control system.