PROBLEMS
4.6 SCALED INSTRUMENTS
4.6.3 Callipers
There are many jobs whose dimensions cannot be accurately measured with a steel rule alone. A typical case in point is a job with a circular cross section. An attempt to take measurement using a steel rule alone will lead to error, since the steel rule cannot be positioned diametrically across the job with the required degree of accuracy. One option is to use the combination set. However, callipers are the original transfer instruments to transfer such measurements on to a rule (Fig. 4.18). They can easily capture the diameter of a job, which can be manually identified as the maximum distance between the legs of the calliper that can just slide over the diameter of the job. Even though callipers are rarely used in production inspection, they are widely used in tool room and related work.
Busch defines callipers as instruments that physically duplicate the separation between the reference point and measured point of any dimension Fig. 4.16 Use of a protractor head for angle
measurement
Fig. 4.17 Use of a centre head for the measurement of diameter
Job
Fig. 4.18 Callipers, the original transfer instruments
Job
within their range. Thus, callipers do only the job of transferring a dimension, but not of measuring instruments on their own. This is illustrated in Fig. 4.19, where a calliper is shown transferring the outer diameter of a job on to a graduated steel rule, to read the dimension accurately and conveniently. The outer diameter of a job is to be measured (Step a). Aligning the ends of the two legs of the calliper to a feature of the part being measured, like the one shown in Fig. 4.19, is accomplished quite easily (Step b) because the calliper provides for easy flexing of the two legs and a means of locking them into position whenever required. Now, simply laying the ends of the calliper on a steel rule facilitates easy measurement of the dimension in question (Step c). Thus, as the definition stated earlier mentions, physical duplication of the separation of reference and measured points is accomplished with a high degree of accuracy.
Callipers are available in various types and sizes. The two major types are the firm joint calliper and the spring calliper. A firm joint calliper, as the name itself suggests, can hold the position of two legs opened out to a particular degree unless moved by a certain force.
This is possible because of higher friction developed at the joint between the two legs of the calliper. They are adjusted closely for size by gentle tapping of a leg. A locknut is needed to lock the calliper in a particular position. On the other hand, a spring calliper can hold a particular position due to the spring pressure acting against an adjusting nut. This permits a very careful control, and no lock is needed. Figure 4.20 illustrates the classification of callipers. Callipers are manufactured in a large number of sizes. They are designated not by their measurement ranges, but by the length of their legs,which range from 50 to 500 mm.
Figure 4.21 illustrates the different types of callipers. These are all simple callipers, whose ends are adjustable to transfer a measurement from the job to a steel rule. Although a member of the calliper family, a divider classified under callipers is simply referred to as a divider.
Callipers
Firm joint callipers Spring callipers
1. Outside calliper 2. Inside calliper 3. Firm joint divider
1. Outside calliper 2. Inside calliper 3. Transfer calliper Fig. 4.20 Classification of callipers
(a) (b) (c) (d)
Fig. 4.21 Types of callipers (a) Outside calliper (b) Inside calliper (c) Divider (d) Hermaphrodite calliper Job
Rule
Step a Step b Step c
Fig. 4.19 Calliper being used to transfer a dimension from a job to a rule
LINEAR MEASUREMENT 93
Outside calliper As the name suggests, an outside calliper is used to measure the outside or external dimensions. While taking measurements, the legs of the calliper should be set gently against the job either by a small physical force in case of a firm joint calliper or by an adjustment of the nut in case of a spring calliper. The user should ensure that a firm contact is established between the calliper and the job before transferring the measurement to a rule. One leg of the calliper is firmly held against a graduation on the steel rule to establish the reference point.
The other leg is now gently transferred on to the rule to capture the measured point. The legs of callipers are made of alloy steel, with the measuring points being heat treated to withstand wear and tear. The legs are of a rectangular cross section, and should be free from cracks and any other type of flaw for longevity. The springs in spring callipers are made of spring steel, which is hardened and tempered. The spring force is adjusted by a knurled nut operating on a precision machined screw.
Inside calliper As illustrated in Fig. 4.21, the measuring ends of the legs of an inside calliper are shaped in the form of a hook. While taking measurements, the calliper legs are initially in a folded condition, which are inserted into the component. Now, the legs are stretched out to butt against the surface of the job. The calliper is carefully rocked over the centre. The feel provided as they pass the centre is the limit of their sensitivity.
Divider As with callipers, the primary use of dividers is to transfer measurements. However, they are used with line references rather than with surface references. Dividers are used for the following tasks: (a) transferring a dimension of a job to a rule for measurement, (b) transferring a dimension from one part to another part, and (c) transferring a dimension from a rule to a job for layout of the part. Scribing arcs during layout work is another chief use of dividers.
Hermaphrodite calliper It is essentially a scribing tool comprising one divider leg and one calliper leg. The scriber can be mounted by means of a locknut, as shown in Fig. 4.21. The chief advantage of a hermaphrodite calliper is that a scriber of any required shape and size can be fitted to it and used.
The proper use of the inside and outside callipers depends to a large extent on the skill of the person taking measurements. Measuring with a calliper consists of adjusting the opening so that its reference points duplicate the features of the job being measured. In other words, there is no other provision in a calliper that helps in its alignment than the reference points. As illustrated in Fig. 4.22, the greatest accuracy is achieved in case of callipers when the line of measurement coincides with a plane perpendicular to the job. The divider provides the best accuracy when the measurements are taken from well-marked lines, as shown in Fig. 4.22.
Many a time measurements need to be taken between edges, in which case care must be exercised in ascertaining the proper way of taking measurements.
Do Di
Plane
perpendicular to axis of the job
Fig. 4.22 Measurement using callipers (a) Outside calliper (b) Inside calliper (c) Divider