Micrometers - Slide Gauges and Calipers - Principles, Construction, Operation and Use of Appliances for Fine Mechanical Measurements

INTRODUCTION

PRINCIPLES OF ENGINEERING MEASURING INSTRUMENTS

THE instruments used for dimensional measurement in engineering practice may be grouped in two main classes, namely, direct registering instruments, and comparators. The former are invariably equipped with some form of graduated scale, which indicates the size of the object to which they are applied, while the latter are not so equipped, but must be set by reference to a scale or master gauge, and used to compare the dimensions of the work with this standard dimension. It is, however, always possible to use a registering instrument as a comparator, and many instruments primarily designed as comparators may be elaborated by the addition of a registering scale.

The Micrometer

The micrometer, in any of its forms as used by engineers, is essentially a registering instrument, and the principle of its operation involves the use of a screw thread, which not only improves its sensitivity of control, but also simplifies the problem of registering very fine measurements, as it is possible to use two separate scales, one recording complete revolutions of the screw (each of which registers a dimension equal to the pitch of the screw) and the other, the finer dimensions equivalent to parts of a revolution. The screw thread may be defined as a continuous inclined plane which, in effect, multiplies the spacing of the graduated scale or scales, and not only makes them easier to read, but also minimises error. Theoretically, there is no limit to the degree of accuracy which can be attained by a micrometer, but practical considerations, one of the most important of which is the effect of expansion and contraction under varying temperature, restrict the application of the ordinary micrometer to the measurement of minimum limits of about one ten-thousandth of an inch, or one four-hundredth of a millimetre.

The Slide Gauge

Slide gauges are generally, but not necessarily, registering instruments, being usually provided with a graduated scale which is identical to that of an ordinary fractional-inch or metric rule. The movable element of the gauge carries a zero mark or pointer, by means of which the size is read against the scale. In order to facilitate the reading of fine measurements, slide gauges are commonly equipped with a vernier scale on the moving part. It is emphasised, however, that the vernier scale is neither an essential feature of the slide gauge, nor yet exclusive to it, as it can be applied to any measuring device, and is extensively used on protractors, height gauges, and also micrometers.

Calipers and Fixed Gauges

Simple calipers are essentially comparators, but as will be seen, they can be, and often are, fitted with registering scales.

Fixed gauges, including plug, ring, slip, feeler or other forms of limit or reference gauges, are outside the scope of this handbook, but the information on the general science of fine measurement contained herein will be found helpful also to users of such gauges.

CHAPTER I

SIMPLE AND REGISTERING CALIPERS

THE simple or firm joint caliper shown in fig. 1 was, in the early days of engineering, the most extensively used instrument for external and internal measurement, but its use for really important work is not encouraged in modern practice. Such calipers have a riveted or bolted joint with friction washers set up sufficiently tightly to prevent inadvertent movement, and are adjusted roughly by hand, fine adjustments being made by tapping either the limb or the pivot on some solid object, to close or open the points respectively. The setting is obtained by reference to a scale or a gauge piece, and a fair amount of skill is necessary, both in setting the instrument and in using it, as the accuracy is dependent on sensitivity of touch. Although the crudity of the means of adjustment is a great disadvantage, the old-time craftsman succeeded in working to fairly close, though undefined limits with simple calipers.

Spring Calipers

Several types of spring calipers have been devised, including one-piece instruments constructed similarly to a draughtsman’s spring bows, but the most popular type in present use employs a separate spring of almost completely circular shape to actuate the legs, as in the examples shown in figs. 2 and 3. The legs are rigid, and entirely separate from the pivot joint, the latter being made by a reel-shaped pin—shown in the section—which fits into semicircular grooves cut in the inner sides of the ends of the legs, and upon which they are free to rotate relatively to each other, being limited in their movements outwards by their projecting ends coming in contact. They are controlled in this same direction by the bow-shaped spring A, the ends of which fit into grooves upon the outside of the legs above the joint. When the legs are wide open, the tension on this spring is then sufficiently strong to prevent its coming out of its place. It will be seen that by the controlling action of the nut on the adjusting screw below the legs can be drawn together or allowed to part, being kept taut by the action of this spring.

Fig. 1.—Outside and inside firm joint calipers

Fig. 2.—Outside and inside spring calipers

Fig. 3.—An improved form of spring dividers, incorporating a self-centring head and a quick-release adjustment nut

The advantage of this form of joint is that, being made up in separate parts, should the spring fly, through rapid change of temperature or other cause, it can be replaced without discarding the other parts of the tool.