Measurement

Depth and Rod Gauges.

It is often necessary to measure the depth of one portion of a piece of work below another part, or the height of one portion relatively to a lower one. To hold a rule perpendicularly and take a sight is not an accurate method, be cause the same objections apply to this as to rule measurement imposed by modern shop ideals, then instruments must be capable of measuring to finer dimensions than this. Hence, while the coarser classes of micrometers read directly to part of an inch, the finest measure up to 1001,000 of an inch, about zoo times as fine as the diameter of a human hair.

Calipers.

Common calipers (fig. 31) are adjusted over or within work, and the dimensions are taken therefrom by a rule or a gauge.

Vernier Calipers.—The vernier fitting, so named after its in ventor, Pierre Vernier, in 1631, is fitted to numerous calipers and caliper rules. It is applied to calipers for engineers' use to read toof an inch without requiring a magnifier. The beam of the is divided into inches and tenths of the inch, and each tenth into fourths and the vernier into twenty-five parts, or the beam is divided into fiftieths of an inch (fig. 32) and the vernier has 20 divisions to 19 on the rule.

Micrometer Calipers are the direct offspring of the Whitworth measuring machine. In the original form of this machine a screw of 20 threads to the inch, turned by a worm-wheel of 200 teeth and single-threaded worm, had a wheel on the axis of the worm with 25o divisions on its circumference, so that an adjustment of of an inch was possible. The costly measuring ma chines made to-day have a dividing wheel on the screw, but they combine modifications to ensure freedom from error, the fruits of prolonged experience. Such machines (fig. 33) are used for testing purposes ; but there are immense numbers of small instru ments, the micrometer calipers (fig. 34), made for general shop use, measuring directly to of an inch, and in the hands of careful men easily to half and quarter thousandths. In these the subdivision of the turns of the screw is effected by circular graduations. Usually the screw pitch is 4o to the inch, and the circular divisions number 25, so that a movement of one division indicates that the screw has been advanced of or of an inch. Provision for correcting or taking up the effects of wear is included in these designs (e.g., at a in fig. 34), and varies with different manufacturers. A vernier is sometimes fitted in addi tion, in very high class instruments, to the circular divisions, so that readings of ten thousandths of an inch can be taken Beam in general. There are many depth gauges made with rule divisions simply. These have the marked advantage of a shouldered face which rests upon the upper portion of the work, from which the rule measurement is taken (fig. 36). These generally have a clamping arrangement. For very accurate work either the vernier or the micrometer fitting is applied. For larger diame ters are the rod gauges (fig. 37) to which the micrometer is fitted.

Screw Thread Gauges.

The taking of linear dimensions, though provided for so admirably by the systems of gauging just discussed, does not cover the important section of screw measure ment. This is a department of the highest importance. In most English shops the only test to-day of the size of a screw or nut is the use of a standard screw or a standard nut. This method may suffice in many classes of work, but it is utterly unsuited to an interchangeable system; and when there is a fair amount of the latter firms sometimes make thread gauges of their own, in general form like the plug and ring gauges, using a hard quality of steel for small sizes or a tough quality of cast iron for the larger. These, though not hardened, will endure for a long time if treated carefully; but though very useful and far better than none at all they lack two essentials. They are simply accommo rather peculiar character. They magnify the most minute error

by adaptations of long and short lever arms. Some simply mag nify inaccuracy, but in one type an index reads to thousandths of an inch (fig. 39). They are used in some lathe chuck work, but their principal value is in fitting and erecting the finer mechanisms.

Surface Plates and Cognate Forms.—Allied to the gauges are the instruments for testing the truth of plane surfaces : the sur vn•rwarrt. •%01,1.0, •• y V A t 1 11,IV dation gauges, made to an existing tap or die, and do not there fore embody any precise absolute measurement, nor do they include any means for measuring variations from standard, nor are they hardened. The essence of a screw gauge is that it measures the sides of the threads without risk of a possible false reading due to contact on the bottom or top of the V. This is fulfilled by flatting the top and making the bottom of the gauge keen. Some gauges are made as a plug and ring (fig. 38), the plug being solid and the ring capable of precise adjustment round it. There is a plain round end, ground and lapped exactly to the standard size of the bottom of the thread, a dimension which is obliterated in the threaded end because of the bottoms of the angles being made keen for clearance. There are three kinds of this class of gauge made; the first and most expensive is face plates, straight-edges and winding strips. The origination of plane surfaces by scraping, until the mutual coincidence of three plates is secured, was due to Whitworth. These surface plates (fig. 4o, A) fill an important place in workshop practice, since in the best work plane surfaces are tested on them and corrected by scraping. To a large extent the precision grinding hardened and ground in the angle, while the second is hardened but not ground. The first is intended for use when a very per fect gauge is required, the second for ordinary shop usage. The third is made unhardened for purposes of reference simply, and it is not brought into contact with the work to be tested at all, but measurements are taken by calipers; in every detail it repre sents the standard threads. An appliance of quite a different character is a micrometer caliper having a fixed V and a movable point between which the screw to be measured is embraced.

Indicators are a small group of measuring instruments of a machines have lessened the value of scraping, but it is still re tained for machine slides and other work of a similar class. In the shops there are two classes of surface plates : those employed daily about the shops, the accuracy of which becomes impaired in time, and the standard plate or plates employed for test and correction. Straight-edges are derived from the surface plates, or may be originated like them. The largest are made of cast _ iron, ribbed and curved on one edge, to prevent flexure, and pro vided with feet (fig. 4o, B). The smaller straight-edges are gen erally parallel, and a similar pair constitutes "winding strips," by which any twist or departure from a plane surface is detected. Squares, of which there are nu merous designs (fig. 4o, C and D) , are straight-edges set at right angles. Bevels or bevel _ squares (fig. 41), are straight edges comprising a stock and a blade, which are adjustable for angle in relation to each other. Shop protractors often include a blade adjustable for angle, forming a bevel with graduations.

Spirit-levels test the horizontal truth of surfaces. Many levels have two bubble tubes at right angles with each other, one of which tests the truth of vertical faces. Generally levels have flat feet, but some are made of V-section to fit over shafting. The common plumb-bob is in frequent use for locating the vertical position of centres not in the same horizontal plane. When a plumb-bob is combined with a parallel straight-edge the term plumb-rule is applied. It tests the truth of vertical surface more accurately than a spirit-level.

(J.

G. Ho.)