In [Fig. 278] f is a jaw fixed by a set screw to the bar of the micrometer, and e is a sliding jaw; these two jaws being fitted to the edges of the triangle or test piece t in the figure which has been made as already described. To the sliding jaw e is attached the micrometer screw c, which has a pitch of 40 threads per inch; the drum a upon the screw has its circumference divided into 250 equidistant divisions, hence if the drum be moved through a space equal to one of these divisions the sliding jaw e will be moved the 1⁄250th part of 1⁄40th of an inch, or in other words the 1⁄10,000th of an inch. To properly adjust the position of the zero piece or pointer, the test piece t is placed in the position shown in [Fig. 278], and when the jaws were so adjusted that light was excluded from the three edges of the test piece, the pointer r, [Fig. 277], was set opposite to the zero mark on the drum and fastened.
To set the instrument for any required pitch of thread of the United States standard form the micrometer is used to move the sliding jaw e away from the fixed jaw f to an amount equal to the width of flat upon the top and bottom, of the required thread, while for the sharp V-thread the jaws are simply closed. The gauge being set the tool is ground to the gauge.
| VOL. I. | MEASURING AND GAUGING SCREW THREADS. | PLATE III. | |||
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| Fig. 279. | Fig. 280. | Fig. 281. | |||
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| Fig. 282. | Fig. 285. | Fig. 286. | |||
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| Fig. 283. | |||||
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| Fig. 284. | Fig. 287. | ||||
Referring to the third requirement, that the tools shall in the case of lathe work be easily sharpened and set to correct position in the lathe, it will be treated in connection with cutting screws in the lathe. Referring to the fourth requirement, that a minimum of measuring and gauging shall be required to test the diameter and form of thread, it is to be observed that in a Whitworth thread the angle and depth of the thread is determined by the chaser, which may be constantly ground to resharpen without altering the angles or depth of the thread, hence in cutting the tooth the full diameter of the thread is all that needs to be gauged or measured. In cutting a sharp V-thread, however, the thread top is apt to project (from the action of the single-pointed tool) slightly above the natural diameter of the work, producing a feather edge which it becomes necessary to file off to gauge the full diameter of the thread. In originating a sharp V-thread it is necessary first to grind the tool to correct angle; second, to set it at the correct height in the latter, and with the tool angles at the proper angle with the work (as is explained with reference to thread cutting in the lathe) and to gauge the thread to the proper diameter. In the absence of a standard cylindrical gauge or piece to measure from, a sheet metal gauge, such as in [Fig. 279], may be applied to the thread; such gauges are, however, difficult to correctly produce.
So far as the diameter of a thread is concerned it may be measured by calipers applied between the threads as in [Figs. 280] and [281], a plan that is commonly practised in the workshop when there is at hand a standard thread or gauge known to be of proper diameter; and this method of measuring may be used upon any form of thread, but if it is required to test the form of the thread, as may occur when its form depends upon the workman’s accuracy in producing the single-pointed threading tools, then, in the case of the United States standard thread, the top, the bottom, and the angle must be tested. The top of the thread may (for all threads) be readily measured, but the bottom is quite difficult to measure unless there is some standard to refer it to, to obtain its proper diameter, because the gauge or calipers applied to the bottom of the thread do not stand at a right angle to the axis of the bolt on which the thread is cut, but at an angle equal to the pitch of the thread, as shown in [Fig. 282].
Now, the same pitch of thread is necessarily used in mechanical manipulation upon work of widely varying diameters, and as the angle of the calipers upon the same pitch of thread would vary (decreasing as the diameter of the thread increases), the diameter measured at the bottom of the thread would bear a constantly varying proportion to the diameter measured across the tops of the thread at a right angle to the axial line of the work. Thus in [Fig. 282], a a is the axial line of two threaded pieces, b, c. d, d represents a gauge applied to b, its width covering the tops of two threads and measuring the diameter at a right angle to a a, as denoted by the dotted line e. The dotted line f represents the measurement at the bottom of the thread standing at an angle to e equal to half the pitch. The dotted line g is the measurement of c at the bottom of the thread.
Now suppose the diameter of b to be 11⁄2 inches at the top of the thread, and 11⁄8 inches at the bottom, while c is 11⁄8 inches on the top and 3⁄4 at the bottom of the thread, the pitches of the two threads being 1⁄4 inch; then the angle of f to e will be 1⁄8 inch (half the pitch) in its length of 11⁄8 inches. The angle of g to e will be 1⁄8 inch (half the pitch) in 3⁄4 (the diameter at the bottom or root of the thread).
It is obvious, then, that it is impracticable to gauge threads from their diameters at the bottom, or root.
On account of the minute exactitude necessary to produce with lathe tools threads of the sharp V and United States standard forms, the Pratt and Whitney Company manufacture thread-cutting tools which are made under a special system insuring accuracy, and provide standard gauges whereby the finished threads may be tested, and since these tools are more directly connected with the subject of lathe tools than with that of screw thread, they are illustrated in connection with such tools. It is upon the sides of threads that the contact should exist to make a fit, and the best method of testing the fit of a male and female thread is to try them together, winding them back and forth until the bright marks of contact show. Giving the male thread a faint tint of paint made of Venetian red mixed with lubricating oil, will cause the bearing of the threads to show very plainly.
[Figs. 283] and [284] represent standard reference gauges for the United States standard thread. [Fig. 283] is the plug or male gauge. The top of the thread has, it will be observed, the standard flat, while the bottom of the thread is sharp. In the collar, or female gauge, or the template, as it may be termed, a side and a top view of which are shown in [Fig. 284], and a sectional end view in [Fig. 285], the flat is made on the smallest diameter of the thread, while the largest diameter is left sharp; hence, if we put the two together they will appear as in [Fig. 286], there being clearance at both the tops and bottoms of the threads. This enables the diameters of the threads to be in both cases tested by standard cylindrical gauges, while it facilitates the making of the screw gauges. The male or plug gauge is made with a plain part, a, whose diameter is the standard size for the bottoms of the threads measured at a right angle to the axis of the gauge and taking the flats into account. The female gauge or template is constructed as follows:—A rectangular piece of steel is pierced with a plain hole at b, and a standard thread hole at a, and is split through at c. At d is a pin to prevent the two jaws from springing, this being an important element of the construction. e is a screw threaded through one jaw and abutting against the face of the other, while at f is another screw passing through one jaw and threaded into the other, and it is evident that while by operating these two screws the size of the gauge bore a may be adjusted, yet the screws will not move and destroy the adjustment, because the pressure of one acts as a lock to the other. It is obvious that in adjusting the female gauge to size, the thread of the male gauge may be used as a standard to set it by.