Fig. 291.

Suppose it is required to make a gauge for a pitch of 6 per inch, then a piece of iron of any diameter may be put in the lathe and turned up to the required diameter for the top of the thread. The end of this piece should be turned up to the proper diameter for the bottom of the thread, as at g, in [Fig. 287]. Now, it will be seen that the angle of the thread to the axis a of the iron is that of line c to line a, and if we require to find the angle the thread passes through in once winding around the bolt, we proceed as in [Fig. 288], in which d represents the circumference of the thread measured at a right angle to the bolt axis, as denoted by the line b in [Fig. 287]. f, [Fig. 288] (at a right angle to d), is the pitch of the thread, and line c therefore represents the angle of the thread to the bolt axis, and corresponds to line c in [Fig. 287]. We now take a piece of iron whose length when turned true will equal its finished and threaded circumference, and after truing it up and leaving it a little above its required finished diameter, we put a pointed tool in the slide-rest and mark a line a a in [Fig. 289], which will represent its axis. At one end of this line we mark off below a a the pitch of the thread, and then draw the line h j, its end h falling below a to an amount equal to the pitch of the thread to be cut. The piece is then put in a milling machine and a groove is cut along h j, this groove being to receive a tightly-fitting piece of sheet metal of which a thread gauge is to be made. This piece of sheet metal must be firmly secured in the groove by set-screws. The piece of iron is then again put in the lathe and its diameter finished to that of the required diameter of thread. Its two ends are then turned down to the required diameter for the bottom of the thread, leaving in the middle a section on which a full thread can be cut, as in [Fig. 290], in which f f represents the sheet metal for the gauge. After the thread is cut, as in [Fig. 290], we take out the gauge and it will appear as in [Fig. 291], and all that is necessary is to file off the two outside teeth if only one tooth is wanted.

The philosophy of this process is that we have set the gauge at an angle of 90°, or a right angle to the thread, as is shown in [Fig. 289], the line c representing the angle of the thread to the axis a a, and therefore corresponding to the line c in [Fig. 287]. A gauge made in this way will serve as a test of its own correctness for the following reasons: Taking the middle tooth in [Fig. 291], it is clear that one of its sides was cut by one angle and the other by the other angle of the tool that cut it, and as a correctly formed thread is of exactly the same shape as the space between two threads, it follows that if the gauge be applied to any part of the thread that was cut in forming it, and if it fits properly when tried, and then turned end for end and tried again, it is proof that the gauge and the thread are both correct. Suppose, for example, that the tool was correct in its shape, but was not set with its two angles equal to the line of lathe centres, and in that case the two sides of the thread will not be alike and the gauge will not reverse end for end and in both cases fit to the thread. Or suppose the flat on the tool point was too narrow, and the flat at the bottom of the thread will not be like that at the top, and the gauge will show it.

Referring to the fifth requirement, that the angles of the sides of the threads shall be as acute as is consistent with the required strength, it is obvious that the more acute the angles of the sides of the thread one to the other the finer the pitch and the weaker the thread, but on the other hand, the more acute the angle the better the sides of the thread will conform one to the other. The importance of this arises from the fact that on account of the alteration of pitch, already explained, as accompanying the hardening of screw-cutting tools, the sides of threads cut even by unworn tools rarely have full contact, and a nut that is a tight fit on its first passage down its bolt may generally be caused to become quite easy by running it up and down the bolt a few times. Nuts that require a severe wrench force to wind them on the bolt, may, even though they be as large as a two-inch bolt, often be made to pass easily by hand, if while upon the bolt they are hammered on their sides with a hand hammer. The action is in both cases to cause the sides of the thread to conform one to the other, which they will the more readily do in proportion as their sides are more acute. Furthermore, the more acute the angles the less the importance of gauging the threads to precise diameter, especially if the tops and bottoms of the male and female thread are clear of one another, as in [Fig. 273].

Referring to the sixth requirement, that the nut shall not be unduly liable to become loose of itself in cases where it may require to be fastened and loosened occasionally, it may be observed, that in such cases the threads are apt from the wear to become a loose fit, and the nuts, if under jar or vibration, are apt to turn back of themselves upon the bolt. This is best obviated by insuring a full bearing upon the whole area of the sides of the thread, and by the employment of as fine pitches as is consistent with sufficient strength, since the finer the pitch the nearer the thread stands at right angle to the bolt axis, and the less the tendency to unscrew from the pressure on the nut face.

The pitches, diameters, and widths of flat of the United States standard thread are as per the following table:—

UNITED STATES STANDARD SCREW THREADS.

Diameter of
Screw.		Threads
per inch.		Diameter at
root of Thread.		Width of Flat.
1⁄420 .1850.0063
5⁄1618 .2403.0069
3⁄816 .2938.0078
7⁄1614 .3447.0089
1⁄213 .4001.0096
9⁄1612 .4542.0104
5⁄811 .5069.0114
3⁄410 .6201.0125
7⁄89 .7307.0139
1 8 .8376.0156
11⁄87 .9394.0179
11⁄47 1.0644.0179
13⁄86 1.1585.0208
11⁄26 1.2835.0208
15⁄851⁄21.3888.0227
13⁄45 1.4902.0250
17⁄85 1.6152.0250
2 41⁄21.7113.0278