Chapter IV: Screw Thread (2)

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.

To produce sheet metal templates such as was shown in Fig. 279, the following method may be employed, it being assumed that we have a threading tool correctly formed.

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 | Threads | Diameter at | Width of |
| Screw. | per inch. | root of Thread. | Flat. |
+-------------+-----------+-----------------+----------+
| 1/4 | 20 | .1850 | .0063 |
| 5/16 | 18 | .2403 | .0069 |
| 3/8 | 16 | .2938 | .0078 |
| 7/16 | 14 | .3447 | .0089 |
| 1/2 | 13 | .4001 | .0096 |
| 9/16 | 12 | .4542 | .0104 |
| 5/8 | 11 | .5069 | .0114 |
| 3/4 | 10 | .6201 | .0125 |
| 7/8 | 9 | .7307 | .0139 |
| | | | |
| 1 | 8 | .8376 | .0156 |
| 1-1/8 | 7 | .9394 | .0179 |
| 1-1/4 | 7 | 1.0644 | .0179 |
| 1-3/8 | 6 | 1.1585 | .0208 |
| 1-1/2 | 6 | 1.2835 | .0208 |
| 1-5/8 | 5-1/2 | 1.3888 | .0227 |
| 1-3/4 | 5 | 1.4902 | .0250 |
| 1-7/8 | 5 | 1.6152 | .0250 |
| 2 | 4-1/2 | 1.7113 | .0278 |
+-------------+-----------+-----------------+----------+

The standard pitches for the sharp [V]-thread are as follows:--

SIZE OF BOLT.
---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---
1/4| 5/|3/8| 7/|1/2|5/8|3/4|7/8| 1 | 1-| 1-| 1-| 1-| 1-| 1-| 1-| 2
| 16| | 16| | | | | |1/8|1/4|3/8|1/2|5/8|3/4|7/8|
---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---
NUMBER OF THREADS TO INCH.
---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---
20| 18| 16| 14| 12| 11| 10| 9 | 8 | 7 | 7 | 6 | 6 | 5 | 5 | 4-| 4-
| | | | | | | | | | | | | | |1/2|1/2
---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---

The following table gives the threads per inch, pitches and diameters at root of thread of the Whitworth thread. The table being arranged from the diameter of the screw as a basis.

+----------+---------+--------+----------------+
| Diameter | Threads | | Diameter at |
| of | per | Pitch. | Root or Bottom |
| Screw. | Inch. | | of Thread. |
+----------+---------+--------+----------------+
| | | Inch. | Inch. |
| 1/8 | 40 | .025 | .0929 |
| 3/16 | 24 | .041 | .1341 |
| 1/4 | 20 | .050 | .1859 |
| 5/16 | 18 | .056 | .2413 |
| 3/8 | 16 | .063 | .2949 |
| 7/16 | 14 | .071 | .346 |
| 1/2 | 12 | .083 | .3932 |
| 9/16 | 12 | .083 | .4557 |
| 5/8 | 11 | .091 | .5085 |
| 11/16 | 11 | .095 | .571 |
| 3/4 | 10 | .100 | .6219 |
| 13/16 | 10 | .100 | .6844 |
| 7/8 | 9 | .111 | .7327 |
| 15/16 | 9 | .111 | .7952 |
| 1 | 8 | .125 | .8399 |
| 1-1/8 | 7 | .143 | .942 |
| 1-1/4 | 7 | .143 | 1.067 |
| 1-3/8 | 6 | .167 | 1.1615 |
| 1-1/2 | 6 | .167 | 1.2865 |
| 1-5/8 | 5 | .200 | 1.3688 |
| 1-3/4 | 5 | .200 | 1.4938 |
| 1-7/8 | 4-1/2 | .222 | 1.5904 |
| 2 | 4-1/2 | .222 | 1.7154 |
| 2-1/8 | 4-1/2 | .222 | 1.8404 |
| 2-1/4 | 4 | .250 | 1.9298 |
| 2-3/8 | 4 | .250 | 2.0548 |
| 2-1/2 | 4 | .250 | 2.1798 |
| 2-5/8 | 4 | .250 | 2.3048 |
| 2-3/4 | 3-1/2 | .286 | 2.384 |
| 2-7/8 | 3-1/2 | .286 | 2.509 |
| 3 | 3-1/2 | .286 | 2.634 |
| 3-1/4 | 3-1/4 | .308 | 2.884 |
| 3-1/2 | 3-1/4 | .308 | 3.106 |
| 3-3/4 | 3 | .333 | 3.356 |
| 4 | 3 | .333 | 3.574 |
| 4-1/4 | 2-7/8 | .348 | 3.824 |
| 4-1/2 | 2-7/8 | .348 | 4.055 |
| 4-3/4 | 2-3/4 | .364 | 4.305 |
| 5 | 2-3/4 | .364 | 4.534 |
| 5-1/4 | 2-5/8 | .381 | 4.764 |
| 5-1/2 | 2-5/8 | .381 | 5.014 |
| 5-3/4 | 2-1/2 | .400 | 5.238 |
| 6 | 2-1/2 | .400 | 5.488 |
+----------+---------+--------+----------------+

The standard degree of taper, both for the taps and the dies, is 1/16 inch per inch, or 3/4 inch per foot, for all sizes up to 10-inch bore.

The sockets or couplings, however, are ordinarily tapped parallel and stretched to fit the pipe taper when forced on the pipe. For bores of pipe over 10 inches diameter the taper is reduced to 3/8 inch per foot. The pipes or casings for oil wells are given a taper of 3/8 inch per foot, and their couplings are tapped taper from both ends. There is, however, just enough difference made between the taper of the socket and that of the pipe to give the pipe threads a bearing at the pipe end first when tried with red marking, the threads increasing their bearing as the pieces are screwed together.

The United States standard thread for steam, gas and water pipe is given below, which is taken from the Report of the Committee on Standard Pipe and Pipe Threads of The American Society of Mechanical Engineers, submitted at the 8th Annual Meeting held in New York, November-December, 1886.

"A longitudinal section of the tapering tube end, with the screw-thread as actually formed, is shown full size in Fig. 291_a_ for a nominal 2-1/2 inch tube, that is, a tube of about 2-1/2 inches internal diameter, and 2-7/8 inches actual external diameter.

"The thread employed has an angle of 60°; it is slightly rounded off both at the top and at the bottom, so that the height or depth of the thread, instead of being exactly equal to the pitch, is only four fifths of the pitch, or equal to 0.8 × 1/_n_ if _n_ be the number of threads per inch. For the length of tube end throughout which the screw thread continues perfect, the empirical formula used is (0.8_D_ + 4.8) × 1/_n_, where _D_ is the actual external diameter of the tube throughout its parallel length, and is expressed in inches. Further back, beyond the perfect threads, come two having the same taper at the bottom, but imperfect at the top. The remaining imperfect portion of the screw thread, furthest back from the extremity of the tube, is not essential in any way to this system of joint; and its imperfection is simply incidental to the process of cutting the thread at a single operation.

The standard thicknesses of the pipes and pitches of thread are as follows:--

STANDARD DIMENSIONS OF WROUGHT IRON WELDED TUBES.

+-----------------------------+-----------+--------------------+
| DIAMETER OF TUBE. | | SCREWED ENDS. |
+---------+---------+---------+ THICKNESS +----------+---------+
| Nominal | Actual | Actual | OF | Number |Length of|
| Inside. | Inside. | Outside.| METAL. |of Threads| Perfect |
| | | | |per Inch. | Screw. |
+---------+---------+---------+-----------+----------+---------+
| Inches. | Inches. | Inches. | Inch. | No. | Inch. |
| 1/8 | 0.270 | 0.405 | 0.068 | 27 | 0.19 |
| 1/4 | 0.364 | 0.540 | 0.088 | 18 | 0.29 |
| 3/8 | 0.494 | 0.675 | 0.091 | 18 | 0.30 |
| 1/2 | 0.623 | 0.840 | 0.109 | 14 | 0.39 |
| 3/4 | 0.824 | 1.050 | 0.113 | 14 | 0.40 |
| 1 | 1.048 | 1.315 | 0.134 | 11-1/2 | 0.51 |
| 1-1/4 | 1.380 | 1.660 | 0.140 | 11-1/2 | 0.54 |
| 1-1/2 | 1.610 | 1.900 | 0.145 | 11-1/2 | 0.55 |
| 2 | 2.067 | 2.375 | 0.154 | 11-1/2 | 0.58 |
| 2-1/2 | 2.468 | 2.875 | 0.204 | 8 | 0.89 |
| 3 | 3.067 | 3.500 | 0.217 | 8 | 0.95 |
| 3-1/2 | 3.548 | 4.000 | 0.226 | 8 | 1.00 |
| 4 | 4.026 | 4.500 | 0.237 | 8 | 1.05 |
| 4-1/2 | 4.508 | 5.000 | 0.246 | 8 | 1.10 |
| 5 | 5.045 | 5.563 | 0.259 | 8 | 1.16 |
| 6 | 6.065 | 6.625 | 0.280 | 8 | 1.26 |
| 7 | 7.023 | 7.625 | 0.301 | 8 | 1.36 |
| 8 | 8.982 | 8.625 | 0.322 | 8 | 1.46 |
| 9 | 9.000 | 9.688 | 0.344 | 8 | 1.57 |
| 10 | 10.019 | 10.750 | 0.366 | 8 | 1.68 |
+---------+---------+---------+-----------+----------+---------+

The taper of the threads is 1/16 inch in diameter for each inch of length or 3/4 inch per foot.

WHITWORTH'S SCREW THREADS FOR GAS, WATER, AND HYDRAULIC IRON PIPING.

NOTE.--The Internal and External diameters of Pipes, as given below, are those adopted by the firm of Messrs. JAMES RUSSELL & SONS, in Pipes of their manufacture.

+---------------------------------------+
| GAS AND WATER PIPING. |
+-------------+-------------+-----------+
| Internal | External | No. of |
| Diameter of | Diameter of | Threads |
| Pipe. | Pipe. | per Inch. |
+-------------+-------------+-----------+
| 1/8 | .385 | 28 |
| 1/4 | .520 | 19 |
| 3/8 | .665 | 19 |
| 1/2 | .822 | 14 |
| 3/4 | 1.034 | 14 |
| 1 | 1.302 } | |
| 1-1/8 | 1.492 } | |
| 1-1/4 | 1.650 } | |
| 1-3/8 | 1.745 } | |
| 1-1/2 | 1.882 } | |
| 1-5/8 | 2.021 } | |
| 1-3/4 | 2.047 } | |
| 1-7/8 | 2.245 } | |
| 2 | 2.347 } | |
| 2-1/8 | 2.467 } | |
| 2-1/4 | 2.587 } | 11 |
| 2-3/8 | 2.794 } | |
| 2-1/2 | 3.001 } | |
| 2-5/8 | 3.124 } | |
| 2-3/4 | 3.247 } | |
| 2-7/8 | 3.367 } | |
| 3 | 3.485 } | |
| 3-1/4 | 3.698 } | |
| 3-1/2 | 3.912 } | |
| 3-3/4 | 4.125 } | |
| 4 | 4.339 } | |
+-------------+-------------+-----------+

+----------------------------------------------------+
| HYDRAULIC PIPING. |
+----------+----------+------------------+-----------+
| Internal | External | Pressure in lbs. | No. of |
| Diameter | Diameter | per Square | Threads |
| of Pipe. | of Pipe. | Inch. | per Inch. |
+----------+----------+------------------+-----------+
| | { 5/8 | 4,000} | |
| 1/4 | { 3/4 | 6,000} | 14 |
| | { 7/8 | 8,000} | |
| | {1 | 10,000} | |
| | | | |
| | { 3/4 | 4,000} | |
| 3/8 | { 7/8 | 6,000} | 14 |
| | {1 | 8,000} | |
| | {1-1/8 | 10,000} | |
| | | | |
| | {1 | 4,000} | 14 |
| 1/2 | {1-1/8 | 6,000} | |
| | {1-1/4 | 8,000 } | 11 |
| | {1-3/8 | 10,000 } | |
| | | | |
| | {1-1/8 | 4,000 | 14 |
| 5/8 | {1-1/4 | 6,000} | |
| | {1-3/8 | 8,000} | 11 |
| | {1-1/2 | 10,000} | |
| | | | |
| | {1-1/4 | 4,000} | |
| 3/4 | {1-3/8 | 6,000} | 11 |
| | {1-1/2 | 8,000} | |
| | {1-5/8 | 10,000} | |
| | | | |
| | {1-3/8 | 4,000} | |
| 7/8 | {1-1/2 | 6,000} | 11 |
| | {1-5/8 | 8,000} | |
| | {1-3/4 | 10,000} | |
| | | | |
| | {1-1/2 | 4,000} | |
| 1 | {1-5/8 | 6,000} | 11 |
| | {1-3/4 | 8,000} | |
| | {1-7/8 | 10,000} | |
| | | | |
| | {1-5/8 | 4,000} | |
| 1-1/8 | {1-3/4 | 6,000} | 11 |
| | {1-7/8 | 8,000} | |
| | {2 | 10,000} | |
| | | | |
| | {1-3/4 | 4,000} | |
| 1-1/4 | {1-7/8 | 6,000} | 11 |
| | {2 | 8,000} | |
| | {2-1/8 | 10,000} | |
| | | | |
| | {1-7/8 | 4,000} | |
| 1-3/8 | {2 | 6,000} | 11 |
| | {2-1/8 | 8,000} | |
| | {2-1/4 | 10,000} | |
| | | | |
| | {2 | 4,000} | |
| | {2-1/8 | 6,000} | |
| 1-1/2 | {2-1/4 | 8,000} | 11 |
| | {2-3/8 | 10,000} | |
| | {2-1/2 | 10,000} | |
| | | | |
| | {2-1/8 | 4,000} | |
| 1-5/8 | {2-1/4 | 6,000} | 11 |
| | {2-3/8 | 8,000} | |
| | {2-1/2 | 10,000} | |
| | | | |
| | {2-1/4 | 3,000} | |
| | {2-3/8 | 4,000} | |
| 1-3/4 | {2-1/2 | 6,000} | 11 |
| | {2-5/8 | 8,000} | |
| | {2-3/4 | 10,000} | |
| | | | |
| | {2-3/8 | 3,000} | |
| | {2-1/2 | 4,000} | |
| 1-7/8 | {2-5/8 | 6,000} | 11 |
| | {2-3/4 | 8,000} | |
| | {2-7/8 | 10,000} | |
| | | | |
| | {2-1/2 | 3,000} | |
| | {2-5/8 | 4,000} | |
| 2 | {2-3/4 | 6,000} | 11 |
| | {2-7/8 | 8,000} | |
| | {3 | 10,000} | |
+----------+----------+------------------+-----------+

The English pipe thread is a sharp [V]-thread having its sides at an angle of 60°, and therefore corresponds to the American pipe thread except that the pitches are different.

The standard screw thread of The Royal Microscopical Society of London, England, is employed for microscope objectives, and the nose pieces of the microscope into which these objectives screw.

The thread is a Whitworth one, the original standard threading tools now in the cabinet of the society having been made especially for the society by Sir Joseph Whitworth. The pitch of the thread is 36 per inch. The cylinder, or male gauge, is .7626 inch in diameter.

The following table gives the Whitworth standard of thread pitches and diameters for watch and mathematical instrument makers.

WHITWORTH'S STANDARD GAUGES FOR WATCH AND INSTRUMENT MAKERS, WITH SCREW THREADS FOR THE VARIOUS SIZES, 1881.

+---------------------+-------------+-------------+
| No. of each | Size in | Number of |
| size in thousandths | decimals of | Threads per |
| of an inch. | an inch. | inch. |
+---------------------+-------------+-------------+
| 10 | .010 | 400 |
| 11 | .011 | " |
| 12 | .012 | 350 |
| 13 | .013 | " |
| 14 | .014 | 300 |
| 15 | .015 | " |
| 16 | .016 | " |
| 17 | .017 | 250 |
| 18 | .018 | " |
| 19 | .019 | " |
| 20 | .020 | 210 |
| 22 | .022 | " |
| 24 | .024 | " |
| 26 | .026 | 180 |
| 28 | .028 | " |
| 30 | .030 | " |
| 32 | .032 | 150 |
| 34 | .034 | " |
| 36 | .036 | " |
| 38 | .038 | 120 |
| 40 | .040 | " |
| 45 | .045 | " |
| 50 | .050 | 100 |
| 55 | .055 | " |
| 60 | .060 | " |
| 65 | .065 | 80 |
| 70 | .070 | " |
| 75 | .075 | " |
| 80 | .080 | 60 |
| 85 | .085 | " |
| 90 | .090 | " |
| 95 | .095 | " |
| 100 | .100 | 50 |
+---------------------+-------------+-------------+

For the pitches of the threads of lag screws there is no standard, but the following pitches are largely used.

+-----------+-----------+
|Diameter of| Threads |
| Screw. | per Inch. |
+-----------+-----------+
| Inch. | |
| 1/4 | 10 |
| 5/16 | 9 |
| 3/8 | 8 |
| 7/16 | 7 |
| 1/2 | 6 |
| 9/16 | 6 |
| 5/8 | 5 |
| 11/16 | 5 |
| 3/4 | 5 |
| 7/8 | 4 |
| 1 | 4 |
+-----------+-----------+

SCREW-CUTTING HAND TOOLS.

For cutting external or male threads by hand three classes of tools are employed.

The first is the screw plate shown in Fig. 292. It consists of a hardened steel plate containing holes of varying diameters and threaded with screw threads of different pitches. These holes are provided with two diametrically opposite notches or slots so as to form cutting edges.

This tool is placed upon the end of the work and slowly rotated while under a hand pressure tending to force it upon the work, the teeth cutting grooves to form the thread and advancing along the bolt at a rate determined by the pitch of the thread.

The screw plate is suitable for the softer metals and upon diameters of 1/8 inch and less, in which the cutting duty is light; hence the holes do not so rapidly wear larger.

The second class consists of a stock and dies such as shown in Fig. 293. For each stock there are provided a set of dies having different diameters and pitches of thread.

In this class of tool the dies are opened out and placed upon the bolt. The set screw is tightened up, forcing the dies to their cut, and the stock is slowly rotated and a traverse taken down the work.

In some cases the dies are then again forced to the work by the set screw, and a cut taken by winding the stocks up the bolt, the operation being continued until the thread is fully developed and cut to the required diameter. In other cases the cut is carried down the bolt, only the dies being wound back to the top of the bolt after each cut is carried down. The difference between these two operations will be shown presently.

The thread in dies which take successive cuts to form a thread may be left full clear through the die, and will thus cut a full thread close up to the head collar or shoulder of the work. It is usual, however, to chamfer off the half threads at the ends of the dies, because if left of their full _height_ they are apt to break off when in use. It is sometimes the practice, however, to chamfer off the first two threads on one side of the dies, leaving the teeth on the other side full, and to use the chamfered as the leading side in all cases in which the thread on the work does not require to be cut up to a shoulder, but turning the dies over with the full threaded teeth as the leading ones when the thread _does_ require to be carried up to a head or shoulder on the work.

To facilitate the insertion and extraction of the dies in and from their places in the stock, the Morse Twist Drill Co. employ the following construction. In Figs. 294 and 295 the pieces A, A´ which hold the dies are pivoted in the stock at B, so as to swing outward as in Fig. 295, and receive the dies which are slotted to fit them. These pieces are then swung into position in the stock. The lower die is provided with a hole to fit the pin C, hence when that die is placed home C acts as a detaining piece locking the pieces A, A´ through the medium of the bottom die.

In other dies of this class the two side pieces or levers which hold the dies are pivoted at the corner of the angle, as in Fig. 296. In the bottom of the stock is a sliding piece beveled at its top and meeting the bottom face of the levers; hence, by pressing this piece inwards the side pieces recede into a slot provided in the stock, and leave the opening free for the dies to pass into their places, when the pin is released and a spring brings the side pieces back. Now, since the bottom die rests upon the bottom angle of the side pieces the pressure of the set screw closes the side pieces to the dies holding them firmly.

In Fig. 297 is shown Whitworth's stocks and dies, the cap that holds the guide die _a_ and the two chasers _b_, _c_ in their seats or recesses in the stock being removed to expose the interior parts. The ends of the chasers _b_, _c_ are beveled and abut against correspondingly beveled recesses in the key _d_, so that by operating the nut _e_ on the end of the key the dies are caused to move longitudinally. The principles of action are more clearly shown in Fig. 298. The two cutting chasers B and C move in lines that would meet at D, and therefore at a point behind the centre or axis of the bolt being threaded; this has the effect of preserving their clearance. It is obvious, for example, that when these chasers cut a thread on the work it will move over toward guide A on account of the thread on the work sinking into the threads on A, and this motion would prevent the chasers B, C from cutting if they moved in a line pointing to the centre of the work. This is more clearly shown in Fig. 299, in which the guide die A and one of the cutting dies or chasers B is shown removed from the stock, while the bolt to be threaded is shown in two positions--one when the first cut is taken, and the other when the thread is finished. For the first cut the centre of the work is at E, for the last one it is at G, and this movement would, were the line of motion as denoted by the dotted lines, prevent the chaser from cutting, because, while the line of chaser motion would remain at J, pointing to the centre of work for the first cut, it would require a line at K to point to that centre for the last one; hence, when considered with relation to the work, the line of chaser motion has been moved forward, presenting the cutting edges at an angle that would prevent their cutting. By having their motion as shown in Fig. 299, however, the clearance of the chasers is preserved.

Referring now to the die A, it acts as a guide rather than as a cutting chaser, because it has virtually no clearance and cannot cut so freely as B and C; hence it offers a resistance to the moving of the bolt, or of the dies upon the bolt, in a lateral direction when the chaser teeth meet either a projection or a depression upon the work. The guide principle is, however, much more fully carried out in a design by Bodmer, which is shown in Fig. 300. Here there is but one cutting chaser C, the bush G being a guide let into a recess in the stock and secured thereon by a pin _p_. The chaser is set in a stock, D also let into a recess in the stock, and this recess, being circular, permits of stock D swinging. At S are two set-screws, which are employed to limit the amount of motion permitted to D. the handle E screws through D, and acts upon the edge of chaser C to put on the cut. The action of the tool is shown in Fig. 301, where it is shown upon a piece of work. Pulling the handle E causes D to swing in the stock, thus giving the chaser clearance, as shown. When the cut is carried down, a new cut may be put on by means of E, and on winding the stock in the opposite direction, D will swing in its seat, and cant or tilt the chaser in the opposite direction, giving it the necessary clearance to enable it to cut on the upward or back traverse. Another point of advantage is that the cutting edges are not rubbed by the work during the back stroke, and their sharpness is, therefore, greatly preserved. A die of this kind will produce work almost as true as the lathe, and, in the case of long, slender work, more true than the lathe; but it is obvious that, on account of the friction caused by the pressure of the work to the guide G, the tool will require more power to operate than the ordinary stock and die or the solid die.

In adjustable dies which require to take more than one cut along the bolt to produce a fully developed thread, there is always a certain amount of friction between the sides of the thread in the die and the grooves being cut, because the angle of the thread at the top of a thread is less than the angle at the bottom. Thus in Fig. 302 the pitch at the top of thread (at A, B) is the same as at the bottom (C, D). Now suppose that in Fig. 303 _a_ _b_ represents the axial line of a bolt, and _c_ _d_ a line at a right angle to _a_ _b_. The radius _e_ _f_ being equal to the circumference of the top of the thread, the pitch being represented by _b_; then _k_ represents the angle of the top of the thread to the axial line _a_ _b_. Now suppose that the radius _e_ _g_ represents the circumference at the bottom of the thread and to the pitch; then _l_ is the angle of the bottom of the thread to the axial line of the work, and the difference in angle between _k_ and _l_ is the difference in angle between the top and bottom of the thread in the dies and the thread to be cut on the work.

Now the tops of the teeth on the die stand at the greatest angle _l_, in Fig. 303, when taking the first cut on the bolt, but the grooves they cut will be on the full diameter of the bolt, and will, therefore, stand at the angle _k_, hence the lengths of the teeth do not lie in the same planes as the grooves which they cut.

In cutting [V]-threads, however, the angle of the die threads gradually right themselves with the plane of the grooves attaining their nearest coincidence when closed to finish the thread.

Since, however, the full width of groove is in a square thread cut at the first cut taken by the dies, it is obvious that a square thread cannot be cut by this class of die, because the sides of the grooves would be cut away each time the dies were closed to take another cut.

Dies of this class require to have the threaded hole made of a larger diameter than is the diameter of the bolt they are intended to thread, the reason being as follows:--

Suppose the threaded hole in the dies to be cut by a hob or master tap of the same diameter as the thread to be cut by the dies; when the dies are opened out and placed upon the work as in Fig. 304, the edges A, B will meet the work, and there will be nothing to steady the dies, which will, therefore, wobble and start a drunken thread, that is to say, a thread such as was shown in Fig. 253.

Instances have been known in the use of dies made in this manner, wherein the workman using a right-hand single-threaded pair of dies has cut a right or left-hand double or treble thread; the teeth of the dies acting as chasers well canted over, as shown in Fig. 305. It is necessary to this operation, however, that the diameter of the work be larger than the size of hob the dies were threaded with.

In Fig. 306 is shown a single right-hand and a treble left-hand thread cut by the author with the same pair of dies.

All that is necessary to perform this operation is to rotate the dies from left to right to produce a right-hand thread, and from right to left for a left-hand thread, exerting a pressure to cause the dies to advance more rapidly along the bolt than is due to the pitch of the thread. A double thread is produced when the dies traverse along the work twice as fast as is due to the pitch of the thread in the dies, and so on.

It is obvious, also, that a piece of a cylindrical thread may be used to cut a left-hand external thread. Thus in Fig. 307 is shown a square piece of metal having a notch cut in on one side of it and a piece of an external thread (as a tap inserted) in the notch. By forcing a piece of cylindrical work through the hole while rotating it, the piece of tap would cut upon the work a thread of the pitch of the tap, but a left-handed thread, which occurs because, as shown by the dotted lines of the figure, the thread on one side of a bolt slopes in opposite directions to its direction on the other, and in the above operation the thread on one side is taken to cut the thread on the other.

These methods of cutting left-hand threads with right-handed ones are mentioned simply as curiosities of thread cutting, and not as being of any practical value.

To proceed, then: to avoid these difficulties it is usual to thread the dies with a hob or master tap of a diameter equal to twice the depth of the thread, larger than the size of bolt the dies are to thread. In this case the dies fit to the bolt at the first cut, as shown in Fig. 308, C, D being the cutting edges. The relation of the circle of the thread in the dies to that of the work during the final cut is shown in Fig. 309.

There is yet another objection to tapping the dies with a hob of the diameter of the bolt to be threaded, in that the teeth fit perfectly to the thread of the bolt when the latter is threaded to the proper diameter, producing a great deal of friction, and being difficult to make cut, especially when the cutting edges have become slightly dulled from use.

Referring now to taking a cut up the bolt or work as well as down, it will be noted that supposing the dies to have a right-hand thread, and to be rotating from left to right, they will be passing down the bolt and the edges C, D (Fig. 308) will be the cutting ones. But when the dies are rotated from right to left to bring them to the end of the bolt again, C, D will be rubbed by the thread, which tends to abrade them and thus destroy their sharpness.

In some cases two or more pairs of dies are fitted to the same stock, as shown in Fig. 310, but this is objectionable, because it is always desirable to have the hole in the dies central to the length of the stock, so that when placed to the work the stock shall be balanced, which will render it easier to start the thread true with the axial line of the bolt.

From what has been said with reference to Fig. 303, it is obvious that a square thread cannot be cut by a die that opens and closes to take successive cuts along the work, but such threads may be cut upon work that is of sufficient strength to withstand the twisting pressure of the dies, by making a solid die, and tapering off the threads for some distance at the mouth of the die, so as to enable the die to take its bite or grip upon the work, and start itself. It is necessary, however, to give to the die as many flutes (and therefore cutting edges), as possible, or else to make flutes wide and the teeth as short as will leave them sufficiently strong, both these means serving to avoid friction.

The teeth for adjustable dies, such as shown in Fig. 293, are cut as follows:--There is inserted between the two dies a piece of metal, separating them when set together to a distance equal to twice the depth of the thread, added to the distance the faces of the dies are to be apart when the dies are set to cut to this designated or proper diameter. The tapping hole is then drilled (with the pieces in place) to the diameter of the bolt the die is for. The form of hob used by the Morse Twist Drill & Machine Company, to cut the thread, is shown in Fig. 311. The unthreaded part at the entering end is made to a diameter equal to that of the work the dies are to be used in; the thread at the entering end is made sunk in one half the height of the full thread, and is flattened off one half the height of a full thread, so that the top of the thread is even with the diameter of the unthreaded part at the entering end. The thread then runs a straight taper up the hob until a distance equal to the diameter of the nut is reached, and the length of hob equal to its diameter is made a full and parallel thread for finishing the die teeth with. The thread on the taper part has more taper at the root of the thread than it has at the top of the same, and the diameter of the full and parallel part at the shank end of the thread is made of a diameter equal to twice the height or depth of a full thread, larger than the diameter at the entering end of the hob. The hob thus becomes a taper and relieved tap cutting a full thread at one passage through the dies. If the hob is made parallel and a full thread from end to end, as in Fig. 312, the dies must traverse up and down the hob, or the hob through the dies to form a full thread.

The third class of stock and die is intended to cut a full thread at one passage along the work, while at the same time provision is made, whereby, to take up the wear due to the abrasion of the cutting edges, which wear would cause the diameter of thread cut to be above the standard.

In Fig. 313 is shown the Grant adjustable die made by the Pratt & Whitney Company. It consists of four chasers or toothed cutting tools, inserted in radial recesses or slots in an iron disc or collet encircled by an iron ring. Each chaser is beveled at its end to fit a corresponding bevel in the ring, and is grooved on one of its side faces to receive the hardened point of a screw that is inserted in the collet to hold the chaser in its adjusted position. Four screws extend up through the central flange or body of the collet, two of which serve to draw down the ring, and by reason of the taper on the ring move the chasers equally towards the centre and reduce the cutting diameter of the die, while the other two hold the ring in the desired position, or force it upward to enlarge the cutting diameter of the die. The range of adjustment permitted by this arrangement is 1-32 inch. The dies may be taken out and ground up to sharpen.

The object of cutting grooves in the sides of the chasers is that the fine burrs formed by the ends of the set screws do not prevent the chasers from moving easily in the collet during the process of adjustment; the groove also acts as a shoulder for the screw end to press the chaser down to its seat. These chasers are marked to their respective places in the collet, and are so made that if one chaser should break, a new one can be supplied to fit to its place, the teeth of the new one falling exactly in line with the teeth on the other three, whereas under ordinary conditions if one chaser breaks, a full set of four new ones must be obtained.

In this die, as in all others which cut a full thread at one passage along the work, the front teeth of the chasers are beveled off as shown in the cut; this is necessary to enable the dies to take hold of or "bite" the work, the chamfer giving a relief to the cutting edge, while at the same time forming to a certain extent a wedge facilitating the entrance of the work into the die.

Fig. 314 represents J. J. Grant's patent die, termed by its makers (Wiley and Russel) the "lightening die." In this, as in other similar stocks, several collets with dies of various pitches and diameters of thread, fit to one stock. The nut of the stock is split on one side, and is provided with lugs on that side to receive a screw, which operates to open and enlarge the bore to release a collet, or close thereon and grip it, as may be required when inserting or extracting the same. The dies are formed as shown in Fig. 315, in which A, A are the dies, and B the collet. To open the dies within the collet, the screws E are loosened and the screws D are tightened, while to close the dies D, D are loosened and E are tightened; thus the adjustment to size is effected by these four screws, while the screws D also serve to hold the dies to the collet B. The collets are provided with a collar having a bore F, through which the work passes, so that the dies may be guided true when starting upon the work; but if it is required to cut a thread close up to a head or shoulder, the stock is turned upside down, not only to have the collet out of the way of the head or shoulder, but also because the thread of the dies on the collet side are chamfered off (as is necessary in all solid dies, or dies which cut a full thread at one traverse down the work) so as to enable them to grip or bite the work, and start the thread upon it as before stated.

In Fig. 316 is shown Stetson's die, which cuts a full thread at one passage, is adjustable to take up its wear, and has a guide to steady it upon the work and assist it in cutting a true thread. The guide piece consists of a hub (through which the work passes) having a flange fitting into the dies and being secured thereto by the two screws shown. The holes in the flanges are slotted to permit of the dies being closed (to take up wear) by means of the small screws shown at the end of the die, which screws pass through one die in a plain hole and screw into the other.

In Fig. 317 is shown Everett's stocks and dies. In this tool the dies are set up by a cam lever, the dies being set to standard size when the lever arm stands parallel with the arm of the stock. By turning the straight side of the cam lever opposite to the dies, the latter may be instantly removed and another size of die inserted. The dies may be used to cut on their passage up and down the bolt or by operating the cam. When the dies are at the end of a cut the dies may be opened, lifted to the top of the work and another cut taken, thus saving the time necessary to wind the stock back. When the final cut is taken the dies may be opened and lifted off the work.

The hardening process usually increases the thickness of these dies, making the pitch of the thread coarser. The amount of expansion due to hardening is variable, but increases with the thickness of the die. The hob as a rule shortens during the tempering, but the amount being variable, no rule for its quantity can be given.[12]

[12] See also page 108.

Stocks and dies for pipe work are made in the form shown in Fig. 318, in which B is the stock having the detachable handles (for ease of conveyance) A, H, the latter being shown detached. The solid screw-cutting dies C are placed in the square recess at B, and are secured in B by the cap D, which swings over (upon its pivoted end as a centre) and is locked by the thumbscrew E. To guide the stocks and cause them to cut a true thread, the bushes F are provided. These fit into the lower end of B and are locked in position by four set screws G. The bores of the bushes F are made an easy fit to the outside of the pipe to be threaded, there being a separate bush for each size of pipe.

The dies employed in stocks for threading steam and gas pipes by hand are sometimes solid, as in Fig. 318 at C, and at others adjustable. In Fig. 319 is shown Stetson's adjustable pipe die containing four chasers or toothed thread-cutting tools. These are set to cut the required diameter by means of a small screw in each corner of the die, while they are locked in their adjusted position by four screws on the face.

The tap is a tool employed to cut screw threads in internal surfaces, as holes or bores. A set of taps for hand use usually consist of three: the taper tap, Fig. 320; plug tap, Fig. 321; and bottoming tap, Fig. 322. (In England these taps are termed respectively the taper, second, and plug tap.) The taper tap is the first to be inserted, and (when the hole to be threaded passes entirely through the work) rotated until it passes through the work, thus cutting a thread parallel in diameter through the full length of the hole. If, however, the hole does not pass through the work, the taper tap leaves a taper-threaded hole containing more or less of a fully developed thread according to the distance the tap has entered.

To further complete the thread the plug tap is inserted, it being parallel from four or five threads from the entering end of the tap to the other end. If the work will admit it, this tap is also passed through, which not only saves time in many cases, by avoiding the necessity to wind the tap back, but preserves the cutting edge which suffers abrasion from being wound back. To cut a full thread as near as possible to the bottom of a hole the bottoming tap is used, but when the circumstances will admit, it is best to drill the hole rather deeper than is actually necessary, to avoid the trouble incident to tapping a hole clear to the bottom.

On wrought iron and steel, which are fibrous and tough, the tap, when used by hand, will not (if the hole be deeper than the diameter of the tap) readily operate by a continuous rotary motion, but requires to be rotated about half a revolution back occasionally, which gives opportunity for the oil to penetrate to the cutting edges of the tap, frees the tap and considerably facilitates the tapping operation, especially if the hole be a deep one.