Modern Machine-Shop Practice Part 24

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 1-1/2 inches at the top of the thread, and 1-1/8 inches at the bottom, while C is 1-1/8 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 1-1/8 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 exact.i.tude 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 ill.u.s.trated 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 ab.u.t.ting against the face of the other, while at F is another screw pa.s.sing 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 a.s.sumed that we have a threading tool correctly formed.

[Ill.u.s.tration: Fig. 288.]

[Ill.u.s.tration: Fig. 289.]

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 pa.s.ses through in once winding around the bolt, we proceed as in Fig. 288, in which D represents the circ.u.mference 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 circ.u.mference, 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.

[Ill.u.s.tration: Fig. 290.]

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 s.p.a.ce 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.

[Ill.u.s.tration: Fig. 291.]

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 pa.s.sage 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 pa.s.s 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.

[Ill.u.s.tration: Fig. 291_a_.]

"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.