[Ill.u.s.tration: Fig. 16. Cross-slide equipped with Stop for Regulating Depth of Cut when Threading]

=The Acme Standard Thread.=--The Acme thread is often used, at the present time, in place of a square thread. The angle between the sides of the Acme thread is 29 degrees (see Fig. 21) and the depth is made equal to one-half the pitch plus 0.010 inch to provide clearance and insure a bearing upon the sides. The thread tool is ordinarily ground to fit a gage having notches representing different pitches. An improved form of Acme thread gage is shown in Fig. 17. The tool point is first ground to the correct angle by fitting it to the 29-degree notch in the end of the gage, as at _A_. The end is then ground to the proper width for the pitch to be cut, by testing it, as at _B_. The numbers opposite the shallow notches for gaging the width represent the number of threads per inch. With this particular gage, the tool can be set square by placing edge _D_ against the turned surface to be threaded, and adjusting the tool until the end is in line with the gage, as at _C_. By placing the tool in this position, the angle between the side and the end can also be tested.

[Ill.u.s.tration: Fig. 17. Gage for grinding and setting Acme Thread Tools]

In case it should be necessary to measure the end width of an Acme thread tool, for a pitch not on the regular gage, this can be done by using a vernier gear-tooth caliper, as indicated in Fig. 18. If we a.s.sume that the caliper jaws bear on the sides of the tool at a distance _A_ from the top, equal to 1/4 inch, then the width of the tool point equals the caliper reading (as shown by the horizontal scale) minus 0.1293 inch. For example, if the caliper reading was 0.315 inch, the width at the point would equal 0.315 - 0.1293 = 0.1857 inch, a.s.suming that the sides were ground to the standard angle of 29 degrees. The constant to be subtracted from the caliper reading equals 2 _A_ tan 14 30' or, in this case, 2 0.25 0.2586 = 0.1293.

[Ill.u.s.tration: Fig. 18. Measuring Width of Acme Thread Tool with Vernier Gear-tooth Caliper]

=The Whitworth Thread.=--The Whitworth (or British Standard Whitworth) thread, which is used princ.i.p.ally in Great Britain, has an included angle of 55 degrees, and the threads are rounded at the top and at the root, as shown in Fig. 23. The shape of the tool used for cutting this thread is also shown in this ill.u.s.tration. The end is rounded to form the fillet at the root of the thread, and the round corners on the sides give the top of the thread the required curvature. Every pitch requires a different tool, and the cutting end is given the curved form by milling or hobbing. The hob used for this purpose is accurately threaded to correspond with the pitch for which the tool is required, and then it is fluted to form cutting edges, and is hardened. The hob is then used like a milling cutter for forming the end of the thread tool. The tool is sharpened by grinding on the top. The method of cutting a Whitworth thread is, of course, similar to that followed for a U. S. standard or V-thread, in that the tool is set square with the unthreaded blank and at the same height as the lathe centers, in order to secure a thread of the proper form. Care should be taken to turn the blank to the right diameter so that the top of the thread will be fully rounded when the screw is the required size.

[Ill.u.s.tration: Fig. 19. United States Standard Thread]

[Ill.u.s.tration: Fig. 20. Standard Sharp V-thread]

[Ill.u.s.tration: Fig. 21. Acme Standard Thread]

[Ill.u.s.tration: Fig. 22. Square Thread]

[Ill.u.s.tration: Fig. 23. Whitworth Standard Thread]

[Ill.u.s.tration: Fig. 24. Standard Worm Thread]

=Worm Threads.=--The standard worm thread has an angle of 29 degrees between the sides, the same as an Acme thread, but the depth of a worm thread and the width of the flat at the top and bottom differ from the Acme standard, as will be seen by comparing Figs. 21 and 24. The whole depth of the thread equals the linear pitch multiplied by 0.6866, and the width of the thread tool at the end equals the linear pitch multiplied by 0.31. Gages notched for threads of different pitch are ordinarily used when grinding worm thread tools.

When it is necessary to cut multiple-threaded worms of large lead in an ordinary lathe, difficulty is sometimes experienced because the lead-screw must be geared to run much faster than the spindle, thus imposing excessive strains on the gearing. This difficulty is sometimes overcome by mounting a belt pulley on the lead-screw, beside the change gear, and connecting it to the countershaft by a belt; the spindle is then driven through the change gearing from the lead-screw, instead of _vice versa_.

=Coa.r.s.e Threading Attachment.=--To avoid the difficulties connected with cutting threads of large lead, some lathes are equipped with a coa.r.s.e screw-cutting attachment. The arrangement of this attachment, as made by the Bradford Machine Tool Co., is as follows: On the usual reversing shaft, and inside of the headstock, there is a sliding double gear, so arranged as to be engaged with either the usual gear on the spindle, or with a small pinion at the end of the cone. The gears are so proportioned that the ratio of the two engagements is as 10 to 1; that is, when engaged with the cone gear (the back-gears being thrown in) the mating gear will make ten revolutions to one of the spindle, so that when the lathe is ordinarily geared to cut one thread per inch, it will, when driven by the cone pinion, cut one thread in ten inches. This construction dispenses with the extra strain on the reverse gears due to moving the carriage at the rapid rate that would be necessary for such a large lead, when not using an attachment. These attachments are not only extensively used for the cutting of coa.r.s.e screws but for cutting oil grooves on cylindrical parts.

When cutting a thread of large lead or "steep pitch," the top of the thread tool should be ground so that it is at right angles to the thread; then the thread groove will be cut to the same width as the tool.

=Testing the Size of a Thread.=--When the thread tool has been fed in far enough to form a complete thread, the screw is then tested for size.

If we a.s.sume that a bolt is being threaded for a standard nut, it would be removed from the lathe and the test made by s.c.r.e.w.i.n.g a nut on the end. If the thread were too large, the nut might screw on very tightly or not at all; in either case, the work would again be placed in the lathe and a light cut taken over it to reduce the thread to the proper size. When replacing a threaded part between the centers, it should be put back in the original position, that is, with the "tail" of the driving dog in the same slot of the faceplate it previously occupied.

[Ill.u.s.tration: Fig. 25. Testing Diameter of Thread with Calipers and Micrometer]

As it is difficult to tell just when a thread is cut to the exact size, special thread calipers having wedge-shaped ends are sometimes used for measuring the diameter of a V-thread or a U. S. standard thread, at the bottom of the grooves or the root diameter, as shown at _A_ in Fig. 25.

These calipers can be set from a tap corresponding to the size of the thread being cut, or from a previously threaded piece of the right size.

=The Thread Micrometer.=--Another form of caliper for testing threads is shown at _B_. This is one of the micrometer type and is intended for very accurate work. The spindle of this micrometer has a conical end and the "anvil" is V-shaped, and these ends bear on the sides of the thread or the surfaces which form the bearing when the screw is inserted in a nut or threaded hole. The cone-shaped point is slightly rounded so that it will not bear in the bottom of the thread. There is also sufficient clearance at the bottom of the V-shaped anvil to prevent it from bearing on top of the thread. The diameter as indicated by this micrometer is the "pitch diameter" of the thread and is equal to the outside diameter minus the depth of one thread. This depth may be determined as follows:

Depth of a V-thread = 0.866 No. of threads per inch;

Depth of a U. S. standard thread = 0.6495 No. of threads per inch;

Depth of Whitworth thread = 0.6403 No. of threads per inch.

The movable point measures all pitches, but the fixed anvil is limited in its capacity, for if made large enough to measure a thread of, say, 1/4-inch pitch, it would be too wide at the top to measure a thread of 1/24-inch pitch, hence each caliper is limited in the range of threads that the anvil can measure. When measuring the "angle diameter" of a thread, the micrometer should be pa.s.sed back and forth across the thread, in order to make sure that the largest dimension or the actual diameter is being measured. If the micrometer is placed over what seems to be the center of the screw and the reading is taken by simply adjusting in the anvil or point against the thread, without moving the micrometer back and forth across it, an incorrect reading may be obtained.

If standard threaded reference gages are available, the size of the thread being cut can be tested by comparing it with the gage.

Micrometers having small spherical measuring ends (see sketch _A_, Fig.

26) are sometimes used for this purpose. The ball points are small enough to bear against the sides of the thread and the diameter, as compared with the reference gage, can be determined with great accuracy.

[Ill.u.s.tration: Fig. 26. (A) Testing Size of Thread with Ball-point Micrometer. (B) Testing Size of V-thread by the Three-wire System. (C) Testing the Size of a U. S. Standard Thread]

=Three-wire System of Measuring Threads.=--A method of measuring threads by using an ordinary micrometer and three wires of equal diameter is ill.u.s.trated at _B_ and _C_, Fig. 26. Two wires are placed between the threads on one side and one on the opposite side of the screw. The dimension _M_ over the wires is then measured with an ordinary micrometer. When the thread is cut to a standard size, the dimension _M_ for different threads is as follows:

For a U. S. standard thread:

_m_ = _d_ - 1.5155_p_ + 3_w_

For a sharp V-thread:

_m_ = _d_ - 1.732_p_ + 3_w_

For a Whitworth standard thread:

_m_ = _d_ - 1.6008_p_ + 3.1657_w_

In these formulas, _d_ = standard outside diameter of screw; _m_ = measurement over wires; _w_ = diameter of wires; _p_ = pitch of thread = 1 number of threads per inch.

To ill.u.s.trate the use of the formula for the U. S. standard thread, let us a.s.sume that a screw having 6 threads per inch (1/6-inch pitch) is to be cut to a diameter of 1-1/2 inch, and that wires 0.140 inch diameter are to be used in conjunction with a micrometer for measurement. Then the micrometer reading _m_ should be

1-1/2 - 1.5155 1/6 + 3 0.140 = 1.6674 inch

If the micrometer reading were 1.670 inch, it would indicate that the pitch diameter of the screw was too large, the error being equal to difference between 1.667 and the actual reading.

[Ill.u.s.tration: Fig. 27. Rivett-Dock Circular Threading Tool in Working Position]

=Rivett-Dock Threading Tool.=--A special form of thread tool, which overcomes a number of disadvantages common to an ordinary single-point thread tool, is shown in Fig. 27. This tool has a circular-shaped cutter _C_, having ten teeth around its circ.u.mference, which, beginning with tooth No. 1, gradually increase in height, cutter No. 2 being higher than No. 1, etc. This cutter is mounted on a slide _S_, that is fitted to the frame _F_, and can be moved in or out by lever _L_. The hub of this lever has an eccentric stud which moves slide _S_ and locks it when in the forward or cutting position. The action of the lever in moving the slide engages the cutter with pawl _P_, thus rotating the cutter one tooth at a time and presenting a different tooth to the work for each movement of the lever. When the slide is moved forward, the heel or underside of the tooth which is in the working position rests on a stop that takes the thrust of the cut.

When the tool is in use, it is mounted on the tool-block of the lathe as shown in the ill.u.s.tration. The cutter is set for height by placing a tooth in the working position and setting the top level with the lathe center. The cutter is also set square with the work by using an ordinary square, and it is tilted slightly from the vertical to correspond with the angle of the thread to be cut, by adjusting frame _F_. At first a light cut is taken with lever _L_ moved forward and tooth No. 1 on the stop. After this cut is completed, the lever is reversed which rotates the cutter one tooth, and the return movement places tooth No. 2 in the working position. This operation is repeated until the tenth tooth finishes the thread. It is often necessary, when using a single-point thread tool, to re-sharpen it before taking the finishing cut, but with a circular tool this is not necessary, for by using the different teeth successively, the last tooth, which only takes finishing cuts, is kept in good condition.

=Cutting Screws to Compensate for Shrinkage.=--Some tool steels are liable to shrink more or less when they are hardened; consequently if a very accurate hardened screw is required, it is sometimes cut so that the pitch is slightly greater than standard, to compensate for the shrinkage due to the hardening operation. As the amount of contraction incident to hardening is very little, it is not practicable to use change gears that will give the exact pitch required. A well-known method of obtaining this increase of pitch is by the use of a taper attachment.

For example, suppose a tap having 8 threads per inch is to be threaded, and, owing to the contraction of the steel, the pitch must be 0.12502 inch instead of 0.125 inch. The lathe is geared to cut 8 threads per inch or 0.125 inch pitch, and then the taper attachment is set to an angle _a_, Fig. 28, the cosine of which equals 0.125/0.12502; that is, the cosine of angle _a_ equals _the pitch required after hardening_, divided by the _pitch necessary to compensate for shrinkage_. The angle is then found by referring to a table of cosines. The tap blank is also set to the same angle a by adjusting the tailstock center, thus locating the axis of the work parallel with the slide of the taper attachment.

When the carriage moves a distance _x_, the tool point will have moved a greater distance _y_ along the work, the difference between x and y depending upon angle _a_; hence the tool will cut a thread of slightly greater pitch than the lathe is geared to cut.

To ill.u.s.trate by using the preceding example, cosine of angle _a_ = 0.125/0.12502 = 0.99984. By referring to a table of cosines, we find that 0.99984 is the cosine of 1 degree, approximately; hence, the taper attachment slide and the work should be set to this angle. (The angle _a_ in Fig. 28 has been exaggerated in order to more clearly ill.u.s.trate the principle.)

[Ill.u.s.tration: Fig. 28. Diagram Ill.u.s.trating Method of Cutting a Thread to Compensate for the Error in Pitch due to Shrinkage in Hardening]

As is well known, it is objectionable to cut a thread with the tailstock center offset, because the work is not rotated at a uniform velocity, owing to the fact that the driving dog is at an angle with the faceplate. For a small angle such as 1 degree, however, the error resulting from this cause would be very small.

If a thread having a pitch slightly less than standard is needed to fit a threaded part which has contracted in hardening, the taper attachment can also be used provided the lathe is equipped with special gears to cut a little less than the required pitch. Suppose a screw having a pitch of 0.198 inch is required to fit the thread of a nut the pitch of which has been reduced from 0.200 inch to 0.198 inch. If gears having 83 and 84 teeth are available, these can be inserted in a compound train, so as to reduce the 0.200 inch pitch that would be obtained with the regular gearing, to 83/84 of 0.200 or 0.19762 inch. This pitch, which is less than the 0.198 inch pitch required, is then increased by using the taper attachment as previously described. (This method was described by Mr. G. H. Gardner in MACHINERY, February, 1914.)

=Calculating Change Gears for Thread Cutting.=--As previously mentioned, the change gears for cutting threads of various pitches are shown by a table or "index plate" attached to the lathe. The proper gears to be used can be calculated, but the use of the table saves time and tends to avoid mistakes. Every machinist, however, should know how to determine the size of gears used for cutting any number of threads to the inch.

Before referring to any rules, let us first consider why a lathe cuts a certain number of threads to the inch and how this number is changed by the use of different gears.