Part 10 (1/2)
[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.