To follow the motion of a train of gears, take a stick (or your finger if they are not running) and trace the motion from the driver to the end as shown by the dotted lines in A, B, C and D. When a lathe is compound geared the stud gear drives an auxiliary gear as 4, which multiplies or reduces the motion as the case may be. It will readily be seen, if the stud drives A and B drives L, the motion will be reduced one-half because A has one-half the number of teeth in B. A SCREW-THREAD ANGLE TABLE THE accompanying table gives the angle of helix of various pitches and diameters with respect to a line perpendicular to the axis. These angles were worked out with the idea of using them for grinding thread tools for threads of various pitches upon different diameters of work. This table will enable one to set the protractor at the proper angle of side clearance for the work in hand and grind the thread tool correctly without guesswork. -54 -3I 27°. 6° -37 41 11° -58 23°- 512°- I 80-86°-4 4-52 4 3 50-12 4-10 17-39 9° 26°- 2 40-33 30-39 15°-49 8°. 4 5-23 4 4 -48 -25 -39 -55 2° -19 -28 II°-59 60- 4 4° 3 3 3 2°-26 2°- 2 50-36 30-44 10°-26 5°-16 3°-29 -49 -15 1° 2° 37 2°- 5 -37 280-3 34° -52 30-15 30-13 10. 54 -37 20-19 20-18 2° 41°-5 37 2 44 -36 1°. 29 -19 -13 10-15 -14 7 8° 84° 9 2°-42 2°- 2 944-34 3 3 2°-18 9°-39 40-52 3°-15 2°-26 -15 30-39 2°-26 1°-49 1°-28 1°-37 37 30-19 20-13 1°-40 10-22 1° 7 3 3 2 2 10-31 10-13 1° 1 55 49 45' 40' 41' 49 36' -21 -29 1°-18 1°-10 1°-23 10-13 1° 51°- 4 1-18 1° 81°- 11°- O 81°- I 10- 2 58' Angle of Helix While the table is worked out for single threads, it can be used for double or triple threads by considering the lead equal to the advance of the work in one revolution instead of as given in the table. I P It is customary in many shops to have several thread tools in stock to cut these various thread angles, each cutting within a certain range of angles. This table will be useful in determining the best range for each thread tool. Figs. 2 and show side and front elevations of the thread tool and of the protractor as applied to obtain the proper angle of side clearance to cut a right-hand screw thread. The front edge of the thread tool is used to determine the angle of side clearance. Fig. 4 shows a section taken along the line a b, Fig. 2. It will be noticed that line e f is shorter than G H to give clearance to the cutting edges of the thread tool, and also that G R is equal to HR and e S is equal to f S. The angle of the helix at half the depth of the thread, Fig. 5, can be used, if desired, and can be approximated to from the table, or figured exactly by the method given at the top of the table. THE accompanying table will be found useful when cutting multiple threads. When one thread is cut, the feed nut may be opened (the spindle of course being stopped) and the carriage moved along by hand the distance given in the table; the nut is then closed on the screw and the next thread cut. This is a quick and sure method of starting the second, third or fourth thread where the lead screw of the lathe is of the pitch given in the table. Say we wish to cut a 31-pitch double-thread screw: the lathe must be geared the same as for a single, triple or quadruple thread. The tool will of course have to be the same width and the depth of cut the same as for a 7 per inch screw. After the first thread is cut it will appear very shallow and wide. With the lathe spindle idle, the nut is opened and the carriage moved (in either direction) I inch; the nut is then closed on the lead screw and the tool is in the proper position to make the second cut. If the carriage were moved 2 inches, the tool could follow exactly the first groove cut. In the case of a triple-thread screw, if the carriage were moved 3 inches, the tool would follow its original path, and it would do the same in the case of a quadruple thread if moved 4 inches. The carriage can, of course, be moved 1 inch and the nut closed no matter what the pitch of the lead screw may be (unless it is fractional), but in order to close the nut after moving inch, the screw must have some even number of threads per inch. As will be seen by referring to the table, a lead screw with any even number of threads per inch is used in a number of cases, while in several other instances the screw may be of any pitch either odd or even. In certain cases 4 and 8 per inch lead screws are specified; and in cutting triple threads a 6 per inch screw is required. FIG. 6.- Face-Plate for Multiple Thread Cutting FACE-PLATE FOR MULTIPLE THREAD CUTTING FIG. 6 shows a face-plate fixture used on various numbers of threads. On an ordinary driving plate is fitted a plate having, as shown, twelve holes enabling one to get two, three, four or six leads if required. This ring carries the driving stud, and is clamped at the back of the plate by two bolts as an extra safeguard. All that is necessary in operation is to slack off the bolts, withdraw the index pin, move the plate the number of holes required, and re-tighten the bolts. It is used on different lathes, as occasion requires, by making the driving plates alike and drilling a hole for the index pin. It is found that the index pin works best when made taper, and a light tap is sufficient to loosen or fix it. CUTTING DIAMETRAL PITCH WORMS IN THE LATHE THE accompanying table is to be used in cases where fractional worm-thread cutting is necessary for diametrical pitch worm threads to mesh into diametral-pitch worm gears. |