66. Screw Cutting. When threads are cut in a lathe, it is necessary to have a definite speed ratio between the lead screw and the work (or lathe spindle), this speed ratio depending upon the number of threads per inch which it is desired to cut. The correct speed ratio in any case is obtained by means of a train of gears between the lead screw and the spindle. In modern lathes these gears are usually mounted in a gear box at the headstock, and the proper combination for cutting any particular thread is effected by the manipulation of levers or knobs according to the directions mounted on the machine. There are, however, many lathes in use which have a set of separate "change gears" from which the machinist must select the proper combination to be used for cutting any particular thread. Such lathes are fitted with a name plate stating the gears to use for each thread. But these plates are occasionally missing from old lathes, or some of the gears have been broken or lost which makes it necessary to use a combination not listed on the plate. In such cases the machinist must calculate the proper gear combination. In any case, a mechanic should have sufficient knowledge of gearing and arithmetic to enable him to make the necessary selection. The lathe carriage and tool are moved by a "lead screw" having usually 2, 4, 6, or 8 threads per inch. If the lathe has a lead screw with 6 threads per inch, 6 revolutions of the lead screw will move the carriage and tool 1 in. By referring to Fig. 22 it can easily be seen that if the spindle, or work, revolves once while the lead screw makes 6 revolutions, the tool will cut FIG. 22.-Simple gears for thread cutting. one thread per inch; if the spindle revolves twice during this time the tool will cut 2 threads per inch, etc. The ratio of the spindle speed to the speed of the lead screw will, therefore, be = Speed of spindle Threads per inch to be cut In the simple-geared lathe, the gears which are changed to obtain the proper ratio are the "stud gear" and the "lead screw gear," as shown in Fig. 22. The intermediate gear is an idler which has no effect on the speed ratio. The stud gear is connected to the spindle or work by a fixed train of gears such that the spindle may or may not have the same speed as the stud gear. The fixed-speed ratio of the spindle to the stud gear should first be determined by actually counting the number of teeth on the spindle and fixed stud gear, or by placing equal gears on the stud and lead screw and determining what thread would be cut. In the latter case the stud makes the same number of revolutions as the lead screw, and the ratio is found by substituting "speed of stud gear" for "speed of lead screw" in the formula just given. Calling this ratio R, we can write the following formula to determine the desired change gears for a simple-geared lathe. Threads per inch to be cut Lead screw gear teeth Threads per inch on lead screw RX = Stud gear teeth To cut 12 threads per inch with a lathe that has a 6-pitch lead screw and a fixed ratio, R, of 1 to 1, requires 1 12 12 60 teeth on lead screw gear X = 1 6 6 or 30 teeth on stud gear Any other two gears the ratio of which is 12:6 or 2:1 may be used, provided that they are otherwise suitable. When it is impossible or undesirable to use the gears required in simple gearing to cut a certain thread, the intermediate or idler gear is replaced by two gears, as in Fig. 23, giving what is called "compound gearing." The ratio R is obtained for compound gearing in the same manner as before, using equal gears on the stud and lead screw and also a pair of equal gears on the idler stud. The following formula for the change gears is then used: Lead screw gear X Driven compound gear Taking the case just worked out for simple gearing, we have be seen that there are a number of combinations which would satisfy the requirements. It is simply a matter of selection, depending upon the available supply of gears. 125. In Fig. 18 if we removed the 48-tooth gear and put a 64-tooth gear in its place, what would be the speed ratio of A to B? 126. If B in Fig. 19 makes 6 revolutions, how many will C make and how many will A make? 127. A machinist wishes to thread a pipe on a lathe having 2 threads per inch on the lead screw. There are to be 11 threads per inch on the pipe. What is the ratio of the speed to the spindle and lead screw? 128. The power shear of Fig. 24 makes 20 strokes per minute at its maximum speed with the motor running at a speed of 1200 r.p.m. The small chain sprocket on the motor has 20 teeth, the large sprocket has 80 teeth, and the pinion meshing with the large gear has 10 teeth. Calculate the number of teeth required for the large gear. Note. The large gear revolves once every stroke, and the two sprockets can be considered as two gears in mesh. 129. Two gears are to have a speed ratio of 4.6 to 1. If the smaller gear has 15 teeth, what must be the number of teeth on the larger gear? 130. If you were to cut a 20-pitch thread on a lathe having a 4-pitch lead screw, what would be the ratio of the speeds of the spindle and the lead screw? |