Decimal Equivalents of 6ths, 7ths, 8ths, 12ths, 16ths, 24ths, etc. 336, 337 Circumferences and Areas of Circles from 4 to 1 inch. Circumferences and Circular Areas of Numbers from 1 to 520, THE AMERICAN MACHINISTS' HANDBOOK SCREW THREADS CUTTING SCREW THREADS NEARLY all lathes are geared so that if gears having the same number of teeth are placed on both stud and lead screw, it will cut a thread the same pitch as the lead screw. This is called being geared "even." If the lathe will not do this, then find what thread will be cut with even gears on both stud and lead screw and consider that as the pitch of lead screw. In speaking of the pitch of lead screw it will mean the thread that will be cut with even gears. In cutting the same thread with even gears, both the work and the lead screw are turning at the same rate. To cut a faster thread, the lead screw must turn faster than the work, so the largest gear goes on the stud and the smaller on the lead screw. To cut a slower thread (finer-pitch or less lead), the larger gear goes on the screw and the smaller on the stud. Calling the lead screw 6 to the inch, what gears shall we use to cut an 8 thread? Multiply both the lead screw and the thread to be cut by some number (the same number for both) that will give two gears you have in the set. If the gears vary by 4 teeth, try 4 and get 24 and 32 as the gears. If by 5, you get 30 and 40 as the gears. Then as 8 is slower than 6, the large gear goes on the lead screw and the small one on the stud. = Cut an 18 thread with a 5-pitch lead screw and gears varying by 5 teeth. 5 X 5 25 and 5 X 18 = 90. There may not be a 90 gear, but you can use a 2 to 1 compound gear and use a 45 gear instead. That is, put the 25 gear on the stud, use any 2 to 1 combination between this and the 45 gear on the screw. The 25 gear must drive the large gear of the 2 to 1 combination and the smaller gear drive the 45-tooth gear, either directly or through an intermediate. = = = To cut 24 and In cutting fractional threads the same rule holds good. II threads with gears that change by 4 teeth, use 4 X 6 4 X II 46, with the 24 gear on the stud and the 46 on the screw. With gears changing by 5 this is not so easy, as 5 X 11 571, an impossible gear. Multiplying by 10 would give 60 and 115, not much better. Multiply by 6 and get 6 X 6 36 and 6 X 111 = 69, neither of which is in the set. It seems as though 35 and 70 would come pretty near it, but they will cut a 12 thread instead. To find what thread any two gears will cut, multiply the pitch of lead screw and the gear which goes on it and divide this by the gear on the stud. Suppose we try 40 on the stud and 75 on the lead = = 480; screw. Multiply 75 by 6 450 and divide by 40 which gives 11 as the thread that will be cut. Try 45 and 80. 6 x 80 = divided by 45 10, showing that the 40 and 75 are nearest and that to cut it exactly a special gear will have to be added to the set. In reality the gears would not change by 5 teeth with a 6-pitch lead screw. = Rules for screw cutting may be summed up as follows, always remembering that the lead screw is the thread that will be cut when gears having the same number of teeth are placed on both screw and stud. GEAR trains for screw-cutting are usually arranged similarly to the illustration, Fig. 1. If the gear E on the lathe spindle has the same number of teeth as the gear H on the stud S, the lathe is geared even, i.e., gears having the same teeth placed on both stud and lead screw will cut a thread like the lead screw. As shown, the gears are out of mesh because the tumbler gears F and G do not mesh with E; but moving the handle I down throws F into mesh with E so the drive is through E, F, G, H, S and intermediate to L, driving it so as to cut a right-hand screw if it is a right-hand thread, as is usually the case. Raising handle I cuts out F entirely and reverses the direction of the lead screw. |