LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
machinist wants to cut a screw with 10 threads per inch on a lathe having a
lead screw with 4 threads per inch. The procedure would be to multiply 10
and 4 by any convenient number, say 6. Then, 6X4=24 and 6X10=60. The stud
gear should have 24 teeth and the lead screw gear should have 60 teeth to
produce the desired ratio to cut 10 threads per inch. If gears of 24 and 60
teeth are not available, multiply 10 and 4 by another number until the
products coincide with the number of teeth that are on available gears.
3 Whenever the thread to be cut is finer than the thread of the
lead screw, the gear with the fewest teeth will be the stud gear. If the
thread to be cut is coarser than the lead screw, the gear with the fewest
teeth will be the lead screw gear.
(b) Compound Gear Train.
1 If the proper ratio between the spindle and the lead screw cannot
be obtained by simple gearing, a compound gear train must be used (figure 45
on the previous page). For example, if it is desired to cut 80 threads per
inch with a lead screw having 8 threads per inch, and the smallest change
gear available has 24 teeth, the lead screw gear must have 240 teeth which
would be too large in diameter to fit the lathe. By compounding the gears,
it would be possible to cut 80 threads per inch with the gears generally
available.
2 In the compound gear train, two intermediate gears replace the
idler gear of the simple gear train. The intermediate gears are mounted to
the same shaft and are keyed together. The gears driven by the stud gear is
known as the first intermediate gear and the gear that drives the lead screw
gear is known as the second intermediate gear. An idler gear can be used if
necessary in this gear train, but will reverse the direction of the lead
screw gear, and make reversal of the stud gear-to-spindle connection
necessary.
3 To compute compound gear arrangements, the following rule should
be applied: establish the ratio between the number of threads per inch to be
cut and the number of threads per inch of the lead screw. Factor each term
of the ratio; that is, determine two numbers for each term which, when
multiplied by each other, result in the number of the ratio term.
The
resulting four numbers, when
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