Compound gear train is employed when the gear ratio

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Compound gear train is employed when the gear ratio is not possible to obtain with a simple gear train out of the given set of change gears. In modern lathes, quick change gears are available to obtain the correct gear ratio to cut a particular thread quickly by simply shifting the lever into different positions which are marked on the instruction plates of the machine.
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Cutting metric thread on English standard lead screws or Vice Versa To cut metric threads on lathes having British standard lead screw: To cut British standard threads on a lathe having lead screw of metric pitch:
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Exercise 1. Calculate change gears for cutting R.H. threads of 2 mm pitch on a lathe having lead screw of 6 mm pitch. Soln.: Like this, we can have number of solutions and every solution will give the desired result.
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Exercise … 2. Calculate change gears for cutting L.H. threads of 1.5 mm pitch on a lathe having lead screw of 6 mm pitch. Soln.: Simple train with two idlers
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Exercise … 3. Calculate change gears for cutting R.H.B.S.W. “V” threads of 12 TPI on a lathe having lead screw of 8 TPI. Soln.: 1 Driver TPI of Screw to be cut Change gear ratio =
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  • Winter '15
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  • Lathe, Screw thread, Unified Thread Standard, Lathe Operations, L.H. Threads, R.H. Threads

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