Simple Gear Train (figure 45).

Lathe Operations
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1 The basic gears which control the ratio between the spindle speed

and the lead screw speed are the stud gear and the lead screw gear.

The

stud gear mounts to a shaft which revolves at the same speed as the spindle

and, therefore, can be considered in this discussion as representing spindle

speed in revolutions per minute. The lead screw gear is usually connected

directly to the lead screw and, therefore, moves at the same speed as the

lead screw. In a simple gear train, the stud gear and the lead screw gear

are meshed together or coupled by an idler gear. If the idler gear is used,

it can be of any size or number of teeth convenient for coupling since it

only transmits motion from one gear to the other and does not affect the

ratio of the stud or the lead screw gears.

2 The threads per inch of the lead screw must be known to compute

the gearing for a specific ratio. The rule for determining the number of

teeth of the stud gear and the lead screw gear for a simple gear train is as

follows: Multiply the number of threads per inch of the lead screw and the

number of threads to be cut by a common number. The products will be the

number of teeth that the stud gear and the lead screw gear should have,

respectively. For example, suppose that a

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