(9) Quick-change Gear Mechanism.

removing and replacing change gears, most modern lathes have a self-

contained change gear mechanism, commonly called a "quick-change gear box."

There are a number of types used on different types of lathes, but they are

all similar in principle.

(b) The quick-change gear box mechanism consists of a cone-shaped group

of change gears.

One can instantly connect any single gear in the gear

train by a sliding tumbler gear controlled by a lever. The cone of gears is

keyed to a shaft which drives the lead screw (or feed rod) directly or

through an intermediate shaft.

Each gear in the cluster has a different

number of teeth and hence produces a different ratio when connected in the

train.

Sliding gears also produce other changes in the gear train to

increase the number of different ratios one can get with the cone of change

gears.

All changes are made by shifting the appropriate levers or knobs.

An index plate or chart mounted on the gear box indicates the position in

which to place the levers to obtain the necessary gear ratio to cut the

threads or produce the feed desired (see figure 7 on the following page).

(c) Figure 7 depicts the rear view of one type of gear box. The splined

shaft turns with gear G, which is driven by the spindle through the main

gear train mounted on the end of the lathe. Shaft F in turn drives shaft H

through the tumbler gear T, which can be engaged with any one of the cluster

of eight different size gears on shaft H by means of the lever C. Shaft H

drives shaft J through a double-clutch gear, which takes the drive through

one of three gears, depending on the position of lever B (right, center or

left). Shaft J drives the lead screw through gear L.

(d) Either the lead screw or the feed rod can he connected to the final

driveshaft of the gear box by engaging the appropriate gears.

The lathe

gear box shown in figure 7 has no feed rod.

(e) Twenty-four different gear ratios are provided by the quick-change

gear box.

The lower lever has eight positions, each of which places a

different gear in the gear train and hence produces eight different gear

ratios. The three positions

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