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Machinery Repairman

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lower lever has eight positions, each of which places<br />

a different gear in the gear train and hence produces<br />

eight different gear ratios. The three positions of the<br />

upper level produce three different gear ratios for each<br />

of the 8 changes obtained with the lower lever, thus<br />

making 24 combinations in the box alone. You can<br />

double this range by using a sliding compound gear that<br />

provides a high- and low-gear ratio in the main gear<br />

train. This gives two ratios for every combination<br />

obtainable in the box, or 48 combinations in all.<br />

The index chart on the gear box also shows the<br />

various rates of power longitudinal feed per spindle<br />

revolution that you can get by using the feed<br />

mechanism of the apron. For example, in figure 6-11,<br />

note that the finest longitudinal feed is 0.0030 inch<br />

per revolution of spindle, the next finest is 0.0032<br />

inch, and so on. To arrange the gear box for power<br />

longitudinal feed, select the feed you wish to use and<br />

follow the same procedure explained for cutting<br />

screw threads, except that you engage the power feed<br />

lever instead of the half-nuts. Cross-feeds are not<br />

listed on the chart but you can determine them by<br />

Figure 6-12.-Compound rest.<br />

6-10<br />

multiplying the longitudinal feeds by 0.375, as noted<br />

on the index plate.<br />

On a lathe with a separate feed rod, a feed-thread<br />

shifting lever located at the gear box connects the<br />

drive to the feed rod or the lead screw as desired.<br />

When the feed rod is engaged, the lead screw is<br />

disengaged and vice versa.<br />

You can cut metric threads on some lathes that<br />

have an English lead screw (threads per inch<br />

machined on it) by transposing a set of gears in the<br />

lead screw to the spindle gear train that provides the<br />

correct conversion ratio. You can find information on<br />

this in handbooks for machinists, in the equipment<br />

technical manual, and through direct correspondence<br />

with the equipment manufacturer.<br />

COMPOUND REST<br />

The compound rest provides a rigid, adjustable<br />

mounting for the cutting tool. The compound rest<br />

assembly has the following principal parts (fig. 6-12):

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