Discussion
A mild steel rod having 50 mm dia and 500 mm length is to be turned on a lathe. What is the machining time to reduce the rod to 45 mm in one pass when cutting speed is 30 m/ min and feed is 0.7 rev/ min?
Answer: B
The machining time is 3.74 minutes.
First find the spindle speed for 30 m/min on the 50 mm rod. The feed, printed as 0.7 rev/min, is taken as 0.7 mm per revolution, the usual unit of turning feed:
\[ \begin{aligned} N &= \frac{1000 \times 30}{\pi \times 50} = 191\ \text{rpm} \\ T &= \frac{L}{f N} = \frac{500}{0.7 \times 191} \\ &= \frac{500}{133.7} = 3.74\ \text{min} \end{aligned} \]
The 45 mm finish diameter sets the depth of cut, not the time, since it is one pass. Basing \(N\) on 45 mm gives 212 rpm and about 3.4 minutes, which is not an option.
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