Discussion
In a single pass drilling operation, a through hole of 15 mm diameter is to be drilled in a steel plate of 50 mm thickness. Drill spindle speed is 500 rpm, feed is 0.2 mm/rev and drill point angle is 118°. Assuming 2 mm clearance at approach and exit, the total drill time (in seconds) is
Answer: A
The total drilling time is about 35.1 s.
The drill must travel the plate thickness, the two clearances, and the height of its own conical point to break through fully:
\[ \begin{aligned} h &= \frac{D/2}{\tan(118^{\circ}/2)} = \frac{7.5}{\tan 59^{\circ}} = 4.51\ \text{mm} \\ L &= 50+2+2+4.51 = 58.51\ \text{mm} \end{aligned} \]
\[ \begin{aligned} f N &= 0.2 \times 500 = 100\ \text{mm/min} \\ t &= \frac{58.51}{100} = 0.585\ \text{min} = 35.1\ \text{s} \end{aligned} \]
Leaving out the cone height gives 54 mm and 32.4 s, and leaving out the cone and one clearance gives 52 mm and 31.2 s. The break-through allowance for the drill point is what the question tests.
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