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A hole of 20 mm diameter is to be drilled in a steel block of 40 mm thickness. The drilling is performed at rotational speed of 400 rpm and feed rate of 0.1 mm/rev. The required approach and over run of the drill together is equal to the radius of drill. The drilling time (in minute) is

1. 1.00
2. 1.25
3. 1.50
4. 1.75

Correct answer: 1.25

Solution

The drilling time is calculated based on the total distance the drill must travel, including the hole depth and the approach and overrun, divided by the feed rate. Given the parameters, the total distance is effectively 40 mm (depth) plus 10 mm (approach and overrun), and with a feed rate of 0.1 mm/rev at 400 rpm, the total time comes out to 1.25 minutes.

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