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A Φ–20 mm through hole is to be drilled in a 30 mm thick plate using a double fluted, 120° lip angle drill. The drill tip is at a distance of 3 mm from the plate surface when cutting is started and an over travel of 2 mm is recommended as a margin to ensure drilling through the full thickness of the plate. If the drill rotates at 500 rev/min and the feed per tooth is 0.01 mm, the machining time of the operation (in min) will be
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- 4.5
- 7.5
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- 10.5
Correct Option: A
D = 20 mm
Plate thickness = 30 mm
Double fluted
AP = 3 mm
OR = 2 mm
N = 500 rpm
Feed/tooth = 0.01 mm 
Total travel distance = 30 + 3+ 2 + 0.25 × 20 = 45 mm
Double fluted = so feed = 0.01×2 = 0.02 mm
Machining time
| = | = | = 4.5 min | ||
| fN | 0.02 × 500 |
