Discussion
What will be drill size for reaming a 10 mm hole, if undersize is 0.2 mm and oversize is 0.05 mm
Answer: A
The drill size is 9.75 mm.
Before reaming, the hole is drilled smaller than the finished size to leave a reaming allowance. Using the workshop formula
\[ \begin{aligned} \text{drill size} &= \text{reamed size} - (\text{undersize} + \text{oversize}) \\ &= 10 - (0.2 + 0.05) = 9.75\ \text{mm} \end{aligned} \]
9.8 mm subtracts only the undersize and leaves out the oversize a drill tends to cut. 10.25 mm is larger than the reamer, which would leave nothing to ream, and 9.85 mm subtracts the difference of the two allowances (0.2 − 0.05) instead of their sum.
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