Drilled hole locations and size variations are cumulative of several manufacturing variables. Tool sharpness, accuracy, tool and machine rigidity, machine spindle bearings wear, general quality of machine, use of drill fixtures, material thermal expansion, and material density variations are some to the reasons hole features will never be manufactured perfect.
Most commercial twist drills have a diameter slightly smaller than the stated nominal size, however the actual hole feature will likely be manufactured larger than the nominal size of the drill tool. Compensating for manufacturing tool and process variability is a significant challenge for tight toleranced holes.
For Drill size chart for both Metric and ANSI sizes see:
Machinist Drill Sizes - ANSI Size Drills, ISO Metric Drill Sizes |
Engineering Design for Manufacturability
Comprehensive guide for proper design of geometry, process capabilities & proper mechanical tolerance specifications.
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