Drill Size Factors

Tapped holes deeper than 1.5 diameters often call for a larger tap drill.
Blind holes often require larger tap drills to reduce loads on the tap caused by chip buildup in the hole.
Materials that tend to gall when tapped or when fasteners are installed should have larger drilled holes.
Under tapping pressure, soft materials tend to extrude and enter the root area, necessiating a larger drilled hole.
Materials that don't readily dissipate heat, should have larger holes to reduce the tooth contact area and minimize heat build up.
When making threads with high helix angles using a larger tap drill will help reduce tap breakage.