Choosing hole sizes that are cheap to make

A hole made with a standard drill is drilled in one quick move. An odd size has to be milled out with an end mill or needs a special tool, which costs time on every hole.

Use standard sizes

Metric drills come in 0.1 mm steps up to about 13 mm and 0.5 mm steps above that. A 6.35 mm hole (¼ inch) on a metric part is a typical example of a size the shop may not have.

• For bolts, use the ISO 273 clearance sizes, for example Ø6.6 for M6 (medium).
• For threads, model the hole at the tap drill size, for example Ø5.0 for M6.
• For precision fits, use a standard reamer size with a tolerance such as H7.

Depth

Keep blind holes within about 4–6× the diameter. Drills have a 118° or 135° point, so the bottom of a drilled blind hole is cone-shaped. Only ask for a flat bottom when needed; it requires a second tool.

Position

Holes too close to an edge can break out or deform. Keep at least half a diameter of material, and more for tapped holes.