Tolerances explained: how tight is tight enough?

What a tolerance is

No machine makes a part exactly to its nominal size. A tolerance says how far a dimension is allowed to be from nominal and still work. A shaft that is 20 mm ±0.05 mm can be anywhere from 19.95 mm to 20.05 mm.

Why tighter costs more

Tighter tolerances need slower cuts, more careful setups, better machines, more inspection and sometimes grinding or other finishing. Scrap also goes up. The cost usually rises steeply as the tolerance gets smaller.

Where to be tight, and where not to

Only features that fit with other parts, seal, slide, or carry load need close tolerances. For everything else, a general tolerance on the drawing is enough.

• Mark the critical dimensions clearly, and give a general tolerance for the rest.
• Think about how the part will actually be assembled, and tolerance the fits, not every face.
• Ask the workshop what they can hold on their machines. Sometimes a small design change removes the need for a tight tolerance.

Tell the workshop what is critical

A note such as “this bore fits a bearing” tells a workshop far more than a tolerance alone. It helps them plan the machining and the inspection, and gives you a more accurate quotation.

Kinds of work

• Die making
• Mold making
• CNC machining
• Casting
• Fabrication