The target dimension written on a drawing, from which the tolerance is applied. A 0.500 in nominal shaft with a tolerance of plus 0 minus 0.001 is good anywhere from 0.499 to 0.500. Machinists usually aim for the middle of the tolerance band, not nominal, so tool wear and measurement scatter have room on both sides.
Heard on the job
Nominal is an inch, but the band is all minus, so shoot for nine-nine-nine-five and you're safe.
Insider tip
Aim for mid-tolerance, not nominal; on a one-sided tolerance nominal is sitting right on the reject line.
Also heard as
- basic size
- nominal size