A diameter tolerance is a distance between two points
Under ISO 14405-1, a size with no further specification is interpreted as a two-point size: the distance between two opposite points, as measured with a micrometer or caliper, must fall within the tolerance.
Under the independency principle of ISO 8015, unless the drawing states otherwise, size and form tolerances apply independently. Meeting the diameter tolerance says nothing about whether the section is round (roundness) or the axis is straight (straightness); those are separate requirements.
A three-lobed section: the same diameter in every direction
A three-lobed (constant-width) section gives the same two-point reading in every direction yet is not round. Sections with an odd number of lobes are known to arise in processes such as centreless grinding. They are hard to find with two-point measurement and are checked by other means, such as roundness measurement or measurement in a V-block.
Straightness follows the same principle
A bar can be bent along its length while every cross-section still meets the two-point size tolerance. Bending of the axis is specified as straightness, separately from the diameter tolerance.
When the envelope requirement is used
Adding the envelope requirement Ⓔ to a size on a drawing ties size and form together, so that the feature may not exceed the boundary of perfect form at maximum material size. Whether it applies, and how it is read, is confirmed against the drawing and applicable standard.
Items stated separately in an order
- Roundness: how far the section departs from a circle
- Straightness: bending of the axis along the length
- Surface condition: roughness and acceptance criteria for defects such as seams or decarburization
- Hardness: required range and test method, together with the heat-treatment condition