GlossaryKind of term: Standard and conventionIndependency principle (ISO 8015)
What is the independency principle?
The independency principle in ISO 8015 makes every requirement on a drawing apply on its own: a size tolerance limits size, not form.
It is a fundamental rule of ISO 8015 and holds unless an indication links requirements, such as the envelope requirement Ⓔ. A size tolerance then limits only size, by default the local two-point size of a cylinder or of two opposite faces under ISO 14405-1. Form, such as roundness, straightness or flatness, is limited only when it is specified separately on the drawing or follows from a general tolerance. A shaft can therefore be within size and still be bent.
What does the standard say about the independency principle?
The 2011 edition of ISO 8015 also states that the ISO geometrical product specification system applies as a whole as soon as a drawing invokes any part of it; before that, the drawing had to refer to ISO 8015. Exceptions to the principle are shown on the drawing itself, such as Ⓔ after a size or Ⓜ and Ⓛ in a feature control frame. ISO 8015 sets further fundamental rules, such as the reference temperature of 20 °C. ASME Y14.5 uses the envelope by default, Rule #1; there independency is the exception, with its own indication. On older German drawings without ISO 8015, DIN 7167 often applied, and with it the envelope.
How does the independency principle work in practice?
The principle decides what may be rejected, and so what you have to measure. Under ISO 8015 a size is inspected with two-point measurements; a slightly oval or bent shaft is good as long as no form requirement is given. Under ASME Y14.5 a ring gauge at maximum material size can reject the same shaft. Two drawings with the same size and tolerance can therefore call for a different measurement and sometimes a different route. The governing standard is usually stated in or near the title block; if nothing is stated, ask before quoting.
What is the independency principle often confused with?
The independency principle is not a general tolerance: ISO 2768 gives values for dimensions and features without a tolerance of their own, ISO 8015 says how requirements relate to each other. The envelope requirement is an exception to the principle, marked with Ⓔ.
Frequently asked questions
What does ISO 8015 in the title block mean?
That the drawing follows the ISO tolerancing rules, including the independency principle: size and form are inspected separately unless Ⓔ, Ⓜ or Ⓛ links them. Since 2011 this also applies without the note, once an ISO tolerancing standard is invoked.
Can a shaft be within size and still be rejected?
Yes, if a form requirement is given separately on the drawing or follows from the general tolerance. Under ASME Y14.5 also on the envelope: a bent shaft that does not pass through a ring at its largest size fails there.
What if the drawing names no standard?
Ask. Where the customer is based proves nothing: a European customer can draw to ASME too. Inspection and sometimes the route depend on it, so settle it before you quote.