CONSEQUENCES ON THE TOLERANCING OF FEATURES OF SIZE
Application and limits of the envelope requirement
21 October 2013: Gunter Effenberger
After the GPS standard for dimensional tolerancing of linear sizes (ISO 14405-1) such as circle, cylinder, ball, pair of parallel lines and pair of parallel opposite surfaces had been published in April 2011, the German Institute for Standardisation withdrew DIN 7167 “Relationship between tolerances of size, form, and parallelism; envelope requirement without individual indication on the drawing” in November of the same year. The envelope requirement in drawings based on authorised DIN standards was thus no longer universally valid. People being less acquainted with this topic, however, were hardly able to identify the consequences. There is still the risk of agreeing on an entire GPS system (principle 1 according to ISO 8015) by applying the invocation principle and quoting a standard of the system. You thus accept the independency principle (principle 5 according to ISO 8015), which becomes binding, and the envelope requirement becomes invalid. This article informs you about the associated relationships and the predictable consequences.
Former drawing specifications
It was common practice in many German companies to tolerate the envelope requirement for linear sizes of the geometric elements cylinder, circle, ball, parallel lines and parallel opposite surfaces without any drawing indication referring to DIN 7167.
Chapter 2 of this standard says that the envelope requirement without individual indication on the drawing applies to all single geometric elements on drawings based on DIN standards about tolerances and fits that do not include any contrary specifications. A single feature of size must not push or even tear the perfect (geometrically ideal) envelope with the maximum material size of the dimensional tolerance interval.
In order to prove that this construction requirement is met at the part, companies used to apply the Taylor Principle (go plug gauge, go snap gauge, go ring gauge). Negative effects of form and location deviations on mating parts can thus be excluded; especially those effects that cannot be identified in a two-point measurement, such as triangular cylinders, curved cylinders or curved surfaces...