Just because the first hole is 1mm over dim, the second has a tolerance applied to the theorically correct position. If those same holes are dimensioned as 200 and 20mm, the manufacturer can assume that the 2 holes hole an individual fixture or 2 fixtures having a functional relationship between them. Applying a general tolerance of +/-1mm the 2 holes may be 18mm to 22mm apart. ie the 2 holes locate different fixtures correctly relative to the edge. If 2 holes are dimensioned as 200 and 220 from an edge, The manufacturer can only assume that the 2 holes have a relationship to the edge which is paramount to the correct proformance of the part. Notice also that ISO 2768-2 for general goemetrical tolerances mentions nothing about IT grades.Im going to step in here because Im having this discussion at work currently. Linear size tolerances are something completely different to geometrical tolerances. The problem is that these IT grades shall not be applied to geometrical tolerances at all. Thus looking from this perspective a stand alone IT5 or IT6 could be OK, but. However theoretically in case of runout or parallelism geometrical tolerance you would not need to have the fundamanetal deviation identifier because the characteristics do not have their nominal values. It only defines the amount of tolerance allowed (depending on the nominal size interval).Īdditionally there has to be a fundamental deviation identifier (like h or H) in order to define how the tolerance is disposed 'around' the nominal dimension. You are correct, a stand alone IT grade for linear size is not enough. RE: IT# as a tolerance on feature control frame fsincox (Aerospace) 28 Aug 12 10:23 Is anyone out there in the possession of complete book (possibly 2010 version) to clarify how far “general purposes” will extend?

I have to agree that IT grade by itself is NOT a tolerance, so it cannot be used with linear sizes anyway (until accompanied by fundamental deviation). The phrase “standardized selection of tolerance classes for general purposes” looks suspicious to me. It provides a standardized selection of tolerance classes for general purposes from amongst the numerous possibilities.Īdditionally, it defines the basic terminology for fits between two features of size without constraints of orientation and location and explains the principles of “basic hole” and “basic shaft”. ISO 286-1:2010 defines the basic concepts and the related terminology for this code system. ISO 286-1:2010 establishes the ISO code system for tolerances to be used for linear sizes of features of the following types: a) cylinder b) two parallel opposite surfaces.

Iso 2768 hole tolerance it6