0218// Can Position Tolerance Larger Than Feature of Size Tolerance?

30 Jul 2019| Guan Boon, Wong

Recently I receive a question from New York friend regarding "Virtual Condition/Resultant Condition. I'm reading a drawing that says feature of size diameter .012-.108 with positional tolerance value .030 LMC (least material condition). I couldn't find anything to refute these numbers. I had a thought that positional tolerance value should be equal or smaller than the feature of size tolerance. I.E. The tolerance of feature of size is .006 with positional tolerance .030 LMC. Is there such thing that positional tolerance can be larger than feature of size tolerance? Thank you." In short, his/her question is "Is possible to have a positional tolerance larger than feature of size tolerance?" My answer is "Yes!". Why? First we need to know what is Positional Tolerancing is and is any "overlap" between feature of size tolerance which will cause redundancy or ambiguity.

According Y14.5M-2009 7.2 Positional Tolerancing, Position is the location of one or more feature of size relative to one another or to one or more datums. A positional tolerance defines either of the following: a) a zone within which the center, axis, or center plane of a feature of size is permitted to vary from a true (theoretically exact) position b) (where specified on an MMC or LMC basis) a boundary, defined as a virtual condition, located at the true (theoretically exact) position, that may not be violated by the surface or surfaces of the considered feature of size. Basic dimensions establish the true position from specified datums and between interrelated features. A positional tolerance is indicated by the position symbol, a tolerance value, applicable material condition modifiers, and appropriate datum references place in a feature control frame. So, objective for feature of size tolerance is to defined the max and min limits for feature of size. However, positional tolerancing is to defined the permitted variation vary from true (theoretical exact) position where RFS, LMC and MMC. If Position Tolerancing is specified with material condition modifier, MMC or LMC perfect form may affecting the position tolerance size. But, not feature of size tolerance. That means smaller feature of size tolerance compare to position tolerance, not an issue at all.

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You may ask, is Rule 1: Envelope Principal which mentioned in Y14.5M-2009 2.7.1 Variation of Form will affecting positional tolerancing? Answer is no, because that is irrelevant between positional tolerancing and Rule 1. According to standard (Y14.5M-2009 2.7.1) The form of an individual regular feature of size is controlled by its limits of size to the extent prescribed in the following paragraphs and illustrated in Fig. 2-6. (a) The surface or surfaces of a regular feature of size shall not extend beyond a boundary (envelope) of perfect form at MMC. This boundary is the true geometric form represented by the drawing. No variation in form is permitted if the regular feature of size is produced at its MMC limit of size unless a straightness or flatness tolerance is associated with the size dimension or the Independency symbol is applied per para. 2.7.3. See Fig. 2-7.

(b) Where the actual local size of a regular feature of size has departed from MMC toward LMC, a local variation in form is allowed equal to the amount of such departure. (c) Where is no default requirement for a boundary of perfect form at LMC. Thus, a regular feature of size produced at its LMC limit of size is permitted to vary from true form to the maximum variation allowed by the boundary of perfect form at MMC.[photo]

(d) In cases where a geometric tolerance is specified to apply at LMC, perfect form at LMC is required. See para. 7.3.5.

******************************************* You may ask again, how about Surface Interpretation which mention in Y14.5-2009 Fig. 7.6 Illustration of Difference Between Surface and Axis Interpretations of Position Tolerancing for a Cylindrical Hole? As illustration, surface interpretation can "supersede" axis interpretation only if functional requirement met (such as size and virtual condition). So, again, positional control tolerance is allow to be larger than feature of size tolerance.

******************************************* An industrial example for feature of size tolerance smaller than position tolerance, is shrink fitting in shipyard. When I was in the shipyard, assemble between turbine shaft and bush often done by shrink fitting. In this application, we may have a very high requirement in feature of size (for precise interference fit by thermal expansion or contraction) with less stringent requirement in term position tolerancing (such as location of shaft interrelated between different diameter)