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Don’t Get Too Lazy Using Default Tolerances
(In accordance with the ASME Y14.5-2009 standard)

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I am seeing a lot of companies relying on general default tolerances.  A large default tolerance that is related to a datum reference frame, stated on the drawing and with one clear meaning is great.  I usually recommend a general profile of a surface tolerance which includes datum referencing.  This is because profile of a surface is the only geometric tolerance that directly locates surfaces.  Since every feature has a surface, this tolerance works for every feature on a part except the datum features which should have individual controls.

Unfortunately, I see people relying on general plus minus linear and angular tolerances which do not relate to datum reference frames. Also, I am seeing drawings referencing ISO 2768, which is a general default tolerance standard. The trouble with this standard is, it is open to interpretation. Here is a figure from that standard. The tolerance values are found in tables depending on the tolerance class specified. In this case it is mH.

The upper figure is the drawing.  The lower figure shows all of the implied geometric tolerances according to this Standard. The tolerances in this Standard are based on machining but I see it applied to parts that are stamped, molded, cast, etc.

Imagine applying this to a complex part and trying to have one clear meaning.   Or doing a "first article inspection" and trying to figure out what to report.  There are a lot of open questions like: Which feature serves as the datum feature for parallelism? or How do you know to make the hole datum feature ’C’?

This standard is for machining but I see it used as a general tolerance on sheet metal, plastic molded and cast part drawings.

Usually less than half of the features on a part require "tight" tolerances.  Defaulting to tight general tolerances on the entire part can only increase the cost.

Back to Tips Tip added Oct 2012