CNCCookbook Beginner's Guide to GD&T: Rules & Philosophy

Basic Rules of GD&T

The proper application of GD&T requires that these rules be followed:

• All dimensions must have a tolerance. Since all real physical objects are subject to variation, we must specify the limits of allowable variation. We can do this either by applying plus and minus tolerances directly to dimensions or by using a tolerance block or note. For Basic Dimensions, GD&T applies tolerances using a related Feature Control Block.
• Dimensions define the nominal geometry and allowable variation. It is not allowed to derive that information through measurement or scaling of the drawing except in certain cases. This is important as casual CAD users are quite used to just adding dimensions to a model or measuring the model directly, which is not allowed under GD&T.
• Every dimension and tolerance required to define the finished part shall be shown on the drawing. If additional dimensions would be helpful, but are not required, they should be marked as reference.
• Dimensions should be applied to features and arranged in such a way as to represent the function of the features. Additionally, dimensions should not be subject to more than one interpretation--they must be unambiguous.
• Descriptions of manufacturing methods should be avoided. The drawing should strictly limit itself to describing the geometry and not the method of manufacture.
• If certain sizes are required during manufacturing but are not required in the final geometry (perhaps due to shrinkage) they should be marked as non-mandatory.
• All dimensioning and tolerancing should be arranged for maximum readability and should be applied to visible lines in the drawing.
• When geometry is normally controlled by gage sizes or by code (e.g. stock materials), the dimension(s) shall be included with the gage or code number in parenthesis following or below the dimension.
• Angles of 90 degrees are assumed when lines (including center lines) are show at right angles, but no angular dimension is explicitly shown. (This applies to any other orthogonal angle of 0, 180, 270, etc.)
• Dimensions and tolerances are valid at 20 degrees C and 101.3 kPa (standard conditions) unless stated otherwise.
• Unless explicitly stated, all dimensions and tolerances are only valid when the item is in a free state.
• Dimensions and tolerances apply to the length, width, and depth of a feature including form variation.
• Dimensions and tolerances only apply at the level of the drawing where they are specified. It is not mandatory that they apply at other drawing levels, unless the specifications are repated on the higher level drawings.

The basic rules tell us how to apply GD&T to a drawing.

Next Article: GD&T Datums

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