Geometric Dimensioning & Tolerancing

Technical Drawing Program

GEOMETRIC DIMENSIONING & TOLERANCING

The following terms and conditions are based on the ANSI Y14.5M - R1988 Standard and should serve as guides to good geometric dimensioning and tolerancing techniques:

Geometric dimensioning and tolerancing clarifies the dimensioning of parts in relation to their assumed geometry, function and relationship to other parts.
Example: The top and bottom surfaces of a part are drawn are as two horizontal lines; given one height dimension; and assumed to be parallel. The surfaces of the actual part may or may not be parallel but may still be usable if given a geometric dimensional tolerance.
The five (5) types of geometric tolerances are form, profile, orientation, location and runout.
The four (4) characteristics of "Form" are straightness, flatness, circularity and cylindricity.
The two (2) characteristics of "Profile" are profile of a line and profile of a surface.
The three (3) characteristivcs of "Orientation" are angularity, perpendicularity and parallelism.
The three (3) characteristics of "Location" are true position, concentricity and symmetry.
The two (2) characteristics of "Runout" are circular and total.

"Modifiers" are used to clarify the of status of the material when a tolerance is applied.
There are four (4) types of "Modifiers": maximum material condition, least material condition, regardless of feature size and projected tolerance zone.

A "Feature Control Frame" is drawn as a rectangle .375" high and long enough to include a Geometric Dimensioning & Tolerancing Symbol, a total tolerance value and a datum letter.
"Feature Control Frames" are are attached to edges and surfaces by extension lines and to features by a leader.

"Datums" are edges, surfaces or features that are considered accurate for locating other features or surfaces.
"Datums" are labeled with a rectangle .375" high by .75" long that contains a letter preceeded by a dash (-) and followed by a dash (-).
"Datums" are attached to edges and surfaces by extension lines and to features by a leader.

NOTE: The American National Standard, ANSI Y14.5M, for the "Dimensioning and Tolerancing of Engineering Drawings and Related Documentation Practices" should be adhered to for uniformity and acceptance by other concerns. It must be remembered that there are no absolutely hard and fast rules, nor any practice, not subject to change or modification under special conditions or requirements of a particular industry. When there is a variation of any rule, there must always be a reason which can be completely justified.