Geometrical Tolerancing

Introduction

The notes below relate to use of geometric tolerances on drawing.  The notes are outline in nature to provide general guidance.  This subject is complex and it is wise to consult the standards and have a clear understanding of what is required when including geometric tolerances on technical drawings.  There are clear differences between ANSI Y14.5 and the above noted ISO standard.   The notes below do not address this difference or refer to the ANSI standard in any way.

Geometric Tolerances

Geometric tolerances specify the maximum variation that is allowed in form or position from true geometry.   The geometric tolerance is, in essence, the width or diameter of tolerance zone within which a surface or axis of hole or cylinder can lie which results in resulting feature being acceptable for proper function and interchangeability.

If a tolerance of form is not specified on a drawing for a feature, then the feature as made will be acceptable regardless of form variation.   The tolerances of form control straightness, flatness, parallelism, angular displacement etc. etc.

The tolerance zone will be one of the following:
• The area within a circle
• The area between two circles
• The area between two equidistant lines or between two parallel straight lines
• The space within a cylinder
• The space between two coaxial cylinders
• The space between two equidistant surfaces or two parallel planes
• The space within a bent pipe

Tolerance Frame with Symbol identifications

Indication of datum

Supplimentary Symbols

Tolerance Frame Variations

The tolerance frame can be divided into two or more compartments.
These compartments include from left to right

The symbol for the feature to be toleranced The tolerance value..If the tolerance zone is circular or cylinderical it is preceded with a    Letters for datums when the toleranced feature is specified in relation to one, or more datums.

When multiple datums are referenced in the tolerance box they are indicated as below:

Maximum Material Indication in Tolerance Frame

The maximum material condition, when used, is indicated by a symbol placed after the tolerance value, after the datum letter, or both.

See the figures below: 

Additional Frames- Notes

If a single frame cannot convey sufficient information it is acceptable to stack additional frames and/or provide additional notes..

Positioning of Frames /Datum triangles

The datum triangle is placed on a feature or on an extension of the outline ( but clearly separated from the dimension line) when the datum feature is the line or surface itself

The datum triangle is place on the extension of a dimension arrow when the datum feature is the axis or medium line.
When two datums are indicated they relate to the common axis of the two features
The datum triangle can replace a dimension arrow if there is not enough room.

The datum can be located on the axes or median line when 

The axis or median line is clearly for a single feature The axis or median line is clearly formed by two features