Geometric Controls (ISO)
The table below shows the geometrical characteristic symbols (GCS) supported by the ISO 1101 tolerancing standard. Each geometric characteristic symbol represents a geometric control that can be used to specify a constraint on one or more geometry attributes (e.g., size or form) of a feature.
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Application
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Type
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Characteristic
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Symbol
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|---|---|---|---|
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Individual Features
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Form
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Individual or Related Features
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Profile
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Related Features
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Orientation
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Location
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Concentricity Note
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Coaxiality Note
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Run-out
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The same geometrical characteristic symbol is used to indicate tolerances of concentricity and coaxiality. Where concentricity is specified, the ACS modifier is automatically placed near the tolerance frame.
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Each GCS is represented with an icon and its name in the menu. The colors of those icons have the following meanings:
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Color
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State
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General Explanation
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|---|---|---|
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White on Green
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Suggested
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The geometric characteristic symbol is applicable to the feature in accordance with ISO 1101, it is compatible with the selected scheme, there is a compatible DRF available, and it is likely to properly constrain the feature.
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Black on White
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Legal, but not suggested
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The geometric characteristic symbol is applicable to the feature in accordance with ISO 1101 and it is compatible with the selected scheme, but selecting it is likely to lead to constraint problems.
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Gray on white
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Not allowed
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The geometric characteristic symbol is not allowed for one of the reasons mentioned in the table below.
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When you hover your mouse over one of the geometric characteristic symbols in the menu, a tooltip is displayed that explains the specific reason for the state of the symbol. The tooltip will show one of the following messages:
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State
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Message
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Specific Explanation
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|---|---|---|
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Suggested
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GCS is a suggested constraint for this feature.
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The geometric characteristic symbol applies at least some of the required constraint to the feature.
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Legal, but not suggested
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GCS is under-constraining.
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The geometric characteristic symbol does not apply the required constraint to the feature. For example, if the location of the feature must be constrained, the orientation geometric controls are under-constraining.
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GCS is not compatible with existing DRFs.
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The geometric characteristic symbol is not compatible with any of the DRFs that precede this feature. For example, in order to apply perpendicularity to a planar surface feature, there must be a preceding DRF whose primary plane or axis is perpendicular to the planar surface feature.
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GCS is potentially over-constraining.
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The geometric characteristic symbol represents a constraint on DOF of the feature that has already been constrained by a similar geometric control. For example, for a simple hole a first segment position to DRF A|B|C constrains all DOF of the feature. If you add a second segment to the tolerance frame, position is indicated as potentially over-constraining.
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Not allowed
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GCS is not applicable to this feature type.
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It is not legal to specify the geometric characteristic symbol for the feature. For example, you cannot specify circularity for a planar surface feature.
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GCS is not applicable to the selected scheme.
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The geometric characteristic symbol is not applicable to the selected scheme. For example, for a simple hole, if the selected scheme is 'Toleranced Size', 'Surface Profile' is not compatible with the scheme because that geometric control constrains the size of the feature and it would not be permissible to specify a toleranced size dimension.
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GCS is restricted in the application options.
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The application administrator can specify geometric characteristic symbols that are restricted in the system application options.
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GCS is not compatible with the constrainable variables of the feature.
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For the first feature in the model, only its size and form may be constrained. Its location and orientation may not be constrained because there are no other features to which the feature may be constrained. There may be similar issues for the first few features in the model.
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GCS is not compatible with the previous segment.
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For multi-segment tolerance frames, the geometric characteristic symbol selected in the lower segments must be compatible with the geometric characteristic symbol in the preceding segment. By convention, the segments must be ordered by descending degree of constraint (i.e., location or profile, then orientation, then form).
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GCS requires a direction indicator that is not supported.
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Some geometric characteristic symbols, such as straightness applied to a planar surface, require a direction indicator (e.g., a surface line) to specify the direction along which the constraint applies (e.g., see ISO 16792:2006, Fig. 55). However, GD&T Advisor does not provide any means to indicate the direction. Therefore, geometrical tolerances that require a direction indicator are not supported.
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Generally, you should select one of the 'suggested' geometric characteristic symbols from the menu (i.e., those symbols that are represented with a white on green icon). Often there is only a single 'suggested' geometric characteristic symbol available, so the decision is easy. In some cases, there is more than one geometric characteristic symbol available in the menu. In such cases, you should consider the function of the feature and the meaning of each of the available geometric characteristic symbol when deciding which symbol to apply.
When specifying a geometrical tolerance value, you should consider the functional requirements for the feature. A tolerance value is often determined based on:
• A corporate design guideline
The tolerance values that you specify may have a significant impact on the cost of the workpiece. You should generally specify the largest tolerance value that will meet the functional requirements of the feature.