Geometry abstraction, polygon geometry, and convergent geometry > Polygon geometry and abstraction

Modeling tolerance values

Some feature creation methods in Modeling approximate the input geometry. Tolerances control this approximation process. You can set these tolerance values in the Modeling application on the General tab of the Modeling Preferences dialog box.

Distance tolerance

All modeling approximation techniques use the software's Distance Tolerance. The Distance Tolerance is the maximum allowable distance between the true theoretical sheet body and the resulting body that the system creates to approximate it.

Recommended distance tolerances

It is best to establish a fixed, single value for the Distance Tolerance, based on the type of parts you model. A common Distance Tolerance value for consumer parts and automobiles is 0.01mm. The best tolerance for other uses may vary from this recommended standard.

The typical recommended value is 0.01 mm
Commonly set values may range from 0.1 mm to 0.001 mm

Ensure that the fixed tolerance you specify results in data that meet the accuracy standards for the model. You should then leave the calculations of local tolerances to the software. The software tries to ensure that the local tolerances are appropriate.

Note:

Setting the Distance Tolerance value to 0.0 may produce strange display effects in shaded mode. In these cases, some facets of a solid may appear to be missing. The best way to avoid this effect while in shaded mode is to reduce the tolerance on the affected vertices.

Angle tolerance

Some techniques that approximate curves also require the Angle Tolerance to define the maximum allowable angle between the body normals at corresponding points. Curve approximation can be adversely affected by the Angle Tolerance. Even when the Distance Tolerance has been met, it is possible that more segments would have to be added to meet the Angle Tolerance. If you find that a body has an excessive amount of data, or that body creation is taking too long, you may want to increase the Angle Tolerance, or possibly make the Angle Tolerance very large to, in effect, remove that tolerance from consideration.

Specifying tolerances

When using a creation technique that requires a tolerance, make certain that the tolerance is reasonable for the model. Very small tolerances produce bodies with large amounts of defining data and may impair the smoothness of the body

There is a maximum amount of data that can be used for the definition of a body, but this maximum size is so large that it should rarely, if ever, be exceeded. If you do exceed the maximum size, the system displays an error message and does not create the body. A common cause for errors of this type are approximation tolerances that are set too small. Increasing these tolerances, especially the Angle Tolerance, should correct the problem.

Natural increases in tolerance

In some cases, certain modeling operations may cause the software to automatically increase the maximum value that you specify for a tolerance. Most commonly, such an increase may occur when creating multiple approximations for the same curve, and then sewing together the resulting geometry. In this case, there are two approximations, each created (for example) to an accuracy of 0.01 mm. These two approximations can only be guaranteed to match each other to an accuracy of 0.02 mm, so that is the tolerance applied.

Offset operations can also result in changes in tolerance, where the angular deviation between surfaces that are being offset result in a distance deviation. In these cases the system uses the modeling tolerance is used as a guide to what is acceptable.

Verifying tolerance with Examine Geometry

Use Analysis→Examine Geometry→Tolerance in Modeling to verify that natural increases in tolerance do not become unacceptable. In the Examine Geometry dialog box:

In the Edge Checks/Post Check Status group, select the Tolerances check box.
In the Check Criteria group, specify a Distance value that is a small multiple of the current Distance Tolerance. For example, a Distance value between 0.03 mm and 0.05 mm would be applicable for use with a fixed modeling Distance Tolerance of 0.01 mm.

If Examine Geometry indicates that the model exceeds the specified Distance threshold, you should closely check the area and the faces that adjoin the edges that are out of tolerance to see if there are problems in the modeling techniques.

For more information on natural increases in tolerance, see How edges are represented in a solid model. For additional information on using Examine Geometry to validate your model, see Solid Analysis Errors.

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