How to Add Geometric Tolerances in BricsCAD

Quick answer: A geometric tolerance specifies the maximum allowed variation in form or position from true geometry, essentially the zone a surface, axis, hole, or cylinder can lie within and still function correctly and remain interchangeable. BricsCAD draws these using a feature control frame, a rectangle divided into compartments.

The Feature Control Frame Structure

Every feature control frame has at least two compartments:

1. Geometric characteristic symbol: indicates which characteristic the tolerance applies to, location, orientation, or form (a form tolerance might indicate flatness or roundness, for example).
2. Tolerance value: preceded by a diameter symbol when appropriate, and followed by a material condition symbol for features that can vary in size.

From there, the tolerance value can be followed by primary, secondary, and tertiary datum reference letters, each with its own material condition. Datum reference letters typically reference one of up to three perpendicular planes a measurement is made from, though they can also indicate an exact point or axis.

Composite and Projected Tolerances

When two tolerances apply to the same geometry, a composite tolerance combines a primary tolerance value with a secondary one. For even more specificity, a projected tolerance adds a height value followed by a projected tolerance symbol, useful for indicating something like the perpendicularity of an embedded part.

Creating a Geometric Tolerance Frame

1. Click Tolerance on the Dimensions toolbar or ribbon panel, choose it from the Dimensions menu, or type tolerance and press Enter. The Geometric Tolerance dialog opens.
2. Compose the frame in the dialog.
3. Click OK; the dialog closes.
4. Specify a point to insert the frame in the drawing.

Editing a Geometric Tolerance Frame

1. Double-click the frame; the Geometric Tolerance dialog opens.
2. Edit the settings.
3. Click OK; the dialog closes and the frame’s content updates.

Frequently Asked Questions

What’s the minimum a feature control frame needs to contain? At least two compartments: a geometric characteristic symbol and a tolerance value.

What does the material condition symbol actually indicate? It applies to features that can vary in size, following the tolerance value or a datum reference.

When would I use a composite tolerance instead of a single one? When two tolerances apply to the same piece of geometry, a composite tolerance combines a primary and secondary tolerance value together.

How do I go back and change a tolerance frame after placing it? Double-click it to reopen the Geometric Tolerance dialog, make your changes, and click OK.