What are the best practices for applying GD&T to curved surfaces and profiles?

1 Choose the right datum

A datum is a reference point, line, plane, or surface that establishes the coordinate system for the part. Choosing the right datum is crucial for applying GD&T to curved surfaces and profiles, as it affects how the tolerances are measured and interpreted. For example, if you have a cylindrical feature, you should choose a datum that is perpendicular to the axis of the cylinder, not parallel to it. This way, you can control the roundness, straightness, and coaxiality of the cylinder more effectively. Similarly, if you have a conical feature, you should choose a datum that is parallel to the axis of the cone, not perpendicular to it. This way, you can control the taper, angle, and concentricity of the cone more accurately.

2 Use the proper modifiers

Modifiers are symbols that modify the meaning or interpretation of a geometric tolerance. They can indicate whether the tolerance applies to the entire feature or only a portion of it, whether it is related to the datum or not, whether it is controlled by a maximum or minimum material condition, and so on. Using the proper modifiers is essential for applying GD&T to curved surfaces and profiles, as it can avoid ambiguity and confusion. For example, if you have a spherical feature, you should use the all-around modifier (a circle around the geometric tolerance symbol) to indicate that the tolerance applies to the entire surface of the sphere, not just a section of it. Likewise, if you have a spline feature, you should use the unequally disposed modifier (a U with a circle around it) to indicate that the tolerance is distributed unevenly along the profile of the spline.

3 Avoid unnecessary tolerances

Tolerances are the allowable variations in the shape, size, orientation, and location of a feature. They are necessary to ensure that the part meets the functional requirements and can be manufactured and inspected within reasonable limits. However, adding too many or too tight tolerances can also increase the cost, complexity, and difficulty of the part. Therefore, you should avoid unnecessary tolerances when applying GD&T to curved surfaces and profiles. For example, if you have a cylindrical feature that is already controlled by a diameter tolerance, you do not need to add a roundness tolerance, unless it is critical for the function of the part. Similarly, if you have a conical feature that is already controlled by an angle tolerance, you do not need to add a taper tolerance, unless it is important for the fit of the part.

4 Use composite tolerancing

Composite tolerancing is a method of applying GD&T that combines two or more geometric tolerances in a single feature control frame. It allows you to specify different levels of control for different aspects of a feature, such as form, orientation, and location. Composite tolerancing can be beneficial for applying GD&T to curved surfaces and profiles, as it can simplify the drawing, reduce the number of datums, and improve the quality of the part. For example, if you have a cylindrical feature that needs to be controlled for both straightness and position, you can use composite tolerancing to specify a tighter straightness tolerance in the upper segment of the feature control frame, and a looser position tolerance in the lower segment of the feature control frame. This way, you can ensure that the cylinder is straight enough to function properly, and positioned close enough to the datum to fit with other parts.

5 Check the manufacturability and inspectability

The final and most important best practice for applying GD&T to curved surfaces and profiles is to check the manufacturability and inspectability of the part. Manufacturability refers to how easy or difficult it is to produce the part with the specified tolerances, using the available tools and processes. Inspectability refers to how easy or difficult it is to verify that the part meets the specified tolerances, using the available instruments and methods. You should always consult with the manufacturing and inspection personnel before finalizing your GD&T, and make sure that they can produce and measure the part with reasonable accuracy and efficiency. You should also consider the trade-offs between cost, quality, and performance, and optimize your GD&T accordingly.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?