In GD&T, a profile tolerance is a tolerance used to define a uniform boundary around a surface within which the elements of the surface must lie. It is a versatile and complex tolerance that controls the shape and orientation of a feature, and sometimes its size and location simultaneously.
Understanding Profile Tolerance
Profile tolerance is a fundamental concept in Geometric Dimensioning and Tolerancing (GD&T) used to control the profile of a feature relative to a datum or another feature. Unlike simpler tolerances that might control only form or size, profile tolerance offers comprehensive control over a feature's shape and position in 3D space.
As stated in the reference: "In GD&T, profile tolerance defines a uniform boundary around a surface within which the elements of the surface must lie. Profile is a complex tolerance that simultaneously controls a feature's form, size, orientation, and sometimes location."
This uniform boundary is essentially an envelope that surrounds the ideal or basic profile. Every point on the actual surface must fall within this specified boundary, which is defined by the tolerance value.
What Profile Tolerance Controls
Profile tolerance is powerful because it can control multiple characteristics of a feature simultaneously. The reference highlights these key aspects:
- Form: Controls the shape of the surface, ensuring it conforms to the nominal profile.
- Size: When applied without a datum, it can indirectly control the size variations of the profile.
- Orientation: Controls the angle or orientation of the profile relative to specified datums.
- Location: When related to datums, it controls the position of the profile relative to those datums.
Here's a breakdown of what profile tolerance can control, depending on its application:
| Aspect | Control Type | Description |
|---|---|---|
| Form | Profile of a Surface / Profile of a Line | Limits variations in the shape along the surface or a cross-section. |
| Orientation | Profile of a Surface / Profile of a Line | Controls the angle relative to datum features. |
| Location | Profile of a Surface (with datums) | Controls the position relative to datum features. |
| Size | Profile of a Surface / Profile of a Line | Limits the overall dimensions of the profile (especially without datums). |
Practical Application
Profile tolerance is commonly used for complex shapes, curves, and surfaces that are difficult to control using standard dimensional tolerances or simpler geometric tolerances like flatness or straightness.
- Examples of Use Cases:
- Controlling airfoil shapes on aircraft wings.
- Defining the contour of a car body panel.
- Specifying the shape of a plastic injection molded part with complex curves.
- Ensuring the form and location of sealing surfaces.
Using profile tolerance helps engineers define precise manufacturing requirements and ensures that parts fit and function correctly. It replaces multiple simpler tolerances with a single, comprehensive control.
Profile Tolerance vs. Other Tolerances
While profile tolerance controls multiple aspects, other GD&T tolerances control specific characteristics:
- Form Tolerances (Flatness, Straightness, Circularity, Cylindricity): Control only the shape of a feature.
- Orientation Tolerances (Parallelism, Perpendicularity, Angularity): Control the angle of a feature relative to a datum.
- Location Tolerances (Position, Concentricity, Coaxiality): Control the position of a feature relative to datums.
- Size Tolerances (Dimensional Callouts): Control the overall size of a feature.
Profile tolerance integrates the control of several of these aspects into one powerful requirement.
