How to Calculate Positional Tolerance

What is Positional Tolerance?

Positional tolerance refers to the maximum allowable deviation in the location of a feature relative to a datum or another feature. It's a key aspect of dimensional tolerancing in engineering drawings and is used to specify how precisely a part must be manufactured to ensure proper assembly and function.

In manufacturing, positional tolerance is typically represented using geometric dimensioning and tolerancing (GD&T) symbols. The most common symbols used for positional tolerance are:

• Position (⊕): Specifies the allowable variation in the position of a feature relative to a datum reference frame
• Profile of a Surface (⊥): Controls the form and orientation of a surface
• Profile of a Line (⊥): Controls the form and orientation of a line

Positional tolerance is distinct from dimensional tolerance, which specifies the acceptable size variation of a feature. While dimensional tolerance focuses on the size of a feature, positional tolerance focuses on the location of the feature relative to other features or datums.

How to Calculate Positional Tolerance

The calculation of positional tolerance involves several steps and considerations. Here's a step-by-step guide to calculating positional tolerance:

1. Identify the Datum Reference Frame: Determine the primary, secondary, and tertiary datums that establish the coordinate system for the part.
2. Determine the Feature's Position: Measure the actual position of the feature relative to the datum reference frame.
3. Calculate the Deviation: Subtract the actual position from the nominal position to determine the deviation.
4. Compare to Tolerance Zone: Check if the deviation falls within the specified positional tolerance zone.

For more complex parts with multiple features, the calculation may involve vector analysis to determine the combined effect of deviations in multiple dimensions.

Factors Affecting Positional Tolerance

Several factors influence the calculation and interpretation of positional tolerance:

• Datum Selection: The choice of primary, secondary, and tertiary datums significantly impacts the positional tolerance calculation.
• Feature Relationships: The relationship between features (e.g., concentric, parallel, perpendicular) affects how positional tolerance is applied.
• Manufacturing Process: The capabilities of the manufacturing process determine the achievable positional tolerance.
• Material Properties: The material's properties can affect dimensional stability and positional accuracy.

Example Calculation

Let's walk through an example calculation of positional tolerance for a simple part with one feature:

1. Nominal Position: The feature should be located at 50mm from the datum.
2. Tolerance Value: The positional tolerance is ±0.1mm.
3. Actual Position: The feature is measured at 49.95mm from the datum.
4. Deviation: 50mm - 49.95mm = 0.05mm (within tolerance).

In this example, the feature is within the specified positional tolerance. The calculation shows that the feature's position deviates by only 0.05mm from its nominal position, well within the ±0.1mm tolerance.