How can you calculate position tolerance for a feature using GD&T?

2 How to calculate position tolerance?

To calculate position tolerance, you need to know the nominal dimension of the feature, the actual measured dimension of the feature, and the DRF. The nominal dimension is the ideal or target size of the feature, as specified on the drawing. The actual measured dimension is the size of the feature as observed on the part. The DRF is the set of datums that define the coordinate system for the part. Datums are usually labeled as A, B, and C, and they represent primary, secondary, and tertiary reference planes or axes. The DRF determines how the position tolerance is applied and measured.

The formula for calculating position tolerance is:

Position tolerance = |Actual position - True position|

The actual position is the location or orientation of the feature as measured from the DRF. The true position is the location or orientation of the feature as defined by the nominal dimension and the DRF. The difference between the actual position and the true position is the deviation of the feature from its ideal position. The position tolerance is the maximum allowable deviation for the feature to be acceptable.

3 How to measure position tolerance?

Position tolerance can be measured using various methods and tools depending on the type, shape, DRF, and accuracy required. For example, a coordinate measuring machine (CMM) uses a probe to measure the coordinates of a feature in three dimensions and compare them with the nominal coordinates to calculate the deviation from the true position. A functional gauge is also used to simulate the mating part or function of a feature and verify whether it fits within the tolerance zone. Additionally, a dial indicator measures the linear displacement of a feature from a reference point by moving it along the DRF and recording the variation from the true position. All of these methods and tools help measure position tolerance accurately.

4 How to interpret position tolerance?

To interpret position tolerance, you need to understand the meaning and implications of the GD&T symbols and modifiers that accompany the position tolerance. Material condition modifiers indicate whether the position tolerance applies to the maximum material condition (MMC), the least material condition (LMC), or the regardless of feature size (RFS) of the feature. MMC means that the position tolerance increases as the feature size decreases, while LMC means that it decreases as the feature size increases. RFS, on the other hand, means that it is constant. Composite tolerance is a type of tolerance that consists of two or more position tolerances that apply to the same feature, but at different levels of control. Projected tolerance extends this zone beyond the surface of the feature, allowing for more flexibility and variation in the location or orientation of the feature.