GD&T Position and True Position Definitions, Calculations & Differences Between Them

The position is a useful and complex tolerance in GD&T, another term true position is often used, how are they defined and how do they differ? Here we’ll get into the position and true position definitions, calculations, and differences.

What is the Position in GD&T?

In GD&T (Geometric Dimensioning and Tolerancing), the position is a tolerance that defines tolerance zones depending on the feature, it specifies the acceptable range of variation for the location of a feature relative to its intended position on a part. It can be used to control the location of features such as holes, bosses, pins, and other geometric features that have a specific location requirement on a part. This tolerance is typically used when the location of the feature is critical to the function or assembly of the part.

The position tolerance specifies two values: a tolerance value and a datum reference. The tolerance value is the maximum allowable deviation from the true position, while the datum reference is the point, line, or plane used as the basis for the measurement. Position can be applied to both 2D and 3D features and can be evaluated using various measurement tools, such as coordinate measuring machines (CMMs), laser scanners, or optical comparators.

What is True Position in GD&T? - True Position Definition and Formula

The true position in GD&T describes the exact coordinate, or location defined by basic dimensions or other means that represents the nominal value. It is similar to position tolerance, but it provides more detailed information about the location and orientation of the feature. It defines a circular tolerance zone that is centered on the true position of the feature and has a diameter that represents the maximum allowable deviation from the intended location. The diameter of the tolerance zone is specified in the feature control frame and represents the positional tolerance. When it comes to the symbol of position and true position, the crosshair symbol (⌖) was referred to as position in the ASME Y14.5 standard, but now it is used for the true position in GD&T as well.

The true position is usually applied to features that have a high degree of positional accuracy, such as holes, pins, or other features that require precise alignment or mating with other parts. It ensures that the feature is located within the specified tolerance zone relative to the datum or reference point.

True position can be calculated using the following formula:

– True position = 2 x (dx^2 + dy^2)^1/2. (dx is the deviation between the measured x coordinate and the theoretical x coordinate, dy is the deviation between the measured y coordinate and the theoretical y coordinate.)

Differences Between Position and True Position

True position refers to the exact position of a feature as defined by basic dimensions, while the position defines the allowable variation of that feature from the true position.

– Calculation method: Position tolerance is calculated as the distance between the feature’s actual location and its intended location projected onto a plane parallel to the specified datum. True position tolerance is calculated as the distance between the feature’s actual location and its intended location projected onto a cylinder whose diameter is equal to the specified tolerance value.

– Range of deviation: Position tolerance specifies the maximum allowable deviation of the feature’s location in both the X and Y directions, while true position specifies the maximum allowable deviation in any direction from the true position.

– Application: Position tolerance is often used when the feature’s location is important, but its orientation is not critical. The true position is used when both the location and orientation of the feature are critical.

– Datum reference: Position tolerance requires at least two datum references, while true position requires a single datum reference.

– Complexity: True position is generally more complex and more difficult to apply than position tolerance, due to the need to calculate deviations in all directions from the true position.

GD&T Position and True Position Definitions, Calculations & Differences Between Them

What is the Position in GD&T?

What is True Position in GD&T? - True Position Definition and Formula

Differences Between Position and True Position

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