True position is not the same as concentricity. While they may seem similar at first glance, they have distinct differences in terms of what they control and how they are measured.
True position is a geometric tolerance that controls the location of a feature relative to a specified datum or reference point. It ensures that the feature is within a certain tolerance zone, which is typically defined as a cylindrical volume around the theoretical perfect position. The tolerance zone is determined by the specified tolerance value and the geometric control frame (usually a perpendicularity symbol). True position is commonly used to control the location of holes, pins, or other features that need to be accurately positioned in relation to other parts.
Concentricity, on the other hand, controls the derived median points of surface elements of a feature, such as a cylindrical or spherical surface. It ensures that these median points, which may not be perfectly aligned due to manufacturing variations, are within a specified tolerance zone relative to a datum axis. Concentricity controls the centerline or axis of a feature, rather than its positional location. It is often used to control the alignment of rotating parts, such as shafts, bearings, or gears.
To better understand the difference, let’s consider an example. Imagine a wheel with bolt holes. The true position of the bolt holes would ensure that they are located accurately in relation to the wheel’s center or a specific reference point. This means that the bolt holes would be within a certain tolerance zone, allowing for slight variations in their exact location.
On the other hand, concentricity would control the alignment of the bolt holes’ centerlines with the wheel’s axis of rotation. It ensures that the median points of the bolt holes are perfectly aligned with the axis, even if they are not located precisely at the intended position. This is important because if the bolt holes are not concentric, it can result in imbalance, vibration, or other issues when the wheel is rotating.
In terms of measurement, true position is typically evaluated using basic coordinate measuring tools, such as calipers or coordinate measuring machines (CMMs). The actual position of the feature is compared to the theoretical position specified on the drawing, and any deviations are measured and recorded.
Concentricity, on the other hand, is more complex to measure. It requires specialized equipment, such as a concentricity gauge or a CMM with specific software capabilities. The equipment measures the surface elements of the feature and calculates the derived median points. These points are then compared to the theoretical axis or centerline, and any deviations are evaluated against the specified concentricity tolerance.
While true position and concentricity may seem similar at first, they are different geometric tolerances that control different aspects of a feature. True position controls the location of a feature relative to a specified datum, while concentricity controls the alignment of the derived median points with a specified axis. Understanding these differences is crucial in engineering and manufacturing to ensure the accurate and functional assembly of parts.