How To Measure Concentricity Tolerance

In the field of Geometric dimensioning and tolerancing (GD & T), concentricity is one type of complicated tolerance. It is usually used to establish the tolerance boundaries (otherwise called tolerance zone) where the median points of a spherical or cylindrical product design or feature are. It is often used for high precision components and when median points will need to be controlled. However, since measuring and verifying concentricity tolerance is a complex and time-consuming process, many engineers and product designers prefer and are usually advised to employ runout or position tolerance.

What Is the Tolerance Zone?

Establishing the tolerance zone before the manufacturing process is critical as this will determine the cost incurred and the success of your project. It is defined as the pre-determined horizontal length that extends from the outer edge wall to the opposite side. Concentricity establishes a 3-D cylindrical or spherical tolerance zone surrounding the datum axis, and all the central points will lie within the boundary. The diameter of this zone is considered the permissible and appropriate value for this callout.

When is Concentricity Tolerance Used?

Concentricity tolerance is complex and complicated; therefore, it is difficult to measure and calculate. It is usually used in transmission shafts, gear, or balancing equipment. This concentricity tolerance will determine the dimensions and size of the driving shaft to prevent any wobbling. First, the part’s real median axis must be determined to ensure concentricity by computing the midpoints of diametrically opposite locations. The median axis is obtained by connecting all such median sites. For the part to be approved in a standard engineering and designing process, all points on the median axis must be inside the cylindrical tolerance zone.

How is Concentricity Tolerance Measured?

The concentricity tolerance can be evaluated or measured in four basic steps. First, you will have to establish and identify where would the point of the datum plane, surface, or axis lies. Next, you will plot the points on the outer profile’s controlled surface, which can be determined using the CMM or a coordinate measuring machine. Then, you will have to measure and calculate where the central points and axis of the plotted profile are at different cross-sections. Lastly, you will need to verify where the positions of the central points within the cylindrical tolerance zone are.

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by George Chitos

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