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# What is Concentricity in GD&T – How to Measure, Calculate Concentricity & Concentricity vs Runout

2023.4.13

Concentricity is a complex tolerance that is used to ensure the precision of quality in manufacturing including CNC machined parts. What is concentricity in GD&T and how is it measured and calculated? Let’s get into some details and the difference between concentricity and runout.

## What is Concentricity in GD&T?

Concentricity in GD&T (Geometric Dimensioning and Tolerance) is a tolerance that specifies the allowable deviation of an axis or center plane of a cylindrical or spherical feature from a datum axis or center plane. Concentricity defines the tolerance zone within which the center points of two or more features must lie. It is often used to ensure that two rotating parts are accurately aligned and to prevent vibration or wobbling. The tolerance zone is defined by two concentric circles or spheres, with the center of the tolerance zone representing the ideal center point of the feature.

The concentricity symbol in GD&T is the same in both ISO and ASME standards, which is two concentric circles along with, at a minimum, a value for the tolerance, and a reference datum. Concentricity is measured in units such as millimeters or inches. It is generally used for high-precision parts, and only when there is a need to control median points.

## How to Measure Concentricity?

Measuring concentricity requires a measurement device capable of accurately measuring the position and orientation of the feature being evaluated relative to a datum feature. Here are the basic steps.

1) Identify the datum feature: The datum feature is the reference feature to which the concentricity tolerance is applied. It is usually a cylindrical or spherical feature that provides a stable reference point.

2) Mount the part: Secure the part to be measured in a suitable fixture or holding device that allows access to the feature being evaluated.

3) Select the measurement device: Use a measurement device such as a coordinate measuring machine (CMM), optical comparator, or laser scanner capable of accurately measuring the position and orientation of the feature.

4) Align the part: Orient the part so that the datum feature is aligned with the measurement device.

5) Measure the feature: Take multiple measurements of the feature, ensuring that the measurement device is positioned at different angles around the feature.

6) Analyze the data: Use specialized software to analyze the measurements and determine if the feature falls within the acceptable tolerance zone defined by the concentricity symbol.

7) Adjust if necessary: If the feature falls outside the acceptable tolerance zone, adjust the manufacturing process or part to bring it within the specified tolerance.

## How to Calculate Concentricity?

Before you can calculate the concentricity, you need to establish the datum and measure all required features. The datum feature is usually a cylindrical or spherical feature that provides a stable reference point. It is recommended to use a measurement device such as a coordinate measuring machine (CMM), optical comparator, or laser scanner to measure the position of the axis or center plane of the cylindrical or spherical feature relative to the datum axis or center plane.

The following formula is used to calculate the concentricity:

C = Wmin/Wmax * 100%.

Wmin is the minimum width. Wmax is the maximum width. C is a percent. 100% means the tubing is fully concentric.

## Concentricity vs Runout, What's the Difference?

Concentricity and runout are two related but distinct geometric tolerances in GD&T. While both tolerances are used to evaluate the circularity or coaxially of cylindrical or spherical features, they have different meanings and applications.

Concentricity refers to the allowable deviation of the axis or center plane of a cylindrical or spherical feature from a datum axis or center plane. It is used to ensure that two rotating parts are accurately aligned and to prevent vibration or wobbling. Runout, on the other hand, refers to the allowable variation in the circular path of a feature as it rotates. It is used to evaluate the roundness, straightness, or flatness of a surface as it rotates around an axis.

Key differences between concentricity and runout:

– Concentricity evaluates the axis or center plane of a feature, while runout evaluates the circular path of a feature.

– Concentricity is used to ensure rotational alignment and prevent vibration, while runout is used to evaluate surface shape.

– The GD& T symbol of concentricity and runout is different, and how they are shown in a drawing.