# Concentricity Measuring

## What is concentricity measuring?

Concentricity measuring, also known as the coaxially measuring, refers to the process of measuring the concentricity, which is a three-dimensional cylindrical tolerance zone, and is determined by the median points on the cross-sections of the datum axis. In other words, it is a set of median points that makes the theoretical datum axis in the cylindrical object that is supposed to be measured.

The concentricity measure is often seen in the application of tube installation, which requires to take the inside diameter and outside diameter as the references. When it comes to the application of tubes, the concentricity determines the constancy of the central axis, which is the datum axis, which then influences whether the tubes can be perfectly fitted and properly used or not.

Measuring the concentricity of a cylindrical or spherical object is not as simple as measuring the length of a straight line, which only requires aligning a ruler along the line. In fact, the theoretical central axis is an imaginary line that doesn’t really exist. Therefore, it is really a challenge in the measuring of concentricity.

There might exist the confusion between the concentricity and the circularity of an object. To measure the concentricity requires looking in the three-dimensional perspective, while the measuring of the circularity is the indication that determines the roundness of a circle in the two-dimensional perspective.

In other words, the circularity is measured when it is necessary to see if an object is out of round around the edges or not, instead of comparing the interrelation between the inside diameter and outside diameter to get the theoretical central axis as in the concentricity measuring.

## What is concentricity?

As far as we know, the concentricity is determined by an imaginary central axis that exists in the cross-section that is connected by the median points on both sides of a cylindrical or spherical object. The concentricity is usually seen in the measurement of the tubes, which are commonly applied in the water supply systems, for example.

The formation of a tube requires certain thickness of the wall, and the thickness represents the variation between the inside diameter and outside diameter. Thus, through the gauging of the inside and outside diameters, the median points along the central line of the part can be obtained.

Once connect the median points, the axes of datum and the referenced feature can be obtained. Comparing the interrelation of these axes, we can get the tolerance zone. Simply put, it is the median points, the datum axis and the referenced feature, and the tolerance zone that define the concentricity of the object being measured.

## When is concentricity measuring required?

The concentricity measuring is required when there is the need to install a tube inside an opening, or something is about to fit into the tube. The former is related to the outside diameter, and the latter is related to the inside diameter. These two parameters, in combination with the concentricity of the tube, have something to do with the fitness of the tube with its applications.

In other cases, when the tubes are applied for the transmission of media like water, oil, or gas, the concentricity is also required to be measured. Although whether the concentricity is in the tolerance zone or not doesn’t influence the flowing of these media, the other parameter, which is the pressure that these media give as they pass through the tubes, is an important reference to see if the wall thickness can withstand the given pressure.

Despite the fact that the wall thickness of tubes is not a direct indication that defines the concentricity of them, the concentricity can still be taken advantage of, because the parameters that are calculated in the measurement of concentricity are related to each other.

## How does concentricity measuring work?

To measure the concentricity of an object, there are two methods that adopt different tools and have different features, but are both available in the measurement, which are the dial gauge or the coordinate measuring machine (CMM).

● measuring with a dial gauge
The measurement that is conducted with the dial gauge is done when the dial gauge is placed on the vertex of the circumference for the targeted object, and the operator rotates the object to let the gauge measure around the circumference. This method may cause some concerns, because there might be unreliable results when the gauge is placed on the wrong angle, or the pressure that the operator gives on the tube, which might be elastic, is too much.

● measuring with a CMM
If the dial gauge is not available or the result of the measurement using the dial gauge is what may be concerned, the CMM is another choice. With a CMM. the stylus, which is the measuring tool, only comes in light contact with the targeted object, and is automatically controlled, which ensures the better accuracy of the measurement compared to its counterpart.

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