How to check the concentricity of two diameters (and other factors in pipe procurement)

The question of how to check the concentricity of two diameters often arises in pipe procurement. It involves determining the wall thickness (the measurement between the outer diameter (OD) and the inner diameter (ID) of the tube) at different points to see how constant it is with respect to the central axis.

 The challenge is to establish a theoretical central axis that serves as a reference point (datum axis). This makes perfect concentricity almost as difficult to measure as it is to achieve.

 Therefore, the following points should be kept in mind before specifying tubing OD and ID measurements with concentricity requirements.

 1. What is concentricity?

In GD&T tolerances, concentricity is complex – a type of "circular symmetry" that is used to establish a tolerance zone for the midpoint of a cylindrical or spherical part. What is the difference between cylindricity and concentricity? Concentricity is an indicator of roundness and straightness along the full axis of a 3D part, while concentricity compares the outer and inner diameters, or the roundness of two different points.

GD&T concentricity, sometimes referred to as coaxiality, "controls the central axis of the reference feature, down to the datum axis," according to the GD&T Basics website.

 To put it more simply, you can define concentricity as a measure of the constancy of the wall thickness of a tube, pipe, or other cylinder. Thus, concentricity controls the central axis derived from the midpoint of the part, measured in cross-sectional units.

 This means that if the concentricity is "perfect", then the wall thickness between the outer and inner diameters is the same on every cross-section at every point around the pipe diameter.

 What makes concentricity such a complex feature?

Tubing concentricity relies on measurements from derived shafts, rather than tangible surfaces. In other words, it creates a theoretical 3D cylindrical tolerance zone into which all derived midpoints of the tube must fall.

 This is why concentricity is usually reserved for high-precision parts that need to control these midpoints.

 What is eccentricity vs concentricity?

When the wall thickness of the tube changes, you get an eccentric tube where the center of the circle formed by the outer diameter is at a different point than the center of the circle formed by the ID (in other words, the two circles are not concentric.

 Eccentricity is measured by looking at the cross-section to determine the minimum and maximum wall dimensions of the tube. The difference between the minimum and maximum thickness is then calculated and this number is divided by two halves.

 2. OD/ID concentric annotation

Tubing OD/OD concentricity requirements can be indicated on the drawing in a few different ways, including:

 GD&T concentric symbols, i.e. circles within circles

 Eccentricity percentage

TIR (Total Indicator Reading)

A written statement, e.g. "OD and ID must be concentric within 0.00X inches"

Another term that is sometimes used when talking about concentricity is wall runout, which is the same thing as TIR. Wall runout is calculated by placing an indicator on the part as it rotates around its axis, measuring not only the concentricity of the part but also the roundness of the part.

 Wall runout is derived from the eccentricity of the tube and describes the change in wall thickness compared to the specified nominal wall. This can also be expressed as:

 The maximum wall thickness minus the minimum wall thickness

Eccentricity x 2 (multiplied by 2)

When using these (and other) terms to describe concentricity requirements in drawings, material suppliers and precision metal cutting shops must not only determine which machine process to use, but also determine how concentricity will be measured to bring it to specification.

 concentricity

Typical concentric callouts

3. The challenge of measuring concentricity

This presents us with the challenge of how to measure concentricity and determine if the specified OD and ID have been achieved. In addition to the problem of establishing a theoretical central axis, measuring concentricity requires:

 Multiple measurements are taken over a series of cross-sections

Accurately draw the surface and determine the midpoint of the cross-section

Plot this series of points to see if they fall within the cylindrical tolerance

You can use a micrometer or an optical comparator to measure the concentricity of some parts. However, it is best to use a coordinate measuring machine (CMM) or other computerized measuring equipment to perform the task. However, CMMs are time-consuming, which in turn means increased costs.

 Another challenge is that in today's micromachining, the parts to be measured are often smaller than ever before. For example, for miniature precision-cut tubes used as components of medical devices, the challenge is how to check the concentricity of two diameters in a very, very small tubing.

 4. When concentricity is required

Because of all these complexities, concentricity is often only used for parts that require very high precision to work properly.

 Whether concentricity is critical depends on the end use, such as whether a physical entity with its own outer diameter needs to be installed into the pipe. For example:

 In general, if one tube needs to go inside the opening and another part needs to fit into the tube inner diameter, the outer diameter, inner diameter, and concentricity may need to be aligned so that all of these parts work together.

However, if the application requires a liquid or gas to pass through the tube, the concentricity may not matter because the non-concentricity of the tube does not impede flow.

However, even if concentricity is not important, it may be important to know the distance of the OD/ID from the concentricity. For example, when a liquid or gas is under pressure through a tube, you may want to specify an acceptable minimum wall thickness to ensure that the pressure does not cause thin spots on the non-concentric tube wall to break.

 To some extent, the choice of material may also be related to concentricity or min/max wall thickness. For example, if you choose to use a welded pipe that has been ground to form a part, you may want to specify a minimum thickness to prevent the pipe wall from being ground too thin and causing the weld to break.

 Similarly, if your end application will use tubing to move the liquid under high pressure, a seamless material that is stretched rather than welded may be a better choice to minimize the risk of breakage. But again, if the tube is simply releasing air into the environment, then a seamless tube would be an over-engineered case.

 5. Another option: concentricity and beat

In some cases, you can avoid the time and cost of verifying concentricity by replacing the concentricity requirement with runout, which is easier to measure and easier to achieve.

 Concentricity looks at how well a cylindrical feature is centered on the theoretical axis, while runout looks at how far the feature deviates from a perfect circle, which is completely centered on the axis of rotation. In other words, runout is a combination of concentricity and roundness – if the part is perfectly round, the runout will be equal to concentricity.

 While concentricity and runout are not the same thing, they are often used interchangeably to achieve the same basic end result.

 The biggest difference is that with runout, you can physically touch and measure the surface of the part. Controlling the runout will also control concentricity, although it is undeniable that it is not to the same degree as when concentricity is applied alone.

 (Learn more about runouts, including circular routs, total runouts, and TIR.)

 Tubes are designed with achievable tolerances in mind

Keep in mind that the feasibility of producing parts within acceptable tolerances is a crucial consideration when drawing drawings. This is why most machinists, measurement technicians, and design engineers recommend avoiding OD/ID concentricity as much as possible.

 Instead, you can use other applicable GD&T symbols in your tube drawings and designs, thus not designing them into the part from the start, preventing OD/ID concentricity challenges.

To learn more about machining and to request a quote, please contact

Zach Pan, Sales Manager, Email: pan@casting-aluminum.com，TEL/Whatsup:+8615632028516