The Basic “Geometric Dimensional Tolerance” series! Introducing essential fundamental knowledge for Accurate Measurement!

In the previous installment (the first article of this series), we explained “position”. In this second installment, we will clarify the definitions of "concentricity" and "coaxiality," two location tolerances that are often difficult to understand and easily confused. This time, ACCRETECH (Thailand) Sales Engineer Hitomi and Sales Manager Pom will be your guides.

I came across the ◎ symbol in a drawing, and I remember it represents two different characteristics. However, I’m unsure how to distinguish them based on the drawing...!

The ◎ symbol represents "concentricity" and "coaxiality." Do you know the definition of each?

It's quite difficult to explain the difference just by looking at the drawing... I've also had customers who perform measurements tell me that they often struggle to determine which one it refers to.

Both concentricity and coaxiality are types of location tolerances and are represented by the same JIS symbol "◎" (double circle).　Fundamentally, these two tolerances are evaluated in similar way, indicating how much the center of an object's axis deviates. However, there are subtle differences between them, which we will now explain.

Concentricity | Evaluates the Deviation of the Center Points

First, concentricity evaluates how closely the centers of two circles align. In other words, it focuses solely on the position of the center points when viewed from directly above.

【Concentricity】

The degree of deviation in the center position of a circular feature relative to the center of the datum circle.

For example, when looking at the center of a cylindrical workpiece in a drawing, concentricity refers to the deviation between the centers of two cylinders. Simply put, in the right diagram, the offset between the red circle and the black circle represents concentricity. This drawing indicates that the deviation must be kept within a φ0.1 range.

Coaxiality | Evaluates the Deviation of the Central Axis (Center Line)

Next is coaxiality. It evaluates how closely two axes (such as the central axes of holes or shafts) align. It takes into account not only the position of the center but also the inclination of the central axis.

【Coaxiality】

The degree of deviation of an axis, which should be aligned with the datum axis, from the actual datum axis.

For example, in a workpiece where cylindrical sections of different diameters are joined, coaxiality refers to the deviation between the axis of the larger-diameter cylinder and the axis of the smaller-diameter cylinder. In the diagram on the right, the red centerline represents the evaluated axis, and the requirement specifies that the deviation must be kept within a φ0.1 range. Coaxiality is typically evaluated at the end of the axis that is farthest from the datum axis.

I see! The key point is that concentricity evaluates the "center point" while coaxiality evaluates the "central axis"!

The shape of the part is also an important factor to consider! Coaxiality generally involves a stricter evaluation

This is a slightly more advanced example: a drawing with three cylindrical parts connected in series. In this case, axes are established using datum A and datum B. The evaluation is generally based on coaxiality. The drawing instructs that the deviation from the reference axis, which connects A and B, must be within φ0.1.

I see! I hadn't really paid attention to it, but now that you mention it, it's something I often come across in drawings.

Although there is no strict rule for when to use concentricity and coaxiality, in general, concentricity is used to evaluate flat workpieces like washers with holes, while coaxiality is more commonly used for shaft-like components with an axis. Both tolerances define the deviation of the axis' center, but since coaxiality tends to be a stricter evaluation, it is often safer to base measurements on coaxiality when in doubt. This approach can help prevent defects.

A single machine covers a wide range of measurement tasks, including roundness, diameter, and surface roughness! Roundness and Cylindrical Shape Measurement Machine - RONDCOM NEX Series

The measurements of "concentricity" and "coaxiality" discussed here are primarily performed using roundness and cylindrical shape measurement machines. Accretech (Tokyo Seimitsu) recommends the RONDCOM NEX series, which was released in 2014.

The product lineup is characterized by a wide variety of models that can be selected based on factors such as load capacity, column size, and required measurement items. It offers customizable measurement functions for roundness, diameter, surface roughness, and more, all within single machine. By combining labor-saving options such as the “AFD (measuring force control detector)” and the “XY-axis automatic stage,” processes can be automated, significantly improving measurement efficiency.

With just one RONDCOM NEX, accurate roundness measurements can be easily performed!

That's correct! We have demo units available at our showrooms in Bangkok and Chonburi, where customers can consult about testing with their workpieces or receive usage training. We also have stock machines in Thailand for quick delivery.

▲From left to right: Pom, Sales Manager; Hitomi, Sales Engineer Manager; Asano, Sales Manager at Accretech (Thailand)

▶ Learn the Basics in Geometric Dimensioning and Tolerancing (GD&T) Course Vol.1: True Position

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