Geometric Dimensioning and Tolerancing, or GD&T, is a language used in manufacturing to make sure parts meet design specs and work as intended. Think of it as a set of rules that ensures everything fits together correctly.
Concentricity is one of those GD&T controls. It makes sure that one part of a feature is centered around an axis. It’s a three-dimensional control, ensuring that the mass is evenly distributed around the center axis.
Measuring concentricity can be tricky and often requires specialized equipment. Because of these challenges, other GD&T controls like circular runout or true position are often used as alternatives.
This article provides a comprehensive overview of concentricity, discussing its applications, how it’s measured, and some of the alternatives you can use.
What is Concentricity?
In engineering terms, concentricity refers to how well the median points of a circular feature line up with a central axis, called the datum. If you were to measure the distance from the center of the circle to the datum axis at many points around the circle, those center points would all need to fall within a specified zone of tolerance.
That tolerance zone is shaped like a cylinder, and its size determines how much variation is acceptable. The smaller the tolerance zone, the more perfectly concentric the feature needs to be.
The datum axis is a crucial reference point. It’s an imaginary line established by another feature (the datum feature) on the part. Concentricity is always measured in relation to this datum axis. Without a well-defined datum, it would be impossible to determine whether a feature is truly concentric.
Concentricity vs. Other GD&T Callouts
Concentricity is just one tool in the GD&T toolbox. There are other callouts that might be more appropriate, depending on the design requirements.
Concentricity vs. Circular Runout
Circular runout is an alternative that’s easier to measure than concentricity. While concentricity focuses on the central points of a feature, circular runout looks at the variation of a surface as it rotates around a central axis. Think of it like checking how wobbly a spinning top is.
Runout measurements can also be taken very quickly, sometimes in under 10 seconds using special gauges.
The main difference is that circular runout is a surface control, whereas concentricity controls the median points. Because it’s easier to measure, circular runout is often preferred.
Concentricity vs. True Position
True position, often just called “position,” is another alternative. It controls how accurately a feature is located relative to a set of reference points (a “datum reference frame”).
The key difference here is that true position is a location control, while concentricity is, again, a median point control. True position is generally favored because it’s easier to inspect and understand.
In fact, the ASME Y14.5-2018 standard, which governs GD&T, recommends using position and runout over concentricity whenever possible.
Understanding the Concentricity Feature Control Frame (FCF)
The Feature Control Frame, or FCF, is the language that GD&T uses to convey all the information you need to know about a particular requirement for a manufactured part. Let’s break down what each section of the FCF is telling you.
Geometric characteristic block
This first section of the FCF contains the geometric characteristic symbol, which, in the case of concentricity, is a single circle neatly tucked inside another circle.
Feature tolerance block
The feature tolerance block tells you the tolerance value. This value is the size of the cylindrical tolerance zone.
Datum block
The datum block shows the datum reference. That reference indicates the axis to which the concentricity is being referenced. In other words, it tells you which axis the controlled feature should be concentric to.
How to measure concentricity
To determine the concentricity of a part, specialized equipment is used. Two common tools are coordinate measuring machines (CMMs) and optical shaft measuring systems.
Measuring concentricity using a CMM
A CMM uses a probe to take precise measurements of an object’s surface. Here’s a simplified version of the process:
- Secure the part and fix the datum axis. Proper fixturing is essential to make sure the part doesn’t move during measurement. The datum axis, or theoretical axis, is established as a reference point.
- Find the center point for one cross-section. The CMM probe touches the surface at multiple points on a single cross-section. Sophisticated software then calculates the center point of that cross-section.
- Repeat for multiple cross-sections. This process is repeated at several cross-sections along the length of the cylindrical part. Taking multiple measurements helps to establish the median axis, which is the average of all the center points.
- Check whether the measured axis lies within the tolerance zone. The CMM software compares the measured axis to the datum axis. The software then determines whether the measured axis falls within the specified tolerance zone, indicating whether the part meets the concentricity requirements.
While this is a simplified explanation, it highlights the key steps involved in using a CMM to measure concentricity. This process ensures that manufactured parts meet the required specifications for proper function and performance.
Uses of Concentricity
Whenever uniform mass distribution is important, concentricity is, too. Here are a few examples:
Precision ball bearings
In ball bearings, concentricity ensures smooth rotation and reduces wear and tear. If the bearing isn’t perfectly concentric, it won’t rotate smoothly, and it will wear out much faster.
Transmission gears
With transmission gears, concentricity ensures proper meshing and reduces noise. Gears that aren’t concentric won’t mesh properly, which can cause loud noises and accelerate wear.
Medical-grade tubing
Concentricity in medical-grade tubing helps ensure consistent fluid flow rates and prevents blockages. If the inner and outer circumferences aren’t aligned, it can disrupt the flow or even cause a blockage.
High-pressure piping
For high-pressure piping, concentricity is essential for structural integrity. It ensures even stress distribution and prevents leaks. If the pipe isn’t concentric, stress will be unevenly distributed, which can lead to leaks or even a rupture.
Practical Considerations and Alternatives to Concentricity
Measuring concentricity precisely is tricky. It demands specialized equipment and, often, a metrology expert to interpret the results. This difficulty leads many engineers to look for simpler, more practical ways to control the relationship between features.
Controls like circular runout and true position offer excellent alternatives. They’re generally easier to measure and understand, providing a clearer picture of whether a part meets design requirements. The key is to understand the trade-offs. While concentricity might seem ideal for certain applications, the difficulty in measuring it might outweigh the potential benefits.
In fact, the ASME Y14.5-2018 standard acknowledges these challenges. It no longer includes concentricity as a tolerance and recommends using position and runout controls instead. This shift reflects a move towards more practical and measurable approaches to geometric dimensioning and tolerancing.
Eccentricity Explained
Eccentricity is the opposite of concentricity. If concentricity describes how well two circles share the same center, eccentricity describes how off-center they are from each other. Think of it as a measure of how lopsided something is.
You can calculate eccentricity by measuring the distance between the centers of the two circles or cylinders in question. The greater the distance, the greater the eccentricity.
While concentricity is often desired, sometimes eccentricity is intentionally designed. Camshafts and eccentric gears, for example, rely on controlled eccentricity to function properly. In these cases, precise control of eccentricity is just as important as precise control of concentricity in other applications.
In Summary
Concentricity, a GD&T control, is all about making sure a feature is perfectly centered around a specific axis. This is especially important when you need even weight distribution in a part.
However, measuring concentricity can be tricky and often needs specialized equipment. That’s why circular runout or true position are often better choices.
When you’re deciding which GD&T control to use, think carefully about what your application needs and how easy it will be to measure. Keep in mind that modern GD&T standards, like ASME Y14.5-2018, are moving away from using concentricity, so it’s worth considering the alternatives.