Ever stared at a technical drawing and wondered what the concentricity callout actually means—or how to use it correctly? You’re not alone. Getting concentricity right is crucial to ensure that parts fit and function flawlessly, especially in precision engineering.
Understanding how to apply the concentricity callout can make or break your project’s success. In this article, we’ll break down exactly what it is, when to use it, and step-by-step guidance for accurate callouts—along with expert tips to boost your confidence.
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How to Use a Concentricity Callout in GD&T
Concentricity is one of those geometric dimensioning and tolerancing (GD&T) concepts that can sound complex, but its goal is simple: making sure the center points of different features line up perfectly. Whether you work in design, machining, or quality control, understanding how to apply a concentricity callout is a must when producing high-precision parts.
In this article, you’ll learn a clear step-by-step explanation of concentricity callouts, discover the challenges and best uses, and pick up practical tips to apply in your projects.
What Is a Concentricity Callout?
A concentricity callout is a GD&T symbol used on technical drawings to control the central axes of two or more cylindrical or spherical features, ensuring they share the same centerline.
In simple terms:
If you have two circles or cylinders that are supposed to share the same middle point, concentricity is what verifies they really do.
Key Points:
- Used mainly for features such as OD/ID (outside diameter/inside diameter) of tubes, shafts, and bores.
- Focuses on the central axes rather than surface points.
- Helps avoid imbalance, vibration, or uneven wear in rotating parts.
The GD&T Symbol for Concentricity
- Symbol: ⦿ (two concentric circles)
- You’ll find this symbol in a feature control frame on engineering drawings, along with a tolerance value.
Example Feature Control Frame:
| ⦿ | 0.01 | A |
This tells you the derived median axis of the feature must fall within a cylindrical tolerance zone—here, 0.01mm—centered on datum A.
When Should You Use Concentricity?
Concentricity is rarely used compared to similar controls like position or runout. You should only call for concentricity when you:
– Need strict control over the distribution of material around the central axis.
– Cannot accept variations even if the surface locations are nearly perfect.
– Are manufacturing high-precision rotating components (such as spindles or bearings).
Inappropriate Use
Overusing concentricity can increase cost and inspection time. For most shaft or hole alignments, consider using:
– Position (for hole center locations)
– Runout (controls both roundness and axis alignment)
These are easier to measure and sufficient for most needs.
Step-by-Step: How to Apply a Concentricity Callout
Here’s how to correctly use concentricity on your design:
1. Identify Which Features Need to Be Concentric
For instance, if you have a shaft with two diameters or a part with an outside and inside diameter, determine which cylindrical features’ axes must align.
2. Choose a Datum
Pick a feature to act as the reference (the “perfect” axis), such as the inner diameter or a critical bore.
3. Place the Feature Control Frame
On your engineering drawing, add the concentricity symbol (⦿) next to the controlled feature. Specify the allowable tolerance and the associated datum.
4. Specify the Tolerance
Set the tolerance as tightly as needed for function—not tighter. Tighter tolerances mean higher cost and more inspection effort.
5. Communicate the Requirement
Make sure your manufacturing, inspection, and quality teams understand why it’s being used and how to check it.
How Is Concentricity Measured?
Measuring concentricity is not easy or quick compared to other GD&T controls. Here’s the typical measurement process:
-
Setup
Mount the part so that the datum feature aligns with the measuring instrument axis. -
Section the Feature
For the feature being checked, measure an array of points along the length and around each cross-sectional slice. -
Calculate Median Points
At each cross-section, determine the median point of the material. This is NOT the same as the geometric center or the physical surface.
-
Construct the Median Axis
Connect the median points from all cross-sections to create the derived median axis. -
Compare Axes
Check that the derived axis lies within a tolerance zone (a cylinder around the datum axis with the diameter you specified).
Why Is This Hard?
- Labor-Intensive: You need many measurements to find all median points.
- Equipment: Requires advanced metrology tools like coordinate measuring machines (CMMs).
- Interpretation: Median points rely on the material distribution, not on easy-to-see surfaces.
Benefits of Using Concentricity
- Ensures even wall thickness, preventing weak spots.
- Critical for high-precision, high-speed rotating parts.
- Reduces vibration, wear, and imbalance in assemblies.
Challenges and Drawbacks
Using concentricity introduces several difficulties:
- Measurement Complexity: Requires specialized equipment and skilled inspectors.
- Cost: Tight concentricity tolerances are expensive to produce and verify.
- Misunderstandings: It’s often misapplied where simpler controls like runout or position would work.
Practical Tips & Best Practices
Follow these best practices to make concentricity work for you:
- Use Only When Necessary: If you just need the feature circles in alignment, consider position or runout.
- Clarify Functional Need: Clearly state why concentricity is critical—such as uniform material thickness.
- Don’t Overtolerance: Avoid setting an unnecessarily tight tolerance, which can greatly increase cost.
- Standardize Inspection: Work with your quality team to establish the measurement procedure before production starts.
Reducing Production and Inspection Costs
If your application involves shipping parts to or from your facility, or outsourcing manufacturing:
- Specify Acceptable Tolerances: Only as tight as needed to avoid unnecessary scrap and rework costs.
- Avoid Over-Inspection: Rely on runout or position controls for most features.
- Communicate Clearly: Share inspection procedures and expectations with suppliers to avoid costly misunderstandings.
- Use GD&T That’s Commonly Understood: Remember, not all shops have experience or equipment for concentricity.
Conclusion
Concentricity callouts in GD&T are powerful but demanding. They center on the idea that certain features must have perfectly aligned axes, keeping the distributed material even on every side. Due to difficulty in measurement and added cost, only specify concentricity if your design absolutely requires it. For many applications, simpler controls like runout or position provide the same functional result with less hassle.
If you’re unsure, consult with your manufacturing and inspection teams before making a concentricity callout. Proper application ensures quality without adding needless complexity or expense.
Frequently Asked Questions (FAQs)
What is the difference between concentricity and runout?
Concentricity controls the median axis of a feature to ensure its center aligns with a datum axis, focusing solely on even material distribution. Runout, on the other hand, is easier to measure and controls both surface roundness and axis alignment in a single operation.
When should concentricity be used instead of other controls?
Use concentricity only when you need to ensure the central axes align AND the material is evenly distributed around the axis—such as for critical rotating parts where imbalance could cause failure. For most cases, runout or position is sufficient and much easier to inspect.
Why is concentricity rarely used in modern manufacturing?
Because it’s difficult, time-consuming, and expensive to measure. Simpler, more practical controls like runout or positional tolerancing can almost always achieve the same result with less effort.
How do you measure concentricity practically?
You measure concentricity by using specialized equipment (like a CMM) to find and connect median points through every cross-section, then verify if that derived median axis stays within a defined cylindrical tolerance zone centered on the datum axis.
How does concentricity affect part cost and inspection?
Specifying concentricity can significantly increase both manufacturing and inspection costs due to tighter process controls, the need for skilled metrology, and the time spent on detailed measurements. Always weigh cost versus necessity before choosing this control.
With this knowledge, you’re ready to apply—and just as importantly, question—the use of concentricity callouts in your next engineering project. If you need perfect central alignment, know that you’re prepared to specify, measure, and justify its use with confidence!