Ever wondered why your parts sometimes don’t fit quite right—even when everything measures up? The answer might lie in understanding the difference between concentricity and true position. Knowing how these two concepts differ can be the key to better assembly, fewer rejections, and improved product performance.
In this article, we’ll break down what sets concentricity and true position apart, explain why the distinction matters, and share practical tips to help you make the right call on your next project.
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Understanding Concentricity vs. True Position in GD&T
When you’re working with precision machining, engineering drawings, or quality control, two terms often pop up: concentricity and true position. While they may sound similar—and are both crucial to quality—they serve different purposes in Geometric Dimensioning and Tolerancing (GD&T). Understanding the distinction can help you choose the right tolerance, avoid costly mistakes, and ensure your parts fit and function as intended.
The Clear Answer: Concentricity vs. True Position
- Concentricity controls the median points of all diametrically opposed elements of a cylindrical feature to a common central axis. Simply put, it focuses on how “centered” one feature is inside another—like making sure a hole is perfectly centered in a rod.
- True position refers to the allowable location deviation of a feature (like a hole or slot) from its exact, theoretically perfect position. It’s all about placement—in 2D or 3D space—relative to datums set on the part.
In summary:
- Concentricity is about aligning axes.
- True position is about locating features.
Breaking Down Each Concept
What Is Concentricity?
Concentricity is a tolerance that ensures the average points (median points) of all diametrically opposed elements of a cylinder or circular feature are aligned to a common axis. It is a form of symmetry, but specifically applies to circular features.
Key Points:
– Checks the median points, not the surface itself.
– Used when the function of the part demands high precision (e.g., rotating parts).
– Common in applications where dynamic balance is needed.
How It Works:
Imagine two cylinders—one inside the other. Concentricity ensures the central axes of both cylinders are exactly the same. Even if the surfaces are slightly wavy, as long as their median points align, the cylinders are concentric.
What Is True Position?
True position tolerance defines how far a feature (like a hole’s center) can be from its theoretically exact location. It tells you the “zone” in which the center of the feature must be found.
Key Points:
– Refers to the location, in 2D or 3D.
– Used for most placement-related tolerances.
– Defined by a positional tolerance zone, often shown as a circle or cylinder.
How It Works:
Picture a hole on a plate that must line up with other holes elsewhere. True position creates a small area (imagine a “target zone”) where the center of your hole must fall. As long as it’s within this area, it meets the tolerance.
Detailed Comparison: Aspect by Aspect
| Aspect | Concentricity | True Position |
|---|---|---|
| What it controls | Center axes of two or more circular features | The location of a feature |
| Type of feature | Circular, cylinders | Any feature: holes, slots, bosses, tabs |
| Application | Rotating parts, shafts within bores | Hole grids, part-to-part assembly |
| Complexity | Difficult to measure and interpret | Easier, straightforward measurement |
| Measurement | Requires special equipment, often interpreted via median | CMMs, micrometers, and gages |
| Inspection time/cost | High | Relatively low |
| Modern usage | Seldom used (superseded by runout or position) | Widely used |
The Benefits & Drawbacks of Each
Benefits of Concentricity
- Ensures true balance for rotating assemblies.
- Guarantees centers are aligned, essential in applications like high-speed bearings.
Challenges of Concentricity
- Measurement difficulty: You can’t just use a dial indicator—measuring median points requires complex gaging or CMM (Coordinate Measuring Machine).
- Inspection time & cost: Much more time-consuming and expensive to verify.
- Often redundant: In many cases, runout or true position give similar results and are easier to use.
Benefits of True Position
- Flexibility: Suitable for nearly all feature locations.
- Ease of measurement: Simple equipment—coordinate measurement or even manual.
- More lenient: Allows for part form and orientation errors, as long as the center is in position.
- Cost-effective: Faster and less expensive inspection.
Challenges of True Position
- Doesn’t specifically control the center median of circular features—so if balance or symmetry is absolutely critical, it may not be suitable on its own.
When to Use Each
When to Specify Concentricity
- Only when your part’s performance depends on the median axes of features—typically in high-precision rotational assemblies where even slight imbalance leads to failure or vibration.
- For example, in gyroscopes or high-speed motor spindles.
When to Use True Position
- Locating holes, slots, or features relative to datums for assembly, fitting, or function.
- Most general manufacturing calls for true position—you’ll see it far more often.
How to Measure Each Tolerance
Measuring Concentricity
Concentricity requires capturing the average (median) points of every cross-section along a feature and checking how they align. This is typically done using:
– Coordinate Measuring Machines (CMMs)
– Custom gages designed to “find” midpoint of diametrically opposed points
Due to complexity, this process is slow and expensive.
Measuring True Position
True position is much easier and can be verified by:
– CMMs
– Optical comparators
– Simple go/no-go gages for some features
– Manual measurement by blueprints for larger parts
This ease of measurement is why true position is the preferred tolerance in modern engineering.
Common GD&T Confusions: Concentricity, Runout, and Position
- Runout is about the variation when spinning a part (surface movement due to out-of-roundness or axis misalignment).
- Concentricity checks for alignment of axes at a median level.
- True Position checks for correct location in space.
Modern practice often replaces concentricity with runout or true position because these are easier to inspect and result in fewer errors in manufacturing and verification.
Practical Tips & Best Practices
- Think Function First: Ask, “What is the function of the feature?” If it must spin precisely with no vibration, concentricity might be warranted. Otherwise, consider true position.
- Favor Simplicity: If runout or true position meet your needs, use them instead—they’re cheaper and easier to check on the shop floor.
- Discuss with Inspection Teams: Before specifying concentricity, ensure your inspection team has the tools and expertise to measure it accurately.
- Use Proper Datums: For true position, set clear, unambiguous datums on your drawings to avoid confusion and errors.
- Review Industry Standards: Many companies now recommend avoiding concentricity unless absolutely necessary.
Cost Tips: Saving on Inspection and Manufacturing
Choosing easier-to-inspect tolerances (like true position or runout) can reduce:
– Machine setup times
– Inspection labor hours
– Scrap rates (due to easier verification of bad parts)
If you’re sourcing parts or getting them shipped to you:
– Request inspection reports using true position where possible—these are easier to audit and confirm.
– Ask vendors about their inspection capabilities; avoid concentricity for cost-sensitive or high-volume parts unless required by function.
Summary
Concentricity and true position serve essential but distinct roles in GD&T. Concentricity deals with the alignment of median axes and is vital for high-precision, rotating parts but is hard to measure. True position lets you locate features accurately and is easier, faster, and cheaper to measure, making it the modern industry standard for most applications. Be strategic about which to use—choose the tolerance that best meets your part’s function and keeps inspection practical.
Frequently Asked Questions (FAQs)
What is the biggest difference between concentricity and true position?
The biggest difference is what they control. Concentricity aligns the median axes of circular features, ensuring they share the same center. True position simply controls where a feature’s center sits in space relative to datums—it’s about placement, not alignment of axes.
Why is concentricity rarely used in modern manufacturing drawings?
Concentricity is complicated and expensive to inspect, requiring specialized equipment and lots of time. Most functional requirements can be met using runout or true position tolerances, which are easier and faster to measure, reducing production costs and inspection errors.
How do runout and concentricity differ?
Runout looks at how much a part “wobbles” or deviates when rotated around an axis, combining roundness and axis alignment in one measurement. Concentricity is purely about the average center axes of features—wobble or runout can still happen on a perfectly concentric part if the surface isn’t round.
Can I measure concentricity using a dial indicator?
No. A dial indicator only tells you about surface locations (runout), not the median axes required for concentricity. Measuring concentricity demands complex gaging or a CMM to evaluate the central axis at every cross-section.
Is true position suitable for all features?
True position is extremely versatile and works for most features, including holes, tabs, slots, and bosses. However, if your project demands the highest precision in balancing rotating axes, you may still need to consider concentricity or runout for those specific features.
Empowering your design and manufacturing work with this understanding ensures parts that not only fit and function but are also economical to produce and easy to inspect. Use concentricity where it’s truly necessary, and lean on true position for reliable, efficient control of your features.