Concentricity in GD&T
By Lucas Lo | Updated: May 14, 2025
Table of Contents
As we all know, concentricity is a critical concept in GD&T, used to control the alignment of axes for two or more circular features (e.g., shafts, gears). It ensures precise coaxiality of rotating components, which is vital for assembly and functional performance.
1. What is the symbol for concentricity?
Symbol: ◎ (a circle with a central dot).
Definition: In GD&T, the concentricity symbol ◎ specifies that the theoretical center points of cylindrical or rotational features (e.g., cylinders, cones) must lie within a cylindrical tolerance zone centered on a datum axis. It controls the alignment of center points, not axes (axial alignment is governed by coaxiality).
2. What is concentricity in GD&T?
Definition: Concentricity is a position tolerance that measures the degree to which the center points of circular features (cylinders, cones, spheres) coincide relative to a shared datum axis or point.
Purpose: It ensures alignment within a specified tolerance zone, reducing vibration, wear, and misalignment in rotating parts, thereby improving assembly quality and extending equipment lifespan.
3. What is the role of the concentricity symbol?
Function: The symbol ◎ controls the alignment of a cylindrical feature’s theoretical center points with a datum axis, ensuring precision in assembly and preventing eccentric rotation.
Standard Updates (ASME Y14.5-2018):
-Deprecation: Concentricity (◎) was removed due to its complexity (requiring center-point measurement) and the industry’s preference for controlling actual axes.
-Replacement: Use position tolerance (⌖) for coaxial control. For example, ⌖⌀0.1 A means the feature’s axis must lie within a cylindrical tolerance zone of diameter 0.1 mm, centered on datum axis A.
Key Notes
-Legacy Drawings: Concentricity (◎) may still appear in older designs; follow original standards for inspection (e.g., CMM-based center-point measurement).
-ISO Standards: Concentricity (◎) remains valid under ISO 1101, unlike ASME.
-Modern Practice: New designs should use position tolerance (⌖) or coaxiality controls for simplicity and alignment with current standards.
4. How to show concentricity on drawings?
The labeling of concentricity on drawings is shown below:
Symbol + Tolerance: Write the concentricity symbol “⨀” and the tolerance value in the tolerance box, e.g. ⨀ 0.05, which means that the axial deviation must not exceed 0.05 mm.
Datum Description: A datum must be specified (e.g., “⨀ 0.05 A”) to tell the machinist to “align the axis with A as the reference standard”.
5. How to measure GD&T concentricity?
Some practical tools for measuring are listed below:
Professional Concentricity Gauge: Direct inspection like calipers for quick measurement of simple parts.
Coordinate Measuring Machine (CMM): By scanning the part, the center of each circular feature is accurately calculated and then compared to the axis deviation, with an error range accurate to the micron level.
Measuring steps:
-Place the part firmly on the measuring platform.
-Scan the feature with the instrument to obtain the center data.
-Compare the center position to see if the deviation is within tolerance.
6. Importance of Concentricity
In the mechanical world, concentricity is the “invisible guardian”:
-Precise fit: gears bite, bearings rotate, couplings connect, etc. For these rotating parts to run smoothly, it all depends on the concentricity gatekeeper. Once the axis offset, gears did not do card alignment, squeaking.
-Reduce wear and tear: different centers will lead to uneven force, equipment operation vibration, noise soaring, doubling the speed of wear and tear of parts, life greatly shortened.
-Performance guarantee: high-precision concentricity can make the equipment run more stable, reduce the failure rate, extend the service life.
7. Common Concentricity Problems and Solutions
In actual machining, the following problems may be encountered:
-Axis offset: the machine tool is not precise enough or the fixture is not clamped.
Solution: Calibrate the machine tool regularly and fix the parts with high-precision fixture.
-Workpiece deformation: Cutting force is too high or clamping method is not right.
Solution: Optimize the machining process, try flexible fixtures and fine machining in steps.
-Tool wear: tool wear, resulting in cutting accuracy decline.
Regular inspection and replacement, choose carbide tools to improve accuracy.
-Material stress release: internal changes in the material affect the shape.
Release stress through heat treatment or aging treatment to stabilize the shape of the part.
8. Tips for Controlling Concentricity Throughout the Process
If you want to make good concentricity, you have to control the whole process from design to production:
Design stage: choose datums reasonably, avoid excessive constraints on parts, and assign tolerances scientifically.
Machining preparation: check the status of the machine tool to ensure accurate positioning of the fixture.
Machining process: Plan the machining sequence to reduce the deformation of parts and stabilize the axis.
Inspection and debugging: Use CMM and other equipment to monitor in real time and make timely adjustments when problems are found.
9. Future Challenges and Technology Prospects
With the development of science and technology, there are new goals for concentricity control:
Micron-level precision: in the field of aerospace and precision instruments, how to control the concentricity error to a few tenths of a hair is one of the future research directions.
Intelligent optimization: with the help of digital twin technology to simulate the machining process, predict the problem in advance; adjust the parameters in real time with intelligent manufacturing, so that the production is fast and accurate.
10. Conclusion
Concentricity is always the key to ensuring product quality, whether in traditional mechanical manufacturing or in cutting-edge technology. Armed with this knowledge, you can easily cope with various challenges in design, machining, and inspection.
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Lucas is a technical writer at ECOREPRAP. He has eight years of CNC programming and operating experience, including five-axis programming. He also spent three years in CNC engineering, quoting, design, and project management. Lucas holds an associate degree in mold design and has self-taught knowledge in materials science. He’s a lifelong learner who loves sharing his expertise.
