1. What Is Concentricity?
Concentricity is complex— a sort of “circular symmetry”used to establish a tolerance zone for the median points of a cylindrical or spherical part.
You can define concentricity as a measure of the constancy of the wall thickness of a tube, pipe, or other cylinder.
If concentricity were “perfect,” then the wall thickness between the OD and the ID would be the same in every cross section, at every point around the diameter of the tube.
2. OD/ID Concentricity Callouts
Tubing OD/OD concentricity requirements can be indicated on a drawing in several different ways.
- GD&T concentricity symbol
- Eccentricity percentage
- TIR (Total Indicator Reading)
- Written statement
3. Challenges of Measuring Concentricity
Measuring requires:
- Taking many measurements across a series of cross sections
- Mapping out the surface and determining median points of all cross sections
- Plotting the points to see if they fall within the cylindrical tolerance zone
Another challenge is that with today’s micromachining, the parts to be measured are often smaller than ever before.
4. When Concentricity Is Needed
Concentricity is usually only used for parts that require a very high degree of precision in order to function properly.
But even where concentricity is not critical, it may be important to know how far out of concentric the OD/ID can be.
5. An Alternative: Concentricity vs. Runout
In some cases, you can avoid the time and cost of verifying concentricity by replacing concentricity requirements with runout, which is easier to measure and more readily achievable.
Runout is a combination of concentricity and circularity — and if a part is perfectly round, the runout will equal the concentricity.
Design Tubing with Achievable Tolerances in Mind
Remember, the feasibility of producing parts that are within your acceptable tolerances is a critically important consideration when doing your drawings.
That is why most machinists, measurement techs, and design engineers recommend avoiding OD/ID concentricity whenever possible
