Concentricity is the relationship that makes rotating and mating parts work: the feature that must spin true, the bore that must align with the shaft, and the register that must seat the assembly. In turning, concentricity is produced by the setup, because features machined in one chuck share the rotation axis, and it is lost by re-fixturing, because every re-chuck introduces error. This guide explains what concentricity means, how the setup controls it, how it is measured, and the design and inspection rules that keep it real.
What Concentricity Actually Means
Concentricity describes the relationship between a feature and a datum axis. A bore is concentric with a shaft when its center coincides with the shaft's axis, and a diameter is concentric with the turning axis when every point on the surface is the same distance from the axis. The practical measurements are runout, the total indicator movement when the part is rotated, and position, the deviation of the feature's center from the datum.
The terms are often confused. Runout is a direct measurement: rotate the part and read the indicator, and the total movement is the total runout. Concentricity is a derived condition, the coaxiality of the feature's centers, and it is measured indirectly from the runout or by coordinate measurement. The callout should name what is being controlled, because a runout callout and a concentricity callout are not the same requirement.
| Term | What it controls | How it is measured |
|---|---|---|
| Runout | Surface variation relative to the axis | Rotate against an indicator |
| Coaxiality | Center-to-center alignment | CMM or derived measurement |
| Total runout | Surface plus axial variation | Indicator over the surface |
How the Setup Creates Concentricity
The turning setup is the source of concentricity. A part machined completely in one chucking has all its features on the same rotation axis, because the chuck defines the axis and every cut references it. The bore, the diameters, and the thread share the axis, and the concentricity between them is a property of the setup, not a tolerance the machinist fights.
The advantage disappears at the first re-chuck. When a part is removed and gripped again, the new chuck centers the part within its own accuracy, and the error, the distance between the two chuck centers, becomes runout in the second operation. The re-chuck error is why parts with critical concentricity are designed to be finished in one setup, and why the datum strategy is part of the drawing.
The workholding choice sets the base accuracy. A collet grips the bar on its true diameter and centers repeatably; a three-jaw chuck centers within its jaw accuracy and can re-center differently each time; and a mandrel or a fixture holds a feature that is already machined, referencing it as the datum. The drawing should identify which feature is the datum, because the setup references it.
Long Parts and the Deflection Problem
Long slender parts add a second problem: deflection. A shaft that is long relative to its diameter bends under cutting force and under its own weight between supports, so the cut diameter is not concentric along the length, and the part runs out when rotated between centers. The fix is support: a steady rest, a tailstock, or a sub-spindle holds the part near the cut and keeps the axis straight.
The support strategy belongs in the DFM conversation. A 200 mm shaft with a 0.02 mm runout callout needs a different setup than a 50 mm stub, and the drawing should flag the long unsupported section so the machinist plans the support. The first article is where the runout is verified, and the inspection should use the same support as the machining, because a part measured between centers differs from one measured in the chuck.
Measuring Concentricity and Runout
The measurement method is part of the specification. The common methods are:
- Dial indicator runout: rotate the part and read the total indicator movement; the standard production check
- Between centers: mount the part on its center holes and rotate it, removing the chuck's influence
- CMM measurement: measure the features and compute the coaxiality from the measured centers
- Air and bore gauges: check the diameter and the taper in the same axis
The method changes the number. A part measured in the chuck shows the chuck's contribution; the same part between centers shows the part's own geometry. The datum and the measurement setup belong in the callout, because the acceptance depends on both.
The inspection temperature matters for the same reason as every precision callout. A shaft measured at 20 °C and at 40 °C differs in diameter and runout, and the material's expansion belongs in the tolerance conversation. The measurement condition is part of the acceptance, because the number changes with the temperature and the setup.
The achievable concentricity is bounded by the machine and the workholding. A precision chuck and a stiff machine hold tighter runout than a worn chuck on a light machine, and the tolerance callout should be matched to the equipment. The DFM review should confirm the machine class before quoting, because a runout callout that the equipment cannot hold is a risk, not a spec.
Design Rules for Concentric Parts
- Identify the datum feature, the axis of rotation, on the drawing
- Mark the features that must be concentric to the datum with a runout or position callout
- Design for one setup: finish the critical features without re-chucking where possible
- Flag long unsupported sections so the support strategy is planned
- State the measurement method and the datum setup for acceptance
- Confirm the tolerance against the chuck accuracy and the part length
- Keep the number of critical callouts small, because every concentric feature adds inspection
Improving Concentricity in an Existing Design
When a part fails runout, the fix is usually the setup, not the tolerance. Re-check the workholding: a collet or a soft jaw often centers better than a worn three-jaw chuck. Review the operation order: cut the datum and the critical features in the same setup, and use a mandrel or between centers for the second operation. Check the support: a steady rest removes the deflection that causes taper and runout on long parts.
The first article is the diagnostic. The measured runout tells which operation introduced the error: a constant runout suggests the chuck, a runout that grows along the length suggests deflection, and a runout that appears after a specific operation points to the re-chuck. The diagnosis directs the fix, and the fix is usually cheaper than the tolerance change.
The same diagnosis applies across the production run: the runout trend from the in-process checks catches the chuck wear and the tool condition before the parts leave tolerance.
Conclusion
Concentricity in turning is produced by the setup and measured by the method: one chucking holds the axis, re-chucking adds error, and long parts add deflection, so the drawing, the workholding, and the inspection must agree on the datum. Design for one setup, support the long sections, and state the measurement method on the drawing. A CNC machining partner that plans the chucking and the support delivers runout that holds, and the tolerance reference keeps the callouts realistic.
FAQs
What is the difference between runout and concentricity?
Runout is the direct measurement of surface variation when the part rotates against an indicator. Concentricity, or coaxiality, is the derived relationship between feature centers. A runout callout and a concentricity callout control different things, so the callout should say which is required.
How does re-chucking affect concentricity?
Every re-chuck re-centers the part within the new chuck's accuracy, and the difference between the two centers becomes runout. Features finished in one setup share the axis; features finished after a re-chuck carry the re-chuck error, so critical concentricity is designed into one setup.
How is concentricity measured on a turned part?
By rotating the part against a dial indicator for runout, between centers for the part's own geometry, or on a CMM for derived coaxiality. The method changes the number, so the datum and the measurement setup should be named in the callout.
Why does my long shaft have runout?
Long slender parts deflect under cutting force and their own weight, so the diameter and the axis wander along the length. A steady rest, tailstock, or sub-spindle supports the part near the cut, and the support strategy should be planned in the DFM.
Sources
- 6CProto CNC Machining Services
- 6CProto Standards and Tolerances
- 6CProto Industrial Equipment Manufacturing
- ISO 2768-1:1989 – General tolerances for linear and angular dimensions
- ISO 9001:2015 – Quality management systems