Runout and Concentricity Tolerances: A Practical Technical Guide for CNC Machining

What is Actually Being Measured or Controlled

To start, let’s define core terms in plain language:

• Concentricity: A geometric tolerance that measures how well two or more circular features (e.g., a shaft’s outer diameter and its central bore) share a common central axis. It is a form and position tolerance, meaning it controls both the shape of individual features and their relative alignment.
• Runout: A simpler, more commonly used tolerance that measures the total variation in a feature’s surface as it rotates around a reference axis. Runout has two primary variants:
  • Circular runout: Measures the variation of a single, circular cross-section of a feature as it rotates. It does not compare cross-sections along the feature’s length.
  • Total runout: Measures the variation of the entire length of a feature’s surface as it rotates. It combines the shape of the feature (e.g., straightness of a shaft) and its alignment relative to the reference axis.

A key distinction: Concentricity is a relative tolerance between two features, while runout is a relative tolerance between a feature and a reference axis (which can be defined by a separate feature, such as a shaft’s journal). For example, if you specify a shaft’s outer diameter (OD) has 0.02 mm total runout relative to its central bore, you are controlling both how round the OD is and how aligned it is to the bore’s axis.

Why It Matters

Runout and concentricity directly impact part function, assembly compatibility, and long-term performance:

1. Rotating component performance: For parts that spin (e.g., motor shafts, gear shafts, turbine blades), excessive runout causes dynamic imbalance. This leads to vibration, noise, premature bearing wear, and reduced efficiency. For example, a high-speed spindle shaft with 0.05 mm runout will generate significant centrifugal force at 30,000 rpm, increasing fatigue failure risk.
2. Assembly fit and load distribution: In mating parts, poor concentricity leads to uneven load distribution. For example, a gear with misaligned teeth will experience concentrated wear on one side, reducing service life. A bearing inner race with excessive runout will not seat evenly, leading to early bearing failure.
3. Sealing performance: For fluid or gas seals, even minor runout can create gaps that cause leaks. A shaft with 0.03 mm runout, for example, can allow a lip seal to lose contact with the shaft surface during rotation, leading to fluid loss.
4. Cost implications: Overly tight tolerances increase machining time, inspection costs, and scrap rates. For example, specifying 0.002 mm total runout for a 1-meter-long shaft requires high-precision equipment and multiple finishing operations, which can double part cost.

How It Is Measured or Achieved

Measurement Methods

Runout and concentricity are measured using rotating inspection setups, as their core definition relies on rotation around a reference axis. Below are practical shop-floor methods:

1. Bench Center Inspection
   • Setup: Mount the part between two precision bench centers (ground to a common axis) to define the reference axis. A dial indicator (0.001 mm resolution for high-precision work) is positioned to contact the feature being measured.
   • Operation: Rotate the part 360 degrees by hand or with a low-speed motor. For circular runout, record the maximum indicator variation at a single cross-section. For total runout, move the indicator along the feature’s length while rotating the part, recording the total variation across all cross-sections.
   • Accuracy: ±0.002 mm for well-maintained bench centers and indicators.
2. CNC Machine On-Machine Inspection
   • Setup: Use the CNC machine’s spindle as the reference axis. Mount the part in a fixture aligned to the spindle’s axis, or use a collet/chuck to hold the part directly in the spindle. A dial indicator or touch probe is mounted in the machine’s tool changer.
   • Operation: For circular runout, program the machine to rotate the spindle 360 degrees while the probe contacts the feature at a fixed cross-section. For total runout, program the probe to move along the feature’s length while the spindle rotates.
   • Accuracy: Dependent on the machine’s spindle runout and positioning accuracy. For example, Gree CNC’s GA-DV750 vertical machining center has a spindle runout of <0.002 mm and positioning accuracy of 0.003 mm, enabling on-machine inspection accuracy of ±0.005 mm for runout.
3. Coordinate Measuring Machine (CMM) Inspection
   • Setup: Mount the part on a CMM table, and define the reference axis by measuring three or more points on a reference feature (e.g., a shaft’s journal).
   • Operation: The CMM’s touch probe measures multiple points on the feature being evaluated. Software calculates circular runout by comparing points at a single cross-section, and total runout by comparing points across the entire feature length.
   • Accuracy: ±0.001 mm for high-precision CMMs, suitable for parts requiring sub-micron tolerances.

Achieving Tolerances in Machining

To meet runout and concentricity requirements, follow these machining best practices:

1. Reference Axis Alignment: Always machine the feature relative to the same axis used for inspection. For example, if a shaft’s runout is measured relative to its central bore, machine the OD while the part is held in a fixture aligned to the bore’s axis.
2. Machine Selection: Match the machine’s capabilities to the tolerance requirement. For example:
   • Tolerances of 0.01 mm or looser: Suitable for machines like the GA-MV856 (positioning accuracy 0.010 mm) or GA-VT540 (positioning accuracy 0.006 mm).
   • Tolerances of 0.005 mm or tighter: Require high-precision machines like the GA-DV750 (positioning accuracy 0.003 mm) or GA-UHD500 (positioning accuracy 0.006 mm).
3. Tool and Spindle Condition: Use sharp, balanced tools and ensure the spindle is clean and properly lubricated. A worn spindle or unbalanced tool will introduce runout during machining.
4. Workholding: Use rigid fixtures or collets to minimize part deflection. For example, a collet holding a shaft should have a runout of <0.002 mm to avoid introducing error into the machined feature.

Sources of Error

Runout and concentricity errors originate from three main categories:

1. Part Design Errors: Poor design can make it impossible to achieve tight tolerances. For example, a long, thin shaft with a length-to-diameter ratio of 10:1 will deflect under machining forces, leading to runout even with a high-precision machine.
2. Machining Errors: These include:
   • Machine inaccuracies: Spindle runout, axis positioning error, and thermal drift. For example, the GA-MVD2560 dual-spindle machining center has a positioning accuracy of 0.015 mm for the X-axis, which can contribute to runout errors if not compensated for.
   • Tool errors: Tool wear, unbalanced tools, and tool deflection. A worn end mill will produce a shaft with uneven diameter, leading to circular runout.
   • Workholding errors: Fixture misalignment, collet runout, and part deflection. A fixture that is not aligned to the machine’s spindle axis will produce parts with poor concentricity.
3. Inspection Errors: These include:
   • Indicator resolution: A dial indicator with 0.01 mm resolution cannot accurately measure 0.005 mm runout.
   • Reference axis error: If the bench center or spindle used for inspection has runout, this error will be added to the measured runout of the part.
   • Operator error: Incorrect indicator positioning or inconsistent rotation speed during inspection can lead to inaccurate readings.

Honest Limitations

Runout and concentricity tolerances have important limitations that are often overlooked:

1. Circular runout does not control straightness: A shaft with 0.01 mm circular runout at every cross-section can still be bent (poor straightness), leading to high total runout. For example, a 1-meter-long shaft with 0.01 mm circular runout at each cross-section but a 0.05 mm bend will have 0.05 mm total runout.
2. Total runout does not distinguish between shape and alignment errors: Total runout measures the combined effect of a feature’s shape (e.g., roundness, straightness) and its alignment to the reference axis. It cannot tell you if a 0.02 mm total runout error is due to a bent shaft, an out-of-round shaft, or misalignment to the reference axis.
3. Concentricity is difficult to measure accurately: Concentricity requires measuring the center of each feature, which is prone to error. For example, a 0.01 mm concentricity tolerance is often replaced with a 0.02 mm total runout tolerance in practice, as runout is easier and more cost-effective to measure.
4. Rotating inspection does not capture non-rotating use cases: Runout and concentricity are measured under rotation, but some parts are used in non-rotating applications. For example, a piston that reciprocates (but does not rotate) may have low runout but still experience uneven wear due to poor straightness, which is not captured by runout measurements.
5. Over-confidence in machine specs: Machine positioning accuracy is not the same as runout capability. For example, the GA-MV856 has a positioning accuracy of 0.010 mm, but its spindle runout is <0.005 mm, so it can achieve tighter runout tolerances than its positioning accuracy suggests.

Summary Table

Practical Checklist for Applying This Correctly

1. Define the reference axis clearly: Specify which feature (e.g., bore, journal) is the reference for runout or concentricity measurements.
2. Choose the right tolerance type:
   • Use circular runout for parts where only cross-sectional roundness and alignment matter (e.g., a bearing race’s inner diameter).
   • Use total runout for parts where the entire length of the feature matters (e.g., a shaft’s outer diameter).
   • Use concentricity only if you need to control the relative position of two features’ centers (e.g., a gear’s pitch circle and its bore).
3. Match tolerance to machine capabilities: Ensure the machine’s spindle runout and positioning accuracy are at least 50% tighter than the required tolerance. For example, to achieve 0.01 mm total runout, use a machine with spindle runout <0.005 mm.
4. Specify inspection method: State whether runout will be measured on a bench center, on the machine, or on a CMM to avoid ambiguity.
5. Account for thermal effects: For long parts or high-precision work, specify that inspection should be performed at a consistent temperature (e.g., 20°C) to avoid thermal expansion errors.

FAQ

1. What is the difference between runout and concentricity? Runout measures the variation of a feature’s surface as it rotates around a reference axis, while concentricity measures how well two features share a common central axis. Runout is easier to measure and more commonly used in practice.

2. Can I measure runout without rotating the part? No. Runout is defined as the variation of a feature under rotation, so rotating inspection is required. Non-rotating methods (e.g., measuring diameter variation at multiple points) can approximate roundness but cannot measure runout.

3. What is the tightest runout tolerance I can achieve with a Gree CNC machine? The GA-DV750 and GA-UHD500 have spindle runout of <0.002 mm, enabling total runout tolerances of 0.005 mm for small parts. For larger parts, tolerances of 0.01 mm are achievable with proper machining practices.

4. Why is total runout often specified instead of concentricity? Total runout is easier and more cost-effective to measure, and it controls both the shape and alignment of a feature, which is sufficient for most applications. Concentricity is more difficult to measure accurately and is reserved for cases where precise center alignment is critical.

5. How does part size affect runout tolerance? Larger parts are more prone to deflection and thermal expansion, so runout tolerances are typically looser for larger parts. For example, a 1-meter-long shaft may have a total runout tolerance of 0.05 mm, while a 100 mm shaft may have a tolerance of 0.01 mm.

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