Measurement Terms

Roundness

What is Roundness?

Roundness is defined as follows.

• Definition of roundness: Roundness is the magnitude of deviation of a circular body from a geometrically correct circle.
• Indication of roundness: roundness is when a circular form (C) is sandwiched between two concentric geometric circles, it is expressed as the difference in radius between the two circles (f) when the distance between the two concentric circles in minimized, and is displayed as roundness mm or roundness _μm. (Figure 1)
• It is indicated in the drawing by the symbol "O" and the unit as mm. (Figure 2)

Figure 1　　　　　            　　　　　　　　　　　　　　　Figure 2

Applications Where Roundness Evaluation Is Essential

• Fitting Parts: Components requiring strict dimensional accuracy for proper assembly and function.
• Rotating Parts: Bearings and similar components where imperfections can lead to vibration and energy loss during high-speed operation.
• Sealing Parts: Hydraulic valves, packing, and other parts where airtightness is critical to performance.
  

Factors Contributing to Poor Roundness

• Machine tool vibration
• Deterioration of rotating parts of machine tools
• Improper center hole shape
• Preprocessing deformation in centerless grinding
• Workpiece distortion caused by improper holding tools or methods for ring-shaped parts.
• Chatter due to abrasion or poor installation of cutting tool
• Deformation due to application of heat after finishing  

Evaluation Methods

①Diameter Method

• Measure the diameter of a single bearing section using a micrometer, and use the maximum and minimum diameter values to calculate roundness.
  *Formula: (Maximum Diameter Value - Minimum Diameter Value) ÷ 2
• For drilling applications, a cylinder gauge is commonly used.
• In cases of circular distortion with equal diameter (see Figure 4), the calculated value tends to be smaller than the actual value.
• The actual cross-sectional shape cannot be determined using this method.

Figure 3　　　　　　　　　　　　　　　　　　　　Figure 4

②Three-Point Method

• Place the shaft on a V-block and apply a micrometer and measure the deflection as the shaft is rotated.
  The amount of deflection is used to determine roundness (refer to Figure 5).
• Sensitivity may vary depending on the cross-sectional shape of the shaft (refer to Figure 6).
• The actual cross-sectional shape cannot be determined using this method.

Figure 5　　　　　　　　　　　　　　　　                　　　Figure 6

③Radius Method

• Utilize a roundness measuring machine or similar device to obtain the unevenness of the entire circumference.
  Calculate roundness using the collected data (refer to Figure 7).
• Roundness is determined from the cross-sectional shape of the entire circumference, allowing evaluation based on JIS standards (refer to Figure 8).
• When using a three-dimensional or image measuring machine, diameter measurement can also be performed simultaneously.

Figure 7　　　　　　　　　　　　　　　　　　　　Figure 8

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