Overview
ISO 4291:1985 - Methods for the assessment of departure from roundness: Measurement of variations in radius specifies how to determine departures from ideal roundness by measuring variations in radius using contact (stylus) instruments. The standard covers instrument types and requirements, recommendations for use, procedures for calibration and verification, and guidance on error analysis and plotting. It applies to profile transformations obtained under reference conditions and includes annexes addressing measured-profile assessment, measurement procedure, calibration, systematic rotation errors, polar-plot rules and a formula for the least‑squares centre.
Key topics and requirements
- Instrument types: Stylus instruments with either a rotating stylus around a stationary workpiece or a rotating workpiece with a stationary stylus/transducer. Outputs can be profile recordings or direct parameter displays (or both).
- Stylus selection: Recommended stylus radii include 0.25, 0.8, 2.5, 8 and 25 mm; stylus geometry examples (spherical, cylindrical, toroidal/hatchet, ovoidal) are described.
- Stylus force: Adjustable up to 0.25 N and should be set to the lowest value that maintains continuous contact.
- Frequency/response: Defines instrument response to sinusoidal undulations per revolution (upr). Band-edge transmission at rated termination should be ~75% of maximum; recommended filters are equivalent to two C–R networks of equal time constant, with allowance for phase‑corrected filters where required.
- Error statements: Overall instrument error expressed as a percentage of the upper limit of the measuring range. Distinguishes radial and axial instrument errors, with requirements to state measurement positions and rates of change along the axis.
- Reference centres for assessment: Roundness may be evaluated relative to the least squares centre, minimum zone centre, minimum circumscribed circle centre, or maximum inscribed circle centre (each has specific applications).
- Annex coverage:
- Annex A: Departure from roundness of measured profile
- Annex B: Measurement procedure and setup guidance
- Annex C–D: Calibration and determination of systematic rotation errors
- Annex E: Rules for plotting and reading polar graphs
- Annex F: Explicit equation for the least‑squares centre
Applications and users
ISO 4291 is directly relevant to:
- Metrology and calibration laboratories performing roundness inspection
- Quality control and inspection engineers in precision machining, bearing manufacture, automotive, aerospace and toolmaking
- Manufacturers of roundness measuring instruments and stylus probes
- Research and development teams concerned with surface properties and functional form
Practical uses include evaluating cylinder roundness, bearing raceways, shafts, and any rotating components where geometric form affects performance.
Related standards
- ISO 6318: Measurement of roundness - Terms, definitions and parameters (referenced by ISO 4291)
Keywords: ISO 4291, roundness measurement, departure from roundness, stylus instruments, profile transformation, calibration, errors of rotation, least squares centre, polar graphs.
