What is cylindricity tolerance in GD&T?
Cylindricity tolerance in gd&t is a type of foam control tolerance, that defines the allowable deviation in a feature from actual true cylinders. It controls the circularity and surface straightness of a controlled feature in a part. In this way, cylindricity controls a condition where all points on the surface of revolution are at that same distance from the actual central axis.
Circularity is used to control required cross sectional area only. Whereas cylindricity is the combination of circularity and surface straightness and controls the entire surface. Don’t miss this article on the basics of “Geometric Dimension and Tolerance”.
Cylindricity Tolerance Representation
Cylindricity tolerance in gd&t is represented as a circle with two line without any datum feature or MMC/LMC modifier. It’s value is always less than the linear tolerance of circle diameter. For example, for a circular feature with diameter 16±0.2 mm. Circularity tolerance value can not exceed ±0.2 mm. Click this link to know the symbols used in geometric and control dimensional tolerances.
Cylindricity in gd&t creates a 3-dimensional tolerance zone with two co-axial cylinders at a distance equal to the cylindricity control tolerance. In the example above radial distance between cylinders will be 0.1mm. All points of the surface to be controlled must lie within defined 3D tolerance zone.
How to measure GD&T Cylindricity Tolerance?
Cylindricity of a part is measured along the entire controlled surface. CMM and VMM machines, 3D scanners, height gauge or dial gauge are used to measure Cylindricity. Following steps are used to measure cylindricity of a feature using height or dial gauge.
- Part is constrained about the central axis on the turntable or any fixture.
- Probe is moved along the complete controlled surface while part is rotated about its axis.
- Any deviation in the surface at required plane measures the cylindricity of that controlled surface.
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