Roundness Defined

The roundness (or sphericity) of a sphere is a measure of how spherical it is. The roundness forms two perfect spheres, one inside the other, which together form a spherical zone. That zone width is the roundness value.

So, Roundness = Zone Width →  The roundness value is the total zone width — the radial gap between the two concentric spheres.

Example 1:

Given a zone width or roundness of 2.5 microns (approximately 0.0001 inches), it can also be expressed as bilateral = ±1.25 microns (approximately 0.00005") from the best-fit sphere center radius

Example 2:

So if your best-fit sphere has radius R, then:

Outer sphere = R + 1.25 microns

Inner sphere = R − 1.25 microns

All measured points fall within that 2.5 micron-wide shell or zone.

GD&T Roundness

GD&T uses tolerance values as total zone widths, not ± values.

Sphere Roundness Diagram

Instructions

1 - Click on the Measured surface word in the black box to toggle the measured surface on and off.

2 - Click on the Outer and Inner sphere words in the black box to toggle the zone surfaces on and off.

3 - Turn the outer and inner spheres on.

4 - Turn the measure surface off.

4 - Adjust the roundness error to visualize the two spheres, creating a zone for that roundness deviation.

5 - Turn the measured surface on to see a sample measured sphere with deformations.