M2 thread disc stylus with a disc diameter of 18 mm. It is made from silver steel with a disc thickness of 2.2 mm and a length of 2.6 mm. BR stands for balls/rollers followed by yes (Y) or no (N).
These styli are sections of highly spherical balls used to probe undercuts and grooves within bores, which may be inaccessible to star styli. Although probing with the spherical edge of a simple disc is effectively the same as probing on or about the equator of a large stylus ball, only a small area of this ball surface is available for contact. Hence thinner discs require careful angular alignment to ensure correct contact of the disc surface with the feature being probed.
A simple disc requires datuming on only one diameter (usually in a ring gauge) but limits effective probing to only X and Y directions. Adding a radius end roller allows you to datum and hence probe in the Z direction, providing the centre of the 'radius end roller' extends beyond the diameter of the probe. The radius end roller can be datumed on a sphere or a slip gauge. Rotating and locking the disc about its centre axis allows the 'radius end roller' to be positioned to suit the application.
Disc may also have an M2 threaded centre to allow the fixing of a centre stylus, giving the additional flexibility of probing the bottom of deep bores (where access for the disc may be limited). These styli are available in various diameters and thicknesses and can be made from steel, ceramic or ruby.