spherical anvils micrometers

Micrometers with one or two spherical anvils are designed for measuring thickness of tubular walls. When the anvils are closed, the micrometer must read zero. Zero setting is done just like any other outside micrometer. Similarly, gage blocks or micrometer standards are used to verify accuracy. Refer to the information outlined in the section for outside micrometers. Parallelism is not an issue with these micrometers.
If you have installed a "snap-on" spherical ball to your standard micrometer anvil, then you must take the diameter of this addition into consideration, not only when calibrating, but also when measuring. It stands to reason that an 8 mm diameter ball attached to the surface of the anvil will mean that the spindle will read 8 mm when closed.