1. A method of calibrating measuring machines, comprising the steps of: positioning a reference device having a sphere within a measurement space by an object three-dimensional measuring machine having a spherical probe; contacting said spherical probe with six or more measurement points uniformly distributed on the spherical surface of said sphere to measure central coordinates of said sphere of said reference device by said object three-dimensional measuring machine; and calibrating said object three-dimensional measuring machine based on said central coordinates obtained.