The output data is coordinates of the probe sphere center when the curve or surface is measured by CMM (coordinate measuring machine)

so the radius compensation for the probe must be done. On the basis of summarizing the methods of the radius compensation for the probe at present

measurement of the free surface is classifi ed into two modes as inspection and reconstruction according to the measurement purpose. The compensation is fi nished in different ways on measurement modes. In the inspection mode

it is done by comparing the distance from the measured points to the theoretical surface with the radius of the probe to judge the measured surface is qualified or not. In the reconstruction mode

it is done by fi tting the surface of the probe radius center fi rst

followed by searching for the equidistance surface to gain measured surface. The methods for calculating the minimum distance from point to surface and equidistance surface are introduced.