Download "Intro to Coordinate Metrology"

CMMs generally gather their data by touching the workpiece with a probe (either a solid probe or an electronic touch trigger probe) attached the machine's measuring axis.  Although the tip of the probe is very accurate, once the probe is attached to the CMM, the location of the tip to the machine's coordinate system must be determined prior to measuring.  Since it's the tip's circumference that touches the part, the probe's center and radius are determined by measuring a very accurate sphere (requalification sphere).

Once the center and radius of the tip are known, when the probe contacts a workpiece, the coordinates of the tip are mathematically "offset" by the tip's radius to the tip's actual point of contact (Figure 14).  The direction of the offset is automatically determined by the alignment procedure.

We do a similar procedure when we park a car.  The better we can estimate our offset from the exterior of the car, the closer we can park it to the curb.