The probe tip offset calibration is based on the position of the tool on the stage. When a tip is calibrated and the tool is said to have moved, the tool position on the stage is determined based on the tip offset. If the tip has not yet been calibrated, then the nominal tip offset from the probe.dat data is used.
It may be important to maintain a common frame of reference for the tip offset calibrations. When multiple tips are calibrated using a common tool, the tips have the same offset frame of reference. This frame of reference can be extended to a second tool by saying the second tool moved and doing a tip offset calibration with a tip calibrated on the first tool. Feature locations measured with tips in the same frame of reference should yield the same answer (within the equipment measurement capability). If you calibrate a tip on a tool that is not in the same frame of reference and do not say the tool moved, the tip calibration frame of reference is changed to the tool. Features measured with tips calibrated in different frames of reference may yield dramatically different answers.
Consider a new system where no probes or tools have been calibrated where a sphere and a ring tool are used for tip calibration. Calibrate the contact probe using the sphere tool and say the tool moved. Then calibrate the same contact probe on the ring gage and say the tool moved. The two calibrations for the contact probe tip establish the reference between the tools and the contact probe tip. Now, calibrate the vision probe tip on the ring gage. The contact probe tip and vision probe tip will now have the same offset calibration frame of reference. The offset calibrations of the two probes with the two tools are linked because the probe that had its offset calibrated on the sphere tool was calibrated on the ring tool when the ring tool was said to have moved. Because the ring tool was said to have moved (or its position is unknown), when the contact probe tip was calibrated using the ring tool, the position of the ring tool on the stage was determined based on the contact probe tip’s measured offset. The contact probe tip's offset was used to determine the stage position of both tools and then the vision probe offset was based on the stage position of one of these tools.
The two probe tips would not be cross referenced if the contact probe tip had been calibrated on the sphere tool and then the vision probe tip had been calibrated on the ring. If the contact probe tip were calibrated on the sphere tool, the vision probe tip calibrated on the ring tool, and then the contact probe calibrated on the ring tool, the two probe tips would be in the same frame of reference, but this would be a different frame of reference than the sphere tool or any probe tips previously calibrated on the sphere tool. This is because the vision probe tip was used to determine the ring tool’s position when it was said to have moved, but the vision probe tip had not been calibrated on the sphere tool. The contact tips frame of reference was changed to match the ring tool. To maintain the linkage of tips across tools, whenever a tool is said to have moved (which also means a tool whose position is unknown), the calibration tip used on the just moved tool must be in the frame of reference of the first tool.
You can only calibrate the bottom tip of a star tip contact probe on the ring gage. A sphere tool or a sphere tool in combination with a ring gage can be used to provide cross referencing between the probe star tips and the vision probe. Normally this cross referencing would be done by calibrating all contact probe star tips on the sphere tool. Then calibrate the bottom tip on the ring tool saying that the tool had moved. Then calibrate the vision probe(s) on the ring tool. You can then calibrate contact tips on the sphere tool and vision probes on the ring tool.