There are three main uses for the looping functionality:
You have a multiple part fixture that holds a grid of parts. The fixture should use consistent spacing between the rows. The translation / rotation offsets allow you to index from one part to the next in the grid of parts.
You have a fixture that holds one part and you want to swap in a new part before each loop of the measurement routine. A COMMENT command is helpful to stop the CMM when the part is being replaced with a new one. The command can be at the beginning or end of the loop.
You want to use the loop functionality to rotate the measurement routine to measure a different portion of the same part. For example, you could create a measurement routine to measure a complicated hole pattern that was duplicated ten times on the part. Your measurement routine would only need to measure one of the hole patterns. You could then use the loop functionality to offset the measurement routine to measure the other nine occurrences of the pattern.
If you're using an alignment inside of a loop, PC-DMIS allows you to use the active alignment in the ALIGNMENT/START command line instead of always recalling a previously stored alignment. See the "Using an Alignment Inside Loops" topic in the "Creating and Using Alignments" chapter.
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