OK - Here is the scoop - If something such as pressure is critical to a process the gage that measures it has to be calibrated. For example, in injection molding typically pressure, temperature and hold time are typically critical to the process. Because they are considered critical to the process, gages that measure these have to be calibrated. One way to know what critical parameters are (or should be) on the Control Plan.
In the above example, I would not think pressure would be a critical parameter in the bending process. But, I'm not experienced enough in bending metal to know for sure. Others may be able to add their

about pressure as a critical parameter in the bending process.