Question 13: What is the difference between tolerance limits and control limits?
An important difference between tolerance limits and control limits is that the former are used to determine whether individual manufactured components are acceptable, whereas the latter are used to control the manufacturing process.
One of the risks of using tolerance limits as a control method during ongoing production is that if you find a part that lies near tolerance limit, you may react too late. There is a high probability that you have already been producing outside the tolerance for some time.
The purpose of control limits is firstly to centre the process around the target value, which is usually the same as the middle of the tolerance range, and secondly to show where the limits of a stable process lie. That means that you normally have no cause to take action until you get readings outside the control limits.
Control limits are sometimes wrongly viewed as too tight, especially in relation to tolerance limits. Control limits follow the process; they are calculated on the basis of measurements from the process in question, and serve as guidelines for controlling it. So control limits cannot, and must not, be used as a substitute for tolerance limits.