Knowledge zone

SPC – the control limit is not tolerance!

One of the most common errors in SPC is confusing control limits (UCL/LCL) with specification limits (USL/LSL). Control limits (process stability limits) are determined on the basis of special formulas, depending on the type of control card

What are control limits (UCL / LCL)?

The control limits (in their standard form) do not have any relation to the customer’s requirements presented, for example, in the technical drawing. Control limits (process stability limits) are determined on the basis of special formulas, depending on the type of the control card (“pure mathematics” based on a given distribution of a random variable, eg normal, log-normal, binomial distribution, etc.).

Can the control limits be wider than the tolerance limits?

Yes, although it is a very disturbing situation! It is not uncommon to come across such situations where, due to the behavior of the process (eg “bad” state of the machine), the control limits are much wider than the specification limits. For example, the “drawing” requirements (of the customer) regarding the shaft diameter are 10.3-10.7 mm, and the control limits are between 10.1-10.9 mm. This happens when the process has a large, constant variation (such is its “nature”), and the customer expects high precision. In such a situation, the process will be stable – it will not exceed the control limits, although it will be very incapable – it will cause many non-conforming products.

Summing up:

• tolerance limits (specifications) show what the customer needs,
• control limits show what the process “can do”.

Tomasz Greber [PROQUAL]