Role
Quality: is the process within requirements?
Quality needs to know whether the process meets the requirements, understand why it deviates, and be able to show the customer and the auditor what has been done.
Questions and decisions
From question to evidence
Each question needs certain evidence to be answered. Together they lead to what the role needs afterwards.
Quality asks
What is needed to answer
Afterwards
Question 1: "Is the process within requirements?"
Needs: Trends and spread against limits
Question 2: "Why did it deviate?"
Needs: The context around a deviation
Question 3: "Can we show the requirement was met?"
Needs: Evidence per order
Afterwards
Reports and history that can be shown to the customer and that hold up in an audit.
Situations
Where the questions arise
Each situation is described once. Here you see the question and the decision for this particular role.
Situation
Values creep towards the limit
Read about the situation"Is the process still stable and capable?"
Decision: Whether the process needs action and whether parts already made are affected.
Situation
A part measures outside tolerance
Read about the situation"How many parts are affected, and is it the measurement or the process?"
Decision: Quarantine, sort, remeasure or release.
Situation
Two operators, same part, different values
Read about the situation"How much of the variation comes from the measurement?"
Decision: Whether a measurement system analysis is needed, and whether the measurement system is fit for purpose.
Situation
Changeover to the next part
Read about the situation"Has the first-piece inspection been done and documented?"
Decision: Whether production may be released.
Situation
The customer wants to see the requirement was met
Read about the situation"Can we show that the requirement was met for this particular order?"
Decision: What is sent to the customer, and whether more orders are affected.
Situation
Incoming material does not meet requirements
Read about the situation"Is it the batch or our own process?"
Decision: Quarantine the batch, file a complaint or release.
Situation
Quality work rests on a local system and a few key people
Read about the situation"What do we need to keep from what we have today?"
Decision: What must be carried over.
Behind the questions
Does the process meet the requirements, and is it stable?
Cp and Cpk summarise how well the spread fits within the tolerance and where the process lies. Whether the process is stable, or has started to change, is shown by SPC: the control limits describe what the process normally does, and a pattern of values is a signal before anything falls outside.
Why can Cp look good when Cpk is poor?
Illustration · Example dataCp only compares the spread with the width of the tolerance. Cpk also takes the position of the mean into account, and drops when the process shifts towards a limit.
Step by step
- Two processes with the same spread and the same Cp, 1.58.
- The centred process has Cpk 1.58.
- The off-centre process has Cpk 0.79, because the mean is close to the upper limit.
How does SPC tell normal variation from a signal?
Illustration · Example dataThe control limits describe what the process normally does. A pattern, such as six rising values in a row, signals that something has changed, even when all values are within the limits.
Step by step
- On the left the values vary randomly around the centre line.
- On the right six values rise in a row.
- That is a signal to investigate, although no value is outside the control limits.
Other roles, same situations
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