Knowledge
The methods behind quality control
Quality work in production rests on a few proven methods. Here we explain them briefly and correctly, without you needing to know statistics in advance, and show where they are used in practice.
What you will find here
- Methods and analyses: longer articles that answer a question from production, with figures and sources.
- Concepts: short explanations of a single concept, such as a measure or a term.
- The questions: common questions with a short answer and a way forward.
- Concepts by area: the definitions grouped by process control, measurement, risk and production.
- The glossary: all technical terms and abbreviations, searchable.
In depth
Methods and analyses
Each article starts from a question in production and explains the method step by step.
Article
Calibration: was the gauge right when the measurement was made?
How do you know a gauge measures correctly, and what do you do when it turns out it did not?
Read the articleArticle
Can the process meet the requirements?
What does it mean for a process to be capable?
Read the articleArticle
Traceability: can you show what happened?
Can you trace a result back to part, order, person, gauge and time when the customer asks?
Read the article
Explanations
Concepts
Short explanations of individual concepts. Each explanation links on to the method where the concept is used.
Concept
Annotations, metadata and trace fields: giving a measurement its context
Why does a measured value need information about who, where and on what it was measured?
Read the explanationConcept
Coordinate measuring machines (CMM): from probed points to a result
How does a coordinate measuring machine measure a part, and what decides whether its results can be trusted?
Read the explanationConcept
Control limits: what UCL and LCL are, and how they are calculated
Where do the upper and lower control limits on a control chart come from, and why are they not the tolerance?
Read the explanationConcept
Control plan: what is checked, how often and what is done when it deviates?
How do the risks in a process FMEA become concrete checks in production?
Read the explanationConcept
Digitalisation in quality and production: record once, use everywhere
What does digitalisation actually change for measurement and quality data on the shop floor?
Read the explanationConcept
FMEA: finding failure modes before they reach the customer
How does a Failure Mode and Effect Analysis work, and what should come out of it?
Read the explanationConcept
Industry 4.0 in an ordinary factory
What does Industry 4.0 mean in practice, beyond the buzzwords?
Read the explanationConcept
X, Xbar and Xdbar: single values, subgroup means and the grand mean
What do X, X̄ and X̿ mean, and why does a control chart plot means instead of single values?
Read the explanationConcept
Measurement uncertainty: how certain is the measured value?
How much can you trust a single measured value, and when does the uncertainty matter for the decision?
Read the explanationConcept
Measurement system analysis (MSA) and Gage R&R
How do you know whether the variation you see comes from the process or from the measurement?
Read the explanationConcept
Nominal value and target value: why they are not always the same
What is the difference between the nominal value on the drawing and the value you aim for in production?
Read the explanationConcept
OEE: where the planned production time goes
What does Overall Equipment Effectiveness measure, and how do availability, performance and quality combine?
Read the explanationConcept
Process capability: Cp, Cpk, Pp and Ppk explained
Can the process meet the tolerance, and what do the four capability indices actually measure?
Read the explanationConcept
Production monitoring: turning stops and counts into improvements
What should production monitoring record, and how does it lead to better decisions?
Read the explanationConcept
Quality assurance: building confidence that requirements are met
What is quality assurance, and how does it differ from checking parts at the end of the line?
Read the explanationConcept
R, Rbar and Rmax: how the range shows the spread
What is the range of a subgroup, and how does the R chart show that the spread is changing?
Read the explanationConcept
Statistical process control (SPC): normal variation or a signal?
How do you tell the variation that is always there from a change that needs action?
Read the explanationConcept
Standard deviation (sigma): measuring the spread
What does sigma tell you about a process, and how is it estimated from measured data?
Read the explanationConcept
Statistical quality control: three questions, one toolbox
What does statistical quality control cover, and which method answers which question?
Read the explanationConcept
Tolerance limits: what LSL and USL decide
What do the lower and upper specification limits tell you, and what do they not tell you?
Read the explanation
The questions
Start with the question
Open the short answer, or go on to the explanation.
SPC
What is the difference between normal variation and a signal?
Short answer
Statistical process control: using statistical methods to tell normal variation from signals that need action, before the process produces defects.
Cpk
Can the process be within tolerance and still be too close to the edge?
Short answer
Like Cp, but also takes centring into account: the distance from the mean to the nearest tolerance limit divided by 3s.
MSA
How do you know the variation you see comes from the process and not from the measurement?
Short answer
Measurement system analysis: how much of the variation you see comes from the measurement system and not from the process.
FMEA
Which failures do you want to find before they reach the customer?
Short answer
Failure mode and effects analysis: identifying possible failures, their effects and the actions needed, in good time.
OEE
How much of the planned time turns into good parts?
Short answer
Overall equipment effectiveness: availability × performance × quality.
CMM
Where does the result go when the measuring program has finished?
Short answer
A coordinate measuring machine that measures many points on a part automatically, guided by the coordinates of the drawing.
Situations
Start from what happens
Common situations in production, and which questions different roles need answers to.
Situation
Values creep towards the limit
Every part passes, but the measured values drift in one direction as the tool wears.
Read about the situationSituation
A part measures outside tolerance
At inspection a value lands outside the limit. Is it the part, the process or the measurement?
Read about the situationSituation
Two operators, same part, different values
The same part gives different results depending on who measures or with what.
Read about the situationSituation
The machine is down
A machine stops. How often, for how long and why, and do you find out in time?
Read about the situation
Area 01
Process control and capability
Concept · article
SPC
Statistical process control: using statistical methods to tell normal variation from signals that need action, before the process produces defects.
Read the articleConcept · article
Control chart
A chart where measured values or means are plotted against control limits, so that trends and outlying points show at once.
Read the articleConcept · article
Cp
A capability index that shows how many times the process spread (6s) fits within the tolerance.
Read the articleConcept · article
Cpk
Like Cp, but also takes centring into account: the distance from the mean to the nearest tolerance limit divided by 3s.
Read the articleConcept · article
Pp and Ppk
Performance indices calculated on the total spread, even when the process is not stable.
Read the articleConcept · article
Capability
The ability of the process to hold the tolerances, expressed in indices such as Cp, Cpk, Pp and Ppk.
Read the article
Area 03
Measurement system analysis
Area 04
Quality assurance, risk and traceability
Concept · article
Quality assurance
Systematic work to develop, document and follow the processes that assure quality.
Read the articleConcept · article
FMEA
Failure mode and effects analysis: identifying possible failures, their effects and the actions needed, in good time.
Read the articleConcept · article
Annotation
Information linked to a measured value, such as operator, serial number or order, which makes traceability possible.
Read the article
Area 05
Production and digitalisation
More ways in
Glossary, situations and roles
Glossary
All terms and abbreviations
Search the technical terms. Where there is an article, the term leads there.
To the glossarySituations
What happens in production
Variation, deviations, stops and changeovers, and who needs to know what.
To the situationsRoles
Same situations, different questions
The questions and decisions for operator, quality, production, engineering, IT and management.
To the roles
Do you have a question about the methods in your production?
Talk to us about your process, your measurements and your requirements.