S11 - Quality management Flashcards
What are the different costs of quality?
Prevention costs: training, continuous improvement programs
Appraisal costs: inspection, measures
Internal failure costs: before delivery to the customer, rework, waste of parts and time
External failure costs: after delivery to the customer, returns, recalls warranty
What is acceptance sampling? What are the risks associated with this method?
A procedure to determine wether a batch is to be accepted or rejected.
N = lot size
n = sample size (random)
c = acceptance number
d= number of defective items in sample
Decision:
- If d <= c, accept lot
- If d>c, reject lot
Risks:
- The sample is better than the lot -> Consumer’s risk (we accept a lot with a quality that we should not tolerate)
- The sample is worse than the lot -> Producer’s risk (we reject a lot with an acceptable level of quality)
How do you conclude wether the process is in control or not?
- Interpret the charts
- Check the criteria:
- Extreme points
- Symmetry
- Distribution centred around the axis
- Random distribution (no cycle, no trends)
The process is in control if BOTH charts are ok,
Out of control doesn’t necessarily mean that we need to change the process
What do we do when a process is not in control? What about the control limits?
The operator continues to take samples at the intervals prescribed.
At each sampling, s/he calculates the average and the range of the sample and adds the points at the end of the chart.
With each new point, s/he evaluates if the process is still in control.
In case of doubt, of a point out of limit or beginning of trend, one can accelerate the sampling.
The limits remain the same since they indicate the natural behaviour of the process.
They are recomputed if the process is modified.
What are the differences between the process capability and the process variability? Between the tolerances and the control limits?
Process capability: Capacity of a process to produce goods and services inside specified tolerances/specifications.
Process variability: Natural or inherent variability in a process
Tolerances: Specifications imposed on the process by the customer, internally or legally.
Control limits: Statistical limits used with the control chart
How do you interpret Cp?
If Cp<1 : The process is not capable to meet the specifications. Equivalent to a +/-3σ confidence interval and more.
1 < Cp < 1.33 : Minimal capability to meet the specifications. Equivalent to a +/-3σ confidence interval and more.
If 1.34 < Cp < 2 : Process is capable of meeting specifications. Equivalent to a +/-4σ confidence interval and more.
If Cp > 2 : Process is capable of meeting specifications and can exceed them. Equivalent to a +/-6σ confidence interval
What is ISO 9000?
A quality certification: doesn’t guarantee good quality but guarantee that there is a process in place to control quality.
