SPC

Question 1

What does SPC tell us?

Answer 1

whether a process is under control

allows us to monitor the quality over a period of time.

Question 2

Common causes of variation

Answer 2

• All processes vary to some extent, imperfection of humans and machines
• People perform tasks slightly differently each time.

Question 3

define process capability

Answer 3

a measure of the acceptability of the variation of the process.

Question 4

process capability equation

Answer 4

Cp = UTL - LTL /6 s

UTL = the upper tolerance limit 
LTL = the lower tolerance limit 
s = the standard deviation of the process variability

Question 5

Why is variation not good?

Answer 5

• signal that something has changed in the process,
• process performance violating the lower control limit has increased,
• The narrower the natural variation of a process, the more obvious are changes in the behaviour of that process.
  
  • The more obvious are the process changes, the easier it is to understand how and why the process is behaving in a particular way.
  • Accepting any variation in any process is, to some degree, admitting to ignorance of how that process works.

Question 6

drawing an SPC chart

Answer 6

1. . Select the data, at least 20 readings
2. Plot the data (“chartsline charts”) and work out moving range
3. Calculate and plot the mean on the chart [“average(range)”]
4. Calculate the standard deviation (SD)
5. Use the SD to compute the control limits: UCL & LCL and plot those on the chart
6. Interpret behaviour using your computed limits
7. Check the variation rules
8. Re-plot the chart if needed
9. Interpret behaviour using your computed limits

Question 7

how to calculate the UCL and LCL

Answer 7

UCL = mean + 3SD

LCL mean - 3SD

Question 8

When is a process capable

Answer 8

(1) It is under control
- when 99.7% of points lie within control limits
(2) It is accurate
(3) It is precise

Question 9

define specification

Answer 9

what the process should achieve to satisfy the customer (internal or external),

e.g. the nail should be long “3 cm ± 0.0001”

Question 10

define performance

Answer 10

what the process is actually capable to achieve, e.g. the nail is actually long 2.999 cm

Question 11

define accuracy

Answer 11

A process is accurate when the mean is included within the specification limits

Question 12

define precision

Answer 12

If the process is accurate, a process is also precise when the control limits are included within the specification limits

If the process is not accurate, a process may be precise if the standard deviation is “significantly” low

Question 13

What is capability

Answer 13

Capability = Accuracy + Precision + Control

Control: We compare observations with control limits

Accuracy: We compare mean with specification limits

Precision: We compare control limits with specification limits

Question 14

What are the Variation Rules?

Answer 14

Rule 1: A point is outside the UCL or LCL

Rule 2: There is a run of 7 points or more in one direction, OR a run of 7 or more points above/below the mean

Rule 3: There is an unusual trend or a cyclical pattern

Rule 4: There is an unusual distribution