Lecture 3/4

Question 1

Examples of special causes of variation?

Answer 1

Machining problems, operator error, defective material

Question 2

How do you know when a process is in statistical control?

Answer 2

When the control charts only shows chance causes of variation

Question 3

What are the 4 steps of the Quality Loop?

Answer 3

Requirements
Measurements
Comparisons
Corrective Actions

Question 4

What are the different out of control conditions?

Answer 4

Freaks
Runs
Trends
Jumps
Stratifications
Mixtures
Cycles

Question 5

Describe the out of control situation cause: Process

Answer 5

Process carried out before machine has stabilized which may cause large process dispersion in output mean

Question 6

Describe the out of control situation cause: Material

Answer 6

Sudden change in material may cause change in output mean

Question 7

Describe the out of control situation cause: Machine/Equipment

Answer 7

Tool breakage or changeouts may change process mean

Question 8

Describe the out of control situation cause: Operator

Answer 8

New or inexpeirenced operator makes mistakes causing the changes in process mean

Question 9

Describe the out of control situation fixes: Process

Answer 9

Give machine sufficient time to stabilize as it starts up and stop production during machine shut down

Question 10

Describe the out of control situation fixes: Material

Answer 10

Take precautions against dimensional change, spot check material quality to ensure properties are to specification

Question 11

Describe the out of control situation fixes: Machine/Equipment

Answer 11

Check for tool damage or breakage
Adjust processs setting accordingly

Question 12

Describe the out of control situation fixes: Operator

Answer 12

Train operators properly and provide good guidance from others

Question 13

What does SPC work to do?

Answer 13

Prevent defective output using a feedback mechanism, however feedback must be reliable and rapid

Question 14

When can process capability been assessed?

Answer 14

Can only be assessed once:
1. Process is in statistical control
2. All special causes are corrected

Question 15

What are the causes if common cause variaton is excessive?

Answer 15

The process cannot produce output that meets the customers needs. Process must be investigated & management action must be taken to improve the system.

Question 16

What does Cpk relate to?

Answer 16

The scaled distance between the process mean (m) and closest specification limit

Question 17

What is the minimum acceptable value of Cpk?

Answer 17

1.33

Question 18

What are the values of Cpk for:
Not capable process
Just capable
Capable

Answer 18

Cpk < 1
Cpk =1
Cpk > 1

Question 19

What does it mean if Cp = Cpk ?

Answer 19

Process is centered at the midpoint of the specifications

Question 20

What is Attribute Data?

Answer 20

Comparing the process outcome to an acceptance specifications and deciding if it conforms or not.

Conforms = pass
Non-Conform = reject

Question 21

What are the 4 types of Attribute Control Charts?

Answer 21

P Chart - Fraction of Defective Items, varied sample size.
Np Chart - Number of Defective Items, constant sample size.
U Chart - Number of defects per unit , varied sample size.
C Chart - Number of defects per sample, constant sample size.

Question 22

What are the 4 steps to implementing Control Charts?

Answer 22

1. Choose proper type of control chart
2. Determine which process to control
3. Determine where chats should be used
4. Take action to improve (as result of SPC)

Question 23

How to decide where to put the control charts? (6 key steps)

Answer 23

1. Anywhere that is deemed important
2. Charts can then be removed and added as needed
3. Track the number of charts, and what types
4. Use of variable charts should increase, while the attribute charts should decrease
5. Earlier the better in the process for control
6. Ensure that the charts are on the line

Question 24

Key points of control charts for individuals

Answer 24

Repeat measurements will differ from person to person
Automated testing & inspection technology used allowing measurement of every unit
Waiting for large volume of data may be impractical

Question 25

Describe Cusum Charts

Answer 25

They maintain control over ongoing processes Better to pick up small shifts in process mean Easier to determine point at which the shift in mean occured Cumulative sum rather than individual values

Question 26

Describe Moving Average Charts

Answer 26

Moving average is plotted Useful when: Group size is limited Process spread is stable The true mean changes slowly

Question 27

Describe Exponentially-Weighted Moving Average Chart? (EWMA)

Answer 27

Moving average plotted by forming exponentially weighted of new value & previous moving average Useful for: One at a time data Greater precision to detect small changes

Question 28

What are the 2 common mistakes of Variation?

Answer 28

If only special causes are present and the process is in a state of control, then leave system alone. If control chart is not interpreted properly there is a failure to identify special causes of variation.

Question 29

What is Acceptance Sampling?

Answer 29

Process of evaluating a share of the product in a population for purpose of accpeting or rejecting the entire population of either conforming or not conforming to a quality specification. Basically just checking a few to decide if the whole batch is double fucked.

Question 30

Describe a Sampling Plan?

Answer 30

Lot Size = N Sample Size = n Acceptance Number = c e.g. N = 1000, n = 125, c =2 125 out of 1000 units inspected. If more than 2 units are out of specification the lot is rejected.

Question 31

Describe the Operating Characteristic Curve?

Answer 31

Measures the performance of an acceptance plan. Plots probability of acceptance against the lot fraction defective

Question 32

What is the Operating Characteristic Curve used to assess?

Answer 32

Average outoging quality Consumer Risk Producers Risk

Question 33

What are the 3 main aspects of sampling?

Answer 33

1. Purpose of acceptance sampling to sentence lots, not to estimate lot quality 2. Acceptance - sampling plans do not have any direct form of quality control. Just accepts or rejects lots. 3. Most effective use of acceptance sampling is as an audit tool. Ensuring the output meets specifications.

Question 34

What are the 3 approaches to Lot Sampling?

Answer 34

1. Acceptance with no inspection - when vendors process is 'gauranteed' and when inspection is not economic 2. 100% inspection - when component is extremely critical, or when vendors process capability is not adequate 3. Acceptance Sampling - when testing is destructive, cost of 100% inspection is high or is very time consuming, If the vendor has a high quality record, when there is a high rate of inspection error.

Question 35

Advantage of Acceptance Sampling

Answer 35

Less expensive and time consuming than 100% inspection Reduced change of damage due to less product handling Applicable to destructive testing Fewer personnel required Reduces inspection error Rejection of entire lots encourage vendor to improve quality

Question 36

Disadvantages of Acceptance Sampling

Answer 36

Risk of accepting 'bad lots' and reject 'good lots' Less information generated about the product Required planning and documentation of acceptance sampling procedure

Question 37

When and where to inspect the product?

Answer 37

At supplier plant Upon receipt of goods Before costly or irreversible process Upon production completion Before shipment During step-by-step production process

Question 38

What is Source Inspection?

Answer 38

Employees self-check their work and that of employees before them. Assisted by control methods such as Poke-Yoke. Treat the next step in process as the customer, ensuring delivery of good product to each step.