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Quality > Lecture 3/4 > Flashcards

Lecture 3/4 Flashcards

1
Q

Examples of special causes of variation?

A

Machining problems, operator error, defective material

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2
Q

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

A

When the control charts only shows chance causes of variation

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3
Q

What are the 4 steps of the Quality Loop?

A

Requirements
Measurements
Comparisons
Corrective Actions

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4
Q

What are the different out of control conditions?

A

Freaks
Runs
Trends
Jumps
Stratifications
Mixtures
Cycles

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5
Q

Describe the out of control situation cause: Process

A

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

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6
Q

Describe the out of control situation cause: Material

A

Sudden change in material may cause change in output mean

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7
Q

Describe the out of control situation cause: Machine/Equipment

A

Tool breakage or changeouts may change process mean

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8
Q

Describe the out of control situation cause: Operator

A

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

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9
Q

Describe the out of control situation fixes: Process

A

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

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10
Q

Describe the out of control situation fixes: Material

A

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

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11
Q

Describe the out of control situation fixes: Machine/Equipment

A

Check for tool damage or breakage
Adjust processs setting accordingly

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12
Q

Describe the out of control situation fixes: Operator

A

Train operators properly and provide good guidance from others

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13
Q

What does SPC work to do?

A

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

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14
Q

When can process capability been assessed?

A

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

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15
Q

What are the causes if common cause variaton is excessive?

A

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

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16
Q

What does Cpk relate to?

A

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

17
Q

What is the minimum acceptable value of Cpk?

A

1.33

18
Q

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

A

Cpk < 1
Cpk =1
Cpk > 1

19
Q

What does it mean if Cp = Cpk ?

A

Process is centered at the midpoint of the specifications

20
Q

What is Attribute Data?

A

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

Conforms = pass
Non-Conform = reject

21
Q

What are the 4 types of Attribute Control Charts?

A

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.

22
Q

What are the 4 steps to implementing Control Charts?

A
  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)
23
Q

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

A
  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
24
Q

Key points of control charts for individuals

A

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

25
Q

Describe Cusum Charts

A

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

26
Q

Describe Moving Average Charts

A

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

27
Q

Describe Exponentially-Weighted Moving Average Chart? (EWMA)

A

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

28
Q

What are the 2 common mistakes of Variation?

A

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.

29
Q

What is Acceptance Sampling?

A

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.

30
Q

Describe a Sampling Plan?

A

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.

31
Q

Describe the Operating Characteristic Curve?

A

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

32
Q

What is the Operating Characteristic Curve used to assess?

A

Average outoging quality
Consumer Risk
Producers Risk

33
Q

What are the 3 main aspects of sampling?

A
  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.
34
Q

What are the 3 approaches to Lot Sampling?

A
  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.
35
Q

Advantage of Acceptance Sampling

A

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

36
Q

Disadvantages of Acceptance Sampling

A

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

37
Q

When and where to inspect the product?

A

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

38
Q

What is Source Inspection?

A

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.

Quality (5 decks)

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