Six Sigma

Question 1

What is six-sigma?

Answer 1

set of quality management methods and tools, including statistical methods that aim to improve the manufacturing quality by identifying and removing the causes of defects and minimising variability

Question 2

What is a capability study? (3 points)

Answer 2

• A capability study is a statistical tools that measures the variation within a manufacturing process.
• samples of the product are taken, measured and the variation is compared wth a tolerance or specification limit
• this comparison is used to establish how ‘capable’ the process is in producing the product through calculation of process capability

Question 3

What are the two kinds of variability?

Answer 3

Common cause variability:

is due to the set of factors that are inherent in a machine or process by virtue of its design, construction and the nature of its operations. Cannot be changed easily without undue expense or new processes.
Examples: positional repeatability, machine rigidity,

Assignable cause:

is due to identifiable sources that can be systematically identified and eliminated.

Question 4

What does it mean when only common cause variability is present?

Answer 4

Means the process is running at best possible level under current conditions

Question 5

10 things which effect variation?

Answer 5

• Tools and functional accuracy
• operator
• set up error
• deformation due to mechanical or thermal
• measurement errors
• material impurities
• specifications
• equipment - bits getting old
• method / instructions used
• environment

Question 6

What is the difference between bilateral and unilateral tolerances?

Answer 6

Bilateral tolerancing is a method of tolerancing a dimension using equal plus and minus deviations from a normal dimension.

Unilateral tolerancing specify a deviation is in one direction, plus or minus in only one direction

Question 7

When impact does using a bilateral tolerance have on the equation for Cp

Answer 7

Cp = t/3sigma where t = bilateral tolerance

Question 8

Why does the process capability graph drift over time and what can we do to accommodate for it?

Answer 8

• commonly caused by process problems such as tooling settings or new material suppliers
• over large number produced we can get a shift of + or - 1.5sigma
• a second index Cpk can be used to accommodate the shift or scewe

Question 9

What is the difference between process and machine capability?

Answer 9

Process capability is attributed to a combination of the variability in all of the inputs

Machine capability is calculated when the rest of the inputs are fixed

Question 10

How do we measure process capability at the end of a production line?

Answer 10

Carry out capability study and calculate a capability index for the component characteristics of interest. Typical sample size of 30. Typical characteristics of interest: dimension, material properties or surface roughness

Question 11

What is the process capability index Cp?

Answer 11

means of quantifying a process in order to verify that the characteristics are within the specification - process distribution is symmetrical about the target value.

Cp= U - L / 6sigma

Question 12

What is the cause and effect diagram? List the six inputs shown in the notes (EM^5)

Answer 12

All process can be described by the cause and effect diagram where the distribution for a process shows the variability due to its partial elements

Manpower, Environment, Measurement, Method, Machine, Material

Question 13

5 occasions when a capability study should be carried out

Answer 13

1) Before the machine / process is brought in
2) When it is installed
3) Regular intervals to check performance is OK
4) If operating conditions change (i.e. new material)
5) Continous improvement project

Question 14

What does it mean if Cp is less than 1.33 or greater than 2.5?

Answer 14

1. 33 - process is not capable

2. 5 or greater - unnecessary precision may be expensive

Question 15

What does it mean if your process has a Cp between 1.33 and 2.5?

Answer 15

Process is running well

Question 16

What ppm do the following Cpk and Cp values relate to:

Cpk = 1.5, Cpk = 1.33, Cp = 2, Cp = 1

Answer 16

Cpk = 1.5 (3.4ppm)
Cpk = 1.33 (32ppm)
Cp = 2 (0.002ppm)
Cp = 1 (2700ppm)

Question 17

Assignable cause variation

Answer 17

 Tool and functional accuracy
 Operator
 Set-up errors
 Deformation - due to mechanical and thermal effects
 Measurement errors
 Material impurities
 Specifications
 Equipment
 Method or job instructions
 Environment

Question 18

Inherent variability

Answer 18

positional repeatability, machine rigidity, which cannot be removed, without
undue expense and/or process redesign.