Six Sigma Tools (Control) Flashcards
(33 cards)
Balanced Score Card
- A set of measures that gives top managers a fast but comprehensive view of the business
- The balanced scorecard allows managers to look at the biz from 4 important perspectives:
- Customer perspective: How do customers see us?
- Internal perspective: What must we excel at?
- Innovation & Learning perspective: Can we continue to improve and create value
- Financial perspective: How do we look to shareholders?
- The balanced scorecard also forces managers to focus on the handful of measures that are most critical
Dashboard
- A tool used for collecting and reporting information about vital customer requirements and/or your business’ performance for key customers.
- Provide a quick summary of the process and product performance.
Andon
- A Japanese word for lantern
- The board hangs over the aisle between production lines and alerts supervisors to any problem
- A typical tool to apply the Jidoka principle in Lean production
Deming on the variation in a process
- Variation is inevitable in industrial life
- Productivity and quality are linked, not traded off against one another
- Reduce 6Ms
- Common cause variation
- Special cause variation
Common cause variation
- Systemic issues, shared by numerous operators, machines, or products
Special cause variation
- Produce non-random variation in the system, usually confined to individual employees or activities
- Causes can be identified and eliminated.
Quality
- A process needs to be capable and in control to consistently produce to customer requirement
Capability
- The ability of a product, process, person or organisation to perform its specified purpose based on tested, qualified or historical performance, to achieve measurable results that satisfy established requirements or specification.
- Cp and Cpk are computing a single value to determine whether a process is capable.
Cp
- Compares the “natural tolerance” of the process (its natural variation) to the specifications: Cp = (engineering tolerance)/(natural tolerance) = (customer specification range)/ 6 sigma
- A Cp of 1 denotes a capable process at 3 sigma - but to allow for drift, a 1.33 is often used as the acceptable minimum (i.e. 4 sigma)
- The disadvantage of Cp is that it doesn’t account for process centering.
Cpk
- Compares the “natural tolerance” of the process (its natural variation) to the specifications for process centering: - Cpk = Z min - Zu = (upper specification - mean)/3 sigma - Zl = (mean -lower specification)/ 3 sigma
- We look at the difference between the mean and the upper and lower specifications. In a centered process, we’d expect these to be equal.
- At 3 sigma a Cpk > 1 is required (but +1.33 is preferred).
Capable Processes in Control
- When a process is capable, it can produce output that meets customer specifications.
- However, a process is only in control when it behaves as expected, that is it exhibits only random variation.
- When a process is capable and in control, the process is producing output that meets customer specifications, consistently.
Two criteria for capable and in control
- Is the process capable, that is does the output meet customer specifications? (calculate Cpk)
- Is the process in control, that is do the outputs meet customer specifications consistently? (variable: x bar and R charts; attribute: p-chart). (only when a process is in control can you know its true capability)
How do you identify whether a process is in or out of control?
Look at the control chart: if in control, the pattern should be random.
Attribute data (e.g. good or defective)
- Data that count items, such as the number of defective items in a sample
- Need just one chart, because mean determines standard deviation - p chart. - e.g. “good” or “defective”
Variable data (measurement)
- Data that measure a particular product characteristics such as length or width.
- Need two charts because mean and standard deviation are independent: x-bar and R chart
Control chart interpretation: points outside of limits
If there are sample means above UCL or below LCL, investigate for assignable causes.
Control chart interpretation: two-in-a row between 2&3 sigma
Two consecutive sample means between +2 and +3 sigma (or between -2 and -3 sigma), investigate for assignable causes
Control chart interpretation: two-out-of-three consecutive between 2&3 sigma
If two out of three consecutive sample means between +2 and +3 sigma, investigate for assignable causes
Control chart interpretation: 4-out-of-5 consecutive between 1&3 sigma
If 4 out of 5 consecutive sample means fall between +1 and +3 sigma (or -1 and -3), investigate for assignable cause.
Control chart interpretation: 5 in a row on one side of center line
If there are five sample means in a row above or below the center line, investigate for assignable cause.
Control chart interpretation: Trends
If there are 6 in a row steadily increasing or decreasing, investigate for assignable cause.
Control chart interpretation: 8 in a row between 2 and 3 sigma
Meaning none of the 8 within 1 sigma of the center line
Control chart interpretation: 14 in a row alternating
It is likely that one part of the tool is worn out unevenly, or 2 machines are used for the same part.
Control chart interpretation: 15 in a row between +/- 1 sigma
It is likely that the control limits need to be reset.
