Precontrol Overview
Watts Water TechnologiesChesnee, SC
Carroll SamsQuality Manager
Precontrol Presentation
• Discovery of Precontrol• Bell Shaped Curve and Precontrol• Sample Precontrol Chart• Precontrol Rules• Summary of Precontrol
Discovery of Precontrol
• Consulting company of Rath and Strong for fortune 500 company
• Frank Satterthwaite founder
• It penalizes poor quality by shutting down processes
• It rewards good quality by sampling more and more infrequently
Graphical Portrayal of Precontrol
Normal Curve with Precontrol LinesTarget Area86% [12/14]
Green Zone
½ tolerance
YellowZone
¼ tolerance
YellowZone
¼ tolerance
RedZone
RedZone
1.5Ơ1.5Ơ 3Ơ3Ơ
7%[1/14]
7%[1/14]
USL = UPLLSL = LPL P-CLine
P-CLine
Precontrol Sample Chart
1. Draw 2 Precontrol (P-C) lines in the middle half of specification width [divide specification width by four].
2. To qualify the process as ready to run (actually determining an instant process capability), 5 units in a row must be within P-C lines (green zone). If not, use diagnostic tools to reduce variation.
3. In production, sample 2 units consecutively and periodically.
4. Frequency of Sampling: divide the average time interval between adjacent stoppages by 6.
CONDITION ACTION
1. 2 UNITS IN Green Zone Continue
2. 1 unit in Green and 1 unit in Yellow Continue
3. 2 units in same Yellow Zone Adjust
4. 2 units in opposite Yellow zones [to resume production, 5 units in a row must be within the green zone] Stop*
5. 1 unit in Red [to resume production, 5 units in a row must be within the green zone] Stop*
Specification Limits [.246” to .254”]
Precontrol Rules
• To qualify a set-up
• If five consecutive pieces are in Green zone, set-up is ok to run.
• If one yellow, restart counting.
• If two consecutive yellows, adjust the process.
• If one reading is red, adjust the process.
To qualify set-up: 5 Greens in a row.
Summary of Precontrol
• Alpha Risk – risk of overcorrection [stopping a process when it should continue] is around 2% of the time.
• Beta Risk – risk of allowing a process to continue when it should be stopped is close to 1.5% of the time.
• When process width is greater than specification width (<0.8 Cpk), it will stop process 99 times out of 100.
• Cpk of 1.33 (4 Sigma) or more, Precontrol is most productive.
• Cpk of 2.0 (6 sigma), Precontrol allows hundreds of thousands of units to be produced without a single reject.
Precontrol Presentation
• Discovery of Precontrol– Lost after WWII
• Bell Shaped Curve and Precontrol– Minimum 1.33 Cpk desired
• Sample Precontrol Chart– Graphical record is not mandatory
• Precontrol Rules– Simple
• Summary of Precontrol
Thank You