Precontrol Overview

Watts Water TechnologiesChesnee, SC

Carroll SamsQuality Manager

Carroll.sams@wattswater.com

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