Pre-control charting

Pre-control charts have long been misused as a way to introduce “control” of the process without any analysis of the process.  It is believed that establishment of “pre” control limits at points two-thirds the distance either side of the center point of specification limits will somehow keep the process in control.
While this CAN be the case, it is not a replacement for proper statistical analysis using actual data from the process not under the guidelines of pre-control.  Also, my experience has been that use of pre-control charts has allowed operators to “pick and choose” the data points they put on the chart.
PROPER use of “pre-control” limits is when the control limits are calculated from actual data, and set in order to show in real-time when the process is running with only common cause variation and will show special cause variation in real time as well.
There is no “international or regulatory” standard for the use of pre-control limits.
My advice – stick with the “real thing” of  established control limits based on actual statistical evidence and analysis.  Pre-control is one of those systems that belongs in the ASQ museum.

Might do some good to read and “reflect” on the concepts given by T.Pyzdek  and F. Breyfogle in their well known books “Six Sigma Handbook” (pp.661-664)  and “Implementing Six Sigma” (pp. 703-707).   Course they don´t talk about high nor profound stat nor math, but they surely leave us a good PRACTICAL message that could be taken advantage of by whoever wanted to capitalize it.
Regards.   Sammy

RubberDude,
I just stuck to the standard control charts rather than have to listen to the dogma. We basically ran about 80 charts to keep them happy – we actually used about 5 to run the process. I had to carry 3 headcount to maintain and train all these charts and still cough up 5% price reduction per year – very enlightened. We won’t even talk about the trips to the ballet.
We used the PreControl a lot when we were building bowling balls (Motorola West). Worked out ok. Got more mileage from the focused cells.
Regards

RubberDude,
One in the same.

I would recommend you stay clear of pre-control. People who use it do not have a proper understanding of why the customer is requesting SPC. The point of SPC is to optimize a process by reducing the variation to that which is inherent in the process. Too many people have a “make it to print” mentality and don’t appreciate the customer’s need to limit variation.
If the customer merely wants product to print, they are not likely to require SPC (maybe during the 1980’s but not today). GM and Delphi state that QCI characterists (those that require capability) are selected because the product performance deteriorates in proportion to devation from target. Control charting is used to minimize variation and center the process around the customer’s target. Pre-control does not address this issue.
Another comment is that control charts are based upon averages. Averages will follow a normal distribution making it possible for a control chart to show the stability of a process. Pre-control is based upon individual readings. It is not possible to determine stability. Since there is no range, it is also impossible to know the standard deviation.
Finally, I find it sad that so many see SPC as little more than creating paperwork to satisfy the customer. SPC works, especially when used for your own benefit. During the 1980’s, I was QC manager for a Tier One Q1 supplier of exhaust manifolds to Ford. We had about 160 control charts on the shop floor and all of them had Cpk’s of 2.0 or higher. Our charting became so sensitive that we could distinguish between tool wear and clamping or location issues. We also installed acoustic sensors to stop machines before cutting tools broke and we used vibration analysis to identify machine components wearing out and schedule downtime for maintenance. Our emphasis was on process control, not detection of defects.
By doing things right, we had tremndous cost savings, high productivity and a customer ppm < 10 during 1990. Ford's satisfaction was simply a by-product of doing things right. If we had maintained a build it to print attitude, we probably would have lost the business.
 

I agree that it can be easier.  I used it to introduce the concept of control in a setup process as well where calculation of limits by floor people needed some ramping up to.  Walk before you run sometimes.
I believe that Shanin still advocates the use of Precontrol with other tools developed by Dorian (since dead, I think).  GM used it at one time and Shanin tools were part of our BB training at GE.  1996!  Whew! Long time ago.