Information | Understanding | Best Practice.
The Pre Control Chart.Pre Control is a simple, easy to use, very visual method for monitoring processes.
Pre control charts:
i) are simple to set up and use.
ii) can be used with either variable or attribute data.
iii) are useful during setup operations – can determine if the process setup is producing product within required tolerances.
iv) can identify if the process centre has shifted or the spread has changed.
Pre control chart limits are calculated using the product or process specifications.
Pre control charts cannot be used to study process capability.
Pre control charts may generate more false alarms or missed signals than a traditional SPC control chart, e.g. X / Y Charts.
The Pre Control Chart format.Pre control utilizes a chart with three color bands, namely Green, Yellow and Red. Small frequent samples (sample size usually set to two units) are taken from a process and checked versus specification requirements. The measurements recorded will indicate where the sample resides within the colored bands. The results will indicate if the process is operating within control, is deviating out of control or needs to be stopped and corrected.
The green, yellow, red bands create five equal zones. A central green zone, two yellow intermediate zones and two outer red zones. The specification limits are set at the border between the yellow and red zones. In a bi-lateral specification, i.e. upper and lower specifications, the green zone will span 50% of the total specification tolerances. Each of the two yellow zones will span 7% each of the total specification, therefore, it will be expected that all readings should reside within either the green or yellow zones if the process is operating in a capable manner and producing acceptable output.
Note: In a unilateral specification, i.e. only a single specification limit. the green zone will represent 50% of the total tolerance, and the single yellow zone will represent 14% of the total tolerance.
Statistical basis and risks behind Pre Control.In a process with a Process Capability of Cpk=1, the upper and lower specification limits will align with +/- 3 standard deviations from the mean. Assuming a normal distribution of the output from a process, 86% of the readings will be in the green zone and 7% of the readings in either of the two yellow zones. 7% equates to one reading in 14. The probability of getting two consecutive readings in a yellow zone will be 1/14 * 1/14 = 1/196 = 0.51%. Where two samples are continually recorded there are four possible permutations, giving an overall probability of 2%, i.e. Pre Control asks for a process to be stopped and adjusted, even when operating in a capable manner 2% of the time.
Drafting a Pre Control chart.Create the zones to be used in the pre-control study:
Place USL (upper specification limit), LSL (lower Specification limit) and centre line (CL) on chart
Divide (USL-CL) in 2 equal zones: green- yellow
Divide (CL-LSL) in 2 equal zones: green- yellow
Green zones (GO SECTION) are next to centre
Yellow zones (CAUTION) are next to the limits
Zones above or below yellow area are colored in RED (UNDESIRABLE)
Pre Control – Initial and adjustment Qualification.There is a higher sampling requirement with elevated acceptance requirement when a process is initially qualified. Also, whenever a process adjustment is performed the initial pre-control qualification needs to be performed.
For the initial qualification, five consecutive samples from the process are measured. All five should be recorded within the green zone. If one of the five samples is in a yellow zone then a further five samples need to be recorded. If two of the five samples are in the yellow zones, or if a sample is recorded in a red zone then the process must be adjusted and a further new five consecutive samples measured and this sampling process repeats until all five consecutive samples are recorded in the green zone.
This requirement for the five consecutive samples in the green zone, ensures that the process is capable before progressing into routine use.
Pre Control – Ongoing process monitoring.Record two samples from the process on an ongoing basis.
If the first measured sample is:
– in green zone, continue running
– in the yellow zone, check the next sample
– if the second sample is also in the yellow zone, reset process
– in red zone, stop, make corrections & reset process
Whenever the process is reset, you need 5 successive pieces inside the green zone before allowing to process to continue as normal.
Once the process is up and running, randomly sample 2 pieces at intervals to monitor the process.
The sampling interval is normally related to the mean time between process adjustment, with six sample sets recorded between average process adjustments. Initially, there will be a level of judgment applied to determine average process adjustment intervals, but as more data is collected and process knowledge increases, the average interval time will quickly be accurately determined.
For example, if the average process adjustment interval is once per eight hour shift, i.e. every eight hours, then normally six sets of two sample units will be recorded per shift.
SPC & Statistical Methods for Process Improvement.
- Process Capability. Variability Reduction. Statistical Process Control.
- Pre-Control. R&R Studies.
- Process capability indices Cp, Cpk, Cpm, Capability ratio.
- Performance indices Pp and Ppk.
- Variable Control Charts.
- Attribute Charts.
- Pareto Charts.
- Individual – X Charts.
- Histograms / Process Capability Analysis.
- Scatter Diagrams.
- Etc. … Etc. …
- Information | Understanding | Best Practice >>>