Steps of drawing SPC
As I mentioned on my previous post, SPC is the one of the important tools for monitoring the system behaviour. SPC can be seen as a six sigma tool; however, the people can use it without any 6 sigma knowledge as well.
Okay , let’s come to draw a SPC chart.
Assume that we work on the quality department and we take regularly 2 samples for checking the dimensions of the products. And we do this observation 3 times: at 11am, 3pm, and 6.30pm in a typical working day (Totally we can say that we measure 6 products a day). Moreover, customer told us that the parts suppose to be (150,00mm- 151,00mm) in those dimensions. Okay I am going to create my own measurement chart step by step.
I wrote the every sample which is collected from production. Firstly I wrote down the difference between highest number and the lowest in the same day. And then I calculated the average of difference by adding whole ranges of days and then dividing to 6. And it is 1,035(Range_bar). X_bar means that average of samples which are collected in same day. After I calculated X_bar, I can reach X_bar_bar easily which is average of average of samples.
Finally, the Upper Control Limit equals to X_bar_bar+ (0,483*Range_bar). And Lower control limit equals to X_bar_bar – (0,483*Range_bar).
[0,483 is a constant and can be founded on almost every tables.] My n is 6, so it is 0,483
I drew my own SPC chart according to assumed values. Actually we can use this tool for almost every application on the industry. As you can see Brown line was our “upper control limit” and green one is “lower control limit”. And Blue demonstrates the system behaviour.
As you can see from the graph the system behaviour stays in the limit; however, the limits are not capable of customer tolerances which is 150,00-151,00, even though the flow is stable.
On the next post, I am going to calculate capabilities!
Important Note: Before reduce the common variations in the system, system or application should be removed from any special causes.