November 30, 2013

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.

Table


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

table2


SPC


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.




- 2 comments by 2 or more people Not publicly viewable

  1. Shujaat Alikhan

    Nihat, I’ve read several texts as well that say we need to remove special causes, but I just wanted to share my view that i feel we need to keep it, especially for my case, because of two reasons:

    1. Insufficient subgroups (taking out one subgroup will adversely affect my chart)
    2. I feel like it is disregarding data that is important to the true process. If you take out the special causes, what is the point of the graph?

    Hope you can help me come up with some ideas,

    Thanks.

    22 Dec 2013, 02:43

  2. Jaat, the most obvious issue in your chart is the accuracy of data. Although you and your partner dealt hard time by collecting 6 times a day for 5 days, some data looks unreasonable (data which are more than 20 min). I do not suggest you to neglect some of data because it sounds like you do not want to run into with; however, in this case go and finish it and define this project as a milestone of your six sigma career. Plus, if you want to draw a more proper one, next time you should pay more attention to data collection method instead of trying to solve further.

    If you take out the data which directly affect the special variations in the system, it would be seemed as stable and perfect; but unrealistic.

    22 Dec 2013, 04:27


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