3 sigma for tolerances formulation

J

jhaizon_e

#1
Anyone could help me to have a standard computation of tolerance for product weight of plastic component using 3 sigma or related method?
 
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D

D.Scott

#2
Anyone could help me to have a standard computation of tolerance for product weight of plastic component using 3 sigma or related method?
I am not sure I understand your question but if you are trying to determine what tolerance you would need for your current process to achieve +/- 3 sigma capability, try this.

First you must determine what your current standard deviation is. Do a run chart of at least 30 pieces and calculate the mean and standard deviation. Calculate +/- 3 standard deviation from the mean. This will show you the tolerance needed to achieve a 3 sigma process.

You should consider though that this requires perfect centering of the process and allows for no process swing which would be normal over time. You could allow for that swing by adding another 1 1/2 standard deviation to each side of the mean to establish your tolerance.

I hope this helps but feel free to ask again if your question is still unanswered.

Dave
 

Statistical Steven

Statistician
Staff member
Super Moderator
#3
Anyone could help me to have a standard computation of tolerance for product weight of plastic component using 3 sigma or related method?
There are two methods you can investigate. First, if you want a tolerance for the individual weights, then use tolerance intervals (ISO 16269-6). If you want to set tolerance intervals on a mean of n weights, then a confidence interval would be appropriate. Depending on the type of interval (tolerance or confidence), you will need to estimate the variability of the process. Let me know if you have other questions.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#4
I am not sure I understand your question but if you are trying to determine what tolerance you would need for your current process to achieve +/- 3 sigma capability, try this.

First you must determine what your current standard deviation is. Do a run chart of at least 30 pieces and calculate the mean and standard deviation. Calculate +/- 3 standard deviation from the mean. This will show you the tolerance needed to achieve a 3 sigma process.

You should consider though that this requires perfect centering of the process and allows for no process swing which would be normal over time. You could allow for that swing by adding another 1 1/2 standard deviation to each side of the mean to establish your tolerance.
Generally, I add an additional 1 sigma, but 1 1/2 may be more generous, and handy especially if the data used is very short term. This makes sense because it is logical that the distribution will be normal for this kind of process. The key is whether the tolerance developed from the process (which is not a bad idea - better than the usual dartboard approach) is functionally acceptable. Testing product at the outer limits to ensure that they are still functionally acceptable would be highly recommended. :cool:
 
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