Help! How to handle Unequal Bilateral Tolerance using RSS/MRSS method? 4

[Buck61]

Help! I posted the following question last week and basically got nowhere- please submit solid approach/solution only.

Best Regards,
Buck61

Hi,
I need to know how to go about using an unequal bilateral tolerance when calculating total variation using the RSS tolerance analysis method.
All RSS examples I have seen use uniform bilateral tolerances.

A text I have at work "Dimensional Management" (by Mark Curtis c2002) which includes a section on modified MSRR (pg 166) where he uses a correction factor of K = 1.4 x RSS and 1.7 x RSS. However, as shown, you must multiply K by the RSS previously calculated using uniform bilateral tolerances.
??
I appreciate your respective input,
Buck

 

[drawoh]

btrueblood,

I am pretty certain I understand what you are saying. When these discussions get going, it sounds to me like someone (not you) thinks that the drawing tolerances equal 3_sigma, and can be used to define a 99.7% confidence interval. I prepare drawings for a living, and I know better.

If you have collected real data, and verified that it is a Normal curve, everything is fine.

Consider the case that we have a machine with variation well within tolerance, that either drifts out of alignment, or wears out. The thing runs for several weeks, then it is shut down and fixed.

In the long term, we have rectangular distribution with the possibilities you note above. At the moment, we have a bin of parts all at the same end of the tolerance range.

What sort of useful decisions can I make, based on the statistical information. As a designer, 99.7% probability that a connection works is not really acceptable, especially if there are a lot of 99.7% probabilities in the design. I can treat all my tolerances as rectangular, open up clearance holes, and I can re-arrange the design to make the available sloppy tolerances work.

In production, based on statistics, my choices are to accept the entire batch as is, reject it completely, do a complete inspection and reject the individual bad components, or sort the pieces at assembly. Sorting and hammering at assembly is the exact crap we were trying to eliminate when we took up dimensioning and tolerancing.

JHG