RSS using unequal bilateral tolerance

[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

 

[DanStro]

Is there a reason why you can't convert the bilateral into unilateral? In my experience that is what the machine shop is going to do anyway so the most likely value will be in the middle of the range, not necessarily the nominal value on the print.

 

[Buck61]

Hi Danstro,

In lieu of a long explanation, the unequal bilateral tolerance is fixed and must remain on the drawing as .055" +.002"/-.005".

I neglected in the original post to suggest making the unequal bilateral an equal bilateral tolerance so it can be used in calculating RSS, i.e., .0535" +/-.0035"?

 

[drawoh]

Buck61,

Are you doing the calculations on your drawing, or are you using a spreadsheet. It is easy to set up a spreadsheet to accept whatever is marked on the drawing, and convert it to whatever supports your calculations.

Are you correct in assuming that your tolerances reflect ±3[σ]? If you made that assumption from my drawings, you would be wrong at least 90% of the time.

JHG