Effective Training Inc. Message Board

Hi:
I recently bought the book of Tolerance Stack ups using GD&T, and I have a doubt regarding the use of unilateral profile tolerances.
When performing a stack up for an unilateral tolerance it says" the stack indicator is used to determine the sign and to which column the profile tolerance is used", but my question is what happens when the starting point of your stack up is the actual surface with the unilateral profile (excersice 9-1)?
If you start at the sketch line and go to the true profile then the total profile tolerance would be positive?
But if you position your starting point at the true profile, then the total profile tolerance would be negative?

Then, were should you position the origin, the sketch line or the true profile? is it always the same?

Also, one note got me confused on the same excersice "profile of a surface,0.4,a,b;Between points c&D, between pts E&F GAGED SIMULTANEOUSLY"....does this note affects the stack up in some way?

I would appreciate your help

Thank you!

Hi Vacg13,

I advise purchasing the selfstudy textbook as it give more detailed information. The regular textbook is intended to be used in the stacks class with an instructor that will be able to answer these questions. I will do the best I can in this limited space.
The rule for starting a stack on a surface that has a unilateral profile tolerance applied to it is to sketch a phantom line representing the tolerance zone and start at the phantom line with distance A and go back to the true profile surface. The direction that distance A is going according to the direction indicator determines the sign of distance A. If distance A is going in a positive direction then the full profile tolerance goes in the positive column and zero is entered in the negative column. If a unilateral profile tolerance is in the middle of the stack path then you follow the standard stack path conventions, (i.e. where distance A ends distance B begins, where distance B ends distance C begins). This is the practice followed for distance C in example 1 in chapter 9.
I hope this was helpful.
Again, you may want to purchase the self study textbook or attend on of our stack classes.