Tolerance stack help needed. 2

[greenimi]

I need some help here to confirm if I am on the right track or not. I would like to find out if the X min/ X max dimensions (see sketch attached) are correct calculated.
I realize none of the fine gentlemen on this form do not work for me, therefore is more like a favor to ask for someone who has strong knowledge in those kind of calculations (or even maybe some kind of reliable tolerance stackup software Vis-VSA, 3DCS , CeTol or other).
I am trying to understand the concept (how the calculations should be done CORRECTLY) otherwise I will be only a “number generator” and my calculations are worthless sheet of paper.
Note: do not consider the form error on the datum feature A. I know does affect the calculation, but I am not there yet. (I have to walk before I can run)
I got
X min.: .057
and
X max.: .076
(should be no surprise if I am telling you that not everybody around here agrees with those numbers).
Thank you

 

[pmarc]

greenimi,
I don't think waqasmalik meant form error of datum feature. I believe he meant form errors of both toleranced features, that is [Ø].372-.378 and [Ø].495-.505.

waqasmalik,
The results will not change when you take these form errors into account. Because the simultaneous requirement applies between both features (hence no datum feature shift is available), the only thing that needs to be done in order to find Xmin and Xmax is calculating:

1. Radial distance between virtual conditions of both features (Xmin):
Xmin = [(.495-.002)-(.378+.001)]/2 = .057
These two virtual conditions represent extreme boundaries that can never be violated by material of the features.

2. Radial distance between resultant conditions of both features (Xmax):
Xmax = [(.505+.002+.010)-(.372-.001-.006)]/2 = .076
These two resultant conditions represent extreme boundaries within which the material of the features must always be.

 

[axym]

Hi All,

As usual, I agree with pmarc.

When calculating tolerance stackups, maximum values for things like misalignment often occur when the features are at LMC, have perfect form, and are perfectly oriented. Introducing form error adds material and actually decreases the possible misalignment. In other words, the imperfect form (or imperfect orientation) doesn't make the situation more extreme.

In this example, the maximum value of X occurs when the tolerance ID feature and the toleranced OD feature both have maximum misalignment, but in opposite directions.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

Waqasmalik is right in a sense that dimension X itself is ambiguous (pretty common for things like "thickness" or "gap".

Let say, the groove has less than perfect shape (within the tolerances of course).
How do you measure X? from B to C? from A to D? from A perpendicular to (CD)? from C perpendicular to (AB)? from A to D but parallel to upper face? lower face?

Just a thought.