Tolerance Stack

2. Tolerance Stack
3. Chain Dimensioning
4. Base Line Dimensioning
5. Direct Dimensioning
6. Dimensional Limits Related to an Origin
7. Designating a Origin
8. Notation and Problem Formulation
9. Tolerance Stacking Formulas
10. Arithmetic or Worst Case Tolerance Stacking
11. RSS Method or Statistical Tolerancing
12. Normal Distribution over Tolerance Interval
13. RSS Method with Inflation Factors
14. Application
15. Conclusion
16. Sources

Introduction

The reason of this report is to explain diverse tolerance stacking methods and his principals concepts, without going into the theoretical details behind them and comparing the differences between the different standards.

Some of the methods covered are: worst case or arithmetic tolerancing, simple statistical tolerancing or the RSS method.

We also are going to learn which dimensioning standard is right for your organization

Concepts

Rules:All geometric tolerances apply regardless of feature size. The maximum material and least material modifiers may be used as desired and where appropriate. Datums:The most recent version of the Y14.5 standard agrees with ISO in datum identification and meaning. An exception is the application of a datum triangle in a side view of a cylinder. The Y14.5 standard does not illustrate this option. ISO states that the datum is a line tangent to the feature unless the triangle is in line with the size dimension. In either case, datums are used to establish origins of measurement and arrest the six degrees of freedom.

The current standards reflect the way mating parts contact one another. Caution, the new ISO draft would change the definition of a datum to be a mathematical derivation of the actual feature using a least squares algorithm.  Form:For all intents and purposes, flatness, straightness, circularity and cylindricity are identical in both standards. All points of the surface must lie inside the form tolerance. In order to properly inspect a feature, the inspector would have to use an infinitely small indicator or subtract the uncertainty, resulting from surface roughness, from the allowable form tolerance. There is a draft of the ISO standard that would exclude surface roughness from form controls. If this revision were approved, the definitions for form would not agree.

 Orientation:The standards are consistent for perpendicularity, parallelism and angularity.  Profile of a Surface:Simple profile is the essentially same for the two standards. The Y14.5 standard uses profile of a surface to locate flat surfaces as well as contours. ISO traditionally uses position to locate flat surfaces and profile of a surface is used for contours. ISO does not prohibit locating plane surfaces with profile of a surface. There is a move towards minimally dimensioned drawings, making the CAD file BASIC and using a general profile of a surface tolerances to control features unless otherwise specified. This does not violate either current standard. The corner condition of the tolerance zone is not identical.The concept of composite profile is not found in ISO.

Position:Position may be used to locate features of size in ISO and Y14.5.Although composite position is shown in ISO, it is not well developed.

 Runout:The circular and total runout definitions are consistent. Both standards default to the indicator being oriented normal to the feature being controlled. ISO allows dimensioning the angle at which an indicator may be oriented relative to the surface.  Major differences that exist between the two standards include:Definition of size:The Y14.5 standard defaults to the Envelope Principle, which provides the control of form when features are inspected correctly. ISO requires that either the Envelope symbol be placed next to the size dimension or individual form controls be specified.

General and title block tolerances

Most US companies continue to specify general tolerances in a schedule based on the number of decimal places specified on the dimension. This often requires adding trailing zeros to indicate a tighter tolerance. Adding trailing zeros violates ISO and ASME metric dimensioning standards. ISO uses tolerance grades and a letter to indicate position of the tolerance with respect to the basic (not the same basic used for geometric controls) dimension.