Tolerance Stack

Partes: 1, 2

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.

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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.

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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.

Partes: 1, 2

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