# Geometric Dimensioning and Tolerancing (GD&T) by g4BK8ImG

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```									            Geometric Dimensioning and
Tolerancing (GD&T)
Dr. Robert Rizza
Associate Professor
Department of Mechanical Engineering
Milwaukee School of Engineering
Milwaukee, WI

5/2/2012                                             1
Introduction

• GD&T is a method of defining parts by how they
function.

• The method has been developed over the last forty
years.

• It allows a designer to define the features of a part
without increasing tolerances.

• The current standard is ASME Y14.5M-1994.

5/2/2012                                        2
• Suppose we want to manufacture tables with a flat top.
With a traditional approach, how do we achieve this goal?

       With this approach, we make the tolerance tighter.

5/2/2012                                                   3
Specification            Limits           Tolerance
Zone
30 ± 1                31-29                2.0

30 ± .5            30.5-29.5               1.0

30 ± .25           30.25-29.75              0.5

• Making the tolerance tighter is not always good enough.
• In this case, making the tolerance more restrictive may lead to a good
table being rejected.
5/2/2012                                                        4
The GD&T Approach
• With this approach, we directly control the desired feature.
• This allows the tolerance on the size to be relaxed.

5/2/2012                                             5
• The holes in the plate must be located with high precision. How do we
achieve this goal using a traditional approach?

5/2/2012                                                    6
The GD&T Approach
• The same plate dimensioned using GDT. Why is this superior?

5/2/2012                                                   7
Datums and GD&T
•Application of GDT principles begins with the use of theoretical reference
datums (planes, circles, lines and points).
•By using datums, there is no ambiguity as to the reference for the feature
because:
1. The feature is designed with respect to the datum(s).
2. The feature is manufactured with respect to the same datum(s).
3. The feature is inspected using the same datum(s).

A

ASME symbol for a
reference datum. The name
of the datum is A.

5/2/2012                                                              8
Control Symbol
•    Symbols defining the GD&T control have been standardized.
•    Here is a typical control symbol:

5/2/2012                                                          9
The Envelope Principle
• Hole with MMC condition.

    The hole may have a larger diameter than at MMC and still be functional.
    The boundary of the actual hole will never violate the perfect hole
boundary.

5/2/2012                                                          10
Geometric Size Controls
The GDT standard describes several different controls.

• Controls related to the size of a feature may be used in conjunction with
MMC. These controls are:

Control                         symbol          Datum
straightness   (axis)                            No
Perpendicularity       (axis)                    yes
Angularity    (axis)                             yes
Parallelism                                      yes
Position                                        yes

5/2/2012                                                       11
Geometric Controls

• Concentricity is typically used to minimize vibration on spinning
round parts.
• Not all controls need to be referenced to a datum.
5/2/2012                                                      12
Virtual and Resultant Condition
• With GD&T, more than one control may be applied to a feature.
• The overall effect is determined by the RESULTANT CONDITION, a
variable.
• The VIRTUAL CONDITION is a constant value, which takes into
account the maximum allowed variation.
• Consider:

5/2/2012                                                    13
Virtual and Resultant Condition
Fastener
Hole Plate:                                       Plate:
Hole    TOL    V COND   R COND            Fastener    TOL     V COND   R COND

MMC         .5156   .0156   .5000    .5312     MMC     .5000      .0156    .5156    .4844

.5256   .0256   .5000    .5512             .4956      .0200    .5156    .4756

.5356   .0356   .5000    .5712             .4926      .0230    .5156    .4696

.5400   .0400   .5000    .5800             .4896      .0260    .5156    .4636

LMC         .5468   .0468   .5000    .5936      LMC    .4844      .0312    .5156    .4532

•Because  the Resultant Condition of the fastener is always less than the
hole, the fastener will always fit.
5/2/2012                                                                 14
Virtual Condition

Hole basis- hole boundary cannot violate
theoretical boundary.

• MMC is .5312-.0156=.5156
• Virtual condition is .5156-.0156=.5000
Axis basis-axis of hole must fall within
tolerance zone.
5/2/2012                                                      15
Floating Fastener Tolerancing
•In many cases, it is desired that a standard clearance fit exist between a
fastener and a hole. What is the tolerance on the control for such a hole?
•The tolerance is given by the formula: T=H-F, where H is diameter of
the hole at MMC and F is the diameter of the fastener at MMC.

For example say a 0.500-13UNC
machine screw is to be used to mate
two parts. Machinery’s Handbook
gives the clearance hole as .5312 with
an allowable tolerance of .0156.

Suppose the  tolerance on the fastener is     .4922  .0078, then the
control on the parts would require a positional tolerance of:
T=H   - F=(.5312-.0156) - (.4922+.0078)
T=.5156    - .5000=.0156
applied   to each part
hole
5/2/2012                                                              16
Fixed Fastener Tolerancing
•In   many cases, a fastener is restrained such as a screw in a tapped hole.
•The     question is what is the tolerance on the control for such a hole?
•The tolerance is given by the formula: T=(H-F)/2, where H is diameter
of the hole at MMC and F is the diameter of the fastener at MMC.

A case where
a fastener is
restrained.

Using   the numbers from the previous example, the positional tolerance
is:

T=(H   - F)/2
T=(.5156   - .5000)/2=.0078
hole
5/2/2012                                                             17
Summary
• GDT allows the designer to directly control a feature.

• Ambiguities between the designer and manufacturer are
avoided through the use of reference datums.

• The condition of the part is taken into account when a
tolerance is applied.

• Use of the Envelope Principle leads to high precision parts
at low cost.

5/2/2012                                             18

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