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




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                     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.



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                 The Traditional Approach
   • 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.

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             The Traditional Approach
            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.
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                The GD&T Approach
  • With this approach, we directly control the desired feature.
  • This allows the tolerance on the size to be relaxed.




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            The Traditional Approach
• The holes in the plate must be located with high precision. How do we
  achieve this goal using a traditional approach?




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                 The GD&T Approach
   • The same plate dimensioned using GDT. Why is this superior?




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                        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.

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                             Control Symbol
  •    Symbols defining the GD&T control have been standardized.
  •    Here is a typical control symbol:




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                 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.

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                   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




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                   Geometric Controls




   • Concentricity is typically used to minimize vibration on spinning
     round parts.
   • Not all controls need to be referenced to a datum.
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        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:




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         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.
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                    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.
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         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
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            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
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                         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.



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