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      <title>Tolerance Stackup Validation calculation</title>
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      <div class="title_topic3" id="xps10_pagetitle">Tolerance Stackup Validation calculation</div>
      <hr noshade="true">
      <p class="para_topic">With Tolerance Stackup Validation, before doing the calculation, follow these steps:</p>
      <ol type="1" start="1">
         <li>
            <p class="para_item">Define the assembly sequence:</p>
            <ul>
               <li>
                  <p class="para_item">Part (c)</p>
               </li>
               <li>
                  <p class="para_item">Part (a)</p>
               </li>
               <li>
                  <p class="para_item">Part (b)</p>
               </li>
               <li>
                  <p class="para_item">Part (d)</p>
               </li>
            </ul>
         </li>
         <li>
            <p class="para_item">Establish mating conditions for the elements.</p>
         </li>
         <li>
            <p class="para_item">Identify the faces that constitute the gap.</p>
         </li>
      </ol>
      <p class="para_topic">The calculation shows the following results:</p>
      <p align="center"><img align="bottom" src="graphics/qs_cyl_info.gif"></p>
      <p class="para_topic">The Tolerance Stackup Validation variation is smaller than the hand-calculated variation. Only 3 tolerances, instead of the 5 used in the hand calculation, are taken into consideration. The flatness of part (d) and the parallelism of part (a) are not considered. The following sections explain why.</p>
      <div class="title_division">Analysis of flatness of feature (d)</div>
      <p class="para_division">Tolerance Stackup Validation ignored the flatness of feature (d) where it mates to feature (c) at point B, as shown in the <a class="links" href="javascript:void(0)" onclick="top.openFile('quickstack/hand_calc_cylinders.html');return(false);">Vector Loop Diagram</a>. The flatness is applied to the top surface of cylinder (d), which mates with the underside of a counterbore hole in part (c).
      </p>
      <p class="para_division">When flatness is applied to a feature that also has a tolerance of size, the flatness controls the form of the surface, but the entire surface must still lie within the limits of size. The flatness tolerance defines a tolerance zone that has a width equal to the value of the flatness callout, but the entire flatness tolerance zone must lie within the tolerance zone defined by the limits of size. In this case, the flatness controls the form of the surface, but not the location of the surface.</p>
      <p class="para_division">The flatness tolerance defines a tolerance zone the width of the flatness tolerance, which is 0.002 in this example. The Tolerance Stackup Validation analysis assumes that the entire surface falls within the tolerance zone defined by the flatness.</p>
      <p class="para_division">However, the overall location of the surface, which is defined by the high points of the surface, should not be shifted due to the flatness tolerance. Also, the overall profile of the surface, which is defined by the high points of the surface, should not change shape due to the flatness tolerance.</p>
      <p class="para_division">When measuring to a surface with flatness, the flatness can be seen as variation between measurements, but the measurements must still fall within the feature's limits of size if they exist.</p>
      <p class="para_division">However, when a feature is mated to another feature, it will typically mate at or near the theoretical surface defined by the high points of the surface, and the effects of the flatness will not be seen.</p>
      <p class="para_division">The Tolerance Stackup Validation analysis makes these assumptions, resulting in features as shown in the figure below:</p>
      <div class="figure">
         <p align="center"><img align="bottom" src="graphics/qs_cyl_flat_within.gif"></p>
      </div>
      <div class="title_figure">Flatness falls within feature limits</div>
      <p class="para_division">If flatness is to be included in the analysis, as per the hand-calculated vector loop method, the analysis has to assume that the surface will appear as shown in the figures below:</p>
      <div class="figure">
         <p align="center"><img align="bottom" src="graphics/qs_cyl_flatlow.gif"></p>
      </div>
      <div class="title_figure">Flatness included - Minimum value</div>
      <div class="figure">
         <p align="center"><img align="bottom" src="graphics/qs_cyl_flathigh.gif"></p>
      </div>
      <div class="title_figure">Flatness included - Maximum value</div>
      <p class="para_division">The figures above are not typically the design intent of flatness, even though they meet the flatness tolerance. It is not realistic to assume that typical manufacturing processes would result in a feature such as shown in the two figures above.</p>
      <p class="para_division">Tolerance Stackup Validation properly considers form tolerances when parts are mated; the hand calculation based on the vector loop method does not.</p>
      <div class="title_division">Analysis of parallelism of part (a)</div>
      <p class="para_division">Tolerance Stackup Validation ignored the parallelism on part (a) where it mates to part (c) at point E on the <a class="links" href="javascript:void(0)" onclick="top.openFile('quickstack/hand_calc_cylinders.html');return(false);">Vector Loop diagram</a>. This parallelism callout is on the bottom of cylinder (a), and references Datum A, which is the top of cylinder (a). The bottom of cylinder (a) is mated to part (c); it is one side of the gap that is being measured. The top of cylinder (a), which is the datum referenced by the parallelism, is not mated to any other feature; it is not part of the measurement.
      </p>
      <p class="para_division">When cylinder (a) is mated to part (c), the surface with the parallelism callout is oriented to the surface on part (c) that it is mated to. The parallelism callout does not change the surface's orientation with respect to part (c). The orientation of the upper surface of cylinder (a) would vary due to the callout in this case, but that surface is not part of the measurement, so its orientation has no effect on the measurement.</p>
      <div class="title_division">Summary</div>
      <p class="para_division">As shown in this example, a hand calculation based on a vector loop may differ from the Tolerance Stackup Validation calculation, because the assembly sequence and mating conditions are not considered in the vector loop method. Considerations of assembly sequence and mating conditions can have a substantial impact in the stack up analysis, showing the limitations of hand calculations.</p>
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