When you create a tolerance stackup, the Tolerance Analysis panel appears on the right side of the display. You can toggle the panel display to show the individual stackup Details and the Summary information for all stackups.
Select the < symbol at the top of Details panel to display the Summary panel which contains information for all stackups. Select the > symbol next to a stackup in the Summary panel to return to the Details panel and display information for the selected stackup.
Summary of 1D Tolerance Stackups Panel
The following image is a Summary panel that displays multiple stackups. Each stackup has a user-defined name.
- Edit the details of stackup. You can also select the > icon in the row to edit the details.
- Rename the stackup. You can also change the name by selecting the Name field.
- Delete the stackup. This workflow is the only way to delete a stackup.
- >: Click to switch to the Details panel of the selected stackup.
- OK: A check in a green circle indicates that the analysis objectives are met. An x in a red circle indicates that the objectives are not met. An exclamation point in a yellow triangle indicates that there may be 2D or 3D influence in the stackup.
- Name: An editable field that displays the name of the stackup. Tolerance Analysis names each study Stackup followed by a sequential number. We recommend that you change the name to something that describes the tolerance study.
- Nominal: The nominal distance between the selected items around which the stackup analysis is being defined. The value is shown without parentheses if it is part of the objective definition. For example, when using symmetric ± or unique +/- values to define the upper and lower limits of the objective relative to the nominal. For the other types mentioned in Objective, the nominal value is included for reference only and is shown in parentheses.
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Objective type: An unlabeled editable field to the right of Nominal that lets you choose different methods for defining the upper and lower objectives of the stackup. Because most tolerances stackups are performed to ensure clearance for fit, the default type for all new stackups is ≥ and the Objective value is 0. Click the symbol to the left of the Objective column to expand the drop-list that contains the following options:
- Symmetric: Bilateral value applied relative to the nominal value.
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Plus Minus: Nonsymmetric values applied relative to the nominal value.
Note: +/+ and -/- definitions are also supported.
- Limits: Allows the definition of absolute upper and lower limits for the objective independent of the nominal value.
- Upper Limit: Defines a single upper limit.
- Lower Limit: Defines a single lower limit.
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Objective: An editable field that lets you choose the tolerance values associated with the Objective type previously described to define the limits of the stackup results. As mentioned in the Objective type, the default objective for all new stackups is ≥ 0.
Note: Number precision is determined by the number of digits you enter in the value box. If you enter 0 without a decimal, it does not change the precision. Number placement is determined by the sign and value you enter. Tolerance Analysis evaluates the sign and value of the number to determine if it should be placed in the upper or lower segment of +/- and Limits Objective types. If no sign is entered, the sign applied to the field before the edit is restored.
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Target Quality: An editable field that lets you define the type of analysis to perform by selecting the drop-list to the left of the column. You can select the following Target Quality options:
- Worst Case: Assumes that all stackup distance dimensions included in the study are at either the extreme minimum or extreme maximum limit at the same time.
- RSS: Root Sum Squared is a statistical method for calculating the combination of dimensions based on the assumption that not all dimensions involved in the stackup are at their limits simultaneously. The key difference between this approach and the more general statistical approach described in the following section is that RSS assumes each of the contributing dimensions have the same quality level as the calculated result. Tolerance Analysis also treats assembly shift resulting from clearances between parts and datum shift resulting from material modifiers applied to datum features in a Datum Reference Frame as worst-case effects (shifted to one extreme or the other) before calculating the RSS combination of the tolerances involved.
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Statistical: Statistical is based on the same principles used for RSS analysis. The advantage of Statistical is that you can define the target quality level for the stackup regardless of what has been assumed for the part dimensions. It also treats the clearances associated with assembly and datum shifts as statistical contributors having a uniform distribution. The following metrics can be used to define the target quality level for statistical analyses:
- Cpk
- Σ (Sigma), not to be confused with lower case σ often representing the standard deviation of a normal distribution.
- % Yield
- DPMO (Defects per Million Opportunities)
Note: Target Quality and the default values for quality metrics are defined in the Tolerance Analysis Settings.
- Results: The results of the stackup analysis based on the defined Target Quality level. The results are presented in the same format used for the Objective definition (±, +/-, Limits, and so on).
- Predicted Quality: When Target Quality is set to use one of the statistical methods this column shows the calculated quality of the stackup distribution compared to the defined Objective. For RSS, it is reported as Cpk. For the general statistical analyses Tolerance Analysis reports the Predicted Quality using the same metric type as defined for the Target Quality.
- #Dims: This column is included to quickly assess the number of dimensions contributing stackup. This is often helpful when making the determination whether to use Worst Case or one of the statistical options for the Target Quality.
View the Results and Contributions
At the bottom of the Tolerance Analysis panel is the Results pane. When multiple stackups exist but none are selected, the Results pane displays a summary of the number of objectives met or not met, and a quality roll-up using the preferred quality metric specified in the Settings.
When no stackups in the Summary table are selected and you select the Contributions tab, the pane displays a roll-up of the contributions each dimension has across all defined stackups.
View Individual Results and Contributions
To select an individual stackup in the Summary table without activating an edit field, select the icon in the OK column. When you select an individual stackup, the Results or Contributions pane displays information for the selected stackup.
When you define the first stackup analysis, the results are shown in the second to last row of the Stackup Details table. A Results Graph is also shown in the Results pane at the bottom of the table.
The Results graph display varies according to the defined Target Quality. The label at the top of the graph lists the Target Quality analysis type and name of the stackup.
Worst Case Results
The numbers above the graph indicate the upper and lower limits and represent the stackup at the specified Quality Target. The area of the graph that is within the specification limits is shown in green and the area outside the limits is shown in red.
The numbers below the graph indicate the nominal stackup value and upper and lower specification limits for the stackup as defined in the Objective. For unilateral specifications, such as this case in which the results must be greater than 9.90, only one specification limit is shown.
RSS Results
The RSS Results graph displays the calculated mean and standard deviation parameters for the RSS component above the graph. It also displays a normal curve which indicates the statistical nature of the analysis.
The numbers above the graph represent the upper and lower calculated RSS limits of the stackup.
The numbers below the graph indicate the nominal stackup value and upper and lower specification limits for the stackup as defined in the Objective.
Statistical Results
- The graph has the shape of the probability density function, often called a bell curve, associated with the Gaussian or Normal distribution.
- The numbers above the graph represent the upper and lower calculated limits of the curve at the desired quality level listed below the graph, Cpk of 1.0 in this case.
- The predicted quality level based on the percentage of the curve falling outside the upper and lower specification limits is shown above the graph with the calculated Mean and Standard Deviation parameters for the distribution.
- The Target Quality metric and value is shown below the graph.
You can view the contribution of each dimension by selecting the Contributions tab at the bottom of the table to display the contributions graph. This information can help you identify which tolerance to reduce first if too much variation is predicted. The list of dimensions and tolerances that impact the stackup and their relative contribution is sorted from largest to smallest.
- The part and dimension name for standard dimensions.
- The part, name of the feature being controlled, and the feature control frame including referenced datum features. In the Stackup Details table, the feature control frame is in the same row as the basic dimension between the controlled feature and the datum feature.
- Datum Shift contributors from datum features being referenced and Maximum Material Boundary (MMB) or Least Material Boundary (LMB). Included in the row is the name of the datum and the name of the feature being controlled.
- Assembly Shift contributors representing clearances between parts along with the names of the features between which the shift occurs.
Select the > icon on the left side of the Summary table to display the stackup details.
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This icon represents the dimension path through the parts and features. Each feature is represented by a symbol indicating the type; a flat surface, a cylindrical feature of size, or a planar feature of size such a slot. You can change the dimension attachment for features of size located with a linear dimension or stackup measurements to or from a feature of size by selecting the line to the left of the dimension as shown in the following image.
When both the feature above and below the dimension is a feature of size, the nine options shown in the image are available. The first group indicates whether the dimension origin should be attached to the Near-Near, Near-Center, or Near-Far side of the feature immediately above the dimension. The second group is for the attachment to the feature immediately below the dimension. If only one of the features is a feature of size, there are only three options available.
- Name: This editable column displays the name of the part, feature, dimension, calculated assembly shift, and calculated datum shift.
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The shared dimensions icon appears when a dimension is used in more than one stackup. Hold your mouse over the icon to see a list of stackups that include the dimension.
- Sens: The sensitivity of the dimension in the stackup. This field indicates the direction of the dimension. The value is usually 1 unless the dimension loop passes from the outer edge to the center of a feature of size, in which case it is ½.
- Nominal: The nominal value of the dimension between the two features determined by the geometry of the models.
- Type: This editable column lets you choose different methods for defining the upper and lower tolerances for the dimension. Click the symbol to expand the drop-list with all available options. The tolerance types supported are:
- ± Symmetric: Bilateral value applied relative to the nominal value.
- +/- Plus Minus: Nonsymmetric values applied relative to the nominal value. +/+ and -/- definitions are also supported.
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Limits: Defines the absolute upper and lower limits for the dimension independent of the nominal value.
- Geometric: Defines the geometric tolerances per ASME or ISO standards. Not available for size dimensions.
Select Geometric to open the Add Geometric Tolerance dialog box. In the following image, Face1 in the top data field is assumed to be controlled by Face3 in the Feature Control Frame. If this is not correct, use the toggle control
to reverse the them.
From the drop-down at the left of the Feature Control Frame (FCF) choose the control, then set the tolerance value. In some cases you may also specify a material modifier to the feature being controlled or to the datum feature by using the drop-down list next to the tolerance or datum feature name respectively.
Use this dialog box to make modifications to the Geometric Tolerance definition. You can also change the tolerance value in the stackup Details table.
- Tolerance: Choose the values associated with the Tolerance Type to define the limits of dimension.
When you enter values Tolerance Analysis uses the number of digits you type to determine the precision. If you enter 0 without a decimal it does not change the precision.
Tolerance Analysis evaluates the sign and value of the number you type to determine if it is placed in the upper or lower segment of +/- and Limits Objective Types. If no sign is entered, the sign in the field before the edit applies.
- Datum: This column indicates which feature is used as the datum feature for geometric tolerance.
- Linked Tolerance: This unlabeled column displays a closed chain link icon when the tolerance in the analysis is linked to the tolerance in the part file
. As you make changes to the tolerance value in the table, the tolerance value in the part file is updated. Similarly, if you make changes to the tolerance in the part file, the changes are consumed by the tolerance analysis when you enter the environment. You can break the link between Tolerance Analysis data and the part tolerance data by selecting the link icon in the table. The link appearance changes to a broken link
. This action disables the sharing of tolerance information between Tolerance Analysis and the part file. Click the broken link icon to restore sharing the tolerance information between Tolerance Analysis and the part file. When the link is restored, the tolerance value in Tolerance Analysis overwrites the tolerance value in the part file.
- Cp: When the analysis type is RSS or Statistical, this column indicates the Cp values assumed for the parts and the dimensions. For Statistical analysis, Tolerance Analysis incorporates a hierarchy of settings enabling you to have complete control over the assumed statistical distributions. Only Normal, or Gaussian distributions are supported, but you can control the standard deviation defined by the tolerance applied through the specification of an assumed Cp quality metric. These assumptions can be defined at each of the following levels:
- The definition of the Default Cp Model Option in Settings. It is recommended to leave the Default Cp at 1.0 unless you know that most of your parts are being produced at a different quality level.
- Part level on the Stackup Details table as shown in the Cp column of the row containing the part name. Until you specify a part-specific value, the Part assumes the Default application-level value and the value of the current application-level Default Cp setting is shown in parentheses. You can change this for each part by selecting the drop-list to change the setting from Model level Cp to Part level Cp. Doing so removes the Default label and replaces it with the actual value. You can change the value as needed.
- Dimension and Feature level Cp values are also shown in the Cp column of the row containing the dimension with tolerance included in the stackup results. Each assumes the Cp defined at the part level unless you specify a unique value for a specific dimension. When using the part-level value, the entry lists Part in the Cp column again with the current Cp value of the part shown in parentheses. You can change this value for each dimension by selecting the drop-list to change the setting from Part level Cp to Dimension level Cp or Feature level Cp depending on the item being changed. Doing so removes the Part label and replaces it with the actual value. You can change the value as needed.
- Parts: One or more parts included in the stackup definition. For each part there is:
- Features: One or more features on the part in order of the stackup loop passing through the part. Features of size have a size dimension and tolerance shown in the same row.
When there are four or more features included for a part, you can change the order of all but the first and last feature to reflect a different series of dimensions by dragging the row up or down in the table. Release the feature when it is on top of a dimension between two other features. When you release the mouse button, Tolerance Analysis creates dimensions between each of the features in the order they appear in the table.
If you want to remove a feature from the stackup so that two dimensions between the surrounding features change to a single one between them, right-click on the feature you want to remove and select Delete.
- Dimensions and tolerances: When there or two or more part features are used there is a dimension between them indicating how they are located relative to each other.
- Datum shift contributors: When a datum feature is referenced at its Maximum Material or Least Material boundary there is an additional row with a light red background between the datum feature and the row with the Feature Control Frame. The value of tolerance in this row represents the permissible datum shift that can occur when the datum feature is at its ideal size. In other words, halfway between the upper and lower specification limits for the size. Other datum shift effects that can occur are accounted for in the modified sensitivity of the size dimension.
- Features: One or more features on the part in order of the stackup loop passing through the part. Features of size have a size dimension and tolerance shown in the same row.
- Assembly shift contributors: When Tolerance Analysis detects that clearance can occur between the features of size, such as bolts or pins in clearance holes that control the location of two parts relative to one another, the row has a light purple background, an icon with opposing horizontal arrows, and a label beginning with Asm shift followed by the part names. The value of tolerances in this row represents the permissible assembly shift that can occur in each direction when both features are at their ideal size. In other words, halfway between the upper and lower specification limits for their sizes. More assembly shift effects can occur as the features of size vary are accounted for in the modified sensitivity of the respective size dimensions.
The assumption of random placement, referred to as assembly shift or Float, means that there is nothing influencing the position of the parts relative to one another during assembly. However, when trying to determine if two parts can be assembled you should account for the fact that the person assembling the parts moves them around, using the clearances between the features described above, until they fit together. In such cases, instead of assuming random placement of the bolt into the hole you should use the clearance to maximize the distance being analyzed in the stackup for fit. In such instances, the parts interfere and are not able to be assembled when the distance between two nearby surfaces decreases to 0 or less despite the parts being moved as far apart as possible.
To bias the Asm shift row so that such clearances maximize the stackup results, click the double-headed arrow icon on the left side of the Asm shift row and change the designation from Float to Maximize in the drop-list. When Maximize or Minimize is selected, the row label changes to show the selected option. The tolerance displayed also changes to a single value in parentheses which indicates the amount of nominal bias. After you edit, the stackup nominal value changes to a larger value for Maximize and a smaller value for Minimize by the bias amount. If you change your mind and want to assume random placement, select the arrow and select Float in the drop-list.
The upper limit of Worst Case and RSS does not change when changing an Asm shift row from Float to Maximize because it assumes it is already in the maximum extreme position. The lower limit may change.
Likewise, the lower limit of Worst Case and RSS does not change when changing an Asm shift row from Float to Minimize because it assumes it is in the minimum extreme position. The upper limit may change.
- Offsets between parts: If you define offset distances between two parts in the stackup with the Add Offset command, the row is labeled Offset with a dark gray background. Select each object in the row to define a new nominal value, a new tolerance type, and a new tolerance. Right-click the row and select Delete to remove the offset.
- Results: The second to last row at the bottom of the Stackup Details table shows the results of the stackup analysis based on the defined Target Quality level. The results are presented in the same format used for the Objective definition. For example, ±, +/-, Limits, and so on. When Target Quality is set to use one of the statistical methods this row also includes the calculated quality of the stackup distribution compared to the defined Objective. For RSS, it is reported as Cpk. For general statistical analyses, Tolerance Analysis reports the Predicted Quality using the same metric type as defined for the Target Quality. To rename the stackup, select the name in this row.
- Objectives: The last row in the table shows the Objectives for the stackup along with the Target Quality. You can change both the Objectives and Target Quality on this row.