Basic Setup and Creation of Tolerance Stackups
Autodesk recently released the Inventor Tolerance Analysis add-in which is available to Product Design and Manufacturing Collection subscription customers and a 30-day free trial is available to all users of either Inventor 2019.1 or 2019.2.
The purpose of this article is to give a functional overview of the setup for the creation of a tolerance stack up and the process of creating a stack up analysis.
One of the main considerations in preparation of using the tolerance analysis is the methods used to constrain the assembly model. In the help file a key statement is:
“Important: For automatic loop detection, the assembly must contain at least one joint or constraint path in the direction of the stackup, and every joint or constraint in the path must make a valid connection between adjoining parts.”
During the creation, a specific type constraint path must be available to use the Automatic Loop Detection method. This is the most desirable method because it only requires a one click selection. If this constraint path is not available, the stack up path can still be created but it is a manual process normally requiring two picks per component to create the constraint loop path used by the stackup.
The assembly constraints required to use the automatic loop detection are normally called the “real life assembly constraint” method. In other words, the assembly constraints must relate all of the components together not just between two adjacent components.
In the assembly pictured below, each component is related to the common shaft as well as each other using two or three “Mate” constraints. If the Insert constraint had been used, the relationship between two components planes could be established but may not allow the creation of the necessary relationship to the shaft in some cases thus preventing the use of the automatic loop detection routine and require the establishment of the loop path using the manual process.
The manual process is partially automated by the application and, except for numerous sequence picks, is quick and efficient. Below is a short video of this process on the same assembly constrained using the “Insert” assembly constraint. Assemblies created using “joints” will normally require the manual method as well since the common shaft is not a part of each component joint with the exception of the first one on the shaft shoulder end.
Based on our understanding of assembly constrains and their relationship to the loop path selection, we are now ready for a step by step creation of a tolerance analysis stackup.
With the assembly open, enter the “Tolerance Analysis” environment found under the “Environments” tab on the “Begin” panel.
This will evoke the Tolerance Analysis ribbon and results dialog box which can be moved to a separate monitor or closed and reopened later using the panel selection icon.
Before beginning your first experiment, click on the “Settings” icon to open the settings dialog box. Setting these variables will only normally only be required be set once for your tolerance analysis requirements.
The first section - “Default Tolerance” - can be configured for the units of the assembly and are ONLY used if your assembly components do not have applied size or linear MBD tolerance annotation. It should be noted that an applied parameter tolerance is not used by the application.
The “Default Target Quality (Analysis Type)” and the “Default Statistical Quality Metric and Value” setting are left at the default values for this article and are more than adequate for most tolerance analysis stackups.
The “Model Options” “Default Cp” is normally set to 1.0 for most tolerance analysis situations, at least as a beginning value. The “Annotation Scale” can be set to a value that makes the on-screen stackup dimensions easy to read. The “Dimension Annotation Color” and “Stackup Annotation Color” can be set to suit your individual desires using a standard full color palette. Select “OK” to save these settings.
The next step is to select the “New Stackup” command. This will open a mini-toolbar which will guide you through the stackup process. The mini-toolbar is intelligent and will automatically progress from top to bottom and also will only allow reasonable selections.
“Selection 1” and “Selection 2” define the two entities that set up the testing tolerance gap. (You can select several entity types - please consult the help file for a list of these). A common entity to use is component planes, or surfaces, which we will be using for this experiment. The tolerance gap is defined by selecting the component surfaces between the bolting washer and the end of the mounting shaft. In this example the “Width 1” and “Width 2” have been automatically determined and are set because surfaces were selected.
The next requested information is the “Annotation Plane” which is the assembly origin plane that the routine will place the stackup dimensions. This plane must be along the axis of the stackup and there are only two choices available. The application will not allow the selection of the origin plane that is ninety degrees to the stackup axis. In my example I have selected the XY Origin Plane from the Model Browser.
The next step is to place the “Dimension Location” using a left-click of the mouse at the desired location on the annotation plane. This dimension is in the “Stackup Annotation” color defined earlier in the Settings dialog and represent the tolerance gap we are analyzing.
After placing the dimension, the mini-toolbar will switch to the “Assembly Constraints/Joints” mode. If the assembly constraints are consistent with the automatic loop path detection, the “# path found” icon will appear where the “#” sign will be the total number of constrain loop paths available. In this example there is only one, and selecting the icon will reveal the loop path and highlight the model accordingly.
If your model does not contain “real life” assembly constraints, the mini-toolbar will request you to manually select the “Components” and the corresponding “Mating Faces/Points” to define the loop path. Please review the short video earlier in this article for this process.
If you desire the manual method of loop selection, you can select the drop-down arrow in the “Assembly Constraints/Joints” box and check the “Select” option.
Continuing with the automatic loop selection method by left clicking on the assembly constraint path, the stackup is configured by selecting the green check icon on the mini-toolbar.
The dimension annotation of the tolerance analysis assembly components is placed and the stackup values are listed in the Tolerance Analysis dialog box - note the symbols inside the red box. These denote that the nominal and tolerance values have dynamically been applied from the MBD annotation of the individual assembly components instead of the “Default Tolerance” value declared in the Setting dialog earlier. These MBD annotations are dynamic and will update automatically if they are modified in the individual component models.
One note here, assembly MBD annotation cannot be used by the application. Also, by left clicking on the symbols in the red box, you can suppress the connection to the MBD annotation of the individual assembly components.
The stackup is complete and ready for analysis which we will cover in another article but it should be noted that multiple tolerance analysis stackups can be created in the same assembly if needed.