How to use 3D Best Fit
3D Best Fit alignment example in CAPPS software.
The 3D Bestfit alignment function is meant to be used for any type of part. The 3D best fit alignment requires both nominal and actual data to perform a basic fit alignment. A CAD model is required for a “Compare to CAD” type of 3D Best Fit alignment.
Basic fit alignment using nominal and actual data:
Create nominal data: The data should be created on surfaces that represent all three axes to better control the accuracy of the alignment
Measure actual data: Typically, the actual data is measured from the nominal data.
Open the 3D Best Fit alignment and select the features
Choose the Compare to Nominals option and click on Align. The RMS Error and MAX Error will be calculated and displayed in the dialog. Enter a label for the alignment if desired.
If the RMS error is too large, features with large deviations can be removed from the best fit by selecting them from the list window and using the Remove Selected option
Constraints can be applied to the overall best fit alignment in the Global Constraints section of the dialog. Constraints can also be applied to individual features, if desired.
Note: It is recommended to constrain only the axes related to the “check direction” of a feature (i.e. direction normal to a point or plane or the axes corresponding to the work plane of a circle). Also, when applying constraints to individual features, it is possible to “over constrain” the fit alignment by choosing to constrain to many features. An error message will appear if this is the case.
Example of the error produced by over constraining the part during a best fit alignment
In some cases, a “Fit to CAD” is desired. In this case, the original nominal features are removed, and the actuals are dropped onto the CAD model. An iterative fit is performed wherein CAPPSNC fits the actual data to the CAD repeatedly until the best possible fit is achieved. It is required that the actual data and the CAD data are within a reasonable level of alignment before this process can be applied.
Parameters required for this task
Choose the Compare to CAD Method
Choose the # of iterations
Set the Tolerance
Set any necessary constraints
Click the Iterate button to start the process
During this alignment task, CAPPSNC will perform a mathematical iterative fit process until the suggested tolerance has been met or the number of iterations has been performed. If the iterations reach a convergence point before the number of iterations have been performed the process is considered complete and the fit results will be displayed.
When proper best fit alignment has been generated. Click the Accept button. If the alignment should be recorded into the program, make sure the Add to DMIS check box has been checked