### Description of the Best Fit Alignment Dialog Box

Best Fit Alignment Dialog Box Features

Best Fit Alignment dialog box

The following features appear on all Best Fit Alignment tab screens:

Item | Description
Available features list area | The Available features list contains a listing of all the features in your part program before the alignment. For a full description, see Dialog Box Description | Feature List box.
Alignment features selected area | The Alignment features selected area lists the features that have been selected for use in the best fit calculation from the Alignment features list area (shown above). Each column can be sorted by clicking on the column header. The check box to the left of each Feature ID is used to enable or disable the feature from being used in the best fit calculation. If checked, the feature will be used in the calculation, otherwise it isn't.

Note: If the Pause during execution setting is checked, the alignment will be automatically reset to include all features in the best fit calculation during program execution.

If a deviation is calculated to be above the Deviation threshold value, the text for that feature will be displayed in red.

Axes can be included in the calculation or excluded by double-clicking the specific Include XYZ item listing for the feature to display the Include XYZ dialog (shown below). Checking an axis will include the axis into the calculation while un-checking an axis will exclude it.

Deviation threshold field | The Deviation threshold is an editable field used to set the maximum acceptable value for the calculated deviation value. If any of the calculated deviations are above this threshold value, the text for that feature will be displayed in red in the Alignment features selected area.

The units will be the same as what is set in the System Settings.

RMS Error field | This is a non-editable field that displays the RMS Error value from the most recent alignment calculation.
Warnings area | The Warnings area displays any error messages from the most recent alignment calculation.
Compute button | The Compute button is used to update the best fit alignment calculation using the current features and settings.

Best Fit Alignment dialog box - Setup tab

Best Fit Alignment dialog box - Setup tab

The Setup tab contains the most common settings for defining a best fit alignment.

The following are features specific to the Best Fit Alignment dialog box Setup tab:

Item | Description
DOF (Degree Of Freedom) area | Available options: The 3D option causes the resulting alignment to be a 3D alignment that is both Translated and Rotated. The 3D No Translation option causes the resulting alignment to be a 3D alignment that is Rotated but not Translated. The 3D No Rotation option causes the resulting alignment to be a 3D alignment that is Translated but not Rotated.

The 2D option causes the resulting alignment to be a 2D alignment that is both Translated and Rotated in the selected 2D plane.

The 2D No Translation option causes the resulting alignment to be a 2D alignment that is Rotated but not Translated in the selected 2D plane.

The 2D No Rotation option causes the resulting alignment to be a 2D alignment that is Translated but not Rotated in the selected 2D plane.

The 2D Plane drop down list sets the plane in which a 2D alignment will be calculated.

The Tx, Ty, Tz, Rx, Ry, and Rz check-boxes let you select which of the six degrees of freedom (rotation about X, Y, or Z axis, and translation in the X, Y, or Z direction) will be included in the alignment. As the degrees of freedom are modified, the Type (2D, 3D, 3D No Rotation, etc) will automatically update according to the current settings. If the current combination of degrees of freedom does not match any of the pre-defined cases, the type will become User Defined. Conversely, as the Type is changed, the check-boxes will be updated to match.

Note: If the Type is set to User Defined, PC-DMIS will automatically select the Least squares option in the Best Fit Method area. The 2D Plane drop down list and the Rotate About options (Advanced tab) will also be disabled.

The 2D Plane drop down is available only if one of the 2D options is selected from the Type drop down listing. Available options are shown in the image on the left.

The coordinate image will dynamically update based on the vector or coordinate check boxes that are checked.

Best Fit Method area | The Best Fit Method area contains several methods you can use to compute the best fit alignment: The Least squares option causes the average squared error for the input features to be minimized in the alignment. The Vector option snaps the errors in the input features onto the theoretical vectors before minimizing the average squared error.

The Min/Max option attempts to orient the part so that PC-DMIS minimizes the maximum error in all of the input features.

Note: The Min/Max option is only available if a 2D Best Fit option is selected from the Type drop down list.

Iterate and repierce CAD area | This area lets you perform an Iterative Best Fit alignment. Iterate and repierce CAD - If selected, PC-DMIS performs an Iterative Best Fit alignment which pierces the CAD geometry and adjusts the feature nominals with each iteration using Tolerance and Max iterations to control the result. If not selected, PC-DMIS does a single Best Fit alignment.

Tolerance - This box allows you to enter a tolerance value in mm which PC-DMIS uses in searching for CAD surfaces to pierce. The new nominal point will be the CAD point closest to the actual feature, provided it is within this tolerance. If no CAD surface is found within this distance from the actual feature, then that feature is ignored in subsequent iterations.

Max iterations - This determines the maximum number of times the best fit alignment algorithm will iterate.

If the re-pierce operation fails for all of the features included in the Best Fit alignment, PC-DMIS will display an error message in the Warnings section in the lower-right and abort the alignment calculation.

Best Fit Alignment dialog box - Advanced tab

Best Fit Alignment dialog box - Advanced tab

The Advanced tab contains settings that are specialized for several less-common conditions.

The following are features specific to the Best Fit Alignment dialog box Advanced tab:

Item | Description
Rotate About area | The Rotate About area contains two boxes used to define the theoretical and measured center of rotation. You can use the Set button to automatically populate these fields with X,Y,Z values from the feature selected in the Alignment features list, or you can manually type in your own X,Y,Z values:

The Theoretical box contains the theoretical center of rotation for 3D best fit alignments.

This Measured box contains the measured center of rotation for 3D best fit alignments.

The Clear button will clear both fields when clicked.

Use scaling check box | The Use scaling check box becomes available for 2D or 3D alignments when you select the Least squares method. It is not available for alignments with specified constraints. When you use scaling, PC-DMIS computes a transformation (rotation and translation) and a scale factor that optimally matches the nominal data to the scaled measured data.

The scaled alignment also scales all of the measured data and subsequent measured features in the part program, multiplying it by the computed scale factor.

Note: Once scaling has been applied to the measured data and features in a part program, it cannot be undone. Also note that Use scaling should only be turned on in a single alignment in the part program.

You may find this useful, for example, in compensating for expansion or contraction of a part due to temperature.

Create weights check box | When checked, the Create weights scales the values in the weights column located in the Alignment features section of this dialog, by the tolerance values of the dimensions that control each feature. If the Create weights check box is unchecked, the dimension tolerances are not factored into the calculation, just the weights defined by the user.
Pause during execution check box | When checked, the Pause during execution setting will cause the part program to pause execution and display the Best Fit Alignment dialog. This allows the user to examine the magnitude of the error in the alignment solution, identify any features or scan hits that are bad, eliminate them from the alignment, recalculate, and then repeat that process until they are satisfied with the alignment. When the OK button is clicked on, program execution resumes.

Note: This option is only available for Portable-type machines. It is hidden for all other machine types, and ignored during program execution.

Best Fit Alignment dialog box - Transforms tab

Best Fit Alignment dialog box - Transforms tab

The Transforms tab displays the alignment transforms from the most recent best fit alignment calculations. These transforms represent the cumulative state of the program up to and including this best fit alignment, and not the transforms of just this alignment.

The following sections are specific to the Transforms tab of the Best Fit Alignment dialog box:

Item | Description
Machine to Part area | Displays the calculated Machine to Part transform values.
CAD to Part area | Displays the calculated CAD to Part transform values.
Machine to CAD area | Displays the calculated Machine to CAD transform values.