### Description of the Best Fit Alignment Dialog Box

Best Fit Alignment dialog box

The following items appear on the Best Fit Alignment dialog box:

Item | Description
Features list | The Features list contains all the features in your part program.
Input List box | The Input List box contains the features that will be used in the creation of the Best Fit alignment. It is also used to edit the features before creation of the alignment.
Edit Weight button | The Edit Weight button puts the current weight of the selected feature into the New Value box of the Weight Editor area, allowing the weight to be changed.
Create Weights button | The Create Weights button automatically creates weights for each feature in the Input List box.
Rotate About button | The Rotate About button takes the theoretical and measured values from the selected feature in the Input List box and places them in the Theoretical and Measured boxes in the Rotate About area. This value is used as the center of rotation during the calculation of the Best Fit alignment.
Weight Editor area | The Weight Editor area remains unavailable for use unless you select a feature from the Input List box and click Edit Weight. The new weight is entered into the New Value box. The Enter button applies the newly entered weight to the selected feature in the Input List box.
Rotate About area | The Rotate About area contains two boxes to define the theoretical and measured center of rotation: 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.
Orientation area | The 3 dimensional option causes the resulting alignment to be a 3D alignment. The 2 Dimensional option causes the resulting alignment to be a 2D alignment. The Active Plane list sets the plane in which a 2D alignment will be calculated. The Specified Constraints option lets 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 constrain the 3D or 2D alignment. When you select this check box, PC-DMIS will display the Select Axes to be Constrained area in the place of the Best Fit Method area.
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: Min/Max is only valid for 2D Best Fits.

The Rotate & Translate option allows maximum freedom in the calculation of the alignment, letting it rotate and translate freely.

The Rotate Only option limits the alignment to rotation only in its calculation.

The Translate Only option limits the alignment to translation only in its calculation.

Select Axes to be Constrained area | The Select Axes to be Constrained area lets you define the axes about which and along which the rotation and translation will be constrained. This area only appears if you select the Specified Constraints option from the Orientation area. The Rotate about X option constrains the alignment so that it does not rotate about the X axis.

The Rotate about Y option constrains the alignment so that it does not rotate about the Y axis.

The Rotate about Z option constrains the alignment so that it does not rotate about the Z axis.

The Translate along X option constrains the alignment so that it does not translate along the X axis.

The Translate along Y option constrains the alignment so that it does not translate along the Y axis.

The Translate along Z option constrains the alignment so that it does not translate along the Z axis.

For example, to rotate only about the Z axis and translate only in the X direction, you would need to select Rotate about X and Rotate about Y to constrain rotation to only the Z axis. Then you would need to select Translate along Y and Translate along Z to constrain translation to only the X direction.

Use Scaling check box | The Use Scaling check box becomes available for selection 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 that once scaling has been applied to the measured data and features in a part program, it cannot be undone.

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

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

Tol - This box allows you to enter a tolerance 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.