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How 3D Best Fit works

How 3D Best Fit works

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.

 

General 3D Best Fit Dialog Information

 

 

Feature List: This window shows all the features selected for the 3D Best Fit. Each feature can be expanded to control how that feature should be used in the Best Fit alignment. Information in the columns are as follows

 

  • Error: This value differs depending on the feature type

    • Points: This value is the profile deviation of the point

    • Non-Point Features (Circle, Sphere, Slot, Ellipse, Etc): This value is the calculated position of the feature (NOT THE TRUE POSITION) and will be the same as the Dist value

 

EX: The X coordinate error (Nominal – Actual)

 

EX: The Y coordinate error (Nominal – Actual)

 

EX: The Z coordinate error (Nominal – Actual)

 

Profile: Profile deviation of the point. Error calculated along the vector of the feature using standard profile deviation formula

 

Dist: 3D distance error (Nominal – Actual)

Feature List window additional functions: Click on the + sign to the left of any feature to access additional best fit control functions.

 

 

Lock (X,Y,Z,I,J,K): The Lock function will apply the fit alignment in such a way that all error will be removed from the feature in the selected coordinates. This function is designed to be used only when a feature is designated to have zero deviation, not as part of a typical datum feature. If the lock function is applied on too many features, it will result in an over constrained situation and the fit alignment algorithm could fail to align the part.

 

  • Lock X: Lock the X Coordinate of the feature

  • Lock Y: Lock the Y Coordinate of the feature

  • Lock Z: Lock the Z Coordinate of the feature

  • Lock IJK: Lock the IJK of the feature

 

BF (X,Y,Z,Prof): The BF function controls which coordinates should be used when calculating the fit alignment. All three axis coordinates are automatically checked when a feature is added to the feature list window. If only certain coordinates should be used to calculate the fit, check or un-check the boxes accordingly.



 

Right Click menu functionality: The feature list window also has a right click menu. The right click menu gives the user access to functions that can be applied to single features or multiple features based on the user’s preference

 

Delete Selected: Delete the selected features

 

Delete All: Delete all features in the feature list window

 

Expand All: Expand the (+) menu for all features

 

Collapse All: Collapse the (+) menu for all features

 

  • Use in 3DBF

    • XYZ (# of Features Selected): Check the BF (X,Y,Z) for all selected features

    • Profile: Check the BF (Prof) for all selected features

    • X: Check only the BF (X) for all selected features

    • Y: Check only the BF (Y) for all selected features

    • Z: Check only the BF (Z) for all selected features

    • XY: Check only the BF (XY) for all selected features

    • XZ: Check only the BF (XZ) for all selected features

    • YZ: Check only the BF (YZ) for all selected features

    • None: Uncheck all BF (X,Y,Z,Prof) for all selected features

 



Example of using the BF (X,Y,Z,Prof) function to create a constrained fit alignment

 

 

  • P1 – P5 points are controlling the Z (Orientation and Location)

    • Fit will be applied such that Z error is minimized

  • C1 circle is controlling the X&Y location

    • Fit will be applied such that X&Y error is minimized

  • C2 circle is controlling the X location and therefore also controlling the rotation about the Z axis for orientation

    • Fit will be applied such that X error is minimized



 

3D Best Fit Settings

 

# Elements: Number of features selected for the Best Fit

 

RMS Error: Calculated error of the Best Fit using a Root Mean Square calculation after fit is applied

 

MAX Error: Largest error present in the data after the fit is applied

 

# of Iterations: Only active when the Best Fit Method is set to Use CAD. When using CAD, the actual data is compared directly to the CAD model to obtain the smallest RMS value possible. This number determines how many times fit is iterated to reduce this RMS error.

 

Tolerance: Only active when the Best Fit Method is set to Use CAD. This tolerance is used to determine when the RMS value is acceptable.

 

Use Stock: This is an offset value that should be applied to the fit calculation. This is used if the part has additional material and has not yet been machined to the finish size. This value will not be seen as error in the part

 

Note about # of iterations and Tolerance: These two parameters are used to determine when an acceptable RMS error has been achieved. However, if the fit calculation reaches a convergence point (i.e. RMS value is no longer decreasing) the fit calculation will be stopped even if one or both parameters has not been met

 

Label: Label for the fit alignment which will designate it in the datums list

 

Minimize Points Profile: Check this box of the fit should be calculated based on the profile deviation of the points instead of the coordinate deviations

 

Use CAD Selection: Check this box if only selected CAD entities should be used when the Best Fit Method if set to Use CAD

 

Un-Comp Points First: Check this box if the fit should not take probe compensation into consideration when calculating the fit (I.e. a complex surface where the measurement vector of the points may not be correct due to poor part location)

 

Global Constraints: General constraints for the fit alignment

 

After a fit is calculated, the amount of translation for each axis and the rotation about each axis will be displayed in this area

 

If a degree of freedom should not be used in the fit, check the box corresponding to that degree of freedom [translation (X,Y,Z)] or [rotation (A,B,C)] Example: If the fit should not apply a rotation about the Z during calculation, the ‘C’ check box should be checked.

 

If known, the user can enter translation or rotation values into this area and apply those values to the fit. Click on the User Apply button to perform this task

 

 

 

Error distribution graph

 

Shows error before and after fit has been applied

 

 

 

 

Hover the mouse pointer over a point to see the profile deviation of the point

 

 

 

Double Click on a point to access additional functionality

 

  • Delete (Label): Delete point

  • Go to (Label): Expand point in the Feature List Window

  • Zoom to (Label): Zoom to that feature in the graphics

  • Use in 3DBF: Add feature to the fit if not currently used. Options for addition are the same as the right click menu in the Feature List Window



 

More 3D Best Fit Settings

 

Best Fit Method

 

  • Use Nominals: A defined set of nominal features and a measured set actual features are used to calculate the fit alignment. The labels of the features must match for this case

  • Use CAD: A measured set of actual features are compared to a CAD surface model. The fit alignment is created from the best set of nominal data extracted from the CAD model



 

Reporting options: Different options available to output features selected in the Best Fit window after fit has been applied

 

  • No Reporting: Do not report features after alignment

  • Add To Report: Add features to the end of the report

  • Update Report: Update items in the report with new, post-fit values

  • Merge: Update the existing items and add any not currently in the report

  • Update Graphics: Update confetti and graphical report items after the fit

  • Report MIN&MAX Profile In All: Shows the MIN & MAX profile error from all features

  • Template MIN&MAX Profile In All: Show templates for MIN & MAX profile error in all features

  • Report MIN&MAX Profile In Each: Reports profile deviation of all points. Including the MIN & MAX points of geometric features (i.e. MIN&MAX profile deviation of points in a circle)

  • Template MIN&MAX Profile In Each: Reports profile deviation MIN&MAX of points of geometric features (i.e. MIN&MAX profile deviation of points in a circle). Also makes templates for points

 

Select Features: Select features to add to 3d Best Fit window

 

Remove selected: Remove selected features from the Feature List Window

 

Reset: Used to reset fit that has been calculated but not yet accepted

 

Align/Iterate: This button changes depending on which Best Fit Method is selected

 

  • Use Nominals (Align): Click this button to calculate the fit between Nominal and Actual features

  • Use CAD (Iterate): Apply the measured actual features to the CAD model and calculate the fit

 

Accept: Click this button to accept the fit alignment

 

CLOSE: Close the 3D Best Fit alignment. The fit will not be applied unless the Accept button has been pressed