Form Checks

Circularity*

Circularity is evaluated in cross sections along the normal vector of the nominal feature (circle or cylinder). The tolerance establishes the maximum distance allowable between two concentric circles containing the measurements of that part at the cross section location.

Evaluation Process within SA:

  1. Cross sections are established along the normal vector of the nominal feature and the data is divided into groups based upon their position along this vector.

  2. A plane is fit to the first point in each cross section group (using the actual feature direction vector) and then the points are projected onto that plane.

  3. The minimum size bounding circles are then established on the plane containing the data while holding a common center point and the distance between these is used to evaluate the cross section.

  4. The check reports the worst of the cross section results.

 

Cylindricity

Cylindricity checks ensure that all the specified points are the same distance from a common axis. The tolerance establishes the maximum allowable distance between two concentric cylinders containing the measurements of that part.

Evaluation Process within SA:

  1. Two coaxial cylinders are built bounding the extents of the data.

  2. The cylinders are allowed to freely rotate to establish the minimum distance between them while still containing the data.

  3. The final distance between these cylinders is compared to the tolerance for evaluation.

 

Straightness*

Straightness is evaluated in cross sections. It ensure that all the measured points are within a specified proximity to a straight line. This can be evaluated for lines directly or cylinders (axis), which define a zone where the median line must lie.

Evaluation Process within SA:

  1. The direction vector is established for the measured feature (line or cylinder).

  2. Cross sections are established along this direction vector and the data is divided into groups based upon their position along this vector.

  3. A plane is fit to the first point in each cross section group (using the actual feature direction vector) and then the points are projected onto that plane.

  4. Median points are established for each cross section, either directly (for lines) or by fitting circles to the data (cylinder).

  5. These median points are then compared to the nominal axis line and the check reports the worst of the cross section results.

 

Per Unit Length Straightness

Straightness can also be evaluated on a per unit length basis. This setting ensures that the specified straightness is held within an acceptable distance but allows a gradual curve to occur on a large part. When selected the straightness tolerance will be evaluated only within the specified length and then a new segment will be evaluated in sequence down the part. The Per Unit Length Step Over Percentage setting controls how much overlap is used in adjacent evaluations.

 

 

Straightness can also be evaluated using a per unit length setting. Typically all the measurements are used within a common tolerance, but with this setting enabled a tolerance can be applied within a specified length. To enable Per Unit Length open the annotation properties and Check the option. You will need to specify the Per Unit Length Distance, which defines the length used for the evaluation and the Per Unit Length Step Over Percent which determines the extent of overlap in the analysis process.

 

Flatness

Flatness checks ensure that all the specified points lie approximately on a common plane. The tolerance establishes the maximum allowable distance between two parallel planes bounding the associated measurements of the part.

Evaluation Process within SA:

  1. Two parallel planes are built bounding the extents of the data.

  2. These planes are allowed to freely rotate to establish the minimum distance between them while still containing the data and the resulting distance is compared to the tolerance.

Per Unit Area Flatness

Flatness can also be evaluated on a per unit area basis. This setting ensures that the specified flatness is held within an acceptable distance but allows a gradual curve to occur on a large part. To enable this mode, check the Per Unit Area option under the tolerance and additional options will appear.

A separate length and width area designation can be used to apply the tolerance in a larger or smaller area along the two directions. The length direction is defined using the Z-axis of the selected SA Object.

The above example applies the tolerance in a 5 inch area along the length and a 2 inch area along the width assuming the working frame was selected as the direction object.

In the summary data, the origin for each of the zones is NOT the planar zone origin, it is the centroid of the points that were used for testing in the given zone.