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.