Flatness
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.
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Two parallel planes are built bounding the extents of the data.
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These planes are allowed to freely rotate to establish the mini- mum 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.
In the above example is applied to 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.