## Information on FCF Dimension Calculations

PC-DMIS and ASME Y14.5M-1994

- PC-DMIS GD&T follows ASME (ANSI) Y14.5M-1994. The mathematics for this standard are described in "ASME Y14.5.1M-1994 Mathematical Definition of Dimensioning and Tolerancing Principles". This standard is similar to ISO 1101. The main difference is that Y14.5 requires True Position datum fitting to find the candidate datum reference frame that minimizes the deviation of the considered feature. PC-DMIS allows the user to turn this off with the Fit to Datums check box.

Profile Calculations

- In v4.2 and higher, the registry setting UseISOCalculations, found in the Options section of the PC-DMIS Settings Editor, can be set to 1 to report profile as two times the maximum deviation. This setting only affects Profile (not Flatness). Also, when the GD&T Standard on the Advanced tab of the GD&T dialog is set to ISO 1101, the Profile reports two times the max deviation and formonly Profile ignores size.

- In PC-DMIS 2009 and higher, FCF Profile with formonly takes into account size when UseISOCalculations is set to 0 (ASME) or the GD&T Standard selected on the Advanced tab of the GD&T dialog box is ASME Y14.5. This is also true for legacy Profile formonly. A new registry entry called UseSizeForProfileDimensions in the Option section of the Settings Editor, lets you revert to the V42 and earlier behavior for legacy profile dimensions only. This registry entry defaults to 1 (TRUE), so you must use the PC-DMIS Settings Editor to change it to 0 (False). If set to 0, legacy Profile formonly will ignore size. However, the FCF profile will always take size into consideration even when this entry is set to 0 if the GD&T Standard is set to ASME Y14.5.

Some Calculation Differences

- Legacy dimensions for roundness such as a Location dimension's RN line or a legacy Circularity dimension, are computed using the Least Square solution. On the other hand, FCF dimensions for roundness (Circularity and Cylindricity) in version 4.2 and higher are computed using the Tchebychev algorithm (min/max) as required by the Y14.5 standard. Because of the change in calculation, Circularity and Cylindricity FCF dimensions will generally compute to a slightly smaller value than their legacy counterparts.

Form Calculations

- FCF dimensions in PC-DMIS 4.2 and later support the Y14.5 definitions of Circularity and Cylindricity.

- FCF dimensions in PC-DMIS 4.3 and later support the Y14.5 definitions of Flatness. FCF flatness uses a Tchebychev (min/max) algorithm. Legacy flatness still uses the LS method. FCF flatness generally gives a somewhat smaller flatness value than the LS flatness. This is independent of the UseISOCalculations registry setting.

- FCF dimensions in PC-DMIS 2009 and later support the Y14.5 definition of Straightness when using FCF dimensions.

- Legacy form dimensions still calculate the least squares form.

A Note on Parallelism Evaluation

- The evaluation of parallelism is 3-dimensional regardless of the workplane or the feature being dimensioned.