## Creating Customized Datum Reference Frames

PC-DMIS supports the ability to create customized datum references frames (DRFs) for FCF tolerances (for Position, Profile with datums, Symmetry, Coaxiality, Concentricity, and Runout dimensions) as defined in section 4.22 of the Y14.5 2009 GD&T standard. The standard states that if using a customized DRF then the datum features and modifiers listed therein must be followed by lowercase letters inside square brackets. These lowercase letters (x,y,z and u,v,w) represent the designated degrees of freedom to be constrained.

This diagram shows the six degrees of freedom in 3D space (x,y,z,u,v, and w)

Mark the Customized DRF check box on the Feature Control Frame tab of the GD&T dialog box to enable the ability to select degrees of freedom to constrain from the Feature Control Frame editor. (If you do not mark this check box, PC-DMIS will instead assign the degrees of freedom based on the "Y14.5.1M-1994 Mathematical Definition of Dimensioning and Tolerancing Principles" standard.)

Next, in the DRF, after you have specified a datum (and, if appropriate, any modifiers), you can then access a drop-down list with the degrees of freedom you are able to constrain shown as the lower-case letters depicted in the above diagram:

Feature Control Frame Editor Showing Available Degrees of Freedom to Constrain

Finally, mark the check box next to the degree(s) of freedom you want to constrain, and then press TAB or click outside of the drop-down list. The FCF preview will show the customized datum reference frame within square brackets:

Preview Area Showing the Degrees of Freedom Constrained

You can select any degree to constrain for a datum that has not already been specified for a datum of higher precedence. For example, if your secondary datum constrains the x degree of freedom, then x will not be available to the tertiary datum.