Rules for Using Feature Control Frame Dimensions

To properly use the GD&T / Feature Control Frame (FCF) tools provided in PC-DMIS, follow these rules:

• For best results, use CAD data.  

  Explanation:

  While you can technically use these tools without CAD, it may require additional editing of commands to get them to function properly. Dimensions that use datums require correct nominal (THEO) values. When you do not create features from CAD data, you may have to edit the nominal (THEO) values of features to ensure that they are correct.

  This rule also applies to legacy dimensions.

• Use 3D features with vectors for primary datums.

  Explanation:

  When creating an FCF dimension, you should not reference a 2D feature as a primary datum because it does not have vector information.

  When you use a circle as a primary datum, the created datum reference frame alignment will not level to the circle. PC-DMIS uses the circle only to position the Datum Reference Frame (DRF) alignment origin (X, Y). The DRF alignment level in this case is derived from the current active alignment.

  However, if you want the DRF alignment to level to a primary datum circle (for example, the circle is an Auto Circle measured with sample points), you can use the PC-DMIS Settings Editor to set the DatumLevelToCircle parameter to 1 (by default, this is 0).

  This also applies to legacy dimensions.

• DO NOT use the old style PC-DMIS text report with FCF dimensions.

  Explanation:

  While this report functions well with Legacy dimensions, FCF dimensions will experience some data loss in this reporting mode.

  To ensure that you display the report properly:

  1. Right-click on any white space of your report.

  2. Select Edit Object.

  3. In the Report dialog box, clear the Use Text Mode Dimension Reporting check box.

• Use Min/Max reporting for Profile FCF dimensions.

  Explanation:

  Ensure that you are looking at the Min/Max value for Profile dimensions, and not the Measured value.

  To turn on Min/Max reporting:

  1. Press F10 to access the Parameters dialog box.

  2. Select the Dimension tab, and then select the Min/Max check box.

• Use correct nominal X,Y,Z,I,J,K data for Deviation Perp to Centerline functionality.

  Explanation:

  Nominal data is needed for Deviation Perp to Centerline functionality to work properly. If you are using CAD, this happens automatically. If not, you will need to type in the needed nominal values into the THEO field.

  FCF Position dimensions always use the Deviation Perp to Centerline functionality and with the Axis set to WORST; you cannot modify this.

General Rules for Position FCF Dimensions

• Select all the specified datums so that proper fitting is performed.

  Explanation:

  The features selected for datum 1, datum 2, and datum 3 represent the primary, secondary and tertiary datums and are used to constrain up to six degrees of freedom (3 degrees of translation and 3 degrees of rotation).

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

  

  • The primary datum constrains as many of the six degrees of freedom as the primary datum feature allows.

  • The secondary datum constrains as many of the remaining degrees of freedom as the secondary datum feature allows.

  • The tertiary datum constrains whatever degrees of freedom remain. 

  Therefore, the order of precedence of the datums is essential. However, to override precedence you can create a customized datum reference frame in the Y14.5 2009 GD&T Standard. See "Creating a Customized Datum Reference Frame".

  Consider these examples:

  Example 1: Suppose for your datums you have a primary plane, a secondary line, and a tertiary circle (assume that the line and circle lie in the plane). The plane constrains rotation about X and Y and translation in Z. The line, constrains rotation about Z and translation in Y. The circle constrains only translation in X.

  Example 2: Now, suppose for your datums you have a primary plane, secondary circle, and tertiary line (again, assume that the circle and line lie in the plane). The plane still constrains rotation about X and Y and translation in Z. The circle now constrains translation in X and Y. The line constrains only rotation about Z.

• Ensure that feature commands used for the datums and the dimension contain correct nominal values.

  Explanation:

  These commands must contain the correct nominal values (X,Y,Z,I,J,K) in the THEO field. For proper fitting to occur, PC-DMIS references these commands to calculate datum constraints and the dimensional results.

• Ensure that the measured feature command and the associated Position dimension come from the same alignment.

  Explanation:

  This ensures that the nominal values are correct and the same as the basic dimension callouts from the drawing. When using CAD, the nominal values are automatically calculated. Without CAD, you will need to edit all the measured feature commands with their correct nominal values.