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## Defining Datums

A datum is a theoretically perfect plane, point, or axis reference feature. A dimensional measurement is made from a datum. Datums are used by Feature Control Frames (FCFs) when displaying GD&T information. In order to define the datums used in FCFs, you should first create the features that will become the datums. These features can come from auto features, measured features, or constructed features. Once you create the features, use Datum Definition (DATDEF) commands to define them as datums. Each DATDEF command associates a datum letter (for example, datum A) with a feature in your program.

Creating a Datum Definition

To define a datum, select Insert | Dimension | Datum Definition
or click the Datum Definitions button on the GD&T dialog box. PC-DMIS will display the Datum Definition dialog box.

Datum Definition dialog box

The datum letter (in the above example "A"), defaults to the next available datum letter. PC-DMIS labels datums from "A" to "Z" and then from "AA" to "ZZ". Simply select a feature to be associated with this datum letter from the List of Features, and click Create. The Create button becomes activated whenever you select a feature from the list. When you create a definition, PC-DMIS inserts a DATDEF command into the Edit window. For example, if you had selected PLN1 and associated it with the letter "A", the Edit window command would read:

DATDEF/FEATURE=PLN1,DATUM=A

In the Datum Definition dialog box, the Datum box automatically updates to the next available datum letter, and the feature you selected will appear in the List of Features with the associated datum letter in parentheses. From the above PLN1 example, it would now appear in the list as PLN1(A).

To create additional DATDEF commands, continue to select features and click Create. If you don't want to create your datums in the default alphabetical sequence, you can easily modify the letter in the Datum box before clicking Create.

Once you have created all your datums using the Datum Definition dialog box, you can then create Feature Control Frame (FCF) dimensions using the GD&T dialog box and assign your defined datum features to those dimensions.

Bonus Refinement

When calculating the bonus on a datum, if there is a previous Position dimension or orientation dimension on that datum feature, then the Position and/or orientation tolerance is added to the datum bonus to calculate the virtual size of the datum.

Specifying Compound Datums in the FCF

A drawing's callout will show a compound datum using the format A-B (where the datums are labeled A and B). This indicates that the specified datums should be used together as if they were a single datum.

Note: Compound datums only function with a Position FCF dimension or Profile FCF dimension.

To specify compound datums:

1. Create the individual datums. See "Defining Datums" for information on how to do this.

2. Select the menu item or toolbar icon to create a Position or Profile FCF. The GD&T dialog box appears.

3. Specify the compound datum in the <dat> field of the Feature Control Frame Editor area. While the drop down list only shows individual datums, you can, however, type the required datum combination directly. Do this by selecting or typing the first datum's ID, type a dash, and then type the ID for the last datum in the combination. If your compound datum used datums A and B, it would look like this:

4. Select the feature (or features) for your FCF.

5. Fill out anything else needed for your FCF.

6. Click Create.

How Bonuses are Applied for Compound Datums at MMC

Suppose you define a Position FCF for a hole using a compound datum D-E at MMC like this:

Datum A is a surface.
Datum D is a hole.
Datum E is a another hole.

PC-DMIS will calculate the bonus for each of the datums in the compound datum based on the measured size of each datum as well as the plus and minus tolerances for the D2 axis (secondary datum) as specified in the Advanced tab of the dialog box:

Advanced tab showing D2 Axis with +Tol and -Tol Values

The bonus for datum D may be different than the bonus for datum E based on the measured sizes of the two holes. The fitting process will take this into consideration.