Datum Principles

Datum Elements and Datum Systems
Datum elements and datum systems are only specified in case of geometrical tolerancing (not on dimensional tolerancing and except on form tolerancing). You can specify simple datum elements, common datum elements, datum targets or specified datum systems.

The toleranced feature is positioned relatively with the tolerance zone, and this set is positioned relatively with the datum or the datum system. These related positions will be specified using basic dimensions and they are displayed or not.

For more information about datum constitution and specification, see ISO 5459, ASME Y15.5M and ISO 1101.

Datum System Composition
When the identifiers are specified separately in each frame of the tolerance frame, the datum elements represent a datum system. A hierarchy is established between the datum elements. Reference A is the primary datum and reference B is the secondary datum. The datum system fitting would be performed first on datum A, then on datum B, with respect to A.  By the way, a tertiary datum can also be specified.
  The datum elements have to be fitted successively (following their specification order) with respect to the hierarchy for the measurement computations.
When only one identifier is specified in the tolerance frame, the datum is a single datum.
When two identifiers separated by a dash are specified in the tolerance frame, the datum is a common datum. The two datum elements are to be considered simultaneously. The datum system fitting would be performed in the meantime on datum A and on datum B (both datum elements have to be fitted simultaneously for measurement computations).

Datum writing rules
A capital letter is used to identify the datum element in the tolerance frame. The datum triangle may be filled or non-filled.

When the datum triangle is placed on the outline of the element or on its extension line, the datum element represents the surface itself or the 2D representation of the surface, which is a line.

When the datum triangle is placed in the alignment of the dimension line, the datum element represents the median element (usually an axis or a median plane).

When the datum triangle points directly on a median element, the datum element represents either the median element itself (usually an axis or a median plane) or the resulting median element of the collection of the considered elements.
 

Datum Creation
  Apart from creating datum from features, you can create datum from patters of features.

When a datum is created from a pattern of features, a label Nx appears to right side of the datum, where N is the number of features the pattern is made of. The label:
  • Can be moved
  • Can be deleted
  • Reappears and updates when the datum is modified and updated
  • Is invalid if the number of instances in the pattern is changed
  • is removed when the standard of a feature is changed from ISO to ASME using the feature properties

Projected Extension Datum Modifier


The Projected extension area is available in the Datum feature dialog box when the datum feature is any of the following:
  • Thread surface
  • Extended cylinder construction geometry
  • Pattern of threads
  • Pattern of cylindrical holes or pins

At a given time, only one of Projected extension option or Contacting Feature [CF] option is available for selection. Click any of the option to enable it. The other option is automatically disabled. Clicking the selected option again clears the selection and makes both the options available for selection.

Note: The selection field in the Projected extension area is available for creation or edition of extended cylinders. If the selected datum feature is a pattern:
  • The elements listed in the selection field correspond to the instances of the pattern in the same order.
  • You can click Create/Edit to create or edit each extended cylinder separately. You can only create or modify the parameters of the extended cylinder using E=Extended Cylinder Construction Geometry dialog box. See Creating Extended Cylinder Construction Geometry.
  • The length and offset are never available for selection in the Projected extension area. You can choose to view their value in the DRF.
  • An error message appears if any of the listed extended cylinders is invalid.

If the selected datum feature is not a pattern:

  • The selection field is unavailable for selection.
  • The associated construction geometry is listed, if already available. You can click Create/Edit to create or edit one.
  • The length and offset are never available for selection in the Projected extension area. You can choose to view their value.

You can select the Show length and offset in DRF option to view length and offset in all the impacted DRFs and geometrical tolerances. The modification is enabled in the work area and in the tree. For example, if the length of the extended cylinder is 5mm and that of the offset is 6mm, the label is modified as (P)11-6, where 11 is the sum of the length and the offset, and 6 is the offset. Ensure that the datum with projected extension is always secondary or tertiary datum.

The annotation for projected extension datum modifier is invalid when the standard of a feature is changed from ISO to ASME using the feature properties.

Theoretically Exact Size

The Theoretically exact size option is selected, by default, at the time of feature creation in the following cases:

  • Cones
  • Angular tabs and slots
  • Patterns of angular tabs and slots

The Theoretically exact size option is not selected, by default, at the time of feature creation in the following cases:

  • Spheres
  • Cylindrical pins and holes
  • Tabs and slots
  • Rectangular or square pockets, pins, and holes
  • Patterns of spheres, cylindrical pins and holes, tabs, and slots

At a given time, only one of Theoretically exact size option or Contacting Feature [CF] option is available for selection.

When the option is selected, a framed basic dimension is created and associated to the datum feature. Clearing the option, deletes the framed dimension.

In case of a torus, you can choose the following options:
  • Theoretically exact small-diameter size
    Theoretically exact high-diameter size

The annotation for theoretically exact size is invalid when the standard of a feature is changed from ISO to ASME using the feature properties.
 

Datum and Datum Target Leaders
 

Extremity of Datum Target Leaders


A datum target leader can have different types of extremities. The extremity of a leader is decided by the selected datum target feature.


Selected datum feature Leader extremity Illustration
Point or vertex Cross
Curve or wireframe plane Open arrow
All other cases Filled circle

  • You can right-click a datum target in the tree and change the associated geometry of the datum feature using Geometry Connection Management. Doing so does not change the type of leader extremity. You can change the extremity manually.
  • A datum target leader extremity is retained when the standard of a feature is changed from ISO to ASME using the feature properties. This is also true when the standard is changed from ASME to ISO.

Datum Target Indicator for Datums


For datums established using datum targets, an indicating label appears to the right side of the datum.

The label:
  • Can be moved
  • Can be deleted manually
  • Reappears and updates when the datum or the datum targets are updated
  • is removed when the standard of a feature is changed from ISO to ASME using the feature properties