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Pro/Engineer   June 2001   Tip-of-the-Month

Datums "On the Fly" or as Discrete Features?

There are 2 ways to make datum planes:  Explicitly (Feature > Create > Datum > Plane > ...), and Implicitly (Make Datum -- aka "On the Fly").

When should datums be created on "On the Fly", and when should they be placed as Discrete Features?

The big difference between datums created explicitly and those made "On the Fly" is where the datum information resides and how accessible it is later.

June 2001, Fig 1 << Standard Datum
     Creation Menus
Menus for Creating 
      Datums "On the Fly"  >>
(Appears during 
feature creation) 
June 2001, Fig 2

When a Datum Plane is placed explicitly:

  • It is a single model feature.
  • It is visible on the screen (yellow/red rectangle) as well as in the model tree (by name).
  • It is given a name (like DTM5) that can be changed.
  • It has attributes that can be modified as needed.
  • Any dimensions are easily modifiable as part of the datum feature.
  • Most importantly, these datums can be referenced by future features.
When Datum Planes are created on the Fly:
  • Multiple datums can be created within the feature.
  • They are embedded within the feature they were created for.
  • They are not visible (or useful (mostly)) outside just the one feature.
  • They have a name (DTM?) but it can't be changed or seen outside that feature.
  • They have no modifiable attributes.
  • All dimensions for these Make Datums become part of the feature dimensions.
  • These datums cannot be referenced by future features.
One of the big benefits of the Make Datum ("On the Fly") functionality is the datum becomes part of the feature.  This makes finding dimensions easier.  For instance, if a protrusion is created using both an offset and angle datums, the use of Make Datum functionality will embed the 2 dimensions within the protrusion.  When the protrusion is modified, these 2 dimensions and all the feature dims will appear together.  Conversely, if the datums are created as discrete features, the offset dimension is with one DTM feature, the angle dimension is with another, and the protrusion dims are, of course, with the protrusion feature.  If the DTMs are not visible, the dimensions are harder to find -- especially if someone else created the model.  Datums on the fly also make patterning features much easier.

The advantage of using discrete datums is future features can also reference the datum.  If the model will have several features referencing a plane in space, the DTM should be created explicitly.  If a pattern will include several features built from the same plane (i.e. angular or ref patterns, etc.), a plane should be created as the first feature.

Another advantage of Explicit datums is the ability to reference and display them in a drawing.  This can be useful with dimensioning and GDT -- depending on your needs.  Datums created on the fly can't be referenced this way.

A couple disadvantages for Explicit datums are:

  • The datums will show (with a lot of them, the model can get cluttered).
  • Dimensions are found within the DTM feature so if datums are not visible (DTMs off, layers, etc.) control dimensions are harder to find.
The big disadvantage of datums on the fly is that other features can't reference the same datum.  If other features need the same reference, a plane must be created again, and the dimensions (if any) have to be tied together using relations or something.

The answer for when to use each type is fairly simple.  If other features need the same reference (location, patterns, GDT, etc.), or if the plane must be visible, make it explicit.  If the datum dimensions belong within the feature, make it on the fly.

** Another great use for datums on the fly is sketcher orientation.  Sometimes when features are created (especially in complicated geometry) it's hard to find a perpendicular reference.  This is a perfect time to use the Make Datum option and build a quick Datum on the fly.

** An example of creating multiple sequential Datums on the fly is shown in the steps below:

Step 1.  Start the protrusion by specifying appropriate attributes.

The steps taken are:
Feature > Create > Solid > Protrusion > Extrude > Solid > Done > One Side > Done.

June 2001, Fig 3
Step 2.  When the Pro/E asks for a sketching plane select  Setup New > Make Datum > Through > pick on the axis > Angle > pick on plane > Done > Enter Value > Enter a value (-20) > Done. June 2001, Fig 4
Step 3.  When the DIRECTION MENU appears with a request to select orientation, don't select the direction, instead, pick  Setup New (from the SETUP SK PLN menu as shown in the image to the right) > then Make Datum to create another. June 2001, Fig 5
Step 4.  Create another datum using Offset > Pick on the newly created datum (DTM1) > Enter Value > Enter a value (80) > Done. June 2001, Fig 6
Step 5.  Accept the orientation and continue with the feature creation.  When the feature is done, use Modify > Select the feature to display the dimensions.  Notice that dimensions appear for the feature as well as for both datums created on the fly.  This is a great way to place features just where you want them without a bunch of extra planes in the model. June 2001, Fig 7

Have a Great Datum Day!

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