About Xref's

As an alternative to inserting a drawing into another drawing as a block, AutoCAD now provides the facility to attach a drawing to the current drawing as an external reference. The external drawing is then treated like a block except that it is automatically updated each time the drawing to which it is attached is loaded. Naturally, if the external drawing is altered in the meantime, then the external reference will be updated to reflect those changes.

Another important feature of a block (including external reference blocks) is that it can be duplicated many times within a drawing, with each copy pointing to the one graphic description, but having its own unique position, scale and rotation factor. This is known as instancing. There are two advantages to this approach:

• space saving - the graphics description is only stored once;
• ease of re-definition - the graphics description of a block can be altered and then re-defined in order to update each instance in the drawing.

The Z axis of the World Co ordinate system (WCS) of the incoming drawing aligns with the local Z axis (current UCS).

CAD Models are 3D drawing files that will be XREFed into a CAD drawing file, and may be an assembly, sub assembly or a component.

It is suggested that the CAD Model drawings have their own sub directory, and that the final assembly has the same name as the sub directory with the prefix A-

Each component or sub assembly CAD Model may be drawn with its BASE or Datum point on or off the component a different prefix will be required for each case:-

         CAD Model  Component  datum on  component    M-
         
         CAD Model  Component  datum off component    M~
         
         CAD Model  Sub Assy  datum on  component     S-

         CAD Model  Sub Assy  datum off component     S~

         CAD Models should have meaningful names not numerical codes, 
         difficult with the 8 character limit that DOS has.


         Example 

         DOS sub dir  BREAD
         
         A-BREAD
                <<<   S-SHELF 1 OFF
                              <<< M-SHELF  1 OFF
                              <<< M-SUPORT 1 OFF

                <<<   S-SLATS 1 OFF 
                              <<< M-SLAT 17 OFF

                <<<     M-BACK     1 OFF
                <<<     M-BASE     1 OFF
                <<<     M-FRONT    1 OFF
                <<<     M-HBACK    1 OFF
                <<<     M-SIDE     2 OFF
                <<<     M-TOP      2 OFF
         

The local UCS must be set to receive the XREF beforehand.

'Datum off Component' should not mean a random datum point but the PROJECTS datum point. For example the roads, drainage and topography of a built up area may be drawn as separate drawing files with a common datum.

Because this type of CAD Model is more project specific than the 'Datum on Component' type the drawing plane will be very often in the correct mode for insertion. If this is so then set to WCS before XREFing.

In both cases the CAD Model register must state datum points and drawing plane alignment! An AutoCAD help file ACAD.HLP within the projects sub directory is an ideal place for this register.

AutoCAD slides of each CAD Model in WCS is a useful way of seeing what's coming!

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