Examples used come from the field of architecture but the concepts apply to all disciplines. If you wish to learn how to use the commands mentioned then you should refer to specific tutorials. myCADsite is a good source of tutorials.
BLOCKs and INSERTs:
Refer to separate discussion of Blocks and Inserts
XREFs, ATTACHMENTs and OVERLAYs:
Refer also to SHEETSET command.
An ATTACHMENT is an object within the drawing created from an XREF definition read in from an eXternal REFerence file.
The concepts of "XREF" and "ATTACHMENT" are similar in most ways to "BLOCK" and "INSERT" i.e. the XREF (source) is the external file and the ATTACHMENT becomes part of the recipient (host) file. When an XREF is edited its dependent ATTACHMENTs will be updated when the recipient (host) drawing file is next updated. XREFs are simply other drawing files and as such may contain any object with any layer name and may be complete drawings or sub-components of drawings. An OVERLAY is similar to an ATTACHMENT except that nested ATTACHMENTs to the source XREF are not displayed.
The principal advantages of XREFs are the ability to share data from different sources and the associated automatic updating when an XREF file is re-defined. Examples are an architect attaches a structural drawing as a background to his/her architectural drawing or vice versa an engineer uses an architectural drawing as a background when designing the structure. Each are constantly updated and refreshed until the final coordinated plans are produced. To avoid unnecessary transference of irrelevant data e.g. an electrical XREF in the architect's drawing the engineer would OVERLAY rather than ATTACH the architect's drawing to his structural drawing.
ATTACHMENTS and/or OVERLAYS may be BOUND to the host file and become permanent data within the host file and no longer dependent upon access to the source file. BOUND OVERLAYS and ATTACHMENTS do not update automatically but may be RELOADED if the source file is available.
Because ATTACHMENTs are coming from an external source we have to pay attention to any differences that there might be between the layer states and layer definitions of the host file and the imported file and how we would like them to display in the host file bearing in mind that the Layer properties might be different and might also subsequently be changed in the source XREFed or ATTACHED/OVERLAID file. This can be a little confusing and will be the subject of a separate discussion. In the meantime I suggest you have a look at AutoCAD Help, VISRETAIN.
Benefits of Xrefs:
- Reduces host file size.
- Automatic update of Xref data on re-load of host file.
- Allows subdivision of work.
- One Xref file may service several host files.
- Reduces duplication of drawing.
Disadvantages of Xrefs:
- Long search times for object snaps.
- Complicates file management - especially for consultant issues.
- Complicates edits if related objects are spread across two or more files.
- Xrefs must be bound or carefully managed when archiving to avoid loss, path change and anachronistic changes from an altered Xref file.
Typical Uses of Xrefs
A typical generic Xref might be a standard drawing Frame. Copy the frame into the project directory to avoid path complications. This also allows customization of the frame to the project. Refer also to SHEETSET command.
All buildables (the objects to be produced) and reference data (notes, dimensions, labels etc that are directly related to the buildables) are created in the principal model file except selected standard sub-components that may be common to more than one model. These may be modeled in their own separate file and XREFed into the host files e.g. a concrete lift core common to several floors.
The sub-component file will be a model file in its own right complete with all of its own buildables and references and be separately editable. Thus a lift core could be independently edited without suspending work on the principal model file.
A single floor plan may be used as a background for several different versions of the plan eg. Furniture, Ceiling, and Finishes plan files may use an Xref floor plan showing only the buildables. Only the special information is created in the model file in juxtaposition to the buildables displayed from the Xref.
Portions of a Location drawing floor plan may be Xref into fit-out plans. This avoids re-drawing of the plan and also ensures current version of plan is used.
The above descriptions need further expansion. Submissions from readers would be welcomed. Please use the Contact Form to express your interest.