By Dennis Collin
A frequent question I often receive whilst training AutoCAD is the difference between External Reference files (Xrefs) and Blocks. Also, when one should be used in preference over the other. Both entity types often originate from external drawings, and then placed within a container drawing. The main difference is usually the desired behaviour of the element when the original file changes.
When working with drawings of sites or buildings that contain services or elements from other disciplines, layouts frequently change or need to be updated. Whilst services might alter, the underlying building might not, therefore it makes sense to divide a project drawing into separate drawing files where each discipline or contractor for a project will update their own working files and then issue it to the lead consultant. By keeping the discipline specific separate from the parent drawing, it reduces the risk of accidental changes to elements that need to remain ‘fixed’. Similarly, when elements are updated, it is advantageous for each disciplines reference file to automatically update in the lead consultants master drawing. If this is the case, then Xrefs should be used. Typically, Xref drawings could be entire lighting or power layouts for a building, an HVAC system or even an entire portion of a building referenced into a larger site such as a shopping centre or university campus.
When a referenced drawing is altered, and the main drawing is open or accessed, AutoCAD will notify the user that one or more of the Xref drawings have changed and will offer to reload the files. If confirmed, the main drawing will then update to reflect the changes the sub-contractor has made. If using recent versions of AutoCAD, options are also available to physically compare the changes between the original and current versions.
To check whether Xrefs have been used within a drawing, open the Manage Xrefs dialogue, also located under the Insert tab.
Block references are placed in a similar way to Xrefs, except they are not dynamically linked to their original document. Often, they tend to represent smaller elements such as furniture, doors or general equipment. If the original file changes and the user wants other drawings to reflect those changes then a manual update is required. The user will need to reinsert the block and choose the redefine block option. One such method is to use the classic ‘-INSERT’ command via the command prompt.
If using other methods of block insertion such as the Designcentre then a redefine option is available from the context sensitive right-click menu.
In summary therefore, in many drawings both types of references are used. External reference files tend to cover a larger area containing portions of a drawing or element types and are dynamically linked to their original source drawing. Blocks in contrast tend to be individual repeatable components that are embedded within that drawing. Each element type can be easily identified by selecting it and querying its properties.
Blocks can be exploded whereas Xref’s cannot be exploded unless embedded or bound into a drawing. As Xref’s are held externally, care must be taken when issuing drawings because if a container drawing is emailed without the accompanying Xref files, portions of the drawing will be missing, and errors will be displayed reporting that files cannot be found. This will not occur with blocks as they are embedded within their drawing.
Issuing Xref Project drawings is a major topic on Cadline’s Intermediate AutoCAD Course. There are several methods of performing this task either by binding the Xrefs into a drawing as Blocks or using AutoCAD’s Etransmit function. This will be covered in another post.
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