Xrefs for Building Drafting

by Mark Middlebrook

External references, or "xrefs", constitute one of the keys to efficient drafting, especially of building plans. Xrefs help you minimize duplication of geometry, thereby making editing more efficient and helping to keep drawings in synch. Xrefs also can help keep file sizes down when you use the same large components (e.g., elaborate title blocks) in multiple drawings.

This paper describes xref concepts, commands, applications, and tips.

I originally wrote this paper for AutoCAD R12’s implementation of xrefs, and have updated it to include some information about the xref enhancements in R13, R14, and 2000.

AutoCAD R13 added:

• Overlay as an alternative way of incorporating xrefs (in addition to the old attach method).
• A new "Bind as insert" method of binding xrefs (in addition to the old "bind as bind").

AutoCAD R14 added:

• Xref clipping, so that you now can control which parts of an xref display in the parent drawing.
• Xref project files search path, in an attempt to address the "xref in a different directory from the drawing that refers to it" problem.
• An xref demand loading feature for speeding up xref access in situations where only a small part of a large xref is needed.

AutoCAD 2000 added:

• In-place xref editing.

These changes are described at the end of this paper.

Concepts

• Xrefs =~ "live blocks"

• Attaching an xref =~ inserting a block

• "Parent" and "children" drawings

• Named objects (a.k.a. dependent symbols)

• Path problems

• Binding xrefs

• Differences between blocks and xrefs

	Blocks	Xrefs
Child’s geometry:	Copied into parent DWG and stored there.	Not copied into parent DWG; loaded temporarily each time you load parent DWG.
Updating parent when child changes:	Must redefine block (with INSERT blkname= or BLOCK).	Happens automatically.
Named objects:	Parent definitions override child definitions.	Parent and child definitions are maintained separately.
Named object names:	Normal.	Prefixed with xrefname|.
Attribute definitions:	Supported.	Ignored.

Commands

External Reference dialog box:
Type: XREF or XR
Pull-down menu: Insert, Xref Manager
Draw toolbar: External Reference (in the Insert Block fly-out toolbar)

Command line version:
Type –XREF

The External Reference dialog box includes these options:

The –XREF command line version includes a similar set of options.

AutoCAD includes these additional xref-related commands:

When to use xrefs

• Title blocks
• Reference elements that need to appear in multiple drawings (e.g., wall outlines, site topography, column grids).
• Assemblies that are repeated in one or more drawings, especially if the assemblies are likely to change together (e.g., repeated framing assemblies, bathroom layout, modular furniture layout).
• Pasting up several drawings (e.g., details or a couple of plans) onto one plot sheet.
• Temporarily attaching a background drawing for reference or "tracing" purposes.

On the other hand, blocks remain useful in simpler circumstances. Remember that every xref requires keeping track of a separate file. Some situations in which you might stick with a block are:

• Components that aren’t likely to change.
• Small components.
• A simple assembly that’s used repeatedly, but in only one drawing. (You can easily update a block in one drawing by inserting it in a blank area, exploding and editing it, and then redefining it with the BLOCK command.)

Also, it’s important that everyone in the office be reasonably consistent about when to use and when not to use xrefs.

Xref guidelines

• Document xref dependencies for each project. Maintain control of the xref structure. If you can’t keep it in your head, it’s too complicated.
• Attach each xref on an appropriate layer. For example: attach a grid on the S-GRID layer. If you can’t think of an appropriate layer, make one called XREF.
• Be aware of potential problems with xrefs in different directories from the parent drawings. The simplest way to avoid problems is to make sure that parent and child drawings always reside in the same directory.
• Do not bind xrefs in working drawings. The only time you should bind xrefs is when creating a set of drawings for archiving or for sending out to a recipient that requests no xrefs. Always do the binding on a copy of the working drawing (i.e., open the drawing, SAVEAS newname, then bind).
• Set VISRETAIN consistently in your drawings. I prefer when possible to set VISRETAIN to 0 and thereby control the visibility and properties of xrefed layers from the child drawings. You may want to or need to set VISRETAIN to 1 and control xref layer visibility and properties from the parent drawings (in order to be able to assign different properties to the same layer in different parent drawings, for example).
• Before sending out or archiving a set of drawings (assuming that xrefs aren’t bound), always verify that the drawing set includes all xrefs. Use your project documentation as a checklist and/or the Pack ’n Go Express tool.

Tips and tricks

• AutoCAD looks for xrefs first in the saved path, then in the directories that make up the library search path (AutoCAD startup directory, current DWG’s directory, support file search path directories, AutoCAD driver directory, Acad.exe directory).
• To remove a hard-coded path, run the XREF command, select the xref, delete the path (i.e., everything except for filename.dwg) in the Xref Found At field, and press ENTER.
• If AutoCAD can’t find an xref (usually because the DWG file got renamed or moved), AutoCAD displays a warning and writes some text at the xref’s insertion point. To fix the problem, restore the xrefed file and run XREF Reload, or modify or remove the xref’s Saved Path.
• With xrefs, assembling a set of drawings for archiving or sending out becomes trickier. You need to make sure that you include all children of all parent DWGs; otherwise the recipient will end up looking at a partial drawing (and will regard you as inept). To avoid this embarrassment, be organized. Make a map that shows xref dependencies for the . You can use the Pack ‘n Go Express Tools command to help you bundle up a set of drawings with all of the necessary xrefs and fonts.
• When you bind an xref, AutoCAD converts the vertical bar between the xref name and symbol name to $n$, where n is a number (usually 0). The result is a lot of peculiar names for layers, blocks, and other named objects. As a result, you should avoid binding unless you’re archiving drawings or sending out a set to someone who doesn’t want to deal with xrefs and doesn’t mind the weird names.
• To avoid the peculiar named object names, set Bind Type to Insert when you bind. Alternatively, detach the xref and then insert it as a block (or as individual geometry with INSERT *dwgname).
• AutoCAD R14 and previous versions limit the names of layers to 31 characters. AutoCAD won't let you attach an xref if any of the resulting layer names (xrefname|layername) would be longer than 31 characters. Similarly, AutoCAD won't let you bind an xref (in bind "As Bind" mode) if the resulting layer name (xrefname$n$layername) would be longer than 31 characters. The same limitation applies to other named objects. If you encounter this problem, you'll need to use the RENAME command to rename the offending named object(s) in the child drawings to something shorter.

Command line tips and tricks

• All of the -XREF subcommands accept wildcards, the most useful of which is * (for "all xrefs").
• Use -XREF ? to get a list of xrefs that you can copy to a text or word processing document.

Xref features added in AutoCAD R13, R14, and 2000

• Overlay vs. attach (R13): The difference between overlaying and attaching an xref appears only when you have multiple levels of xrefs (e.g., A.dwg references B.dwg, which references C.dwg). If you attach C.dwg in B.dwg, and then attach B.dwg in A.dwg, you see C.dwg’s geometry in both B.dwg and A.dwg. If instead you overlay C.dwg in B.dwg, you see C.dwg’s geometry in B.dwg, but not in A.dwg. In other words, an overlaid xref displays only one level up in the hierarchy. Above that one level, it disappears.
• "Bind as insert vs. "bind as bind" (R13): When you bind as bind, AutoCAD creates a slew of unwieldy names containing $0$ for xref layers and other named objects. When you bind as insert, AutoCAD merges the xref’s named objects with those of the current drawing, thus eliminating the unwieldy names. There is one potential danger with bind as insert, though: If an xref has a layer or other named object that uses the same name as one in the parent drawing, but is defined differently (e.g., S-GRID is red in the parent but gray in the xref), then the properties of the xref objects will change when you bind as insert.
• Xref clipping (R14): Xref clipping allows you to define a clipping boundary that limits which part of the xref displays. For example, you can draw a full plan in one DWG file, xref it into multiple "plot sheet" DWG files, and in each plot sheet clip the xref so that only the desired area displays. This method is similar to the older paper space approach, but it isn’t limited to paper space or rectangular viewports. There’s nothing wrong with using paper space, but the xref clipping procedure is more intuitive than creating paper space viewports and zooming the model inside of them. With xref clipping, you can create everything on your plot sheet – clipped plan, title block, and additional notes – in model space if you want to. Also, you can clip xrefs to irregular polygonal areas, which avoids the rectangular limitation of paper space viewports. See my document "Xref Clipping of Building Plans" for more information. (Warning: AutoCAD LT does not include the XCLIP command. AutoCAD LT will display and plot drawings with clipped xrefs properly, but LT cannot you cannot create or manipulate clipping boundaries with LT.)
• Project xref files search path (R14): You can define various project names in the Preferences dialog box, specify xref search directories for each project, and then specify the project with which a particular drawing is associated (and thus in which additional directories AutoCAD should look for xrefs when opening that file). The project files search path feature can help on one computer, but it doesn’t really address the drawing exchange problem that xrefs in different directories cause. For that reason, I don’t think it’s a very helpful feature for most people. I continue to recommend keeping parent and child drawings in the same folder.
• Demand loading (R14): When you open a drawing containing an xref, AutoCAD normally must load the entire xrefed DWF file from the disk into memory. Demand loading allows AutoCAD to load only those portions of the xref that are displayed in the parent drawing. If you use xref clipping to show a small portion of a large xref, or you freeze layers containing a large amount of xrefed geometry, then demand loading might help improve performance. Taking advantage of demand loading requires that you set the INDEXCTL system variable to a non-zero value in the xrefed DWGs so that AutoCAD creates the necessary indices. You also must set XLOADCTL to a non-zero value so that AutoCAD knows to use demand loading. Warning: The default XLOADCTL setting of 1 is not good in a networked office where one AutoCAD user may need to open a drawing that’s attached as an xref in a parent drawing that another user has open. Change XLOADCTL either to 0 (turn off demand loading) or to 2 (make a local temporary copy of any xrefs).
• In-place xref editing (2000): With the new REFEDIT command, you can edit an xref (or block insert) "in place" – that is, you don’t have to open the child drawing.

Return to the markcad.com series of AutoCAD tutorials on xrefs.