AutoCAD 2005 for Dummies

Each block definition slightly increases the size of your DWG file, as do other named objects such as layers, text styles, and dimension styles. If you delete (or explode) all the block inserts that point to a particular block definition, then that block definition no longer serves any purpose.

You should run the PUrge command periodically in each drawing and purge unused block definitions and other named objects. Choose File>Drawing Utilities>Purge to display the Purge dialog box, as shown in the figure. Click the Purge All button in order to purge all unused named objects in the current drawing.

  If you explode a block that contains attributes, the attribute values change back to attribute definitions. This usually isn’t the sort of change that you want. If you really need to explode the block insert, you’ll probably want to erase the attribute definitions and draw regular text strings in their place. If you’ve installed the AutoCAD Express Tools, you can perform this task automatically with the BURST command (Express>Blocks>Explode Attributes to Text).

Going External

In AutoCAD, an xref, or external reference, is a reference to another, external file — one outside the current drawing — that you can make act as though it’s part of your drawing. Technically, a reference is simply a pointer from one file to another. The xref is the actual pointer, but many people call the combination of the pointer and the external file the xref.

Drawings that you include as xrefs in other drawings often are called child drawings. Drawings that contain pointers to the child drawings are called parent drawings. This family terminology gets a little weird when you realize that a child drawing can have lots of parent drawings that refer to it — apparently it’s the commune version of family relations. If you find such relationships odd, you can, like the AutoCAD online help system, refer to the parent drawing as the host drawing. I prefer the terms parent and child, in part because they’re easily extendable to describing more complex hierarchies, such as a parent drawing, which xrefs a child drawing, which in turn xrefs a grandchild drawing.

Xrefs have a big advantage over blocks: If you change a child drawing, AutoCAD automatically loads the change into all the parent drawings that reference the child drawing.

  AutoCAD loads all xrefs into the parent drawing each time the parent drawing is opened. If the child drawing has been changed, AutoCAD automatically incorporates those changes into the parent drawing.

When you open a drawing containing xrefs, AutoCAD 2004 displays a little symbol (which looks like papers with a binder clip) on the right end of the status bar. This symbol alerts you to the fact that some of the things you see in the drawing are actually parts of other, xrefed drawings. If an xref changes while you have the parent drawing open (because you or someone else opens and saves the child drawing), the status bar xref symbol displays an External Reference Files Have Changed balloon notification. (If you want to change whether the notifications appear and how often AutoCAD checks for changes, look up XREFNOTIFY and XNOTIFYTIME in the online help.) You can use the Reload option in the Xref Manager dialog box to show the updated xrefs. See the “Managing xrefs” section later in this chapter for details.

Another advantage of xrefs over blocks is that their contents aren’t stored in your drawing even once. The disk storage space taken up by the original drawing (that is, the xref) isn’t duplicated, no matter how many parent drawings reference it. This characteristic makes xrefs much more efficient than blocks for larger drawings that are reused several times.

But you can always buy more hard disk space, so the storage issue isn’t crucial. The key benefit of xrefs is that they enable you to organize your drawings in a modular way so that changes you make to a single drawing file automatically “ripple through” all the parent drawings to which it’s xrefed. This benefit is even greater on larger projects involving multiple drafters, each of whose work may be incorporated in part or in whole in the work of others.

  The automatic update feature of xrefs is a big advantage only if you’re organized about how you use xrefs. Suppose that an architect creates a plan drawing showing a building’s walls and other major features that are common to the architectural, structural, plumbing, and electrical plan drawings. The architect then tells the structural, plumbing, and electrical drafters to xref this background plan into their drawings, so that everyone is working from a consistent and reusable set of common plan elements. If the architect decides to revise the wall locations and updates the xrefed drawing, everyone will see the current wall configuration and be able to change their drawings. But if the architect absent-mindedly adds architecture-specific objects, such as toilets and furniture, to the xrefed drawing, or shifts all the objects with respect to 0,0, everyone else will have problems. If different people in your office share xrefs, create a protocol for who is allowed to modify which file when, and what communication needs to take place after a shared xref is modified.

Becoming attached to your xrefs

Attaching an external reference is similar to inserting a block, and almost as easy. Just use the following steps:

1. Set an appropriate layer current, as described in Chapter 4.

I recommend that you insert xrefs on a separate layer from all other objects. Note that if you freeze the layer an xref is inserted on, the entire xref disappears. (This behavior can be either a handy trick or a nasty surprise.)

2. Choose Insert>Xref Manager from the menu bar to start the XRef command.

The Xref Manager dialog box appears (see Figure 13-7).

Figure 13-7: The Xref Manager dialog box.

  Don’t choose Insert>External Reference. This menu choice jumps ahead to Step 4, which will be confusing at this point.

3. Click Attach.

The Select Reference File dialog box appears.

4. Browse to find the file you want to attach, select it, and then click Open.

The External Reference dialog box appears.

5. Specify the parameters for the xref in the dialog box.

Parameters include the insertion point, scaling factors, and rotation angle. You can set these parameters in the dialog box or specify them on-screen, just as you can do when inserting a block, as described earlier in this chapter.

  You can choose the Attachment or Overlay radio button to tell AutoCAD how to handle the xref. The choice matters only if you create a drawing that uses xrefs, and then your drawing is in turn used as an xref. Attachment is the default choice, and it means that the xrefed file will always be included with your drawing when someone else uses your drawing as an xref. Overlay, the other choice, means that you see the xrefed drawing, but someone who xrefs your drawing won’t see the overlaid file. By choosing Overlay, you can xref in a map, for example, to your drawing of a house, but not have the map show up when someone else xrefs your house drawing. (That person can xref the map, if need be.) I recommend that you use the default Attachment reference type unless you have a specific reason to do otherwise.

The Path Type drop-down list provides more flexibility in how the xref’s path gets stored. See the “Forging an xref path” section later in this chapter for more information. For now, I recommend that you choose Relative path instead of the default Full path.

6. Click OK.

The externally referenced file appears in your drawing.