AutoCAD 2005 for Dummies

After you get the lines and text right, you may be justified in thinking that your work in AutoCAD is done. But AutoCAD enables you to do so much more! Blocks and external references help you manage data within drawings, between drawings, and across a network. AutoCAD 2005’s new sheet sets feature bids fair to redefine how people organize drawings on larger, multisheet projects, so this section devotes a new chapter to it. If you plan to share drawings — whether among your own projects, with people in your office, or with folks in other companies, you need to think about consistency in presentation and drawing organization — in other words, CAD standards.  The Internet is the biggest ongoing swap meet in human history, and AutoCAD offers some unique trading possibilities — and potential pitfalls — via e-mail and the Web. With the information in this part, you’ll be teaching AutoCAD how to give and receive in no time.

Chapter 13 Playing Blocks and Rasteroids

In This Chapter

? Introducing blocks, external references (xrefs), and raster images

? Creating block definitions

? Inserting blocks

? Using attributes in blocks

? Attaching and managing xrefs

? Controlling xref paths

? Attaching and managing raster image files

Chapter 6 shows you how to copy objects within a drawing or even to another drawing. That’s one way to use CAD to improve drafting efficiency. You can copy a DWG file and then modify it to create a similar drawing — an even better productivity-booster, as long as you’re in the habit of making similar drawings. But all those are baby steps compared to the techniques that I cover in this chapter: treating drawings, parts of drawings, and raster images as reusable and updateable modules. If you want to make drafting production more efficient with CAD, then you want to know how to use blocks, xrefs, and raster files.

A block is a collection of objects grouped together to form a single object. You can insert this collection more than once in the same drawing, and when you do, all instances of the block remain identical, even after you change the block definition. Although a block lives within a specific drawing, you can transfer copies of it into other drawings. You can add fill- in-the-blank text fields called attributes to blocks.

An external reference, or xref, is like an industrial- strength block. An external reference is a pointer to a separate drawing outside the drawing you’re working on. The referenced drawing appears on-screen and on plots as part of your drawing, but it continues to live as a separate document on your hard disk. If you edit the externally referenced drawing, the appearance of the drawing changes in all drawings that reference it.

A raster file (also called a bitmap file) stores a graphical image as a series of dots. Raster files are good for storing photographs, logos, and other images, whereas CAD vector files are good for storing geometrical objects such as lines and arcs, along with text and other annotations for describing the geometry. Sometimes it’s handy to combine raster images with CAD vector files, and AutoCAD’s IMage command makes the process straightforward.

Blocks, external references, and raster images enable you to reuse your work and the work of others, giving you the potential to save tremendous amounts of time — or to cause tremendous problems if you change a file on which other peoples’ drawings depend. Use these features when you can to save time, but do so in an organized and careful way so as to avoid problems.

  The way you use blocks and especially xrefs will depend a lot on the profession and office in which you work. Some disciplines and companies use these drawing organization features heavily and in a highly organized way, while others don’t. Ask your colleagues what the local customs are and follow them.

Rocking with Blocks

First, a little more block theory and then you can rock right into those blocks. To use a block in a drawing, you need two things: a block definition and one or more block inserts. AutoCAD doesn’t always make the distinction between these two things very clear, but you need to understand the difference to avoid terminal confusion about blocks. (Maybe this syndrome should be called blockheadedness?)

A block definition lives in an invisible area of your drawing file called the block table. (It’s one of those symbol tables that I describe in Chapter 4.) The block table is like a book of graphical recipes for making different kinds of blocks. Each block definition is like a recipe for making one kind of block. When you insert a block, as described later in this chapter, AutoCAD creates a special object called a block insert. The insert points to the recipe and tells AutoCAD, “Hey, draw me according to the instructions in this recipe!”

Although a block may look like a collection of objects stored together and given a name, it’s really a graphical recipe (the block definition) plus one or more pointers to that recipe (one or more block inserts). Each time you insert a particular block, you create another pointer to the same recipe. 

The advantages of blocks include:

? Grouping objects together when they belong together logically. You can draw a screw using lines and arcs, and then make a block definition out of all these objects. When you insert the screw block, AutoCAD treats it as a single object for purposes of copying, moving, and so on.

? Saving time and reducing errors. Inserting a block is, of course, much quicker than redrawing the same geometry again. And the less geometry you draw from scratch, the less opportunity there is to make a mistake.

? Efficiency of storage when you reuse the same block repeatedly. If you insert the same screw block 15 times in a drawing, AutoCAD stores the detailed block definition only once. The 15 block inserts that point to the block definition take up much less disk space than 15 copies of all the lines, polylines, and arcs.

? The ability to edit all instances of a symbol in a drawing simply by modifying a single block definition. This one is the biggie. If you decide that your design requires a different kind of screw, you simply redefine the screw’s block definition. With this new recipe, AutoCAD then replaces all 15 screws automatically. That’s a heck of a lot faster than erasing and recopying 15 screws!

  Blocks aren’t as great for drawing elements used in multiple drawings, however, especially in a situation where several people are working on and sharing parts of drawings with one another. That’s because blocks, after they get into multiple drawings, stay in each drawing; a later modification to a block definition in one drawing does not automatically modify all the other drawings that use that block. If you use a block with your company’s logo in a number of drawings and then you decide to change the logo, you must make the change within each drawing that uses the block.

  External references enable you to modify multiple drawings from the original referenced drawing. You can find out more about external references in the section “Going External,” later in this chapter.

  If all you need to do is make some objects into a group so that you can