370 Part IV: Advancing with AutoCAD

Rocking with Blocks

A block is a collection of objects grouped 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. By redefining the block definition, you can automatically change all instances of the block insertion (officially called a “block reference”) at once. Although a block lives within a specific drawing, you can transfer copies of it into other drawings. And you can add fill-in-the-blank text fields — attributes — to blocks.

You can create single-line or multiline attributes; in addition to having more than one line, multiline attributes have many of the formatting options of multiline text. And blocks both with and without attributes can be defined as annotative objects to boot. (See Chapter 13 for a rundown on annotative objects.)

To use a block in a drawing, you need two things: a block definition and one or more block insertions. AutoCAD doesn’t always make the distinction between these two things very clear, but you need to understand the differ-

ence 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: the block table. (It’s one of those sets of named symbols that we describe in Chapter 6.) 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 in the upcoming section, “Inserting Blocks,” AutoCAD creates a special object called a “block reference.” The block reference 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 references). Each time you insert a particular block, you create another pointer to the same recipe.

The advantages of blocks include

Grouping objects 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.

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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 references that point to the block definition take up much less disk space than 15 copies of all the lines, polylines, and arcs. No matter how big the block definition is, each insertion adds only about 100 bytes to the drawing file size.

The ability to edit all instances of a symbol in a drawing simply by modifying a single block definition: 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!

Varying the appearance of block references by using dynamic blocks:

If your design requires a different kind of screw, you simply change the view of the screw to the other kind (assuming, of course, you’ve defined your screw as a dynamic block). Every instance of the screw in the drawing could show a different kind of screw. And that’s a heck of a lot more efficient than creating 15 different block definitions! For the lowdown on creating, inserting, and manipulating dynamic blocks, see Chapter 18.

Blocks aren’t all that great for drawing elements that might be used in multiple drawings, however, especially if 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. Using external reference (xref) files can overcome this problem. We cover XREFs in Chapter 18.

If all you need to do is group some objects so that you can more easily select them for copying, moving, and so on, use AutoCAD’s Group feature. We describe groupies a little more fully in Chapter 10. Type GROUP (or the command alias G) and press Enter, or simply click the Group button on the Home tab’s Groups panel. Then just select some objects, and you’re done. When you’re editing drawings that contain groups, press Ctrl+Shift+A to toggle “group-ness” on or off. If you’ve toggled group-ness on, picking any object in a group selects all objects in the group. If you’ve toggled it off, picking an object selects only that object, even if it happens to be a member of a group. For more information, refer to Chapter 10 or visit the online help index.

Blocks — along with external references, DWF and PDF underlays, raster images, and DGN files — 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 people’s

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372 Part IV: Advancing with AutoCAD

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.

How you use blocks and xrefs depends 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, but others don’t. Ask your colleagues what the local customs are and follow them.

Creating Block Definitions

To create a block definition from objects in the current drawing, use the Block Definition dialog box. (The other way to create a block definition is by inserting another drawing file into your current drawing as a block, which we explain in the next section.) The following steps show you how to create a block definition by using the Block Definition dialog box:

1.On the Ribbon’s Home tab, click the Create button on the Block panel.

The Block Definition dialog box appears (see Figure 17-1).

Figure 17-1: The Block Definition dialog box.

Pay attention to layers when you create the objects that make up a block. As a rule, block geometry created on most layers retains the color, linetype, lineweight, transparency, and plot style properties of those layers. The exception to the rule is object geometry created on Layer 0. If you create a block by using geometry drawn on Layer 0, the block takes on the features of any layer into which you insert it.

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2.Type the block definition’s name in the Name text box.

If you type the name of an existing block definition, AutoCAD will warn you when you click OK at the end of the process. This isn’t always a bad

thing because AutoCAD will ask whether you want to replace that block definition with the new objects you select. This process is block redefinition. If you have a drawing with a large number of block insertions — for example, numerous bathtubs in a 1,000-room hotel — you can use a simplified block representation during the design process and then redefine it to a more complex definition just before final plot time.

To see a list of the names of all the current blocks in your drawing, open the Name drop-down list.

3.Specify the base point (also known as the insertion point) of the block, using any of the following methods:

•Enter the coordinates of the insertion point in the X, Y, and Z text boxes.

•Click the Pick Point button and then specify a point on the screen.

(In this case, use an object snap or other precision technique, as described in Chapter 7, to grab a specific point on one of the block’s objects.)

The base point is the point on the block by which you insert it later, as we describe in the next section.

Use an obvious and consistent point on the group of objects for the base point, such as the lower-left corner, so that you know what to expect when you insert the block.

4.Click the Select Objects button and then select the objects that you want as part of the block.

AutoCAD uses the selected objects to create a block definition and displays an icon showing those objects next to the block name. Figure 17-2 shows the base point and group of selected objects during the process of creating a new block definition.

Base point

Figure 17-2: Building a block.

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374 Part IV: Advancing with AutoCAD

5.In the Objects area, select one of the radio buttons to tell AutoCAD what to do with the objects used to define the block: Retain them in place, convert them into a block instance, or delete them.

The default choice, Convert to Block, is usually the best. See Step 9 for a description of what happens with each choice.

6.Specify the insert units to which the block will be scaled in the Block Unit drop-down list.

When you or someone else drags the block from one drawing into another via the DesignCenter palette (see Chapter 6) or Tool Palettes (described later in this chapter), the units you specify here and the units of the drawing you’re dragging into will control the default insertion scale factor. The list contains 17 different units from Angstroms to Parsecs, but unfortunately doesn’t include fathoms or furlongs.

Three additional features in AutoCAD’s Block Definition dialog box give you even more control over what happens to your blocks as they’re inserted:

•If the Annotative check box is selected: The block insertion will be scaled to suit the current drawing scale, and additional scales can be applied so the insertion will be scaled differently to suit other drawing scales. This would typically be applied only to symbols (such as north arrows) and not to insertions of blocks depicting actual objects (such as toilets or motors). (We explain the nuts and bolts of annotative objects in Chapter 13.)

•If the Scale Uniformly check box is selected: Blocks will be inserted with the same X, Y, or Z scale factors. (Scale Uniformly is selected automatically if Annotative is selected.)

•If the Allow Exploding check box is selected: Blocks can be exploded during or after their insertion in a drawing.

7.Enter a description for the block in the Description text area.

You don’t have to enter a description to create a block, but it’s not a bad idea. Think like a database manager and enter a useful description that will identify the block to yourself and others.

8.Make sure that the Open in Block Editor check box is deselected.

You don’t need to use the Edit Block Definition dialog box unless you’re going to add dynamic features to the block. (See Chapter 18 to find out more about dynamic blocks.)

9.Click OK to complete the block definition process.

AutoCAD stores the block definition in the current drawing’s block table. The radio buttons you choose from in Step 5 do the following:

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