AutoCAD

LT On-line: Lesson 6

1 Aug, 2000 By: Mark Middlebrook


Make and use blocks

Page 1: Introduction to blocks

"Don't redraw, reuse" is one mantra of efficient CAD users, and AutoCAD LT's block feature is a tool you can use to put that mantra into practice. A block is a collection of objects grouped together to form a single object. A block can live within a specific drawing, or you can export a block so that multiple drawings share access to it.

At any time you can explode a block, that is, divide it back into the objects that make it up, and edit the objects. In addition, you can add fill-in-the-blank text fields called attributes to blocks. (I'll cover attributes in a future AutoCAD LT On-line tutorial, but you can see how they work in the title block tutorial.

The procedures described here work with AutoCAD LT 2000 and AutoCAD 2000. The procedures are similar in AutoCAD LT 98 and AutoCAD Release 14, but the dialog boxes look a little different.

A little block theory
You need two things in order to use a block in a drawing: a block definition and one or more block inserts. AutoCAD LT 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 (a syndrome that I call blockheadedness).

A block definition lives in an invisible area of your drawing file called the blocktable. Think of it as a table of graphical recipes for making different kinds of blocks. Thus each block definition is like a recipe for making one kind of block. When you insert a block, as I describe later in this tutorial, AutoCAD LT creates a special object called a blockinsert. The insert simply points to the recipe and tells LT, "Hey, draw me according to the instructions in this recipe!"

So although a block looks at first like a collection of objects stored together and given a name, it's really a graphical recipe (the block definition), together with 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 logically belong together. For instance, you can draw a screw using lines, polylines, and arcs, and then make a block definition out of all these objects. When you insert the screw block, AutoCAD LT treats it as a single object for copying, moving, and so on.

  • Storage efficiency when you reuse the same block repeatedly. If you insert the same screw block 15 times in a drawing, AutoCAD LT stores the detailed block definition only once. The 15 block inserts that point to the block definition are very compact and 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. For instance, if your design requires a different kind of screw, you simply redefine the screw's block definition. AutoCAD LT then replaces all 15 screws automatically with this new recipe.

Blocks are great for convenience and saving storage space within a drawing. Blocks aren't as great for drawing elements used in multiple drawings, however, especially in a multiuser environment. That's because later modifications to a block definition in one drawing do not automatically modify all the other drawings that use that block. So if you use a block with your company's logo in a number of drawings and then decide to change the logo, you must make the change within each drawing that uses the block.

Xrefs (external references) do let you modify multiple drawings from the original referenced drawing. (You can see an xref in action in the title block tutorial.

Note: If all you need to do is make some objects into a group so that you can more easily select them for copying, moving, and so on, use the AutoCAD LT group feature. Choose Tools|Group Manager to open the Group Manager dialog box. Then select some objects, choose the Create Group button, and type a name for the group. After you've created one or more groups, use CTRL+ H to toggle "group-ness" on or off for all of them. If you've toggled the "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.

How to create a block definition
To create a block definition from objects in the current drawing, use the Block Definition dialog box. Alternatively, you can create a block definition by inserting another AutoCAD DWG file into your current drawing (as described in the next section). To define a block from objects in the current drawing:

1. Click the Make Block button on the Draw toolbar to open the Block Definition dialog box, as shown below.

figure
Figure 1.

Tip: Whenever you create objects that you'll later include in a block definition, pay attention to what AutoCAD LT layers those objects are on. In AutoCAD LT, layer 0 functions as a special construction layer for blocks. Let's say you create geometry on layer 0 and include it in a block definition. When you insert the block, the geometry takes on the layer characteristics, such as color and linetype, of the layer on which the block is inserted. If you create a block from geometry drawn on any other layer, it always retains the color and linetype in effect when it was created. Think of block definition geometry created on layer 0 as a chameleon.

2. Type the block definition's name in the text entry box.

If you type the name of an existing block definition, AutoCAD LT replaces that block definition with the new group of objects you select. This process is called block redefinition. AutoCAD LT first warns you, then updates all instances of the block in the current drawing to match the changed block definition.

3. Specify the base point of the block, also known as the insertion point, by either entering the coordinates of the insertion point at the x, y, and z prompts or selecting the Pick point button and then a point on the screen.

Tip: Try to use a consistent point on the group of objects for the base point, such as the lower-left corner, so that you always know what to expect when you insert a block.

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

Click for larger image
Click for larger image
Figure 2.

AutoCAD LT uses the selected objects to create a block definition. Figure 2 shows a group of selected objects during the process of creating a new block definition. To specify the base point, I used the Endpoint object snap mode to grab the lower-left corner of the rectangle, as shown in Figure 2.

5. Select the Retain, Convert, or Delete radio button to tell AutoCAD LT what to do with the objects used to define the block. The default choice, Convert to Block, is usually the best because it preserves the pointer to the block definition recipe. If you choose Retain, AutoCAD LT leaves the objects on the screen, but as separate objects with no relationship to the block definition.

Warning: AutoCAD LT 98 and Release 14 don't include the Convert to Block option. Your best bet is to turn on Delete selected objects after block creation and then insert the block after you create its definition.

6. Click a radio button to choose either Do not include an icon or Create icon from block geometry.

Go ahead and create the icon. It will help you and others find the right block to use later.

7. Specify the Insert units to which the block will be scaled.

If or when someone drags the block from one drawing into another via the AutoCAD DesignCenter, the units you specify here and the units of the drawing you're dragging into will control the default insertion scale factor.

8. Enter the block Description.

9. Select OK to complete the block definition process.

If you didn't choose the Convert to block or the Retain radio button, your objects disappear! AutoCAD LT stores the block definition in the current drawing's block table, however, and the block is ready to use. If you chose the Convert to Block radio button (the default), AutoCAD LT creates a block insert that points to the new block definition-the objects look the same on the screen, but now they're an instance of the block rather than separate objects. If you chose the Retain radio button, the objects remain in place but aren't converted into a block insert-they stay individual objects with no connection to the new block definition.

Make and use blocks
  Page 1: Introduction to blocks
  Page 2: How to insert a block


About the Author: Mark Middlebrook


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