Understanding Blocks

A block is a collection of individual objects combined into a larger single object. Think of the block as the parent of a family, and think of the individual objects as the parent’s children. Although the children have identities of their own (color, layer, lineweight, and linetype), they are also under the control of their parent, which has its own color, layer, lineweight, and linetype properties.

The fact that both the block (parent) and its individual objects (children) have their own properties makes it important to understand how these properties are affected by certain conditions. For example, assume that a block has been created from several child objects and that each child object was originally created on its own layer. The layers on which the child objects were created can be frozen individually. If one of these layers is frozen, the child object that resides on that layer is also frozen and becomes invisible. However, the other child objects in the block remain visible because the layers they are on are still thawed. In contrast, if the parent block is inserted on a layer and that layer is then frozen, all its child objects will become frozen. This is true even when the layers on which the child objects reside are on and thawed.

For example, Figure 13.1 shows an inserted block made up of three objects: a rectangle, a triangle, and a circle. The block (parent) is inserted on the layer Parent. When the block was defined, the rectangle, triangle, and circle objects (children) were on the layers Rectangle, Triangle, and Circle, respectively.

When the Triangle layer is frozen, as shown in Figure 13.2, only the triangle object disappears, and all other objects remain visible. This is true even though the triangle is part of another object—the inserted block object. In other words, because the Triangle child’s layer is frozen, only it is affected. None of the other children or the parent is affected.

In contrast, all the objects disappear when the Parent layer is frozen, as shown in Figure 13.3. This demonstrates the difference between freezing a child object’s layer and freezing its parent layer.

This example is just one of several conditions that can influence the behavior and appearance of an inserted block. Understanding the rules that govern these conditions is essential to implementing the power of blocks and increasing your productivity.

The New Block Definition Dialog Box

The Block Definition dialog box is the most common method used to create new blocks. With AutoCAD 2000 came an updated dialog box, adding new features and enhancing its functionality. You access the Block Definition dialog box, shown in Figure 13.4, from the Draw pull-down menu by choosing Block, Make.

The following list describes the dialog box’s features:

• Name. This is where you specify the name of the block. To assign a new name, type the name in the edit box.

  This feature is improved over its predecessor by the addition of a drop-down list. The list displays the names of all currently defined blocks. In addition to displaying block names, by selecting a name, you can display all its current settings and then redefine any or all of its values.

• Base Point. Allows you to define the X,Y,Z insertion coordinates for the block. You can enter the coordinate values in the appropriate edit boxes, or you can click the Pick Point button and specify the insertion base point by selecting a point on screen.

  The base point value is saved with the block and represents the point in the block that AutoCAD uses to define the block’s position when it’s inserted in the drawing.

• Objects. This area controls various options when selecting objects that define a new block, and it displays the number of objects selected.

  If you click the Select Objects button, you can then choose the objects that make up the block definition onscreen.

  Clicking the Quick Select button (the funnel/lightning bolt icon at the upper right) displays the Quick Select dialog box shown in Figure 13.5, which allows you to select objects based on filter criteria. For example, you can select all CIRCLE objects whose color is blue.

  Figure 13.5. The Quick Select dialog box.

  

  Refer to Chapter 11, “Advanced Geometry Editing,” for more information on using the Quick Select command.

  The Retain, Convert to Block, and Delete option buttons tell AutoCAD what to do with the selected objects after the block is defined. When you retain the objects, they are left in the drawing and are not converted to a block after AutoCAD defines the new block. When you convert the objects to a block, the original objects are erased and then the block, as a single block, is inserted using the original location. When you delete the objects, AutoCAD erases them from the drawing after the new block is defined.

• Preview Icon. This feature allows you to control whether AutoCAD creates an image of the new block and saves it with the block definition. You can display the image when viewing blocks by selecting a block name or by using the AutoCAD DesignCenter (discussed later in this chapter).

• Insert Units. This feature, introduced in AutoCAD 2000, specifies the units to which the block is scaled when it’s inserted from the AutoCAD DesignCenter. You can choose from numerous unit types including feet, inches, or millimeters. This feature is discussed in the AutoCAD DesignCenter section later in this chapter.

• Description. This feature allows you to provide a detailed description of the block definition. The description is displayed when it’s selected in the BLOCK command and when blocks are viewed using the AutoCAD DesignCenter, as discussed later in this chapter.

Defining Blocks

What happens inside a drawing when a new block is defined? If you have created blocks before, you know that when you select the child objects that make up the block, they all disappear from the screen. This happens because AutoCAD automatically erases them after they have been used to define a block.

There is a logical reason why AutoCAD erases the objects. The BLOCK command is more than another COPY command; it enables you to make copies of a collection of child objects that uses less file space. It minimizes the file size of an AutoCAD drawing by storing each child object’s property data in a place AutoCAD calls the Block Table. It stores this information under the name of the parent block. When you insert a block into AutoCAD, instead of duplicating the property data of each child object (as the COPY command does), AutoCAD simply refers to the property data stored in the Block Table. It then draws the child objects based upon this data. This enables AutoCAD to store each child object’s property data in just one place, the Block Table. You can, therefore, insert multiple copies of a block, duplicating the child objects where needed. In each case, AutoCAD refers back to the Block Table for the data it needs to draw the child objects. Consequently, AutoCAD erases the original objects after they are used to define a block because it assumes you will want to reinsert those objects as a block to reduce the file size of your drawing.

The Effect of the Current UCS on Block Definitions

When you create a block, you must define its insertion base point. This point’s coordinates are relative to the block object and are set to its 0,0,0. Consequently, when you define a block’s insertion base point, even though the current UCS (User-defined Coordinate System) coordinates may be 100,100,100 when you pick them, AutoCAD ignores these values and stores the block’s insertion base point as 0,0,0. This is true in both paper space and model space. If you were then to export this block out as its own drawing using the WBLOCK command, the insertion base point would be at 0,0,0 of the WCS (World Coordinate System) in the new drawing. This feature enables predictable insertion of blocks.

In addition to understanding how AutoCAD deals with the current UCS’s coordinates when defining a block, you must also understand the effect the current UCS’s X-axis orientation has on the angle a block assumes when it is inserted into a drawing.

When creating a block, AutoCAD uses the current UCS to determine its insertion angle. This angle is oriented relative to the current UCS’s X-axis.

To demonstrate the effect of the current UCS’s orientation, the following exercise walks you through inserting two different arrowhead blocks into an existing drawing.

Exercise 13.1 Examining the Effect of the Current UCS’s Orientation

1.	Open the 13DWG01.DWG drawing file on the accompanying CD. The drawing contains two sets of objects that appear on the right side of your screen. Both sets are made up of a closed polygon in the shape of an arrowhead with a text object inside. The first set was used to create a block definition called AR1. The second set was used to create a block definition called AR2. It is important to note the X-axis orientation relative to the two arrowheads. Both arrowheads were defined as blocks with the same UCS orientation. As a consequence, when you insert each block during this exercise, you will see the effect the current UCS orientation has on the inserted objects. Now you will insert the AR1 block.
2.	From the Insert menu, choose Block. The Insert dialog box opens.
3.	Choose AR1 from the Name drop-down list if it is not already displayed.
4.	In the Insertion point area, be sure the Specify On-screen option is selected. In the Scale and Rotation areas, make sure the Specify On-screen option is cleared.
5.	Click OK to close the Insert dialog box. The dialog box closes, and the AR1 Insert object appears. Notice that the arrowhead is oriented in the same direction as the original AR1 object that appears on the lower-right side of your screen.
6.	Choose a location in the lower-left portion of the drawing to insert the arrowhead. Now, you will insert the AR2 block.
7.	From the Insert menu, choose Block to open the Insert dialog box.
8.	Choose AR2 from the Name drop-down list.
9.	Click OK to close the Insert dialog box. The dialog box closes, and the AR2 Insert object appears. Notice that the arrowhead is oriented in the same direction as the original AR2 object that appears on the upper-right side of your screen.
10.	Choose a location in the upper-left portion of the drawing to insert the arrowhead. Your drawing should look similar to Figure 13.6. Figure 13.6. The two arrowhead blocks are inserted. Next, you change the rotation of the WCS about its Z-axis. Then you redefine the AR2 block to see the effect the new UCS has on the block’s rotation.
11.	From the Tools menu, choose New UCS, and then choose Z.
12.	At the Specify rotation angle about Z axis <90>: prompt, type 45 . Notice that the UCS’s X-axis arrow is now rotated parallel to the AR2 objects in the upper-right corner.
13.	From the Draw menu, choose Block, Make. The Block Definition dialog box opens.
14.	From the Name drop-down list, choose AR2.
15.	In the Base Point area, click the Pick Point button. The Block Definition dialog box temporarily disappears, allowing you to pick an insertion point for the block.
16.	Using an Endpoint snap, select the tip of the AR2 arrowhead object in the upper-right corner. AutoCAD captures the coordinates for the pick point and displays the values in the Block Definition dialog box.
17.	In the Object area, choose the Select Objects button. The Block Definition dialog box temporarily disappears, allowing you to select the objects.
18.	Select the two AR2 objects in the upper-right corner and press Enter. AutoCAD displays the Block Definition dialog box, noting that two objects are selected.
19.	Click OK. AutoCAD displays a message noting that the AR2 block is already defined and asks if you want to redefine it.
20.	Choose Yes to redefine the AR2 block and close the warning dialog box. AutoCAD redefines the AR2 block definition using the current UCS’s X-axis orientation and regenerates the drawing. If the AR2 object set was erased from your screen, enter OOPS at the Command prompt: to unerase it. Next, you will set the UCS back to the WCS and insert the AR2 block.
21.	From the Tools menu, choose New UCS, World.
22.	From the Insert menu, choose Block to open the Insert dialog box.
23.	Choose AR2 from the Name drop-down list.
24.	Click OK to close the Insert dialog box. The dialog box closes, and the AR2 Insert object appears. Notice that the arrowhead is now oriented in the same direction as the WCS’s X-axis.
25.	Choose a location near the center of the drawing to insert the arrowhead. Your drawing should look similar to Figure 13.7. Notice that both AR2 block insertions align parallel to the WCS’s X-axis. Figure 13.7. The redefined AR2 block is inserted.
26.	Close the drawing without saving it.

This exercise shows how the current UCS affects a block’s orientation when it is created and then inserted into a drawing. By understanding this behavior, you can better control how blocks are inserted into drawings, and thereby maintain a higher level of productivity.

Inserting Blocks

Several commands can be used to insert blocks. Understanding the unique features of these commands is important in selecting the right tool for a particular task.

INSERT and –INSERT

The INSERT and -INSERT commands are used to insert blocks. The INSERT command prompts for insertion information using the Insert dialog box, whereas -INSERT prompts for information at AutoCAD’s command line. The Insert dialog box interface makes it easy to select blocks already stored in the current drawing’s Block Table. The Insert dialog box also makes it easy to search for blocks stored outside the current drawing by choosing the Browse button.

MINSERT Versus ARRAY

Sometimes it may be necessary to insert a block as a rectangular array. Two options exist to accomplish this. You can use MINSERT or ARRAY.

The MINSERT Command

First, the MINSERT command combines the INSERT and ARRAY commands. When executed, the first command-line prompts are the typical ones for inserting a block. After that, the typical command-line prompts for creating an array appear. The exception is that MINSERT can only create rectangular arrays. Therefore, no option is available for selecting a polar array.

One drawback to this command is that the MINSERT object cannot be exploded, nor can its individual objects be moved or edited. The advantage to using this command, however, is that less file space is required to define the MINSERT object, which reduces the file size of your drawing. It is important to note that the reduction in file size can be dramatic. For example, a simple block inserted as an array of 100100 using the MINSERT command will have little, if any, impact on the file’s size. In contrast, the same number of blocks inserted using the ARRAY command can increase the file’s size by 1/2MB or more.

If you need to insert an array of blocks and you will not need to explode or edit the objects, I suggest that you use the MINSERT command. Otherwise, use the ARRAY command discussed in the next section.

The ARRAY Command

The ARRAY command accomplishes the same thing as the MINSERT command, but it affords you more control over the inserted objects. With the ARRAY command, you can choose between rectangular and polar arrays. Also, after the array is created, you can explode the inserted objects individually or move them independently of the other Insert objects. To use the ARRAY command with a block, however, you must first use the INSERT command to create the first object; then you can use the ARRAY command to create the desired array.

The disadvantage of using the ARRAY command is that multiple insertions of the block object are made, which therefore, increases your drawing’s file size to a degree.

MEASURE and DIVIDE

The MEASURE and DIVIDE commands provide two methods of inserting a block along a path.

The MEASURE Command

The MEASURE command enables you to insert a block in multiple places along a line, arc, or polyline at a given distance. To demonstrate, the next exercise shows you how to create a series of rectangles along the centerline of a street design. By creating a block of a rectangle and inserting it at the appropriate distances along a centerline path, you can quickly create a series of viewport guides. For this particular drawing, the guides can be used to define the various Plan View sections for street improvement plans.

Exercise 13.2 Using MEASURE to Set a Series of Blocks Along a Path

1.	Open the 13DWG02.DWG drawing file on the accompanying CD. This file contains a typical street centerline with right-of-way lines. A block called Viewport exists in the Block Table. This block consists of a rectangle with an insertion base point located in the center of the rectangle. The rectangle is 400 feet wide by 1,000 feet long. You will use the MEASURE command to insert this block every 800 feet along the centerline.
2.	From the Draw pull-down menu, select Point, Measure. AutoCAD prompts for the object to measure.
3.	Select the red centerline.
4.	At the Specify length of segment or [Block]: prompt, type B to select block, and then press Enter.
5.	At the Enter name of block to insert: prompt, type the block name Viewport , and then press Enter.
6.	At the Align block with object? [Yes/No] <Y>: prompt, type Y , and then press Enter.
7.	At the Specify length of segment: prompt, type a segment length of 800 , and then press Enter. AutoCAD creates many Viewport blocks along the centerline path, placing one every 800 feet, as shown in Figure 13.8. Figure 13.8. The MEASURE command places the Viewport blocks along a path.

The DIVIDE Command

The DIVIDE command allows you to insert a block multiple times along a line, arc, or polyline any given number of times. Suppose you must draw a series of manholes along the street centerline in the previous example. In order to place the Manhole block, you can use the DIVIDE command to insert 30 copies of it along the centerline path.

Exercise 13.3 Using DIVIDE to Insert 30 Manhole Blocks Along a Path

1.	Continue with the previous drawing, or open the 13DWG02a.DWG drawing file on the accompanying CD.
2.	From the Draw pull-down menu, select Point, Divide. AutoCAD prompts for the object to divide.
3.	Select the red centerline.
4.	At the Enter the number of segments or [Block]: prompt, type B to select block, and then press Enter.
5.	At the Enter name of block to insert: prompt, type the block name Manhole , and then press Enter.
6.	At the Align block with object? [Yes/No] <Y>: prompt, type Y to align the block with the selected object, and then press Enter.
7.	At the Enter the number of segments: prompt, type a segment number of 30 , and then press Enter. AutoCAD draws 30 evenly spaced Manhole blocks along the centerline path, as shown in Figure 13.9. Figure 13.9. The DIVIDE command evenly spaces the Manhole blocks along a path.
8.	You can close the drawing without saving.

Windows-Based Insertion Features

AutoCAD 2000 introduced a new feature called the Multiple Document Environment (MDE), which allows you to open multiple drawings in a single AutoCAD session. This feature works in the same fashion as other Windows applications, such as Microsoft Word.

There are many advantages to opening several drawings in a session of AutoCAD, and two of them relate to blocks. Along with MDE, AutoCAD 2000 also introduced two new features that took advantage of multiple open drawings. These new features allow you to either cut and paste or drag and drop objects from one drawing to another. Additionally, you are given the option of inserting the objects as a block.

Cut and Paste Block Insertion

The phrase “cut and paste” refers to the process of selecting an object or group of objects and cutting or copying them to the Windows Clipboard. When you cut selected objects, they are copied to the Clipboard and erased from the original document. In contrast, when you copy objects, they are copied to the Clipboard, but the selected objects are left untouched in the original document.

Cut and paste block insertion is not really new to AutoCAD, but its capabilities are greatly enhanced. For example, AutoCAD 2002 allows you to identify the base point of an object when copying it to the Clipboard. This very powerful feature provides you with the control to specifically identify the new insertion point of the selected object.

When you insert the block from the Clipboard, you can paste it using either a selected base point, or the block’s original coordinates. The Paste to Original Coordinates option, found on the shortcut menu, copies the coordinate location of the objects in the original drawing and then pastes the objects in the new drawing using those same coordinates. This option is useful for copying objects from one drawing to another when both drawings use the same coordinate system. You must have previously saved a base point to have access to the Paste to Original Coordinates option. For more information, refer to Chapter 10, “Basic Object Editing.”

Drag and Drop Block Insertion

When you drag and drop objects, you are moving them from one place to another. When you drag and drop objects within the same AutoCAD drawing, the result is the same as using the MOVE command to move the object. When you drag and drop from one drawing to another, the result is similar to copying the object from the original drawing to the target drawing.

The drag and drop feature is useful for quickly moving or copying objects. However, this method does not provide a method for accurately selecting the base point for pasting the objects as a block. Consequently, to select objects in one drawing and paste them as a block in another while controlling the new block’s base point, use the copy and paste feature described in the previous section. For more information, refer to Chapter 10, “Basic Object Editing.”

Block Reference

When you insert a block, you are actually creating an Insert object. The Insert object references a particular set of block data in the Block Table. This is called a Block reference. AutoCAD uses the Block reference to find the data stored in the Block Table. It uses this data to draw the child objects that make up the Insert object.

Although only one set of data in the Block Table is used to define a block, multiple block references can refer to that data. In fact, there is no limit to the number of Insert objects you can create. In each case, AutoCAD uses the Block reference to find the data it needs to draw the Insert object.

Behavior of Block Properties

There are three properties of blocks that behave in different ways depending on their settings when the block is defined. The color, linetype, and lineweight properties can behave in different but predictable ways when defined on the 0 layer as opposed to other layers. In addition, you can define these properties explicitly by selecting particular values, or you can define them implicitly as BYLAYER or BYBLOCK.

The Effect of Creating Blocks on Layers Other Than 0

The simplest way to control the appearance of a block is to define it on a particular layer and explicitly define its color, linetype, and lineweight. For example, suppose you have created a circle object on a layer called Circles. To explicitly define its color, open the Modify menu and choose Properties. When AutoCAD displays the Properties window, select the circle. When the Properties window displays the circle’s properties, select a color from the Color property drop-down list, or choose the Other option from the list to display the Select Color dialog box. The Properties window now lists the color you chose as the property of the circle and changes the circle object’s color to reflect the modified property. You have just defined the color value explicitly. As a consequence, if you use the circle object to define a block and the block is inserted into the drawing, its color will be constant. It will always be the color you explicitly defined.

In contrast, if you implicitly define an object’s color, linetype, and lineweight by choosing BYLAYER, altering the layer properties the original object was on when it was defined as a block will change the object’s appearance in the block. For example, suppose the circle object in the previous example is used to define a block. Also suppose the circle’s color, linetype, and lineweight are defined as BYLAYER, and the circle object is on a layer called Circle. When the block containing the circle object is inserted into the drawing, altering the color, linetype, or lineweight of the Circle layer will change the circle object’s color, linetype, or lineweight. This is true no matter what layer the block is inserted on. When a child object’s color, linetype, and lineweight properties are set to BYLAYER, those properties are determined by the values of the child object’s original layer.

The Effect of Creating Blocks on Layer 0

Layer 0 has a unique feature. When a block is defined from child objects created on the 0 layer, AutoCAD assigns special properties to that block if its color, linetype, and lineweight properties are set to BYLAYER or BYBLOCK. This feature can be very powerful.

If BYLAYER is used to define a child object’s color, linetype, and lineweight, the layer on which the block is inserted controls the child object’s color, linetype, and lineweight properties.

The following exercise demonstrates the effects of inserting a block whose color, linetype, and lineweight properties have been set to BYLAYER.

Exercise 13.4 Inserting a Block with BYLAYER Properties

1.	Open the 13DWG03 drawing file on the accompanying CD. The screen is blank, containing no objects. In this drawing file, two blocks are already defined. The block C1 is a circle created on layer 0 with its color, linetype, and lineweight properties set to BYLAYER. The block C2 is a circle created on layer 0 with its color, linetype, and lineweight properties set to BYBLOCK. Note that on the Object Properties toolbar, the current layer is BLUE, and the layer’s color, linetype, and lineweight properties are set to BYLAYER.
2.	From the Insert pull-down menu, choose Block. The Insert dialog box appears.
3.	From the Name drop-down list, choose the C1 block.
4.	In the Insertion Point area, be sure the Specify On-Screen option is selected. In the Scale and Rotation areas, make sure the Specify On-Screen option is cleared.
5.	Click OK to close the Insert dialog box.
6.	Choose a location on the left side of the screen where you want to insert the block. The C1 block is inserted and assumes the color, linetype, and lineweight of the BLUE layer’s property values. Remember, this occurs because the block’s child objects were created on the layer 0, and their property values for color, linetype, and lineweight were set to BYLAYER.

In contrast, if BYBLOCK is used to define a child object’s color, linetype, and lineweight, the current Object Creation values control the child object’s color, linetype, and lineweight values. This is true no matter which layer the block is inserted on. These values are controlled from the Object Properties toolbar.

The following exercise demonstrates the effect of inserting the C2 block whose color, linetype, and lineweight properties have been set to BYBLOCK.

Exercise 13.5 Inserting a Block with BYBLOCK Properties

1.	Continue with the 13DWG03.DWG drawing file. From the Object Properties toolbar, change the Color property to Magenta , change the Linetype property to Hidden2 , and change the Lineweight property to 0.50mm .
2.	From the Insert menu, choose Block. The Insert dialog box appears.
3.	From the Name drop-down list, choose the C2 block.
4.	Click OK to close the Insert dialog box.
5.	Choose a location on the right side of the screen to insert the block.

Your screen should look similar to Figure 13.10. Notice that the C1 block acquired the color, linetype, and lineweight property values based on the layer’s values, whereas the C2 block acquired the color, linetype, and lineweight property values set via the Object Properties toolbar.