国家制造业信息化AUTOCAD认证考试样题.docx

Chapter 10 Using Block and External References

10.1 Overview

AutoCAD provides several features to help you manage objects in your drawings. With blocks you can organize and manipulate many objects as one component. You can also associate items of information with the blocks in your drawings- for example, part numbers and prices - by attaching attributes. Specification sheets or bills of materials can be created using this information.
You can attach or overlay entire drawings in your current drawing by using AutoCAD external references, or xrefs. When you open a drawing that contains xrefs, any changes that have been made in the referenced drawing appear in the current drawing.

10.2 Working with Blocks

A block is a collection of objects you can associate together to form a single object, or block definition. You can insert, scale, and rotate a block in a drawing. You can explode a block into its component objects, modify them, and redefine the block. AutoCAD updates all current and future instances of that block based on the block definition.
Blocks streamline the drawing process. For example, you can use blocks to

Build a standard library of frequently used symbols, components, or standard parts. You can insert the same block numerous times instead of re-creating the drawing elements each time.
Revise drawings efficiently by inserting, relocating, and copying blocks as components rather than individual geometric objects.
Save disk space by storing all references to the same block as one block definition in the drawing database.

For information about how to create a block, see "Defining Blocks."
When you insert a block in your drawing, you are creating a block instance. Each time you insert a block instance, you assign a scale factor and rotation angle to the inserted block. You can also scale a block instance using different values in any coordinate (X, Y, Z) direction.

Blocks make it possible for you to organize your drawing tasks in a systematic way, so that you can set up, redesign, and sort the objects in your drawings and the information associated with them.

Defining Blocks

You can group objects to create block definitions in the current drawing, or you can save the block as a separate drawing file. When you define a block, you specify the base point, the objects to group, and whether to retain or delete the objects or convert them to a block in the current drawing. You can also enter a text description and specify an icon used to help identify the block definition in AutoCAD DesignCenter. Block definitions are one of many nongraphical objects saved in a drawing. See "Working with Named Objects."

To create a block definition

1 From the Draw menu, choose Block Make.
2 In the Block Definition dialog box, enter a name for the block.
3 Under Objects, choose the Select Objects button to use the pointing device to select objects for the block definition.
The dialog box closes temporarily while you select objects for the block. Press ENTER when you are done selecting blocks. The dialog box reopens.
4 If you want to create a selection set, use the Quick Select button to create or define a filter for your selection set. See "Filtering Selection Sets."
5 Under Objects, specify whether to retain, convert to a block, or delete the selected objects.
Retain: Keeps selected objects in the current drawing, in their original state.
Convert to Block: Replaces selected objects with an instance of the block.
Delete: Removes selected objects after the block is defined.
6 Under Base Point, enter the coordinate values for the insertion base point or choose the Specify Insertion Base Point button to use the pointing device.
7 Under Description, enter text to help identify the block for easy retrieval.
8 Under Icon, specify whether to create an icon from the block definition.
Do not include an icon: Omits preview image from block definition.
Create icon from block geometry: Saves preview image with the block definition.
9 Choose OK.

The block definition is saved in the current drawing.

To save a block or object as a separate drawing file

1 At the Command prompt, enter wblock.
2 In the Write Block dialog box, specify a block or an object to write out as a file.
Block: Specifies a block to save as a file.
Entire Drawing: Selects current drawing as a block.
Object: Specifies objects to be saved as a file.

3 Under Block, select a name from the list to save as a file.
4 Under Base Point, use the Pick Point button to define the base point.
5 Under Objects, use the Select objects button to select the object for the block file.
6 Enter a name for the new file.
If a block is selected, WBLOCK automatically uses that block's name for the new file.
7 In the Insert Units list, select an insert unit to use in AutoCAD DesignCenter.
8 Choose OK.

The block definition is saved as a drawing file.

Inserting Blocks

You can insert blocks or entire drawings into the current drawing with INSERT. When you insert a block or drawing, you specify the insertion point, scale, and rotation angle. This section explains inserting blocks into your drawing using INSERT. See "Using AutoCAD DesignCenter to Insert Blocks."

When you insert an entire drawing into another drawing, AutoCAD treats the inserted drawing like any other block reference. Subsequent insertions reference the block definition (which contains the geometric description of the block) with different position, scale, and rotation settings, as shown in the following illustration. If you change the original drawing after inserting it, the changes have no effect on the inserted block. If you want the inserted block to reflect the changes you made to the original drawing, you can redefine the block by reinserting the original drawing.

If you want to insert a drawing but you don't want the file name to be the same as the block name, you can insert it using a blockname = filename syntax at the Block Name prompt.
By default, AutoCAD uses the coordinate 0,0,0 as the insertion base point for inserted drawings. You can change the insertion base point of a drawing by opening the original drawing and using BASE to specify a different insertion base point. AutoCAD uses the new base point the next time you insert the drawing.

If the drawing you insert contains objects created in a paper space layout, those objects are not included in the current drawing's block definition. To use the paper space objects in another drawing, open the original drawing and use BLOCK to define the paper space objects as a block. The block is now defined within your drawing. You can insert the drawing into another drawing and insert the block defined in your original drawing in either paper space or model space. With AutoCAD DesignCenter, you can display layouts and drag them into your drawings. See "Working with Drawing Content."

To insert a block reference

1 From the Insert menu, choose Block.
2 In the Insert dialog box, specify the block name, where you want to insert it in the current drawing, and whether you want to explode it after insertion.
3 If you have modified the original drawing file for a block, you can redefine the block in the current drawing by choosing File to locate the file for the block.
4 Choose OK.

The block reference is updated in the current drawing.

Exploding a Block

Use EXPLODE to break a block instance. By exploding a block instance, you can modify the block or add to or delete the objects that define it.

10.3 Working with Attributes

An attribute provides a label or tag for you to attach text to a block. Whenever you insert a block that has a variable attribute, AutoCAD prompts you to enter the data to be stored with the block. Examples of data are part numbers, prices, comments, and owners' names.
You can extract attribute information from a drawing and use that information in a spreadsheet or database to produce items such as a parts list or a bill of materials BOM). You can associate more than one attribute with a block, provided that each attribute has a different tag. AutoCAD prompts you for the value of each attribute when you insert the block. You can also define constant attributes: because they have the same value in every occurrence of the block, AutoCAD does not prompt for a value when you insert the block.
Attributes can be invisible, which means the attribute is not displayed or plotted. However, information on the attribute is stored in the drawing file and written to an extract file by ATTEXT.

Creating Attributes

To create an attribute, you must first use ATTDEF to create an attribute definition, which describes the characteristics of the attribute. The characteristics include the tag, prompt, value information, text formatting, location, and any optional modes.
To create an attribute definition
1 On the command line, enter attdef.
2 In the Attribute Definition dialog box, specify the insertion point and set the attribute modes and the tag, prompt, and text options.
3 Choose OK.

After creating the attribute definition, you can select it as an object in a block definition. If the attribute definition is incorporated into a block, whenever you insert the block, AutoCAD prompts you with the text string you specified for the attribute. Each subsequent instance of the block can have a different value specified for the attribute.
If you want to use several attributes together, create each one separately and then include them in the same block. To control the order of prompts for multiple attributes, see OPTIONS in the Command Reference.

System variables AFLAGS sets the ATTDEF mode. ATTDISP globally controls the visibility of attributes. ATTREDEF redefines a block and updates associated attributes.

Editing Attribute Definitions

You can use DDEDIT to edit an attribute definition before it is associated with a block.

To edit an attribute definition before it is associated with a block

1 From the Modify menu, choose Text
2 Select the attribute to edit.
3 In the Edit Attribute Definition dialog box, specify the attribute tag, prompt, and default value. Then choose OK.
Command line DDEDIT
Related CHANGE edits attribute definitions.

Attaching Attributes to Blocks

You can attach attributes to a block when you define or redefine that block. When AutoCAD prompts you to select the objects to include in the block definition, include the desired attributes in the selection set. The order in which you select the attributes determines the order in which you are prompted for attribute information when you insert the block (see "Defining Blocks").

Editing Attributes Attached to Blocks

You can edit attributes that are already attached to a block and inserted in a drawing.
To edit an attribute attached to a block
1 From the Modify menu, choose Attribute Single.
2 Select a block to edit.
3 In the Edit Attributes dialog box, edit the attribute information as necessary.
4 Choose OK.

To control the order of prompts for multiple attributes, see OPTIONS in the Command Reference.

10.4 Using External References

An external reference (xref) links another drawing to the current drawing. When you insert a drawing as a block, the block definition and all of the associated geometry are stored in the current drawing database. The block is not updated if the original drawing changes. When you insert a drawing as an xref, however, the xref is updated when the original drawing changes. A drawing containing xrefs always reflects the most current editing in each externally referenced file.

Like a block reference, an xref is displayed in the current drawing as a single object. However, an xref does not significantly increase the file size of the current drawing and cannot be exploded. As with block references, you can nest xrefs that are attached to your drawing.
By attaching xrefs, you can

Assemble a master drawing from component drawings that may undergo changes as a project develops.
Coordinate your work with the work of others by overlaying other drawings on your drawing to keep up with the changes being made by other users.
Ensure that the most recent version of the referenced drawing is displayed. When you open or plot your drawing, AutoCAD automatically reloads each xref, so the drawing reflects the latest changes to the referenced drawing file.
Display only a specific section of the xref file in the master drawing by creating clipped boundaries of xrefs.

10.5 Clipping Blocks and Xrefs

After attaching a drawing as an xref or inserting a block, you can define a clipping boundary by using XCLIP. A clipping boundary can define a portion of a block or xref while suppressing the display of geometry outside of the boundary. Clipping applies to an individual instance of an xref, not the xref definition itself. The portion of the xref or block within the clipped boundary remains visible, and the remainder of the xref or block becomes invisible. The referenced geometry is not altered, only the display of the xref is edited.

You can use XCLIP to create a new clipping boundary, delete an existing boundary, or generate a polyline object coincident with vertices of the clipping boundary. Xref clipping can be turned on or off. When a clipping boundary is turned off, the boundary is not displayed and the entire xref is visible, provided that the geometry is on a layer that is on and thawed. When a clipping boundary is turned off, it still exists and can be turned on. However, deleting a clipping boundary is permanent. For more information about XCLIP, see the Command Reference.

After an xref or block has been clipped, it can be edited, moved, or copied just like an unclipped xref or block. The boundary moves with the reference. If an xref contains nested clipped xrefs, they appear clipped in the drawing. If the parent xref is clipped, the nested xrefs are also clipped.
If you want to see the clipping boundary, you can turn on the XCLIPFRAME system variable. XCLIPFRAME determines whether the clipping boundary frame is displayed. When the clipping frame is on (set to 1), it can be selected as part of the object and plotted. For more information about XCLIPFRAME, see the Command Reference.

10.6 Editing References in Place

You can modify external references and redefine block definitions from within the current drawing by using in-place reference editing. Both blocks and xrefs are considered references.
Often, a drawing contains one or more xrefs as well as multiple block references. When working with block references, you can select a block, modify it, and update the block definition. When working with xrefs, you can use in-place reference editing to select the reference you want to work with, modify