~|| Lesson 12 of 12 - Blocks ||~


A block in Autocad is a collection of all of the line and arc entities that create an object, 'moulded' together to make one selectable object. Selecting one particular entity (such as a midpoint of a line for example) will select all objects contained within the block. They can be moved, rotated, mirrored etc and will always be treated by Autocad as one whole object.

The only way to 'break' the block apart, and split it into its seperate line entities, is to explode it. If one particular part of a block needs to be changed, the block has to be exploded, the modification made, and the objects created back into a block.

Why use Blocks?

Blocks are an invaluable way of quickly arranging complex pieces of a drawing. Selecting the object is a breeze, it is no longer neccessary to painstakingly select each indivdual line. They also prevent accidental modification of parts of the object, as they cannot be modified unless the whole block has been exploded. Blocks also make it easy to duplicate the same object throughout a drawing, and as Autocad recognises that each copy of a block is identical to the previous, the memory & processing time required byAutocad is reduced.

The most impressive feature of blocks is that if a block detail has to be amended, simply changing the detail and 'redefining' the block applies the changes to EVERY instance of the block in the drawing!

Block Example
A simple example of the use of blocks is in a plan view of a classroom measuring 5.5m x 5.5m. You may be required to find the most suitable layout for 12 desks and 24 chairs. You would of course have to draw the chair and table. Then simply turn them each into a block. They can then be moved, selected and copied simply and quickly.

1) Download the drawing file for your version of Autocad.

Classroom.dwg Autocad 2000 .dwg file
Classroom r14.dwg Autocad 14 .dwg file

2) Firstly we'll turn the table into a block. To open the 'block definition' box type the command block into the Autocad console then hit return.

3) We'll need to name the new block that we create. Type Chair into the name box.

4) Click the select objects button to select the seperate entities that we wish to contain within the block. Select all lines making the table then right-click or hit enter to return to the block definition box. Within the objects section we have 3 options to choose from: Retain, Convert to Block and Delete.

Retain: Once the OK button is hit, the block will be created and stored within the drawing, although the selected entities will not have been turned into the block and will be kept as seperate entities. The block can be inserted from the 'insert -> block ' drop down menu.

Convert to Block: This is the opposite of retain, the selected entitiles will be converted into a block, and will be the first instance of the block within the drawing.

Delete: Not as drastic as it sounds! The block will be created, although not displayed. The seperate entities which make the block will be deleted. The block can be inserted from the 'insert -> block ' drop down menu.

In this instance, choose convert to block.

5) Now we hace to choose a base point for the block. This is simply the insertion point if we decide later to insert the block from insert-> block in the drop down menus. Choose a suitable position, such as the approximate centre of the table, or the midpoint of one of the lengths. Right-click or hit enter to return to the block definition box.

6) Preview Icon simply lets you choose if you want Autocad to create a small preview of the block which will be shown in the insert block box. This function is useful if a large amount of blocks are to be created within a drawing. Whichever option is chosen does not affect the block creation, so choose whichever option you prefer!

7) Insert units should be set to whichever scale we you are drawing in. This drawing is using 1 Autocad unit to represent 1mm, so select millimetres from the drop down box. An optional description of the block can be entered, if desired.

8) That's it! Hit OK to create the block. Try selecting the table, notice how the entire table becomes selected? Also note that the colour and linetype of any of the lines within the table cannot be edited.

Now turn the drawing of the chair into a block using the block command, name the block: chair, and use the convert to block option.

Now that we have the chair and table block, lets see how simple it is to work with blocks.

1) Copy the chair and place the copy to the right of the existing chair, so that that there are two chairs behind the table.

Remember how we said we would arrange the 12 tables and 24 chairs to fit into the room? Lets arrange them by making copies of the original blocks. It would be acceptable to use the copy command to make copies, although it may be a little tiresome to arrange them neatly. By far the quickest way is by using the Array command we looked into in Tutorial 6.

Use an offset of 1500mm for the row and column offsets, and array to 4 rows and 3 columns as shown below.

The finished result:

Redefine the Block

Imagine that the school decides that the chairs aren't to have arms! All of the chairs can be amended by simply exploding 1 chair, modifying it then saving back to the block usig the SAME block name:

1) Zoom in on one of the chairs, and explode it. This will break the block.

2) Delete the arms of the chair, as below.

3) Now use the block command to turn the 'armless' chair back into a block. Make sure that you use the same block name as the existing block to be changed, for block redefine to work correctly.

4) Autocad will warn you: 'Chair is already defined. Do you want to redefine it?'. Choose Yes.

Voila! Every instance of the chair has now been changed by redefining the block. This is where the power of creating blocks really comes into its own.

Wherever your are copying the same information around in a drawing, always ensure that you have turned it into a block first, it can save a lot of headache when you later decide to alter the original detail!

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