Blocks

A block is simply another AutoCAD or IntelliCAD drawing. (IntelliCAD is a much cheaper clone of AutoCAD).

Some examples of blocks in AutoCAD/IntelliCAD drawings are shown in the figure below:

At left is a rather complex block used as part of a title sheet drawing. It is made up of many (primitive) elements; in the middle section is a block used to illustrate the position of a (complex) tree and on the right, a simple tree symbol used in a planting plan. There are of course many other examples of block symbols.

Why should you use blocks in CAD drawings?

Many beginners are puzzled by the emphasis on the use of blocks in the IntelliCAD and AutoCAD manuals. We often hear the comment: "it is possible to simply copy groups of entities (entities) to any position in the drawing. Why the do you need the ability to stamp a symbol into the drawing?"

The answer is threefold.

Using blocks speeds up the CAD drawing process.
Blocks are used because they allow for the creation of smaller drawing files. If blocks are used, the drawing will be much smaller than a comparable drawing where elements have simply been copied.
Finally, blocks can contain non-graphic (text) information (part numbers, installation date, tree size, species name etc.) which can be extracted from the drawing and passed to databases and spreadsheets for analysis.

We cover the use of blocks in our broadband learning series.

A comment on file size

A block is a single element which is stamped into a drawing. Many copies of the same block can be placed throughout a drawing and effectively take up only the room of a single block. Thus, a large architectural plan showing furniture layout with desks, chairs and work stations will be efficient, in terms of file size, if a block called 'chair', 'desk', and 'work station' has been used to insert these items. A drawing with the same desks, chairs and workstations created by copying drawing primitive elements will be much larger in file size.

Preliminaries

The concept of what exactly constitutes a block, when to use them and how to store blocks is difficult for beginners to grasp. Before discussing the use of the BLOCK command to create blocks, we need to cover some background material.

Where are block stored?

In its simplest sense, AutoCAD/IntelliCAD allows you to select a group of entities in a drawing, give them a name and store this named group of entities (a block) in the drawing. It sometimes helps beginners to imagine that an AutoCAD drawing has a shelf on which these blocks can be stored.

Drawings may contain literally hundreds of blocks, but they are 'invisible' until they are required.

Metaphorically speaking, blocks can live on a shelf inside the drawing.

Q. How do you determine if a drawing has blocks on the 'shelf'?

If you open a drawing produced by someone else, you will often find a considerable number of blocks stored on this block 'shelf'. Nowhere on the screen does AutoCAD/IntelliCAD tell you that the drawing you have opened contains numerous block references.

One of the options in the block command is used to determine those blocks stored with the drawing. You issue the BLOCK command by typing it (-BLOCK is the best way), but respond with a question mark when asked for the block name. After you hit the ENTER key, a list of blocks on this shelf is given in the pop up text window.

Where else do blocks 'live'?

As mentioned, blocks can be inserted into the current drawing either if you (or someone else) has used the BLOCK command to create a block. Alternatively, blocks can be inserted in the current drawing (as a standard drawing file) from what is often referred to as a block library. Block libraries are simply collections of AutoCAD drawing files stored in a folder on the hard disk, on a floppy disk, or CD-ROM.

It is a good idea to have a company block library stored on a common (virtual) drive and make this library accessible to all design drafters.

Making a block

Let's imagine that you were a drafter working in the field of process and instrumentation. P&ID drawings often contain a ball valve symbol. A block symbol of a ball valve is required (this will be used over again in many drawings).

Ball Valve for a metric ISO P&ID drawing (metric environment)

Start a brand new (clean) drawing.

Set the grid 2.5 and using the LINE and CIRCLE command build the drawing shown in the diagram below .

Note that the ball valve is built on a 2.5mm grid.

You need to set limits to be 0,0 to 50,50. The grid to 2.5, the snap setting to 2.5 and build the drawing shown aove using lines and circles, ignoring our dimensioning and text.

Save the drawing as (say) 'V-ballvalve' in a named folder. This drawing becomes the first candidate for your P&ID block library.

You will need to specify the insertion point of the block (the midpoint of the left side vertical line). This is done with the BASE command.

A block is just an AutoCAD drawing

How do we use this block? Just open a new drawing and use the INSERT command. Select the browse option and navigate to the folder in which you deposited the V-ballvalve drawing and select it. The image of the ball valve will come into your new drawing. It will be anchored at the correct place and can be easily located on a process line. More information on this topic is given in our CAD workshop course series.

The importance of layer 0

Thus far we have not concerned ourselves with the layer that the block is on, but if in our example we had switched to a layer called ProcessLine, and created our line work on it, the new drawing will contain the layer ProcessLine when the block is inserted.

However, if the entities making up the block are placed on layer 0, when we bring in this block, it will act like a chameleon and change its colour and line type to the layer that is active at the time of insertion. So, for many purposes, it is better to make blocks on layer 0 than on other named layers.