ENGR 150 Engineering Graphics

    Assembly Drawings

 

 

 

Objective

Introduction and Concepts

This lab is an exercise in preparing assembly drawings. The new concepts of blocks and prototype drawings are covered in the lab. You will create your assembly drawings from individual part drawings and notate it with leader arrows.

Assembly Drawings

Assembly drawings are illustrations used to explain how parts fit together in a larger assembly.

The assembly drawing illustrates more than just the location of each part. Each component is identified by a leader, at the end of which is a numbered balloon. Each numbered balloon has a corresponding entry in the parts table. Each row in the parts table corresponds to a different numbered part, and contains the full part description.

There are two kinds of assembly drawings:

In the previous lab you created a three-view orthographic projection of a bracket. That drawing provided detailed information required to produce the part and is called a working drawing. In this lab there will be four working drawings accompanied by an assembly drawing. The working drawings will contain one, two or three orthographic views of each part of the final assembly. Working drawings are not made for standard parts such as bolts and screws that appear in the assembly drawing.

Prototype Drawings

AutoCAD has a feature called Prototype Drawings, in which you can designate any drawing file to serve as a the starting point for a new drawing. This saves time setting up the drawing units, layers, dimensioning styles, line types, etc. Most companies that use AutoCAD have a standard library of prototype drawings for different sizes of paper.

Blocks

If your drawing contains many copies of the same object (such as a chair in an office layout drawing),  that object is a good choice for a block.  Blocks are one of AutoCAD's strategies to keep drawing file size down. The larger and more complex a drawing file becomes, the greater the demand on memory and disk space. If a chair symbol requires 12 objects (line, arc, etc.) to draw and if there are 100 copies of it in a drawing file, there are a total of 1200 objects to store. By storing a master definition of the chair in a special place within the drawing database called the block table,  AutoCAD can reference the chair's common features without the overhead of 99 other copies. Whenever you redefine the chair block, each of its copies, (called instances by AutoCAD) are automatically updated to reflect the changes. 

 How Blocks & Layers Relate To Each Other

Any given block can contain objects drawn on different layers, and can therefore be quite a hassle to manage with AutoCAD's layer controls. Imagine a chair block defined with the seat, armrests, and back all on custom layers of the same name. This block would ignore the layer control settings of whatever layer it was drawn on in favor of the settings on those custom layers. Worse yet, this block would create those layers in any new drawing it was inserted in, much to the chagrin of the hapless person who thought they were inserting a nice inoffensive chair symbol into their drawing! In order for a block to be as well behaved as other AutoCAD objects such as lines, arcs and circles, it must be made from objects drawn on layer 0: Objects living on all other layers will make your block disobedient to the layer controls, and will result in the creation of these layers in any new drawing you insert them in.

Before you create a block from any objects, use the chprop command to move the parts you want to layer 0. This will guarantee that your newly created block contains no undesired blocks or layers, and will therefore behave itself in your drawing's layer table.

The Block Command

Creating a block within AutoCAD is easy. Invoke the block command by typing block at the command line, and provide AutoCAD with a name, an insert point, and then the objects you want to make up the block. The insert point should be chosen correctly bearing in mind that AutoCAD will insert, scale and rotate the block around this point. When you complete the block command, all the objects you have selected will be deleted as they would be transferred to the block table.

The WBlock Command

With the block command described above only creates a local version of the block. It is only available within the drawing it was defined in. To transfer any blocks between drawings, you'll need to use the wblock command. This command opens up a create new file dialog box (The same one you see when starting a new drawing) from which you are prompted to name the block as an AutoCAD drawing file. Next, you are prompted for the block name and/or an insert point followed by the objects you want to make up the block.

The Insert Command

The insert command is used to place instances of the blocks you have defined. The first thing you will be asked when you run this command is the name of the block. Once you have named the block you must choose the insert point, followed by the x and y scale of the block instance. Hitting the <Enter> key twice will select scales of 1 for both x and y. Finally, you must enter the rotation of the block (relative to its orientation when you defined it)...Just remember that angles are measured counterclockwise in AutoCAD.

The Explode Command

Eventually, you'll want to revise or otherwise change a block you have defined. To begin this task you'll need to "break open" one of your block instances in order to be able to get at the lines, arcs, and other objects inside: This is achieved with the explode command.

Once you have exploded a block, you are free to edit the stuff that was once inside it. When your edits are complete, you redefine the block by simply running the block command and giving your new block the same name as the old one you exploded. AutoCAD will update the block definition, replacing all of its instances in your current drawing.

Dim Leader

AutoCAD has an automated dimension command called leader. As a dim mode command, you'll need to type dim at the command line in order for it to work or choose it from the Dimension drop down menu. Once you have started the leader command, AutoCAD will prompt you for the location of the arrowhead, followed by the next point of the leader. When you are tired of mouse-picking the points that define the line segments of the leader, simply hit the < Enter > key to go to the next phase of the command: Entering the leader text. In this lab, the only leader text you require is a number from 1 to 5, so type in the number you want and hit the < Enter > key to end the command. Draw a balloon around the number using the circle command and trim the end of the leader with the trim command

Procedures

Creating the Prototype & Part Drawings

Creating the Assembly Drawing

Verify that everything in your drawing is where it should be by selectively turning off each layer from 1 to 5. If each corresponding part does not turn off with its parent layer, there is an error. Each block must be inserted on the appropriate layer, and exploded and trimmed block contents must be chprop'ed back to this layer.

Deliverables

      All the drawings should be made on a letter size paper of dimensions 11 x 8.5. (you may have to create a new layout depending on what layout(s) you have in your prototype drawing.

      Use mm as the units for all the drawings.

      Each drawing should include the title block.
The assembly drawing should also have the parts table.