Explosive Scenes

1 Oct, 2000 By: John E. Wilson

Figure 1. Exploded view drawings are commonly used to show all of the parts in an assembly and their spatial relationships to each other.
In manufacturing industries, exploded view drawings, such as the one shown in Figure 1, are used extensively in assembly and service manuals because they clearly show the individual parts of an assembly and how they are spatially related. You make exploded view drawings in Autodesk's Mechanical Desktop by, almost literally, exploding the assembly-the parts become spread apart in space. Then, just as you can with an individual part, you can make drawings from the exploded assembly.

Exploded assemblies are referred to as scenes in Mechanical Desktop, and they are created within a special mode: the Scene mode. (Mechanical Desktop scenes have no relationship with AutoCAD rendering scenes, which are for defining a particular view and set of rendering lights.) Working in Scene mode is similar to working in Model mode. The distance between parts is controlled by an explosion factor. You can also tweak the distance between parts and even move them sideways and rotate them. Moreover, to more clearly indicate how parts go together, you can connect them with lines that are called trails.

Explosion Factors
You create a scene by selecting the Scene option of the AMNEW command. (As I typically do in this column, I'll use command names rather than toolbar button or menu option names in describing how to initiate operations because they are more basic.) Mechanical Desktop will prompt for the scene's name, with SCENE1 as the default name, and for an overall explosion factor. Then, you will be asked if you want to activate the scene. If you respond positively, Mechanical Desktop will switch to its Scene mode and activate the scene. If you choose to not immediately activate the scene, you can later invoke the AMACTIVATE command and select the Scene option. You can also have multiple scenes and use AMACTIVATE to move from one scene to another.

The new scene will initially use the Model mode viewpoint, and the parts will be spaced apart according to the scene's explosion factor. Explosion factors rely on assembly constraints that involve opposing planes. For instance, the two planes

Figure 2. Explosion factors establish the spacing between individual parts. They rely on assembly constraints that are based on planes.
specified for an insert assembly constraint will move away from each other by the value of the explosion factor, as shown in Figure 2. Movement is always away from the grounded part of the assembly or subassembly. The units for explosion factors are those of the assembly. Thus, when the parts of an assembly are based on millimeters, an explosion factor of 1 represents 1 mm.

You assign explosion factors with the AMXFACTOR command. This command offers you the option of having the explosion factor apply to all parts in the scene or to a specific part. When you select a part (which is done by picking a point on it) that is a member of a subassembly, Mechanical Desktop will ask if you want the explosion factor to apply to the entire subassembly or to just the selected part.

Applying explosion factors is a straightforward process. Generally you will assign an overall explosion factor when you create a scene, then set a viewpoint that best shows the exploded assembly, and lastly modify the explosion factor of individual parts to accommodate the part size and the viewpoint.

Scenes based solely on explosion factors seldom have all of the display characteristics you desire. Components, whether they are subassemblies or individual parts, might be hidden or not properly aligned; and they might not even respond to explosion factors because their assembly constraints are either not based on planes or offset one another.

You can remedy these problems with the AMTWEAK command. This command enables you to move and even rotate selected components within a scene in any direction, without regard to their assembly constraints. When you invoke AMTWEAK from the command line, you will be prompted to select a part. As with AMXFACTOR, if the part you pick belongs to a subassembly you can specify that you want to select the entire subassembly or just the part that was picked. Once you have specified the component you want to tweak, Mechanical Desktop displays a dialog box that has three radio buttons for you to use in choosing one of three tweak options: Move, Rotate or Transform.

The Move option of AMTWEAK moves the selected component linearly by a specified distance. You will be prompted from the command line to select an object that defines a line. You can select a straight edge or the round edge of a cylindrical or conical feature on a part. When you select a round edge, the axis of the feature defines the move direction. The object you select does not have to be on the part you want to tweak. An arrow pointing in the move direction will appear, and you will be prompted to enter the move distance. If you want to move the component in the opposite direction of the arrow, enter a negative number.

The Rotate option rotates the selected component about an axis. As with the Move option, you will be prompted to select an object that defines a line. The object you select will serve as the axis of rotation. You can select a straight edge or a round edge on any part to define the axis. An arc-shaped arrow will encircle the axis to indicate the rotation direction, and you will be prompted to enter the number of degrees you want the selected component to rotate. You must enter a positive number.

While the Transform option of AMTWEAK performs the same move and rotate operations as the other two options, it gives you more choices in specifying directions and axes. Also, you can perform any number of moves and rotations until you exit the option. When you select Transform, Mechanical Desktop will issue command-line prompts for you to either (1) move the selected component; (2) rotate it; or (3) exit the option (and thereby end the command). When you choose Transform's Move option, Mechanical Desktop displays the command-line prompt:

Specify start point or [Viewdir/Wire/X/Y/Z]:
If you specify a point using any AutoCAD method, it serves as one end of a directional line (or vector), and you will be prompted to specify a point for the line's other end. The x, y and z options move the component in the direction of those User Coordinate System (UCS) axes directions, and the Viewdir option moves the component in the current viewport's line-of-sight direction. The Wire option uses an existing wireframe object to define the tweak's move direction and prompts you to select an object. The object can be an AutoCAD line, polyline, arc, circle or ellipse. When you select a polyline that has multiple segments, the segment the object pick point is in is used to establish the move direction. When you select a circle, an arc or an ellipse, the move direction is perpendicular to the object's plane.

If you specified a line for the move direction by either picking two points or an AutoCAD line or linear polyline segment, the line will also be used to establish the move distance. Otherwise, Mechanical Desktop will issue a command-line prompt for the move distance. An arrow indicating the move direction will appear, and you can either accept or flip the arrow's direction.

When you choose Transform's Rotate option, Mechanical Desktop will ask you to pick a point to serve as the center of rotation. Next, you will be prompted to specify the direction of the rotation axis. Your choices are the UCS x, y, or z axes directions, the current viewport's line-of-sight direction and the direction

Figure 3. Unlike explosion factors, tweaks are not dependent on assembly constraints. You can use them to move individual parts or subassemblies in any direction.
defined by a wireframe object. An arc-shaped arrow will encircle the axis to indicate the positive rotation direction, and you can accept or reverse it. Lastly, you will be prompted to enter the rotation angle.

After you have performed a move or rotate operation, the Transform prompt for selecting an option reappears. Select the Exit option to end both Transform and AMTWEAK. Figure 3 shows an example of a tweak.

You can't edit tweaks; you can only delete them. This is done through the AMDELTWEAKS command, which issues a command-line prompt for you to select one component. All of that component's tweaks will be deleted, and it will return to its pre-tweaked position.

Assembly Trails

Figure 4. Assembly trails, which indicate the explosion factor and tweak movements of an assembly's parts, help viewers understand the spatial relationships of parts.
Assembly trails, as shown in Figure 4, are lines that show the paths used in moving parts away from their assembled positions. You create trails on a part-by-part basis with the AMTRAIL command. Mechanical Desktop places them in the AM_TR layer, which has white as its color and CONTINUOUS as its linetype. Trails follow the paths made by explosion factors and tweaks. They are parametric, and, therefore, they change as the parts and the positions of the parts change.

When you initiate AMTRAIL, a command-line prompt will ask you to select a reference point on a part or subassembly. You must pick a point on the edge of a face. The trail will extend from the center of circular edges and from the endpoint of straight and arc-shaped edges.

Once you have selected a reference point, Mechanical Desktop will display the Trail Offset dialog box for you to use in controlling the relative locations of the trail's endpoints. It is divided into two clusters of edit boxes. The cluster labeled Offset at Current Position is for the reference point end of the trail, while the cluster labeled Offset at Assembled Position is where the reference point would be if the part had not been exploded or tweaked. By default the trail starts and ends at these two points. You can, though, specify Over Shoot and Under Shoot distances for both ends. When you specify an Over Shoot distance, the

Figure 5. Over and under shoots lengthen or shorten a trail. They are based on the current position of a user selected reference point on an exploded or tweaked part and the assembled, or constrained, position of that reference point.
trail will extend beyond the reference point location; and when you specify an under shoot distance, the trail ends before reaching the reference point. Figure 5 shows examples of these options.

Once you have created a trail for a part, you can modify its over and under shoot values by invoking the AMEDITTRAIL command. This command displays the same Trail Offsets dialog box that is used by the command for creating trails. You can remove trails with the AMDELTRAIL command, which will issue a command-line prompt for you to select the trail you want to delete.

Trails follow tweak movements exactly, which can cause problems if you have used multiple tweaks in positioning a part. Suppose, for instance, you used AMTWEAK to move a part 12 units in a particular direction. Then after deciding that that was too far, you added another tweak to move the part six units back toward its original position. The part's trail will follow the 12-unit tweak, and then turn back on itself for another six units.

Managing Scenes
Usually scenes automatically update themselves when changes have occurred. You can, though, force updates by selecting the Scene option of the AMUPDATE command.

When you have set the explosion factors and tweaks within a scene as you want them, you can prevent inadvertent changes to them with the AMLOCKSCENE command. This command displays a command-line prompt with options to either lock or unlock the active scene. When a scene is locked, you can't add or modify explosion factors or tweaks. You can, though, create and modify trails.

Occasionally you may want to turn off the visibility of some parts within a scene. For instance, in an assembly that has numerous pairs of bolts and nuts, you may want to turn off the visibility of all but one representative pair to reduce clutter. You can control the visibility of parts, subassemblies and trails by selecting the Scene tab of AMVISIBLE's dialog box.

You can make a copy of a scene with AMCOPYSCENE. Prompts similar to those used in creating a new scene are issued for you to establish a name and an overall explosion factor for the copy. The copy will have all of the original scene's parts, tweaks and trails. You can also remove an unneeded scene by choosing the Scene option of the AMDELETE command.

The Scene tab of the Desktop Browser lists the names of all parts within the existing scenes in the file. Those within inactive scenes, however, will be grayed out. You can conveniently access the commands related to scenes described in this column through right-click shortcut menus. These shortcut menus are especially useful in changing explosion factors and adding tweaks to subassembly parts. When you select a subassembly name, all of the parts in the subassembly will move. Conversely, when you select a part's name, only that part will move.

About the Author: John E. Wilson

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