Creating the parts | ||
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Context:
The bolted joints, which connect the cross bracing to the truss structures, and the connection at the tip of the truss structures are different from the welded joint connections. Since these joints do not provide complete continuity for all degrees of freedom, separate vertices are needed for connection. Thus, the cross bracing must be treated as a separate part since distinct geometric entities are required to model the bolted joints. Appropriate constraints between the separate vertices must be specified.
We begin by discussing a technique to define the truss geometry. Since the two truss structures are identical, it is sufficient to define the base feature of the part using only the geometry of a single truss structure. The sketch of the truss geometry can be saved and then used to add the second truss structure to the part definition.
The dimensions shown in Figure 2 are relative to a global Cartesian coordinate system. The base feature, however, must be sketched in a local plane. To make the sketching easier, datum features will be used. A datum plane, parallel to one of the trusses (truss B in Figure 1, for example), will serve as the sketch plane. The orientation of the sketch plane will be defined using a datum axis.
The other truss will also be added as a planar wire feature by projecting the truss created here onto a new datum plane.
The final truss part is shown in Figure 1. The visibility of all datum and reference geometry has been suppressed.
Define the geometry of a single truss
Using the Create Datum Point: Offset From Point tool
, create two datum points at distances of (0, 1,
0) and (8, 1.5, 0.9) from the
reference point. These points represent points C and E, respectively, in
Figure 1.
Reset the view using the Auto-Fit View tool
in the
View Manipulation toolbar
to see the full model.
Using the Create Datum Plane: 3 Points tool
, create a datum plane to serve as the sketch plane. Select the
reference point first, and then select the other two datum points in a
counterclockwise fashion. Click mouse button 2 to exit the procedure.
Note: While selecting the points in this way is not required, it will make certain operations that follow easier. For example, by selecting the points in a counterclockwise order, the normal to the plane points out of the viewport and the sketch plane will be oriented automatically in the 1–2 view in the Sketcher. If you select the points in a clockwise order, the plane's normal will point into the viewport and the sketch plane will have to be adjusted in the Sketcher.
Using the Create Datum Axis: Principal Axis tool
, create a datum axis parallel to the
Y-Axis. As noted earlier, this axis will be used to
position the sketch plane.
You are now ready to sketch the geometry. Use the Create Wire: Planar tool
to enter the Sketcher. Select the datum plane as the plane on
which to sketch the wire geometry; select the datum axis as the axis that will
appear vertical and to the left of the sketch. You may need to resize the view
to select these entities.
Once in the Sketcher, use the Sketcher Options tool to modify the display. In the General tab, change the Sheet size to 20 and reduce the Grid spacing to 8. Zoom in to see the datum points more clearly.
Note: If the sketch plane is not oriented in the 1–2 plane, use the Views toolbar to change to the X–Y view.
Using the Create Lines: Connected tool
, sketch the lines representing the main truss, as shown in
Figure 2.
The datum points that were projected are treated as fixed points in the sketch.
Any line connected to one of these points effectively inherits a fixed
constraint at that point.
Figure 2. Main members of the truss.
Next, create a series of connected lines as shown in Figure 3 to approximate the interior bracing of the truss.
Figure 3. Rough layout of interior members.
At this stage, the layout of the interior bracing is arbitrary and is intended only as a rough approximation of the true shape. The endpoints of the lines, however, must snap to the edges of the main truss members. This is indicated in the figure by the presence of small circles next to the intersections of the interior bracing with the main members. Avoid creating 90° angles because that will introduce unwanted additional constraints.
Dimension the vertical distance between the left endpoints of the sketch and the horizontal distance between the reference point and the right endpoint of the sketch, as shown in Figure 4. These dimensions will act as additional constraints on the sketch. Accept the values shown in the prompt area when creating the dimensions. These values represent the dimensions of the part, projected from the global Cartesian coordinate system (depicted in Figure 2) to the local sketch plane.
Figure 4. Dimensioned sketch.
To complete the sketch, recognize from Figure 2 that the interior bracing breaks the main members into equal length segments on both its top and bottom edges. Thus, impose equal length constraints on the segments of the top edge of the main member; repeat this operation for the bottom edge of the main member. The final sketch appears as shown in Figure 5.
Figure 5. Final sketch of single truss structure.
Using the Save Sketch As tool
, save the sketch as Truss.
Define the geometry of the second truss structure
Define three datum points using offsets from the end points of the truss, as shown in Figure 6. The offsets from the parent vertices are indicated in the figure. You may need to rotate your sketch to see the datum points.
Figure 6. Datum points, plane, and axis.
Use the Add Sketch tool
to retrieve the truss sketch. Translate the sketch by selecting
the vertex at the top left end of the new truss as the starting point of the
translation vector and the datum point labeled
P in
Figure 6
as the endpoint of the vector. Zoom in and rotate the view as necessary to
facilitate your selections.
Note: If the points defining either the original or new datum plane were not selected in a counterclockwise order, you will have to mirror the sketch before translating it. If necessary, cancel the sketch retrieval operation, create the necessary construction line for mirroring, and retrieve the sketch again.
Create the cross brace geometry
Context:
Recall that the cross bracing must be treated as a separate part to properly represent the pin joints between it and the trusses. The easiest way to sketch the cross bracing, however, is to create wire features directly between the locations of the joints in the trusses. Thus, we will adopt the following method to create the cross bracing part: first, a copy of the truss part will be created and the wires representing the cross brace will be added to it (we cannot use this new part as is because the vertices at the joints are shared and, thus, cannot represent a pin joint); then, we will use the cut feature available in the Assembly module to perform a Boolean cut between the truss with the cross brace and the truss without the cross brace, leaving the cross brace geometry as a distinct part. The procedure is described in detail below.
The pin locations are highlighted in Figure 1. Use the Create Wire: Point to Point tool
. In the Create Wire Feature dialog box,
accept the default setting of Chained wires and click
to add the cross bracing geometry to the new part, as shown in
Figure 7
(the vertices in this figure correspond to those labeled in
Figure 1;
the visibility of the truss in
Figure 7
has been suppressed). Use the following coordinates to specify a similar view:
Viewpoint (1.19, 5.18, 7.89), Up
vector (−0.40, 0.76, −0.51).
Figure 7. Cross bracing geometry.
Tip: If you make a mistake while connecting the cross bracing geometry, you can delete a line using the Delete Feature tool 
; you cannot recover deleted features.