Autodesk Mechanical Desktop: a Stroll Down the 3D Path
By Bill Fane
In previous tutorials, we have studied some of the four different kinds of 3D sweep paths. Last time we created the twisted part shown in Figure 1 by sweeping a profile along a 3D sweep path.
Figure 1: This part was produced by sweeping a profile along a 3D spline path.
Figure 3: The direction vector for the starting point is 0,0,1.
View Larger Image Figure 2: The dialog box that controls a 3D spline path.
We used the standard AutoCAD? SPLINE command, and then turned it into an Autodesk? Mechanical Desktop? 3D spline path. We then created and constrained a suitable profile and swept it along the path to produce Figure 1.
I ended the previous tutorial by pointing out that you had not actually constrained the spline path in the normal manner and referred you to the spreadsheet-like dialog box shown in Figure 2. This box comes up when you use the AM3DPATH command to turn the spline into a 3D sweep path, or when you double-click the path's entry in the browser to edit it later. As promised, we are now going to delve into the mysteries of the 3D Spline Path dialog box. The More Things Change... Let's start with the three columns labeled Delta X, Delta Y, and Delta Z, which represent the coordinates of the points through which the spline passes. This explains why you do not constrain the path in the normal manner. The constraining is taken care of in this dialog box. In mathematics, Delta means "a change of" or "relative to." In the case of a 3D spline sweep path, it means "relative to the origin of the sketch plane that was active at the time the path was created." This is an important point, so you might want to review it again to make sure you understand its significance. You can edit the value of any cell in the 3D Spline Path dialog box. Simply click the cell and then type in a new value, or use the spinner arrows to scroll the value up or down. The vertex being edited highlights in red in the drawing, and the blue spline curve adjusts in real time to follow your changes.
Now let's look at the three columns headed i, j, and k in the 3D Spline Path dialog box.
They specify the direction vector as the spline passes through each vertex. The values at the start and end of the spline are the only ones that can be changed.
Direction vectors may look a little intimidating, but they are actually quite simple. They show relative movements compared to the previous vertex, with i, j, and k representing the x, y, and z directions respectively. For example, i, j, and k values of 0,0,1 would represent 0 units in the x direction, 0 units in the y direction, and 1 unit in the z direction. The spline would therefore leave the start point normal to the x-y plane. These are relative values, so 1,1,2 would be identical to 2,2,4. The direction vector at the end works exactly the same way except that it shows the arrival direction. Note: Direction vectors are relative to the sketch plane that was active when the spline was created, which may not match the current coordinate system. The direction vectors also cause the work plane to realign itself to be normal to the start of the path. (See the Note at the end of the next section for more information.)
Figures 3 and 4 show the effect of changing the direction vectors. Keep in mind that the vertical face was set as the sketch plane when the spline path was created. In Figure 3, the start direction vector is 0,0,1 and so the spline starts out normal to the face. In Figure 4, that vector has been changed to 0,1,1 and so the spline starts at a 45-degree angle to the face. Notice also that the black work plane has tilted so it is still normal to the start direction vector.
Figure 4: The direction vector has been changed to 0,1,1. The end vector works exactly the same way, but don't forget that its values are relative to the sketch plane.
Figure 5: A low Weight value.
A Heavy Subject The last column in the 3D Spline Path dialog box sets the weighting for the start and end of the spline curve. Initially, the spline leaves the start point in a direction determined by the direction vector. It then begins to curve toward the first vertex point. A low Weight value means the spline begins to curve toward the first vertex almost immediately, as shown in Figure 5.
Figure 6: The same spline as shown in Figure 5, but with a higher Weight value.
A higher value means the spline continues in a straight line in the direction of the direction vector for a longer distance before it begins to curve, as shown in Figure 6.
Figure 7: A bracket, a spline curve, and two work points.
The weighting at the end of the spline curve works exactly the same way except that it sets the arrival weighting from the last vertex to the end.
The blue spline curve sort of follows in real time (see the following note), so a bit of simple experimenting with the spinner arrows will show the effect. Click OK in the 3D Spline Path dialog box when you are ready to commit to your changes, or click Cancel to abandon them.
Note: Except for the starting and ending points, the intermediate vertex x, y, and z values are not "carved in stone." When you finish editing, the spline curve may realign itself so it does not look exactly like the preview curve. Some vertices may move themselves to ensure the spline forms a smooth curve.
Interestingly, if you change the weighting back to a previously used value, you may not return to the exact same spline because the software calculates the change in relation to all current values, without regard for history. Even though Undo does work, you may want to save often if you are experimenting. I would also suggest that you change the weighting in very small increments until you become familiar with it.
All Tied Down Since all constraints are handled in the 3D Spline Path dialog box, how do we constrain a 3D spline curve to an existing feature?
Easy. We use the dialog box.
Figure 7 shows a simple L-shaped bracket, along with a 3D spline path. I set the vertical flat face of the bracket to be the sketch plane and created a work point on it, and then set the horizontal face to be the sketch plane and created another work point on it. The work points were dimensioned to the edges of the face.
Figure 8: The spline path is now constrained to the work points.
Double-clicking the spline path object to edit it brings up the 3D Spline Path dialog box (see Figure 2). Now note the two columns labeled C in that dialog box. Right-clicking either C box in row number 1 displays a shortcut menu. Clicking the Constrain to Work Point option from that menu, prompts you to select a work point. Clicking the work point that was placed on the vertical face causes the start of the spline to jump over until it coincides with the work point. Similarly, the last or ending vertex can be constrained to the work point that was placed on the horizontal face. Clicking OK completes the edit, as shown in Figure 8.
Figure 9: A profile can be swept along the 3D spline curve.
As usual, you can create and constrain a profile and sweep it along the path to produce a 3D swept feature, as shown in Figure 9.
Figure 10: Any editing of the bracket is reflected in the swept feature.
Figure 10 shows how any editing of the bracket or the work points is reflected in the spline path. As I noted previously, intermediate vertices will probably move to accommodate the new shape. If you insist that a spline must pass through a specific point or points, then suitable work points can be created to control the path. At least one end of the spline must be constrained to a work point, and then intermediate vertices can be constrained to the path points. Note: If you want to constrain a spline path to work points, then the work points must exist before the spline. The good news is, if you forget this requirement, you can create the work points later, then drag and drop them in the browser to resequence them so they appear ahead of the spline.
And in Conclusion... As we have seen, 3D sweep paths are quite easy to create and edit. They are particularly useful for creating flexible things like hoses and wiring looms. Be sure to come back next month when we see how simple it is to create 3D piping runs and to sweep a profile along the existing edges of a part.