From the course: AutoCAD: Advanced 3D Modeling

Loft surfaces from cross sections - AutoCAD Tutorial

From the course: AutoCAD: Advanced 3D Modeling

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Loft surfaces from cross sections

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- [Instructor] At this stage, we have a number of cross-sectional circles that line up with the sketch that we have shown in red. Now we need to start considering how this is going to work to wrap around the rear tire. Now we know that this is going to have to start at the seat post here and it's going to have to wrap around the rear wheel and ultimately terminate at some height above the world xy plane over here. So to help us visualize that, let's draw in some temporary geometry. I'll do so by drawing a line from the end of this purple line and the center of the circle here and also the midpoint of this yellow line. I'll draw that straight up using ortho, a distance of 70 millimeters so I'll type 70 Enter and then I'll turn off ortho. And I want this line to go all the way over here and terminate right there at the center of the circle. And then I want that to complete the triangle so I'll type c for close, Enter. So now we have a triangle that represents the situation. We know we have to start here, we have to ultimately get up to this height, so that this can wrap around the rear wheel. But we don't want to go in a straight line. We want that to be curved. So I'm going to need to draw in a curved path and to visualize that, we need to look at that triangle straight on so we need to re-orient the ucs to that plane which is defined by the triangle. So I'm going to type ucs Enter. And this time, I'm going to specify the ucs by clicking three points. The first point will be down here. This represents the origin of the coordinate system. The second point will be along the direction of the x-axis and that will be snapped to the end of this triangle. And the third point will be at the top of the triangle. So now, the xy plane of this new coordinate system is coincident with the plane defined by the triangle. Now I want to look at that dead on and I can use the Plan command for that. I'll type plan Enter and then I'll press Enter again to accept the default option which is the current ucs. So now I'm looking straight at that current ucs. I can see it as a triangle now. Now I'm going to draw a spline, I'll type spl for spline, and start the spline over here at this end point at the top of the triangle. And make sure that ortho is off so that you have freedom to draw this spline however you want, and then click one, two, three points and then the fourth point will be over here, and I'm going to specify the center by typing cen Enter and then I'll click over here and snap to the center of that circle. And then I'll press Enter to end the spline command. So now, the idea here is that this element is going to curve up around the rear wheel and the rear wheel will exist somewhere in this area. And I want this curve to be nice and smooth so I may need to come in here and adjust the position of these grips. What you don't want is it to kind of bow out like that or have any undulations. You want this to be nice and smooth so it may take a little bit of tweaking just to get it right. That's looking good, I want something that just smoothly curves up, goes around the rear wheel, and just kind of continues down like that. And this is obviously very subjective so your curve will doubtless be slightly different than mine and that's fine. I'll press Escape to deselect. I don't really need this line anymore so I'll delete that. Now I need to start moving these circles up to meet that spline. So I'll say move, m for move, select the circle, and move it up. Now I'm going to use ortho and move that straight up and in this case, I know it has to move up exactly 70 millimeters so I'll type 70 Enter. But it's the only one of these circles that I know the exact distance. All the others it's going to be more of a subjective process where I just move it up approximately like that. And I'll just move each one up more or less in the center. So that the center of the circle lines up with the spline. So that's approximately right, however, I'm going to move this down again. However, these are not oriented correctly with respect to this path. We need to also rotate the circles. So I'll use the Rotate command, select this circle, and rotate from its center point, turn off ortho, and then I'm going to use this rubber band line as a guide so that I can use it to make this tangent to the spline. I'll move or rather, I'll rotate the next one in the same way. Just visually rotating each one a little bit so that it lines up better with the spline guideline. And we have to also rotate this last circle as well. We want it to be something like that. Now let's take a look at this in 3D and you can see that these still line up from the top view and they also line up in this view and I think we finally have the cross-sections in the correct locations. So I can delete these guidelines. And now, let's turn this into a surface. I'll use the Loft command on the Surface tab and the way that this works is you simply have to select each cross-section in order and it will automatically blend between those different cross-sectional shapes. When you get to the end, just press Enter twice to complete the command. And there you have it.

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