AutoCAD Tutorial : Session Six

3dface and complex surfaces, edge, splframe

Open your model. Get a plan view, and make sure that all the layers that are not text layers are displayed. Freeze HATCH, GRID, and TRASH as well.

Make a layer called FACES, and make it the current layer.

We have been making combinations of lines, polylines, and 3D faces so far, but let's now make a figure that consists entirely of faces, surfaces. These are the kinds of figures we should make, in fact, when constructing 3D models. So let's begin by making a box, entirely constructed of faces, to contain all the horizontal and vertical rectangles we've drawn so far.

* Use the 3dface command to make a face with the following corners: 2.5, 8.5, -2; 7.5, 8.5, -2; 7.5, 1.5, -2; 2.5, 1.5, -2.
* That surface will lie below all the lines and surfaces of the rectangle we've drawn. Now copy it and put the copy at an elevation of 2.5.
* Now you have a top and bottom surface. Put sides on the "box" by using the 3dface command.
Remember that you will want to use an isometric view and the osnap with endpoint option to make the sides of the box.
* You should now have a six-sided box consisting only surfaces. To prove that there are surfaces, let's move around the box and see how it looks from various angles. Use the vpoint command to make views from 1,1,1; -1,-1,-1; 1,-1,1; and -1,1,-1. After generating each viewpoint, use the hide command to see how the box appears. You will find that the views from below the level of the box (the ones with -1 as the z-coordinate) to be very deceptive. But you should find that, in each view, the rectangles we drew before are hidden and the polyline is not, because it lies entirely outside the box. Part of the polyline will be hidden, because the box will be in front of it.
* So far, we've only worked on four-sided boxes, quadrilateral shapes. Now let's try to make a box with five sides, one that contains the box we just drew (let's call it Box 1) but not the polyline. First, make a new layer called PENT; make it the current layer. Now let's make a 3dface using these 5 points: 1.7,1.3,3; 8.3, 1.3, 3; 8.3, 8.5, 3; 5, 11, 3; 1.7, 8.5, 3. (You will need to use twice to complete the command.)
* The results should be quite unsatisfactory. You will have picked five points, but you will have drawn one quadrilateral face, using the first four points specified, and one triangular face, using the last three points specified. Use the hide command and the list command to see what has happened.
* We can try this again, entering the points in a different order, to get a pentagonal figure. First erase your last surfaces (there are two separate ones) and enter the points in this order: 1.7,1.3,3; 8.3, 1.3, 3; 8.3, 8.5, 3; 1.7, 8.5, 3; 5, 11, 3. (You will, again, need to use twice to complete the command.) What do you have this time? You should have another quadrilateral figure and another triangular one, but this time they should cover the entire area correctly. Try the hide command to be sure.
* However, there are still two figures, with a line separating them, rather than a single pentagonal surface. We can make them appear to be a single surface by hiding the line of separation. Type EDGE and select the line of separation. You will probably have to do that twice, since there as a boundary line on each of the surfaces. In addition, there are still two separate surfaces, even though they appear to be one. If you were to try to erase the pentagonal surface, you would need to erase both the quadrilateral and the triangular ones.
* Let's copy the surfaces you just made and put them at an elevation of -3. Make sure you get both surfaces.
* Now you should have four new surfaces, two at an elevation of 3, and two at an elevation of -3. Choose a viewpoint of 1, 1, 1. Now let's put sides on the box, but let's use more complex surfaces to do that. In order to see what we're doing better, freeze all the layers except PENT. You should see the two pentagonal figures from a reasonable viewing angle.
* As we just learned, we can make figures with more than four sides, but we have to be careful about choosing the points in the right order. We must enter the points so that the first four points will define a quadrilateral figure and the next point (or two points) will make a second figure, either a triangular or a quadrilateral figure, out of points three, four, and five (or three, four, five, and six), in that order. In fact, it is possible to keep adding new points indefinitely, but each time we finish a quadrilateral figure, we start a new quadrilateral or triangular figure based on the last two points entered and the next one or two points. (Only the last figure can be a triangle.) The ordering of the points is crucial; each new figure is based on the last three or four points entered, always in the order entered. Let's see just how effective this command can be by putting sides on our pentagonal box, which only has a top and bottom so far.

* Set the osnap to end so that you will be choosing endpoints in the following. Type 3dface and then select points in this order. Start on the upper pentagon, at the corner furthest from you (the SW corner), then pick the same corner of the lower pentagon, then the next corner of the lower pentagon (going clockwise), then the corresponding corner of the upper pentagon, then the next corner (continuing clockwise) of the upper pentagon, then the corresponding corner of the lower pentagon, the next corner of the lower pentagon (still clockwise), and so on until you come back to the first two points, which you will have to pick again. Type hide to see the results. If there are incorrect surfaces, it is probably the result of selecting points out of order. You should do this over again, if necessary, to get it right.

* Now thaw again the layers HM, HB, HT, VR, VL, and FACES. (Remember, you could use H? and V? to get all the rectangles.) Use the hide command to see what happens.
* Making larger surfaces can become rather complex. Let's see how to approach that task, using a polyline as the boundary so that we can re-work the surfacing possibilities without typing coordinates again and again. Return to the plan view. Draw a closed polyline (still on layer PENT) using these coordinates: 11, 9, 0; 10, 4; 12, 0; 15, 1; 16, 3; 15, 5; 13.5, 7. Now let's try to make a surface within the boundary of that polygon. (Remember that, once you specified an elevation for the first point, all succeeding elevations were determined, and no z-values could be entered after the first point of the polyline.)
* First, zoom in to make the polyline occupy most of your screen area.
* If you look at this figure, I think you will find that it would be impossible to make a surface with this boundary with a single 3dface command. We may be able to do it with only two, but we will want to use a feature we have not previously discussed. When using the 3dface command, typing I with a space or before specifying a point causes the system to make the edge coming up (the one indicated by the point about to be specified and the following one) invisible. We made the common edge of the triangular and quadrilateral surfaces invisible before (but after the fact) to make our pentagon look like a pentagon. We could have done it while drawing the figure. We will do that here.
* Type 3dface and, using your rodent with osnap set to endpoint, select the starting point of the polyline (11, 9, 0), then type I, then select the next point (10, 4), then type I again, then select the point at 15, 1, then select the point at 13.5, 7. Don't complete the command yet, but notice that you have selected four points but specified that the lines from the second to the third and the third to the fourth points be made invisible. Now select the points at 15, 5 and 16, 3. Complete the command.
You have a hexagonal figure, but one edge of it is not visible (and, as you realize, the hexagon is actually two adjacent quadrilateral figures with the common edge not showing.
* Now we will make another figure to complete the surface. But first we should make certain that the surface we have drawn is really a surface. Draw a polyline from 10, 7.5, -1 to 15, 7 to 10, 6 to 15, 3 to 10, 1. Issue the hide command. You can see clearly where there are surfaces.
* Start the 3dface command again. Select the point at 10, 4; then the one at 12, 0; then type I; then select the point at 15, 1. Complete the command. That will make a tringular figure, this one adjacent to the original one. Here again, the common edge has been made invisible. Re-issue the hide command. Now the figure is complete.
* Now zoom back out so that you can see our boxes and the surface(s) we just made. Type splframe. Make the value 1 instead of the default value of 0. You have just changed one of the AutoCAD system variables; this one controls the visibility of the lines we have declared to be invisible. Sometimes we want to see them. Type regen. Notice now that all the invisible lines, the ones in the figures we just constructed and the ones in the boxes, are now visible. Type splframe again, change the value back to 0, and type regen again. The lines are invisible again.
* This may be a good time to check your AutoCAD manual. The 3dface command is not easy to use; more than one explanation may help.

Time to save and stop.

End of Session Six

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source : http://www.digitalcad.com/Htm/tutorials/CSA_beginners_autocad.htm

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