AutoCAD Tutorial : Session Three

Session Three:

3dface (again), layer, ddlmodes, shade, copy (again), extrude rotate, (solmesh), perp, zoom, extend, trim

Open your model again.

3DFACE (again)

* Make a 3dface on the top of Rect. B. Be sure to use the endpoint command to put the corners in the correct spots. Use the hide command to be sure you have the top in the correct spot.


CAD models become very complex. Imagine, for instance, a model of a building with a complicated history of additions and alterations. If you show all the parts, since when some of them existed in the same place at different times, the result would be very confusing. You need to be able to show the parts you want when you want to see them. As you can imagine, there are many other reasons to separate pieces of a model from one another.

To deal with such complexity, CAD programs permit you to divide the model into pieces. All the pieces together make up the model, but not all have to be used at the same time. So the pieces that make up a particular phase of a building can be used at one time, and the pieces that make another phase be used at another time. Some of the pieces will be common to both phases; others not.

These different pieces of CAD models are called layers. (Despite the term, layers have no necessary physical relationship to one another. No layer need be above or below another; each is simply a portion of the model.) Each layer contains drawing entities - lines, surfaces, circles, etc. - which have something in common. A complex model really must be divided into various layers, though a simple model need not be.

When a new AutoCAD file is opened, the only layer is the one called layer 0. Others must be explicitly made. So we have been drawing everything on layer 0. (You will see a 0 displayed in the appropriate place to indicate the current layer; the place differs with different versions of AutoCAD.)

Even though our model is not really that complex, let's create some other layers so that we can begin to divide up the entities in the model. We can create any number of layers, and they can be named or numbered. We will name them, and our names will turn out to be very important; so be sure to type correctly. Everything we have drawn thus far will stay on layer 0 until we explicitly move something to another layer. (You can find out what layer anything is on with the List command.)

Type layer and then m to make a new layer. Make the name of the new layer VL (for Vertical rectangle, Left; we'll add a VR later), and type to exit the command. You will see that the box in the upper left now has VL in it. When you make a new layer with that process, it becomes the current one, meaning that any new model entities will be placed on that layer. VL is the current layer now instead of layer 0.

* Now type layer again, but this time follow with n to make new layers. Type HT,HM,HB,VLT,HTT,HMT,HBT,EXP to make eight new layers at once. (Note that you could not put any spaces in that sequence.) You can add several new layers at once with the new command but not the make command.

* Now let's put the model entities on different layers rather than leaving them on layer 0 where they are. You need to use the chprop command again. Since it will be easier if you work in the plan view; type vports and then si (for single viewport; oddly enough, "1" won't work here, though I've certainly tried it often enough). Note that if you had the 3D view in the active viewport, you now have a 3D view only. If so, type plan.
* Select Rect. A; then type chprop.
* Type la to indicate that you want to change the layer for everything selected.
* Then give the system the new layer name, VL and then another to complete the task.

Follow the same process for the other rectangles, using the appropriate layer name for each. HT for horizontal rectangle at the top of the drawing, HM for horizontal rectangle in the middle, and HB for horizontal rectangle at the bottom. As you can tell, using the names in this way will make it relatively easy to know what is on each layer.

Now you will find some difficulty selecting the rectangle you want; so it's time to talk more seriously about how to select entities for editing, moving, copying, etc. (Make sure you get the top of Rect. B on the correct layer with the rest of the figure. You may need to read the following to be sure.)

Earlier we talked about the difference between noun-verb commands and verb-noun commands. If you recall, you can state the process and then select the items to be processed (verb-noun) or do the reverse, select the items and then name the process (noun-verb). The items selected are called a selection set. The advantage of using the verb-noun system is that AutoCAD offers a number of ways to create a selection set, and the selection set can be augmented or reduced rather easily until you have everything you want - and nothing more than you want - and are ready to initiate the process. If you start with the object selection, on the other hand, you can't use all these helpful tools. You can add and delete items from your selection set with windows and direct selection only (to delete an item from the selection set, hold down the shift key while making the selection).

Let's start with the basics. You know how to select something. Just click on it. You also know that you can select something by using a window. But the window is more powerful and flexible than you probably realize. If you make a selection window by starting on the left, only objects entirely contained in the window will be selected. If, however, you start on the right and move left, any object even partially included in the window will be selected. That makes it possible for you to select objects much more effectively. You can also use a free-form polygon to select all objects inside it or a free-form crossing polygon to select all objects even partially within it. There is also a selection system called a fence, a free-form polygon that selects all objects the edge of the polygon passes through.

You can remove objects from the selection set as easily as you add them. You can simply use the shift key as you select to remove something from the set. In addition, only if you are selecting after stating the command, once you have selected some objects, typing r (for remove) will tell the system that your selections are to be removed from the selection set. Any selections you make then - with a direct choice or with a window - will be removed from the selection set, and the removal process will continue until you type a (for add) to return to the process of adding items to the selection set. When you are satisfied that you have the correct items in your selection set, you just type , and the command will be carried out.

One last comment while we're on this subject. For any command that needs a selection set, you can automatically choose the items in your last selection set - if you are using the verb-noun system - by typing p (for previous) when prompted to select an object. That selection set, though, can still be added to or reduced before the command is initiated, just as if the items had been selected with a window or direct selection method.

* We want the text items to be on separate layers, too; so select the text "Rect. A" and change its layer to VLT. Change the label, "Rect. B" to HTT, and so on. The layer name is the same as the name for the rectangle, with the T appended.

What layer is the polyline that we drew last time on now? I can't tell you, because I don't know how you drew the selection window when you were changing layers for the rectangles. If you used the system we just discussed, it should still be on layer 0.

* Let's put the polyline on layer EXP. You can select it by clicking on it with the mouse or by using a window. The easy window would be one made from right to left, just touching a piece of the polyline and nothing else.
* Use the chprop command to change the layer to EXP. (If you don't know what layer the polyline is on, the list command can be used.)

When you have finished moving the polyline to layer EXP, try to change the layer for the north side of Rect. A (the vertical surface on the north side of the box). In the current plan view, you can't see the surfaces; so you must return to a 3D view.

* If your last view in this viewport was the 3D view, you can type z (for zoom) and l (for last) to go to your last view. Otherwise, type vpoint and then 1,-1,1 to recreate the 3D view. (Using the coordinates 1,-1,1 to define an isometric view provides the angle defined by a line from the point 1,-1,1 to 0,0,0.)

* Now let's type chprop and try to select the north side of the box we made out of Rect. B by clicking on its upper edge. We will put it on layer EXP.

What is highlighted? It should be the top surface of the box, not the side surface. Why? Both surfaces (actually the side is a thick line, the top of the box is a 3D face, but we see them both as surfaces) include the edge you selected. When you pick such an edge or line that exists in two entities, AutoCAD will always choose the last one drawn, in this case the top of the box.

* So now you can click on the other edge of the side of the box to select it as well. But what do you do about the top, which is still highlighted?
* Type r (for remove) and select the edge of the top surface again. That will remove the top surface from your selection set, and you will have only the side surface selected. (You could also hold down the shift key while picking the top surface. That would also remove it from the selection set. It's probably easier to do, but it's a newer feature.)
* Now you can continue the command, moving the surface to layer EXP. (Release 13 includes a new way to deal with the problem of always picking the last item drawn - holding down the control key when picking will make AutoCAD choose one object, the last one drawn, but also prepare the system for finding others that share the line. Keeping the control key down and picking again will make AutoCAD select the next oldest object sharing the line, and continuing the process will get the system to cycle through all the objects there so that you can select the one you need.) It is often a good idea to use a different color for each layer (until you run out of colors) so that you can tell the items on diferent layers apart.

* So type layer again, then c (for color), then b for blue, then VL as the name of the layer to make blue, then you can either exit the command or, better yet, repeat the color changes, giving each of our layers a different color (use r [red], g [green], and m [magenta]).
* Before you exit the command, change the color for all the layers that have text on them to white (which will display as white on a black screen or black on a white one) by typing c for changing color, w for white, and then ??T as the layer name. AutoCAD will understand that the command should be applied to all layers with three chracters, the last of which is a T (the ? is a wild-card character, indicating to the system that any character in that spot will do; so two question marks indicate any two characters); so it will make white the color for all entities on layers with two characters followed by a T. Then exit the command.

Now each layer has its own color; so the north side of our box on Rect. B is the wrong color. We should put it back on the correct layer.

* Type chprop but do not select anything.
* Type p instead. That tells AutoCAD to select the last used (previous) selection set. Now you can finish the command.

Now let's suppress the material on layer HB.

* Type layer, f (for freeze), and HB as the layer name. Then press the to end the command. Note that Rect. D is now gone - or at least it should be!

You could use the hide command or draw more objects, even in the space occupied by Rect. D, and Rect. D would not be affected. When you thaw the layer again, the rectangle will reappear.

* Type layer, t (thaw), and HB as the layer name. Then press the to end the command. You're back where you were.

Indeed, having layer HB not showing while the label, "Rect. D," was still showing seemed silly. So type layer, f (freeze), HB*, and to end the command. That will freeze all layers beginning with HB; so both HB and HBT will be frozen. The rectangle and the label will be gone. (The asterisk is another wild-card character, but it indicates any number of unspecified characters, including none.)

* Type layer, t (thaw), *, and to end the command. That will thaw all layers, since the asterisk indicates that any characters fit the pattern.

Using wild cards in layer names is very much like a DOS or UNIX search, but, unlike DOS or UNIX, AutoCAD will also let you search for characters after an asterisk. So, for instance, the character string *P would work to indicate our layer EXP. Using all the possibilities for specifying layers in groups makes it very easy to manipulate layers using typed commands. By comparison, it takes several steps to accomplish the same processes if you use the dialog box that is next on our list of topics.


You can click on the layer push-button to get a dialog box that will let you make the same adjustments to layers (Release 13 only). That dialog box can also be brought up with the command ddlmodes or by using the pull-down menu called settings in Release 12, data in Release 13. (Commands with dd at the beginning all open dialog boxes.)

It is easier, though, to do some operations with the typed command, as we did above.

Layers can be frozen (or thawed), on (or off), and locked (or unlocked). A layer that has been turned off will not be displayed, just like a layer that has been frozen. But the entities on a layer that has been turned off and left thawed will be part of the data the computer must handle as it redraws and regenerates images. A frozen layer, on the other hand, is completely ignored; so I think it's better only to use freeze and thaw, leaving all layers on.

A locked layer cannot be edited. This is very helpful for layers on which you have primary data. Once entered, the data can be more secure if the layers containing that data have been locked. (You can still use the data points on the layer with commands such as endpoint.)


You have used the hide command. Now try shade instead.

If your current view is not our isometric one, return to our isometric view and shade the drawing again. (vpoint 1,-1,1 to get our isometric view back again.) * Type shade. The surfaces are now solid-filled with the colors of the outlines. This is a good way to see that you really have surfaces where you need them. Notice that only the text for Rect. C still shows. It shows because the text is at an elevation of 0, but most of the rectangle is well above that elevation; so it does not hide the text.

It's time to save your work.

COPY (again)

* I want you to make a copy of the box called Rect. A - the basic rectangle without its top - and put it on the right side of Rect. C, just as far to the right of Rect. C as Rect. A is to its left. We will call it Rect. E, and we'll put it on layer VR, with a label on layer VRT. Make the color of layer VR cyan. You should be able to be all that without instructions, but it won't be easy.

(Here's how I did it. I started by copying Rect. A - but it's not easy to copy the rectangle without its top. To select only the rectangle for copying, I issued the copy command, used a right-to-left window to select the rectangle and the top quickly, typed R to remove something, selected an edge of the rectangle, knowing that the top was the last thing I drew and would be removed first, and then finished the process. I placed the rectangle using the corners of Rect. A and C to determine the x- and y-coordinates. I used 0 for the z-coordinate, because the z-coordinate does not affect how it will look to us. We've used the coordinates from other rectangles before. Remember? But that put Rect. E right on the edge of Rect. C; so I used the distance command to find out how far apart Rect. A and Rect. C are, and I moved Rect. E accordingly - moving it only in the x-direction, using only the distance between objects in the x-direction. I used the layer command to make two new layers, VR and VRT, and to make cyan the color of layer VR. Then I used the chprop command to put the new rectangle on layer VR. Finally, I set the current layer to VRT and used the text command to make the label, rotating it as we did with the label for Rect. A. Before putting in the text, I returned to a plan view.)


* If you did what I did and changed to the plan view, return to the isometric view. (You can probably use zoom-previous. Before you make this figure, be sure you are on layer VR. When you extrude a figure, even though the figure already exists on a specific layer, the new figure will be on the current layer.

* Select Rect. E. Change the thickness to 0. (Use chprop, remember?) It is now only a rectangle again.
* Type extrude, and select Rect.E again. (This command does not work as a noun-verb command.) Now give a height of 1 and a taper angle of 0.

Rect. E now looks as it did before, but it is more complex. The new figure has a top and a bottom, and the system understands that it is a solid object, not simply a collection of surfaces. That is an important distinction; this figure is quite different from a series of surfaces. It can be given mass and volume, and many calculations can be performed to determine properties of a solid object with the shape of this figure. (Unfortunately, the solid object has no surfaces if you are working with Release 12. You must issue the command solmesh and select the solid in order to add surfaces to the solid block if you are using Release 12. That is not necessary if you are using Release 13, but please do it now if you are using Release 12.)

* Type hide to see how this looks. See what happens if you use shade instead. Note the effect of the lid we put on Rect. B. Note also that, in the hidden line drawing, AutoCAD does not produce a line where two surfaces intersect (where Rect. E passes into Rect. B).

* Let's put a truncated pyramid atop the box you just made. First make a polyline around the top of the box. You may need to regen first. (Be sure to use the endpoint sub-command to pick the points and to close the polyline.)
* Type extrude and then type l (last) to select the polyline. (It is the last entity drawn.) Extrude it for 1 unit with a taper of -15 degrees. (Use the solmesh command on this figure is you are using Release 12.)


* Now let's move the the truncated pyramid by rotating it.
* Select it. Issue the command, rotate and select the point about which it will be rotated; choose the nw corner of the base of the pyramid. Then enter the rotation angle of -45 degrees (counter-clockwise).

Now you will see that your 3D view seems wrong, but it isn't. Try hide and

shade to see how it looks. * Let's undo the rotation. (Don't forget to regenerate the image if your view is a shaded or hidden-line one.) Then rotate it again using an angle of -60 degrees. That looks better, but both were actually correct.

Now create a new layer, Trash, and put the truncated pyramid on that layer. Then freeze the layer trash.


Set the current layer to EXP.

Sometimes you need to make a line perpedicular to another but can't know the exact numbers to make it work. The perp command makes it easy.

* Draw a polyline starting at 12,5,0. Then type perp and select the polyline we drew earlier on layer EXP. (That should be a white or black line; we did not give the layer an explicit color.). Make the next point 12,2 and again type perp and select the polyline on layer EXP. Finish the command. Note that both line segments are perpendicular to the earlier polyline.

ZOOM - extents, previous, all, vmax, Xscale

Your lines may not show fully; type z (zoom) and then .5x to indicate that you want to zoom to half the scale of your current view.

There are several choices in the zoom submenu; all gives you the entire model, up to the limits, including empty space; extents gives you all the drawn objects and no unnecessary empty space; previous gives you the last view; a number gives you a magnification relative to the zoom-all view; a number with an x (as we just did) gives you a magnification relative to the current view; vmax gives you the largest magnification possible without a regeneration of the image. Zoom-window, of course, allows you to pick a window showing the area to be enlarged; you can specify the window corners with the mouse or with typed coordinates.


* Make a new line from 10,8,0 to 10.2,5.2,0.

* You can extend the new line to meet the polyline you just drew. Type extend and then select the polyline as the boundary.
* Type to indicate that you have no more boundary lines to locate; then select the last line you drew as the one to be extended.

Your line only extended to the first crossing point on the polyline, but if you do the process again, the line will extend to the next crossing point. Don't exit the command but select the line to be extended again, and your line will make it to the last crossing point.

Now make a line that is parallel to the line you've extended.

* Type copy, select the line and make the new copy pass through the first point of the polyline. To do that, make the "base point" of your copy at the first intersection of the line and the polyline (use intersection as you have been using endpoint. Then make the second point of displacement at the first point of the polyline.


* Trim the original line you extended by typing trim. Then select the polyline as the trimming or cutting line and hit (gently!) to indicate that you've finished selecting trimming lines. Next select the line to be trimmed - but where? Try the end that you extended. Then type u and try the middle section of the line. Keep undoing and reselecting until you are comfortable that you know what to expect when you select a part of a line. Then end the command with the two ends of the line still in place but the middle section gone.

* Put the polyline and your two parallel lines on the layer Trash. You will note that they disappear, because that layer is frozen.

Time to save again.

Make a slide of your model again.

Then quit. (Making the slide did not change the model; so we don't need to save again.)

End of Session Three

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