Lecture 9: 3D Modeling Modify Commands 1. LOFTING The loft command is similar to the extrude command, but much more versatile. Instead of extruding a single shape, the loft command allows you to extrude several shapes and make one continuous object. Here's an example of how it works, I have drawn several shapes and aligned them with the same center. Next, they were copied above and below the largest circle. The goal is to create a smooth object from the 7 profiles shown below. Here is a front view of the objects before and after the loft command: The example above is very simple, but think of how you can create complex shapes using this simple command. 2. USING THE LOFT COMMAND The goal of this exercise is to create a "twisted Cube" - a 20x20x20 cube that looks like the top was twisted 45 o. We're going to start with a simple one. Draw a square 20x20. Copy it in the same place. Rotated the 2nd square 45 o using the midpoints of the square as the base point.
Finally, move the 2nd square up 20 units. You should have something like this from the SW view: From left to right, the original square, the copied and rotated square and the moved 2nd square. Copy the 2 rectangles over (out of the way) to use in another exercise. Next you will create a lofted object from the 2 rectangles using the defaults. Command: LOFT Select cross-sections in lofting order: <SELECT BOTTOM SQUARE> 1 found Select cross-sections in lofting order: <SELECT TOP SQUARE> 1 found, 2 total Select cross-sections in lofting order: <ENTER, Then Dialog appears> Enter an option [Guides/Path/Cross-sections only] <Cross-sections only>: <PRESS OK IN THE DIALOG> After hitting OK, you square will be turned into one lofted 3D Solid. Now something doesn't look quite right to me. I would expect that corners of the squares would be even so that the lofted square looks like a 'twisted cube'. Instead, you can see lines going from the middle of the bottom square to a corner of the top square. Use the HIDE
command to see how it looks. It looks a little strange, but you can make out that it has some odd shapes. Take the pair of squares that you copied over earlier and draw lines from lower corner to upper corner as shown in the drawing below: Now start the LOFT command again. This time you'll try something different. Select the squares again as your cross sections, then instead of accepting the default, use the Guides option G. Now select the four lines which will guide the loft to create a shape that better represents what you want. Command: LOFT Select cross-sections in lofting order: 1 found Select cross-sections in lofting order: 1 found, 2 total Select cross-sections in lofting order: Enter an option [Guides/Path/Cross-sections only] <Cross-sections only>: G Select guide curves:1 found Select guide curves:1 found, 2 total Select guide curves:1 found, 3 total Select guide curves:1 found, 4 total Select guide curves: <ENTER> Compare the 2 objects, the 2nd one should look more like the goal of a twisted cube. To change how you view things, use the VISUALSTYLES command to open the Visual Styles Manager palette. This command allows you to quickly change settings for how you view 3D objects that previously needed the knowledge of several system variables. Use the settings highlighted below and press the "Apply to Current Viewport" button (or double click on any of the colored squares at the top. You should now have a clear view that illustrated the differences between the first lofted object and the second one that used guides.
Try creating your own lofted 3D Solids. Anything that can be extruded can be lofted - so any closed shape will work. Try lofting a rectangle to a circle. Try exploding a lofted object. There are many other options to this command, but in a effort to be brief and cover the basics, I will not go further into this command. Remember to pick the cross sections in the correct order, add guides to better define the shape. In review, you can see how extruding and lofting are two simple commands that give you a lot of power in the 3D world. Get familiar with extruding, as it is a great way to build things in 3D. Lofting will allow you to create more 'organic' shapes those previous versions of AutoCAD. 3. Revolved Objects Below is a sample drawing showing 2 different revolved objects (lamp and lampshade). It will also be your goal in this lesson to duplicate similar objects. So far you've only worked with very basic blocks. Suppose that you need to draw something other than a rectangular cube. You did some new shapes in the previous lesson while extruding
and lofting. AutoCAD gives you two commands for those times when you need to draw cylindrical objects. One (revsurf) will give you a complex model comprised of a 3-D surface made up of many facets. The other (revolve) will give you a solid object. The method that you use will once again depend on what you need it for. Take a look at the lamp on this page. This is an example of two different types of objects requiring two types of object construction. The lampshade is a 'hollow' object. Essentially it's just a surface. The lamp base is a solid object. The revsurf command was used to create the lampshade, while the revolve command was used to create the base. It's not the greatest looking lamp ever, so in this lesson, you'll be designing a lamp base and a lampshade. You'll start by defining half the profile of each object, then revolving them. This will also be good practice for viewing your 3-D model. Create 2 layers called SHADE and BASE and give them different colors. Make BASE your current layer. Start the polyline command. Begin the profile for your lamp base. Use Ortho mode to draw a backwards "" shape with the vertical line 10 units up and the horizontal lines can be whatever you like (you're the designer now). Next draw a spline to connect the two ends of your polyline using Osnaps. Hint, when ending the spline command, you will be asked for the start and end vectors - choose the endpoint just to the right of your ends in the vertical line. Draw a short vertical line as shown in step 3 (make sure Ortho mode is ON). Make SHADE your current layer Draw a short angled line as shown in step 4 (this is the beginning of the shade.) Make BASE your current layer. Start the REGION command and create a region from the lines that makeup the base. Don't include the vertical line at the top.
You have drawn everything you need for this lesson and will use modifying commands to complete the lesson. First you'll create the lampshade. Before you do this, though, you'll have to set two of AutoCAD's system variables (SURFTAB1 & SURFTAB2). These variables control how many facets you'll have in your surface. The default is 6, which will give you a very chunky looking shade (like a hexagon instead of a circle). The number you pick will also influence how fast your computer can display the object as well as how round the shade will appear. The sample on the previous page had the SURFTAB1 variable set to 24 to give the lamp 24 sides. To change these, type in SURFTAB1 at the command prompt enter 24. AutoCAD will show you the current setting and give you a chance to reset it. Set the variable and then set SURFTAB2 enter 2 for this variable. Since the vertical shape is a straight line, you only a setting of 2 (the minimum). Now you're ready to create the lampshade. Start the REVSURF (Revolve Surface) command. You will be asked to select the object to revolve. This is the line that represents the lampshade. Then you are asked to select the axis of revolution. Pick the vertical line that you drew up from the base profile. Accept the defaults of 0 for the start angle and 360 for the included angle. This will rotate your line a full 360 degrees. Command: REVSURF Current wire frame density: SURFTAB1=24 SURFTAB2=2 Select object to revolve: <pick the lampshade line> Select object that defines the axis of revolution: <pick the vertical line> Specify start angle <0>: <ENTER> Specify included angle (+=ccw, -=cw) <360>: <ENTER> You'll see that the lampshade looks like a lampshade now. Next you will create the base. Start the REVOLVE command. You'll be asked to select objects. Pick the region for the lamp base. Next you're asked to select the axis of rotation. With your endpoint Osnap on, pick the top and bottom of the vertical line. Accept the default of <full circle> for the angle of revolution. This will revolve the profile around the vertical line 360 degrees and create a solid object. Command: REV REVOLVE Current wire frame density: ISOLINES=4 Select objects: <SELECT THE REGION> 1 found Select objects: <ENTER> Specify start point for axis of revolution or define axis by [Object/X (axis)/y (axis)] <OBJECT>: <ENTER> Select an object: <SELECT THE VERTICAL LINE ABOVE THE BASE> Specify angle of revolution <360>: <ENTER>
VIEWING YOUR LAMP For the next few lessons, you should switch to the 3D modeling workspace. Look for the icon in the bottom right of the AutoCAD screen. Use the HIDE (HI) command to see that you really have 3D objects and that the shade blocks the top part of the base. If you switch to your SW Isometric view, you'll see that the lamp is on its side'. To arrange it to be sitting on its base, you will have to use the ROTATE3D command. Begin the command by typing ROTATE3D and select the objects that you want to modify and press enter. By accepting to default of 2points, you are going to tell AutoCAD what axis you want the lamp rotated about. Pick the points as shown in the example below. Make sure you have your Osnaps (Quadrant) on. Refer to the right hand rule for the correct rotation angle. Command: ROTATE3D Current positive angle: ANGDIR=counterclockwise ANGBASE=0 Select objects: Specify opposite corner: 2 found Select objects: <ENTER>
Specify first point on axis or define axis by [Object/Last/View/Xaxis/Yaxis/Zaxis/2points]: <PICK POINT 1> Specify second point on axis: <PICK POINT 2> Specify rotation angle or [Reference]: 90 <ENTER> Now that you have your own 3D model, try some of the viewing options described in Lesson 3-4. Save your drawing as cool_3d_lamp.dwg as you will need it in a later lesson. You'll be adding materials to it and rendering a visualization of it soon.