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Next, you will draw the seat of the chair. The seat of a butterfly chair is usually made of-canvas and drapes’ from the four corners of the chair legs. You will first define the perimeter of the seat using arcs, and then use the Edge Surface tool on the Surfaces toolbar to form the shape of the draped canvas. The Edge Surface tool creates a surface based on four objects defining the edges of that surface. In this example, you will use arcs to define the edges of the seat.

To draw the arcs defining the seat edge, you must first establish the ucss in the planes of those edges. Remember, in the last example you created a ucs for the side of the chair before you could draw the legs. In the same way, you must create , a ucs defining the planes that contain the edges of the seat.

Since the ucs you want to define is not orthogonal, you will need to use the three point method. This lets you define the plane of the ucs based on three points.

1. Click the 3 Point ucs tool on the ucs toolbar. You may also choose Tools >- ucs >- 3 Point, or type UCS..1 3..1. This option allows you to define a ucs based on three points that you select.

2. At the Specify new origin .point <0,0,0>: prompt, use the Endpoint Osnap to pick the bottom of the chair leg to the far left, as shown in the top panel. This is the origin point of your new ucs.

3. At the Point on positive portion of the X axis: prompt, use the Endpoint Osnap to pick the bottom of the next leg to the right of the first one, as shown in the top panel.

4. At the Point on positive – Y portion of the UCS X-Y plane: prompt,
pick the top comer of the butterfly chair seat, as shown in the top panel. The UCS icon changes to indicate your new UCS.

5. Now that you have defined a UCS, you need to save it so that you can return to it later. Click Display UCS Dialog from the UCS toolbar, or choose Tools :> UCS :> Named UCS. You can also type UC↵. The UCS dialog box appears.

6. With the Named UCSs tab selected, right-click the Unnamed item in the list box, then select Rename.

7. Enter Front Side↵.

8. Click OK to exit the UCS dialog box.

You’ve defined and saved a UCS for the front side of the chair. As you can see from the UCS icon, this UCS is at a non-orthogonal angle to the WCS. Continue by creating UCSS for the other four sides of the butterfly chair.

1. Define a UCS for the side of the chair as shown. Use the UCS Control dialog box to rename this UCS Left Side, just as you did for Front Side in steps 5 through 8. Remember that you renamed the Unnamed UCS.

2. Repeat these steps again for a UCS for the back of the chair, named Back. Use the bottom image for reference.

3. Open the UCS dialog box again, and in the Named UCSs tab, highlight Front Side.

4. Click the Current button, and then click OK. This activates F~ont Side as the current UCS.

5. Choose Draw >Arc :> Start, End, Direction.

6. Draw the arc defining the front edge of the chair.Use the Endpoint Osnap override to pick the top endpoints of the chair legs as the endpoints of the arc.

7. Repeat steps 3 through 7 for the ucs named Side, and then again for the ucs named Back-each time using the top endpoints of the legs for the endpoints eX the arc;

Next, you will mirror the side-edge arc to the opposite side. This will save you from having to define a UCS for that side.

1. Click World UCS on the ucs toolbar. This restores the WCS. The reason for doing this is that you want to mirror the arc along an axis that is parallel to the plane of the WCS. Remember that you must go to the coordinate system that defines the plane you wish to work in.

2. Click the arc you drew for the side of the chair (the one drawn on the Side UCS).

3. Click the midpoint grip of the arc in the Side UCS; then right-click the mouse and select Mirror.

4. Enter C↵ to select the Copy option.

5. Enter B↵ to select a new base point for the mirror axis.

6. At the Base point: prompt, use the Intersect Osnap to pick the intersection of the two lines in the Front plane.

7. Next, use the Intersection override to pick the intersection of the two legs in the Back plane.. The arc should mirror to the
opposite side, and your chair should look like.

8. Press the Esc key twice to clear the grips.

Finally, let’s finish off this chair by adding the mesh representing the chair seat.

1. Click the Edge Surface tool on the Surfaces toolbar, or enter Edgesurf↵ at the command prompt.

2. At the Select edge 1: prompt, pick the arc on the Front UCS.

3. At the Select edge 2: prompt, pick the next arc on the Side UCS.

4. Continue to pick the other two arcs in succession. (The arcs must be picked in a circular fashion, not crosswise.) A mesh appears, filling the space between the four arcs. Your chair is now complete.

S. Use View> Hide to get a better view of the butterfly chair. You should have a view similar.

6. Save this file.

At this point, you’ve been introduced to a few of the options on the Surfaces .’ toolbar. You’ll get a chance to use these later in this chapter. Next, you’ll learn how to edit mesh objects like the Butterfly chair’s seat.

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