40 AutoCAD and Its Applications—Advanced Copyright Goodheart-Willcox Co., Inc. Creating a Helix Creating a Helix A helix is a spline in the form of a spiral and can be created as a 2D or 3D object. See Figure 2-24. It is not a solid object. However, it can be used as the path or frame- work for creating solid objects such as springs and spiral staircases. When the command is initiated, you are prompted for the center of the helix base. After picking the center, you are prompted to enter the radius of the base. If you want to specify the diameter, enter the Diameter option. After the base is defi ned, you are prompted for the radius of the top. You can use the Diameter option to enter a diameter. The top and bottom can be different sizes. Entering different sizes creates a tapered helix, if the helix is 3D. A 2D helix should have different sizes for the top and bottom. After the top and bottom sizes are set, you are prompted to set the height or select an option. To specify the number of turns in the helix, select the Turns option. Then, enter the number of turns. The maximum is 500 and you can enter values less than one, but greater than zero. By default, the helix turns in a counterclockwise manner. To change the direction in which the helix turns, use the Twist option. Then, enter CW for clockwise or CCW for counterclockwise. The height of the helix can be set in one of three ways. First, you can enter a direct distance. To do this, type the height value or pick with the mouse to set the height. To create a 2D helix, enter a height of zero. You can also set the height for one turn of the helix using the Turn height option. In this case, the total height is the number of turns multiplied by the turn height. If you provide a value for the turn height and then specify the helix height, the number of turns is automatically calculated and the helix is drawn. Conversely, if you provide values for both the turn height and number of turns, the helix height is automatically calculated. Finally, you can pick a location for the axis endpoint using the Axis endpoint option. This is the same option available with a cone, cylinder, or pyramid. As an example, a solid model of a spring can be created by constructing a helix and a circle and then using the SWEEP command to sweep the circle along the helix path. See Figure 2-25. The SWEEP command is discussed in detail in Chapter 7. First, determine the diameter of the spring wire and then draw a circle using that value. For this example, you will create two springs, each with a wire diameter of .125 units. Therefore, draw two circles of that diameter, Figure 2-26. Their locations are not important. Next, determine the diameter of the spring and draw a corresponding helix. For this example, draw a helix anywhere on screen with a bottom diameter of one unit and a top diameter of one unit. Set the number of turns to eight and specify a height of two units. Draw another helix with the same settings, except make the top diameter .5 units. HELIX Ribbon Home Draw Helix Type HELIX Figure 2-24. Three types of helices. From left to right, equal top and bottom diameters, unequal top and bottom diameters, and unequal top and bottom diameters with the height set to zero.