Manufacturing

Creating TurboCAD Solids

1 Jun, 2002 By: John E. Wilson


Last month, after comparing the solid-modeling capabilities of IMSI's TurboCAD Professional to those of AutoCAD, I concluded TurboCAD is the more powerful modeler. Even though its operations are somewhat quirky and its visualization tools primitive, TurboCAD can create geometry that is beyond AutoCAD's capacity. This month we'll look at TurboCAD's tools for creating solids. If you would like to experiment with these tools, you can download an evaluation copy at www.turbocad.com.

WorkPlane
The drawing tools of even the most sophisticated and expensive 3D modelers are largely confined to a 2D plane, so you must frequently move and orient this plane in 3D space to create 3D objects. TurboCAD refers to this plane as the WorkPlane, and it refers to its x,y,z coordinate system as the User Coordinate System (UCS). To help you visualize the WorkPlane, you can display an xyz tripod at its origin by selecting Workspace>User Coordinate System from TurboCAD's menu bar, and you can display a rectangular representation of the WorkPlane by selecting Workspace>Display WorkPlane. You can implement TurboCAD's tools for moving and orienting the WorkPlane from Workspace>WorkPlane in the menu bar as well as from the WorkPlane toolbar. Here are the WorkPlane tools you'll often use.

  • Origin moves the intersection of the WorkPlane's x and y axes to the point you specify without changing its orientation.
  • By 3 Points positions the WorkPlane with three user-specified points: the first point establishes the origin of the WorkPlane; the second sets the direction of its x axis; and the third controls the orientation of the WorkPlane as well as the general direction of the y axis. The third point does not have to be 90 degrees from the x axis.
  • By Facet places the work plane on the selected face of an existing 3D solid. The face can be curved, but generally you will select only planar faces. The point you pick in specifying the face serves as the WorkPlane origin, and often you will follow this tool with either the Origin or the By 3 Points tool to position and orient the WorkPlane with the edges of the selected face.
  • By World will return the WorkPlane to the xy plane of the World Coordinate System (WCS).
  • Previous restores the previous WorkPlane.
  • Edit lets you move the WorkPlane's origin to a point relative to the WCS origin, and to rotate the WorkPlane about its x, y, and z axes. The WorkPlane must be displayed before you can use this tool.
  • Named displays a dialog box for assigning a name to the current WorkPlane and to restore one that has been named.

Basic Solid Shapes
TurboCAD has a list of tools for creating the basic geometric shapes of solids, such as bricks (which TurboCAD calls boxes), spheres, cones, cylinders, polygonal prisms, wedges, or tori. These solids are always placed on the current WorkPlane. For each of these tools, the Inspector bar displays entry-fields and option-buttons. You can also access these options through right-click menus. For example, when you use the cone tool without selecting an option, the axis of the cone is perpendicular to the cone's base and you will be prompted to specify the cone's center, its base radius, and its height. You can either use your mouse or enter numeric values in the Inspector bar's fields to define the radius and height. Also, before you begin drawing the cone's base, you can select an option for creating a cone with a slanted or truncated axis.

The torus tool creates donut-shaped solids by default, but you can also use it to create spring-like helix-shaped solids, as shown in Figure 1.


Figure 1. TurboCAD's Torus tool has an option for creating helix springs with round wires. You simply specify the total length of the spring, the number of coils, the radius, and the wire radius. This figure shows a typical spring solid-model in its wireframe, hidden line, and rendered viewing modes.

To create a helix-shaped solid, initiate the torus tool and then open the Properties dialog box from the tool's right-click menu or the Inspector bar. Select Revolution Shape from this dialog box, enter the total height of the spring in the Spiral Pitch edit box, and enter the number of spring revolutions in the Number of Coils edit box.

You can also specify whether the spring is to be coiled clockwise or counterclockwise. TurboCAD will prompt you to specify the centerline of the spring, its radius, and the radius of the spring's wire. If you are not satisfied with your spring, you can use the Select tool to reopen its Property dialog box and change its height and number of coils.

Profile-Based Solids
Another way to create TurboCAD solids is to draw one or more closed 2D objects that represent a cross section, or profile, of the solid. To create a motor shaft, for example, you can draw, using a 2D polyline, the lengthwise profile of the shaft on one side of the shaft axis, and then revolve the profile a full 360 degrees about the axis.

Other tools in this category create solids by either pushing a profile through space or filling the space between two or more separate profiles. Of these tools, Normal Extrude is the most straightforward. You select it by choosing Insert>3D Object>Normal Extrude from the menu bar or by choosing Normal Extrude from the 3D Object toolbar. Next, you select the object to be extruded and then specify the height of the extrusion by either dragging the screen cursor away from the profile and clicking your pointing device, or by entering a z value in the Coordinate edit box. Lastly, you click Finish from the Inspector bar or from the right-click menu. A significant limitation of the extrusion tool--especially if you design parts that are to be manufactured by casting and molding--is that it cannot assign a draft angle or taper to an extrusion.

By default, the extrusion direction is perpendicular to the plane of the profile object, but if you turn the Normal Path option off, through a right-click menu or a button in the Inspector bar, the extrusion direction is on the plane of the profile. Furthermore, you can consecutively select any number of points on the work plane that are not in a line, and the extruded 3D solid will have mitered bends at each point.


Figure 2. A round profile has been created to be extruded along a path made from a TurboCAD 2D polyline (2a). The center of the profile has been selected to be its base point. 2b shows the resulting extruded solid in its wireframe mode. Notice that TurboCAD retains the profile and the path. 2c shows the solid in a hidden-line view.

You can also use a wireframe object, such as a polyline, to define the extrusion path. You must initiate the Select Extrusion Path option from the Normal Extrude Inspection bar or the right-click menu before selecting a profile. You'll be prompted to select the profile object and the path object. The profile object will move to the end of the path object, become perpendicular to it, and slide along the path to its other end. By default, the point on the profile object that you pick to select the profile is used as its point of attachment to the path. However, you can activate the Specify Extrusion Base Point option of the Extrude tool so you can specify the point on the profile to attach to the path, as shown in Figure 2. The path object can be a line or an open or closed polyline. The polyline, however, cannot contain any arc-shape elements. Circles, arcs, and splines can be used as path objects only after you have exploded them. A 3D polyline is the only object you can use as a 3D path.

TurboCAD has a Rigid Extrude tool that works in the same fashion as the Normal Extrude tool; however, in path extrusions, the profile does not reorient itself to become perpendicular with the path. If the path has been drawn to be perpendicular to the plane of the profile, the resulting extrusion is the same as one made by the Normal Extrude tool; if the path is at an angle to the profile's plane, the extrusion will have slanted ends; if the path is in the same plane as the profile, no extrusion will be created.

TurboCAD's Prism tool creates a 3D solid in the space between two 2D profiles that are in different planes. They do not have to be equal in size, but they must have similar shapes. You can't, for example, create a prism between a profile that has five straight sides and one that has six, or between a round profile and a rectangular one. But you can create a prism between a round profile and an elliptical one.

Like its Prism tool, TurboCAD's Lofting tool creates a 3D solid in the space between 2D profiles in different planes. But, unlike the Prism tool, the Lofting tool can handle any number of profiles and the profiles can have dissimilar shapes. The Lofting tool also accepts profile objects that the Prism tool does not, such as those made from splines and polylines containing curved segments. When more than two profiles are used, the solid's sides curve as necessary to blend smoothly according to NURBS mathematics along its length between the profiles. The most complex stage in creating a lofted solid is that of moving and orienting the construction plane and drawing the profiles. On the other hand, in using the Lofting tool, by selecting Insert>3D Object>Lofting from the menu bar, you simply select each profile in its order and click Finish in the Inspector bar to instruct TurboCAD to create the solid.


About the Author: John E. Wilson


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