AutoCAD and Its Applications—Comprehensive 2020, 27th Edition Page 255 (1327 of 1650)

Copyright Goodheart-Willcox Co., Inc. Chapter 10 Advanced Surface Modeling 255 to set the projection method used by AutoCAD when projecting curves onto a surface. The following settings are available. • Automatic. The cutting object is projected onto the surface to be trimmed. The projection is based on the current viewing direction. In a plan view, the projec- tion of the cutting object is in the viewing direction. In a 3D view, the projec- tion of a planar curve is normal to the curve, and the projection of a 3D curve is parallel to the direction of the Z axis of the current UCS. The Automatic option is set by default. • View. The cutting object is projected in a direction based on the current view. • UCS. The cutting object is projected in the positive or negative direction of the Z axis of the current UCS. • None. The cutting object is not projected and must lie on the surface in order to perform the trim. PROFESSIONAL TIP PROFESSIONAL TIP The SURFTRIM command defaults to the Automatic option. Automatically projected geometry is used in most trim situations. NOTE NOTE When using the PROJECTGEOMETRY command, the SURFACEAUTOTRIM system variable controls automatic trimming. The default setting is 0. A setting of 1 enables automatic trimming of surfaces. The current setting is indicated by the Auto Trim button in the Project Geometry panel of the Surface tab on the ribbon. Exercise 10-9 www.g-wlearning.com/CAD/ Complete the exercise on the companion website. Projected curves A B C Surface area to be trimmed Curves to be used as cutting objects Figure 10-22. Using projected geometry with the SURFTRIM command. A—The cell phone case model is a surface created from a mesh. The arcs drawn above the model are used for trimming the sides. B—Selecting the two curves and pressing [Enter] projects the curves to the model surface. The areas within the projected geometry on each side are selected as the areas to trim. C—The model after trimming.

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Copyright Goodheart-Willcox Co., Inc. Chapter 10 Advanced Surface Modeling 255 to set the projection method used by AutoCAD when projecting curves onto a surface. The following settings are available. • Automatic. The cutting object is projected onto the surface to be trimmed. The projection is based on the current viewing direction. In a plan view, the projec- tion of the cutting object is in the viewing direction. In a 3D view, the projec- tion of a planar curve is normal to the curve, and the projection of a 3D curve is parallel to the direction of the Z axis of the current UCS. The Automatic option is set by default. • View. The cutting object is projected in a direction based on the current view. • UCS. The cutting object is projected in the positive or negative direction of the Z axis of the current UCS. • None. The cutting object is not projected and must lie on the surface in order to perform the trim. PROFESSIONAL TIP PROFESSIONAL TIP The SURFTRIM command defaults to the Automatic option. Automatically projected geometry is used in most trim situations. NOTE NOTE When using the PROJECTGEOMETRY command, the SURFACEAUTOTRIM system variable controls automatic trimming. The default setting is 0. A setting of 1 enables automatic trimming of surfaces. The current setting is indicated by the Auto Trim button in the Project Geometry panel of the Surface tab on the ribbon. Exercise 10-9 www.g-wlearning.com/CAD/ Complete the exercise on the companion website. Projected curves A B C Surface area to be trimmed Curves to be used as cutting objects Figure 10-22. Using projected geometry with the SURFTRIM command. A—The cell phone case model is a surface created from a mesh. The arcs drawn above the model are used for trimming the sides. B—Selecting the two curves and pressing [Enter] projects the curves to the model surface. The areas within the projected geometry on each side are selected as the areas to trim. C—The model after trimming.

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256 AutoCAD and Its Applications—Advanced 256 AutoCAD and Its Applications—Advanced Untrimming Surfaces If you need to restore a trimmed surface back to its original shape, use the SURFUNTRIM command. After selecting this command, select the edge of the surface area to untrim. If the surface has multiple trimmed edges, you can use the Surface option. The SURFUNTRIM command untrims surfaces trimmed by the SURFTRIM command. It does not untrim surfaces trimmed using the PROJECTGEOMETRY command. PROFESSIONAL TIP PROFESSIONAL TIP Open the Properties palette if you are unsure if a surface has been trimmed. The Trimmed surface property setting indicates the surface trim status. NOTE NOTE Exercise caution if you are trimming an associative surface using another surface as a cutting object. When using another surface as the cutting object, turn off surface associativity (set the SURFACEASSOCIATIVITY system variable to 0) before trimming to avoid potential problems in future edits. NURBS Surfaces NURBS Surfaces When creating a NURBS surface, you use many of the same commands that you would use to create a procedural surface. You can use splines and various curve shapes to create NURBS surfaces. In addition, you can convert procedural surfaces into NURBS surfaces. As discussed earlier in this chapter, a NURBS surface is created when the SURFACEMODELINGMODE system variable is set to 1. In addition, NURBS surfaces are non-associative. The setting of the SURFACEASSOCIATIVITY system variable has no effect when creating a NURBS surface. The advantage of working with NURBS surfaces is that you use control vertices to control or infl uence the shape of the surface. The ability to edit control vertices provides signifi cant fl exibility in creating and sculpting freeform, organic shapes. For example, in computer animation work, NURBS surface modeling techniques are commonly used for modeling characters to produce the organic shape desired. In AutoCAD, you can create highly sophisticated, freeform shapes using NURBS surface models. This chapter introduces the NURBS surface modeling tools available in AutoCAD. NURBS Surface Modeling Workfl ows There are two common workfl ows used in NURBS surface modeling. You can begin by creating procedural surfaces and then convert them to NURBS surfaces, or you can create the initial surfaces as NURBS surfaces. When you start the modeling process by working from procedural surfaces, the following workfl ow is common: • Create procedural surfaces using commands such as EXTRUDE, REVOLVE, SWEEP, LOFT, PLANESURF, and SURFNETWORK. The SURFACEMODELINGMODE system variable should be set to 0. • Create other surfaces, such as blend surfaces, patches, fi llets, and offset surfaces. Use the commands presented in this chapter. • Convert the surfaces into NURBS surfaces. • Edit the NURBS surfaces as needed to create the desired sculpted shape. SURFUNTRIM Ribbon Surface Edit Surface Untrim Type SURFUNTRIM

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254 AutoCAD and Its Applications—Advanced 254 AutoCAD and Its Applications—Advanced Select the SURFTRIM command, select one or more surfaces or regions to trim, and press [Enter]. Next, you are prompted to select the cutting objects. Select one or more curves, surfaces, or regions. Then, press [Enter]. You are then prompted to select the surface areas to be trimmed. Select one or more areas. As you select each area, it is trimmed by the cutting object(s). If you trim an area that you wish to restore, use the Undo option. When you are fi nished trimming, press [Enter] to end the command. See Figure 10-21. The Extend and Projection direction options are available after selecting the SURFTRIM command. The Extend option determines whether a surface used as a cutting edge is extended to meet the surface to be trimmed. By default, this option is set to Yes. The Projection direction option specifi es the projection method used for projected geometry, as discussed in the next section. Exercise 10-8 www.g-wlearning.com/CAD/ Complete the exercise on the companion website. Using Projected Geometry to Trim Surfaces With the SURFTRIM command, you can trim surfaces using cutting objects other than existing surfaces. Objects used in this manner are referred to by AutoCAD as curves. Selecting a curve, such as an arc or circle, allows you to project the geometry onto the surface and use it as the cutting edge. See Figure 10-22. In the example shown, the cell phone case is selected as the surface to trim. The arcs located above the cell phone case are selected as the cutting edges. When selected, the arcs are projected onto the surface by AutoCAD. See Figure 10-22B. The areas to be trimmed are then selected to complete the sides. See Figure 10-22C. To use a curve instead of an existing surface for trimming, select the curve after selecting the object to trim. You can select lines, arcs, circles, ellipses, polylines, splines, and helices. The Projection direction option of the SURFTRIM command can be used SURFTRIM Ribbon Surface Edit Surface Trim Type SURFTRIM A B Cutting object Surfaces to be trimmed Areas to be trimmed Figure 10-21. Trimming surfaces with the SURFTRIM command. A—The original model consists of three loft surfaces. Two of the loft surfaces are to be trimmed. B—The model after trimming.

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