3D AutoCAD 2010 for Starters and Others. . .

Saturday January 02, 2010

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Surface & Mesh Models

Surface and mesh models are better to visualize than wire frames, as they imitate solid models and can be rendered for presentation with color, textures, lights and shadows.   Such models have no mass properties.  Surface models in AutoCAD receive an ultra-thin computer-calculated surface with two sides like a thin piece of paper added between edges of a wire frame.  This can become a time consuming activity.   For faster results, surface models can be created from 2D objects by extruding and revolving operations and mesh primitives to create a new surface model.  

Mesh primitives used with AutoCAD’s free form modeling features consist of vertices, edges, and faces that define a 3D shape.  Mesh primitives can be modified using AutoCAD’s enhanced modeling capabilities for more fluid and free-form designs.  

Remember, the better you understand wire frames and Surface models the better you understand 3D solid modeling.

AutoCAD offers surfaces tools such as Rulesurf, Tabsurf,  Revsurf, and Edgesurf and applies objects called triangular 3D planar faces to make up so-called faceted surfaces by automatically creating a polygonal mesh and then hides the edge lines to make it look like a rectangular face, but in reality all surfaces are composed of triangular 3D planar faces.

Surface models, like wire frames have no volume or mass but the HIDE command can be used to hide background elements.  Surfaces can be shaded.  The visual style controls are used to display shaded surface models.

Figure 1 Simple flat Surfaces added between the wire frame top edges (2) allows hidden views (3) & (4) when surfaces are shaded or rendered.

3DFACE Command

The 3DFACE command is the simplest form of a surface in AutoCAD.

It creates a true 3D planar or flat surfaces or mesh of three-or four-straight edges to form the shape.  Each surface created is a single object.  Surfaces cannot be joined, subtracted or intersected.  They can be extruded into 3D Solids.  Once extruded, they can be joined, subtracted or intersected.

The first point defines the start point for the 3D surface.  After entering the first point, enter the remaining points in a natural clockwise or counterclockwise order to create a normal 3D face.  If you locate all four points on the same plane, a planar face is created and you are automatically prompted for the third and fourth point of the next surface adjacent to the previous one and so on.  See Figure 3-2.

If you continue the last two corners of the previous face will be considered as the first two corners of the current face.  Therefore you need to specify one or two corners to create the next 3D face.

When you shade or render the object, planar faces are automatically filled. The 3DFACE command is not suitable for objects with curved surfaces. Curved surfaces are created using other mesh commands.  

To use the 3DFACE command, apply the following procedure: 

1.   Invoke the 3DFACE command. 

2.   At the: Specify first point or [Invisible]: prompt, select first point. 

3.   At the: Specify second point or [Invisible]: prompt, select second point. 

Figure 2    3DFACE surfaces are added starting with the first point and continuing to place the first 3D face after the fourth point.   The command automatically prompts for the third and fourth point of the next 3D face and so on.  The HIDE command is used to create the view on the right with only visible edges after placing the top and side surface.

4.   Continue to select points following the prompts.  Notice that adjacent faces need only the third and fourth point to continue. 

Specify third point or [Invisible] <exit>:

Specify fourth point or [Invisible] <create three-sided face>:

Specify third point or [Invisible] <exit>:

Specify fourth point or [Invisible] <create three-sided face>: 

Entering i or invisible before the first point of an edge makes the edge invisible.

SURFACE MODEL EXAMPLE

Construct a surfaced model eye piece roof structure using extruded 2D geometry and planar surfaces and then applying the Union and Subtract command. See Figure 3.   Rectangles may be created in the respective planes using the appropriate UCS placement.

Figure 3  The eye piece geometry and finished surface model in SE Isometric view. 

Start with creating the three vertical planar rectangles using the proper UCS and the given dimensions.  See Figure 4. 1.   

 

Create the top rectangular planar surface and use the UNION command to join it with an arc section to form the closed top.

 

Place the circle into the top surface and convert it to a planar surface using the Object option.   Use the SUBTRACT command to subtract the circle from the top surface to create the opening.

 

Copy the finished top and insert it at the bottom.

 

Place a circle to match the diameter of circular opening at the bottom surface and add a 0.6 thickness to the circle using the Properties dialog box.  This creates the bore hole.

 

Create a 0.6 radius ARC section with a thickness of 0.6 to fit the rounded vertical portion of the model.

 

Convert all planar surfaces to Surface (Extrusions) including the bore hole using the Convert to Surface command and apply a Zero value to the U isolines and V Isolines property using the Properties dialog box.

Figure 4  With the use of the UNION and SUBTRACT commands, surface may be joined for assembly of the model and then converted to planar surfaces.

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