Working with NURBS Surfaces and Surface Tools in AutoCAD 3D

NURBS stands for nonuniform rational B-spline curves. In AutoCAD, you can treat a NURBS surface as a 3D spline with fit points. These fit points can be used to manipulate the geometry. They also enable better control of the surface; and by using different densities of fit points on AutoCAD 3D surfaces, you can control the degree of precision of the surface.

In this article, I will make a simple 3D surface representing the top surface of a computer mouse using NURBS and other surface tools.

Making a Surface with NURBS

To create a NURBS surface, select the NURBS Creation option from the Surface tab and then make any geometry using AutoCAD surface tools. In this case, I will create a surface with the Network Surface tool using the spline profiles shown in Figure 1.

Figure 1. Spline profile for creating a network surface.

A NURBS surface thus created will have lots of control vertices (CVs) along its geometry, which can be used to manipulate it precisely. By default, CVs will remain hidden from the drawing view. In order to make them visible, click on the Show CV icon on the Control Vertices panel of the Surface tab. Select Surface and press Enter. You will see lots of CVs similar to those of a spline on the surface. Now, select any CV from the surface; a gizmo will appear. Click on the Z-axis of the gizmo and move it down slowly. You will notice that the surface will deform accordingly, creating a cavity.

Converting a Simple Surface into NURBS

Keeping the NURBS Creation option on while creating a surface is recommended because it creates a uniform distribution of control vertices. You can also convert a normal surface into a NURBS by selecting the Convert to NURBS option from the Control Vertices panel of the Surface tab. Select Convert to NURBS and then click on the surface and press Enter. The NURBS will be added to the surface but it may not be visible. To make CVs visible, click on the Show CV option on the Control Vertices panel, click on the surface and then press Enter. The CVs will appear on the surface.

Applying Surface Trim Command

So far, we have created the NURBS surface, which has rough edges that need to be trimmed. To trim the surface, make a trimming profile above the surface and select the Trim tool from the Edit panel of the Surface tab. In this case, I am making an ellipse for the trimming profile. Before using the Trim command, make sure the Surface Associativity option is off in the Create panel of the Surface tab.

After making the profile, click on the Trim tool on the Edit panel of the Surface tab and select the surface that you want to trim, then press Enter. Select the trimming curve and then press Enter again. You will see a projection of the trimming boundary on the surface. Click outside the trimming boundary of the surface and press Enter to cut the rough edges from the geometry.

Now, select the curve that was used to trim the surface and then right-click to select Isolate > Hide Objects from the contextual menu. Don’t delete the curve used for trimming the surface, as it will also remove the trim portion from the geometry if the trim is associative. The final trimmed curve will look like Figure 2.

Figure 2. Surface obtained after using the surface trim command.

Adding or Removing CVs

If you want more control over the surface curvature, you can add more CVs to the existing surface. Make sure you have a NURBS surface with the CVs made visible and then click on the Rebuild tool in the Control Vertices panel. Now, select the NURBS surface. You will notice that a Rebuild Surface window will pop up. Change a number of control vertices in the U direction and V direction as required and click on OK. In Figure 3, for example, I have changed the number of CVs in the U direction to eight and V direction to four. U and V directions, in this case, are equivalent to the horizontal and vertical direction for the surface.

Figure 3. Rebuild Surface window.

Add and Remove CVs Selectively

In Figure 3, CVs were added in a proper order and at the uniform spacing on the surface. To provide additional control points on the NURBS surface, you can add CVs selectively. To see the surface clearly, divide your complete drawing area into four model viewports from the Multiple Viewports option of the View panel on the Home tab as shown in Figure 4.

Figure 4. Adding CVs in the existing NURBS surface using model space viewports.

Select the Add option from the Control Vertices panel and click on the surface in any one of the viewports. Move your cursor over the surface and click at a point on the surface. This will add a new CV. In order to remove the CVs, select the Remove option from the Control Vertices panel, click on the surface and then click close to the CV that you want to delete. Removing the CV will reduce the number of control points, thus making simpler curves.

CV Edit Bar

The CV edit bar is required for further modifying the surface. To use the CV edit bar, select the tool from the Control Vertices panel and then click on the NURBS surface. You will notice that two red curves will follow your cursor along the surface. Click at any point on the surface to place the CV edit bar gizmo. Three mutually perpendicular axes representing U, V and W directions will become visible as shown in Figure 5.

Figure 5. CV edit bar on the NURBS surface.

Click on any axis and move it. You will notice that the surface will be stretched that direction. After changing the curve, you can also change the magnitude to modify the extent of deformation applied to the surface. To change the magnitude, click on the arrow grip and select the appropriate value of magnitude. You can also move your cursor dynamically to set the magnitude value of the curve. Figure 6 shows the final shape of our sample drawing’s surface after using the CV edit bar.

Figure 6. Surface obtained after manipulating it with CVs.

Auto Trim Command

Using this tool, we can cut a groove in the existing surface along the UCS, View or two-point direction. To use this tool, create a 2D curve above the surface representing the cutting boundary. Make sure the curve is closed. Click on the Auto Trim tool in the Project Geometry panel of the Surface tab and then select the Project to UCS option. Click on the 2D curve that you made for cutting the groove and press Enter. Then, click on the surface on which you want to cut the groove. This will create a groove, which is a projection of the 2D curve on the selected surface. Figure 7 shows the final geometry.

Figure 7. Final surface obtained after using the Auto Trim tool.

Conclusion

NURBS surfaces are a great way to make organic shapes with AutoCAD. The shapes that can’t be made with 3D modeling tools can be easily made with AutoCAD surface tools. This article introduced you to some of the basic surfacing tools of AutoCAD, including NURBS surfaces and CVs. Do you also use NURBS surfaces for creating 3D geometries in AutoCAD? Let us know in the comments below.