Article Submitted by Design Visionaries.
What about Broccoli?
There are many shapes that are exceedingly difficult to represent with CAD. Ordinary things like a stalk of broccoli or even the shape of a rose bush are common place yet exceedingly difficult due to the varied and “humaniform” nature of their geometry. These shapes are also difficult due their intricate nature and the degree to which everything is different. Capturing these shapes is near impossible. The shape of cedar shingles on a house or large screen panels are also difficult due to shear number of surfaces that are present even when it’s the same shape over and over again.
Figure 1. Cedar shingles on a shed
In these situations many engineers that are defining product definition data are forced to resort to the use of notes on a drawing to define the shape. They can’t model it. In some cases such as those that involve injection molding a standard texture note is enough. Below is a texture that can be specified from the MoldTech company. www.mold-tech.com SED 25020.
Figure 2. A swatch of the Mold-tech SED 25020
A designer will create the overall shape of a product and the injection molder will cut the mold. Then the mold will be sent to the texture people such as Mold-tech who will use one of several proprietary processes that will produce the specified texture on that shape. It’s worked well for many years but it takes some of the control away from the designer who is forced to rely on the mold maker’s judgment and standard catalog of textured shapes.
All of this creates a problem. When defining a new product it is not always easy for a designer to know what’s available without an extensive set of samples from the mold preparation that they happen to choose. Also, due to the fact that textures that are non-standard are difficult to apply to new surfaces especially when they are highly sculpted, designers limit their designs to fit the CAD tools and the textures that they have available. This is an intrusion on the creative process. The design should be driven by the needs of the user alone, not the needs or constraints of the computer program that happens to be employed or the texture house that happens to be selected. And what happens when the part will not be injection molded at all.
There are a few solutions that can be employed. One such solution is to provide a swatch, a small portion of the overall surface with the fully defined pattern or texture on it. This will inform manufacturing of the overall geometric thrust, without creating a massive and cumbersome file.
Another solution is to try to define a mathematical equation that drives the geometry so that you actually create the shape. It can be a bit difficult especially for those who’ve been out of school for a while. Many of the textures that you can achieve with mathematical formula are the result of alternating sinusoids. As shown in figure 2.
Figure 3. A solid model of a screen and on the right, the equations to create it
The model of the mesh shown above is available for download. It’s called meshmodel.prt and you may find it in the “models” section of NXTuroials.com.
Another techniques that you can use when you have a sculpted piece of geometry and you want to actually model the specific geometry is using the Point set command along with Associative copy. First you distribute points on the surface in a pattern that represents the locations of each repetitive portion of the geometry. Then you create a solid that you will serve as a cutter. Using the associative copy command, you duplicate the cutter all along the point set. Then you subtract all the cutters and voila, your pattern is done. For more info download the part file called “patterns on a surface.prt”
Figure 4. A surface with a texture created by the Point set and Associative copy command
Yet another way of creating highly repetitive geometry is using the instance along a guide curve technique. For example, you can create a simple revolved body and placing under it a spiral shaped curve. Then project that spiral along a vector straight up onto the body so you have the spiral shaped curve superimposed on top of the surface. Next you place some sort of cutter solid at the beginning of the curve and use the instance geometry command with the along guide curve option. See the exercise below:
Step 1 Create a revolved feature
Figure 5. A Revolved Solid
Step 2 Create a spiral curve
Figure 6. A spiral sketch on the bottom of the solid
Step 3 Project the spiral up onto the face of the surface
Figure 7. A spiral projected onto a surface
Step 4 Create a sphere at the end of the spiral and instance it along the curve. In this case there are 240 copies of the sphere, evenly distributed along the curve.
Figure 8. Spherical cutters distributed along a projected helix
Step 5 The face of the original shape was offset with a small negative offset, then all the spheres were subtracted.
Figure 9. The spheres are subtracted from the larger solid
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