MCAD Modeling Methods-Best Practices for 3D File Transfers1 May, 2005 By: IDSA ,Mike Hudspeth
Don't just send the file—plan ahead to minimize problems
We've All Been There. You're modeling merrily along and the phone rings. "I have this file my vendor sent me that I can't open," the caller says, "Can you do anything with it?"
Figure 1. Missing faces can bring tooling to a standstill. Check data carefully before sending it anywhere. Entire faces may be missing, or the edges might not meet within tolerances, leaving open gaps that must be filled. Sometimes edges are too small to be of any value to the design and must be deleted.
"Well," you stammer. "I, uh...."
Your e-mail bleeps that you have a message. "There's the file. Thanks!" you hear as your caller hangs up.
You look at the file. It's not a file extension that your software can open directly, but it is a format in your list of translators. You throw it through whatever programs you have and voilà, you end up with something you can open. Unfortunately, that's when the real fun begins. After a very long time the file opens to reveal untrimmed surfaces that shoot off into space, large and small holes that shouldn't be there, wrinkly-looking surfaces that should be smooth—in short, a nightmare of digital proportions. Because you've been here before, you know what long, drawn-out steps you must go through to salvage what useful data has survived.
An ounce of prevention is worth a pound of cure. If designers followed a few simple steps when sending and receiving 3D data, the nightmare might just turn into a dream.
In the old days, transferring data was pretty easy: print and mail. That worked, but incorporating outside data was labor intensive—the receiver had to redraw everything. Computers then automated much of the process. When 3D modeling developed, users no longer had to redraw everything to make changes or additions. They merely changed the model and everything else took care of itself, more or less. Today, designers can bring data into designs from a bewildering number of sources via a simple cut and paste. But they must be careful what data they bring in and how they do so—they can't use just anything.
There are several ways to add outside data to designs. Most need at least a little preparation before being sent. Though some translators are fairly bulletproof, none brings along parametric information. Currently, three types of translators are available: 2D, 3D surfaces and 3D solids.
Polygon-based Data made easy
2D GeometryYes, many uses for 2D geometry remain even today: product manuals, instruction sheets, marketing materials and the like. Bit-map images are commonly used in presentations. Most 3D modeling programs provide some built-in way to save an image at high or low resolution, even if it's just a screen capture. Users can bring these images into almost any software. To go a little further, users can export vector-based geometry, such as CGM (computer graphics metafile), and bring it into a program such as Illustrator or CorelDRAW. These are wire frame images, but they can be edited easily by clicking and dragging. To use a vector-based format, users need to get what they want on the screen and export it. Vector formats present a few more choices than a bit-map. Usually text can be exported as text or lines—it depends on what the user wants to do with the data.
3D SurfacesDesigners need surface data for many reasons. The automotive industry, for example, is a big surface enthusiast. When dealing with surfaces, designers must look out for many more problems. Several different types of surface translations are possible. The most common are faceted, polygonal (very common in the gaming and entertainment worlds) and regular (such as bounded and NURBS). Faceted surfaces generally have a smaller file size and are less precise than regular surfaces. They are good for showing what a model looks like and can be manipulated faster and with less computing horsepower, but they don't carry as much technical information.
Faceted surfaces can also be used for other things besides visualization. Stereolithography files are basically faceted models. Designers can wrap image files around them and assign other properties. When brought into engineering software, they can't really be measured. Users can get an overall idea of how far apart things are supposed to be—just don't expect to make drawings from them.
Regular surfaces can be measured more exactly, but have their own issues. Many times surfaces are missing, so the data isn't usable as planned (figure 1). Users end up spending a lot of time going over the data, trying to find the problems and fix them (figure 2).
Figure 2. Frequently, surfaces lose their boundaries during translation. Modeling software such as VX CAD/CAM provides tools to heal these problems either by trimming the surface or by deleting and recreating it.
3D SolidsThese are probably the easiest and hardest files to transfer. Easiest, because a solid transfers as a solid and is an exact representation of the part. Users can measure it accurately and create drawings and assemblies from it. It's the hardest because of the disparity among modeling kernels. One kernel isn't necessarily compatible with another. Keep in mind that one of the greatest advantages of parametric solid modeling—the parameters—won't survive the transfer. Another advantage of 3D solids is that users can also export them as 2D wire-frame data and 3D surfaces.
Send and ReceiveThat said, what should be the protocol for sending and receiving 3D models? Before sending models, ask recipients what program they intend to use the data in. Is a Unigraphics file moving into SolidWorks? Are Alias surfaces going into Pro/ENGINEER? Knowing that can save everyone a lot of trouble.
Some companies ask for IGES or DXF files without stopping to find out if the sender has the same software as they do. Also, translators may have been written specifically to take one software format into another. These are generally the best way to go because the work is done without reinventing the wheel. Always ask before sending anything.
Next, find out the best file format to send. That decision depends on what the sender and the recipients want to do with the data. If the data will be used for visualizations only, there's probably no need for high accuracy. If the model will be used for general sizing, say for creating packaging, a faceted model should be enough (figure 3). If the data will be used to cut steel for molds, the receiver must have the most accurate model possible.
Most vendors have a list of acceptable formats they can handle. VRML and STL are faceted formats. Parasolid and ACIS are solid formats. DXF can handle just about anything, as can IGES, but be careful with these last two. Depending on the settings, they break a solid down into surfaces.
Figure 3. For rapid prototyping, where the process is somewhat forgiving, it makes sense to use a faceted model. You can see the rough texture caused by the triangular surfaces used to represent the model. Notice the faces that stand out. Edit the surface normals and point them away from the model to smooth out the faces.
The Information TradeIt's interesting that recent studies show that most people use business software such as Microsoft Word and Excel to exchange information. Why? Because it's what they know. That's why Adobe is making such efforts to encourage 3D content within Acrobat 7. Adobe licensed Deep View technology from Right Hemisphere. Right Hemisphere likes to say its product, also available as a stand-alone program, is intended to be "the Swiss Army knife of 3D file formats." It takes a file and converts it to the U3D format, a new polygon-based general format that's quickly gaining acceptance. It lets users rotate and zoom, view the model as wire frame or shaded and more. See the box at left for more on polygons.
Of course, many other techniques help ensure clean data transfer. Some are more effective than others. If you find a method that works, share it with your colleagues. You'll be very popular for it. Without distribution, 3D data is of limited use. To be able to send and receive 3D data is a must. To do it well greases the wheels of your corporate machine and makes it run smoothly, as it was meant to.
Mike Hudspeth, IDSA, is an industrial designer, artist and author based in St. Louis, Missouri.
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