MCAD Modeling Methods--File Translation Options1 Mar, 2006 By: IDSA ,Mike Hudspeth
New feature-based capabilities can translate your design intent where it's needed.
We Work Hard on our designs. We take great pains with accuracy. We agonize over our Class A surfaces to make sure they look their best. We build parametric models so changes occur easily down the road. We do all we can to produce models of exacting quality. That's how we maintain our place in the market. But unless we make every part of our product in-house, something rarely done today, we must pass our data to an outside vendor or bring in outside data.
Online file translation services
One CAD program usually won't read data from a different application. CAD vendors do that for a variety of reasons, but it boils down to two: they don't want us seamlessly transferring our data to a competitor's software, and they don't want someone to tell them how to do things. The first is a legitimate business practice as well as a bit of plain old paranoia. Although CAD vendors would never admit it, they all seem to have a secret desire to corner the market. At the very least, they don't want to see competitors benefitting from their hard and expensive work. So they write their software using proprietary code and their own formats. If no one can read it, no one can steal it. As inconvenient as that can make things sometimes, you can hardly fault them for it. We have a patent process for the very same reasons.
Software vendors tend to start out because they want to do things differently, which is often a good thing. Some branch off from companies or universities because they have a great idea and the company they work for doesn't want to do it that way. However they start, they have a right to go their own route. That's essentially how we got into the mess we're in today.
A fillet is a smooth transition from one surface to another, particularly at a joint. If you were educated on the drafting board, that was the end of it. But now fillets mean 2D representations on a drawing. When one line on a page meets another, the fillet is the arc that joins them.
In the 3D world, things get a little more complicated. Many edges may come together at the joint; you may have multiple radii. All that geometry does the same thing as the fillet, but different CAD vendors call it different things. Some call them blends. Others call them rounds. Who's right (figure 1)? No one's wrong. It's the same with other entities. Some vendors call circles circles, others call them arcs, still others call them curves. Some vendors refer to both lines and arcs as curves and let what they do define their shape. It can become very confusing, especially for translation software. So how do you get from one proprietary CAD format to another?
Figure 1. What is this—a fillet, a round or a blend? It depends on who you talk to.
If you don't speak German, you can't read a Munich newspaper. If you're lucky, you can find someone to tell you what it says. Instead of going from the printed page straight to you, it must first go through someone else. The trouble comes when the other person chooses his or her words. If you ask five people to translate something, you'll end up with five slightly different versions, all technically correct.
In the CAD world, the consequences of this are pretty serious. Back in the early days, we didn't have nearly as many modeling programs. Companies would write a proprietary translator between, for example, Unigraphics and AutoCAD.
As the number of CAD programs multiplied, it became too burdensome for developers to do that, so they've turned to intermediaries. IGES, DXF and STEP are file formats that were set up as independent go-betweens. Each software vendor needs to translate its proprietary format into and out of one of these formats only. But there are limitations. We'll talk about those in a minute.
What do you do if you want to make your data definite? Some software programs use a modeling kernel licensed from another developer. Theoretically, any program that uses the same modeling kernel can import your data. Parasolid and ACIS are two of the most widely used modeling kernels available. SolidWorks can read in Unigraphics files directly because both programs use the Parasolid modeling kernel.
Software or Services
Most modern modelers integrate translators because users are likely to encounter files from many sources. In programs like SolidWorks, you open the file and choose from the list of what it can handle (figure 2). You can bring in only what the software can recognize, so if a file isn't on the list, you have to look elsewhere for translation. My company originally bought its first license of SolidWorks to use as a translator. It had many more import and export formats than our software did. That was actually a good thing for SolidWorks because it got us to look at them for the first time. Now we have several licenses.
Figure 2. SolidWorks lets you open files it can translate. If you don t see your file type on the list, you ll need to find another way to translate it.
Many services on the Internet will translate your files (figure 3 and box). Some are better than others. When an outside vendor handles your translations, you should specify the deliverables clearly. Make sure the vendor can handle what you need. If you need a parametric model, make sure it can provide one. If you just need the shape of the model for general fit, don't pay for a fully parametric model. If you need a solid model, make sure you get one. Sewing a complicated model into a solid can be frustrating and time consuming.
Figure 3. Outside companies can do translations for you. Pay attention to their location if your files are particularly sensitive.
What You Get
The upside to file translation is that you can pull data into and out of your software with far less effort than if you had to remodel it. Many Web sites offer catalogs of parts that you can use in your designs. You just download what you want. Most of the time it's even free!
In the past, if you used a parametric software package, you weren't able to translate your parameters. Today, translation programs are beginning to maintain quite a bit more of your design intent. Programs such as Acc-u-trans from Translation Technologies (figure 4) and Elysium's CAD-Feature are feature-based translators. That means they look deep inside your models and translate on a feature-by-feature basis. Extrudes, blends and even sketches can transfer intact. What they cannot translate, they convert to boundary representations. This process will save hundreds of hours of work.
Figure 4. Acc-u-trans is a feature-based translator that rebuilds your data one feature at a time. You end up with actual design intent, not just a geometric representation or "dumb" solid.
But file translation has a dark side. Depending on the data, its ultimate source, the type of translation format and other factors, you could end up with quite a mess. IGES files are notorious for not handling trimmed surfaces well. You will usually get good data, but every so often you can end up with surfaces that lose their trims and fly out all over the place. Also, because of little differences among software packages, you often see surfaces that don't quite meet where they should. Gaps open up where one program rounds to four decimal places and the other rounds to more. You can fix these problems. Some modeling packages, such as VX CAD/CAM, include healing capabilities. Depending on how you set it up, the program interactively or automatically examines the model and fixes problems by adding patches to cover holes or by altering surfaces to stretch them together (figure 5).
Figure 5. VX CAD/CAM is known for its powerful model-fixing capabilities. VX CAD/CAM identifies problematic areas and then manipulates the surrounding surfaces to seal the gap.
More than ever, excellent options for file translation are out there. With the new feature-based capabilities some programs employ, your design intent can go where it's needed. Whether you do it yourself or hire someone to translate for you, the process is getting more and more reliable. That's a very good thing indeed.
Mike Hudspeth, IDSA, is an industrial designer, artist and author based in St. Louis, Missouri.
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