Don't Draw It-Download It (MCAD Modeling Column)1 Jan, 2007 By: IDSA ,Mike Hudspeth
Software providers and Web sites have part libraries waiting for you.
One element required in most design projects is a full-blown assembly. You need fasteners, motors, actuators and all kinds of parts that you'll be purchasing from outside sources. Most of the time when you create an assembly, you need to either show the parts or at least accommodate their volume. How you do that can affect how long it takes to create your assembly. You have three choices:
- 1. ask or pay someone to model the parts for you,
- 2. model them yourself or
- 3. download a suitable model from the manufacturer's Web site.
Each choice has its own challenges.
First, timing may be a challenge with the first two options. Someone, either you or someone you trust, will need to model the parts. Most parts have drawings available online or in printed catalogs. Take a linear actuator, for example. It's basically a cylinder within a cylinder. If all you need is a visual representation, modeling it will be fairly straightforward. But if you need more detail, you'll have to create pieces that move as the actual part would. You'll probably want to create some halfway-accurate connection geometry at each end to mate up with other parts of your assembly. If weight is an issue, you need to know how much it weighs. You get the idea.
Second, cost is an issue. It will take time to model all the little screws and things that the typical mechanical assembly uses. It's never just one size throughout the job, either. There are any number of different but similar parts. Do you remember the scene in Apollo Thirteen when they needed to figure out how to adapt the lithium hydroxide canisters from the command module to work in the lunar module? The two parts did the same thing, but they had different geometries because they were from different manufacturers. It's the same today. I don't know how much your company pays for modeling labor, but it isn't cheap.
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Third, accuracy will be a problem. Any drawings that are available to you won't be perfect—just basic length, width and height, usually. They will also include dimensions important to the function of the part in question. But although the drawings might depict all the protuberances and undulations of the parts you need, they won't be dimensioned. You'll have to guess. And you're only going to be as accurate as the drawing. If there's a mystery projection on one side of the image, you'll need to make assumptions, a hazardous undertaking in a world clamoring for precision.
So What Do You Do?
Don't lose hope. You have an alternative to modeling purchased parts. Download. You're probably asking where to find such parts and how useful they really are. There are two main choices when it comes to downloaded parts: libraries included with your modeling software and everything available on the Internet.
Most 3D modeling vendors provide libraries of parts with their software. These parts will be the most effective ones you can use within that modeler. Each part is native to the application, so you won't encounter import issues. If your 3D modeler supports assemblies and parametrics, you can get great use out of the parts. Things like dynamic motion and animations will benefit from library parts. Downsides may be the level of detail and where the vendor obtained the information used to model the part. I've seen some models that were just outside geometry with nothing internal—static 3D snapshots. Others are fully parametric. Check the parts out carefully. If they come with the software, you'll be ahead of the game even if you need to fix something.
Solid Edge comes with libraries that contain millions of parts you can pull into its assemblies. They run the gamut from simple screws to standard piping components. Additional libraries are available to supplement the included parts. UGS partners with the original manufacturers, so the libraries are pretty accurate.
SolidWorks has 3D Content Central. It's much the same, except it's an online resource. It also offers millions of parts, and has some additional features of note. SolidWorks lets users post models. The User Library area contains an interesting mix of user-supplied components, ranging from bearings, electrical items and hardware to knobs, miscellaneous and structural. You'll have the most fun in the Miscellaneous area. You can find everything from jewelry to toys to 3D clip art, which includes a parametric model of Homer Simpson complete with doughnut and Duff (figure 1). But hang on to your expectations. Not all of these models are meant to be dead-on accurate. Some are merely fast models made for fun. Being a sci-fi nut, I was hoping for better quality than what I found, but to be fair not everyone has the time to model the U.S.S. Enterprise perfectly (the one they have is pretty good, though).
Figure 1. The SolidWorks Web site offers fun models as well as useful ones. This model is an assembly of several parts. Some people have way too much time on their hands.
Check with your CAD software vendor to see what part libraries are available. Almost all vendors offer something.
Out and About
When you begin to stray from files supplied by software vendors, things get more interesting. The Internet offers models supplied directly from a vendor's Web site as well as online catalogs compiled from various vendors and miscellaneous third parties.
Vendor Web sites are where you'll get the most informative models. For example, Clippard makes air cylinders (figure 2), and just about its entire product line is available to download and use in models. The Web site uses SolidWorks 3D PartStream.NET, which allows you to see the part in a shaded 3D view that you can rotate, pan and zoom. It's very easy to visualize what you will get. A word of advice about the images: If surfaces don't look perfect, don't panic. To keep performance high for rotating and such, the online image is sometimes low resolution.
Figure 2. Clippard uses 3D PartStream.NET by SolidWorks for its model visualization. You can turn the part over and look at it from any side you want.
If you've never used ThomasNet, you really need to check it out. From its Web site, click on Visit PartSpec. You'll find more than a million predrawn mechanical and electrical parts and data that can be downloaded into your CAD system. Do a search for the part you need and select from the results. 3D model files are available in a nice range of software formats (figure 3). Some of the files are available only in 2D, but even that's better than having to model something from a badly pixilated bit-map image (something I've had to do more times than I care to remember).
Figure 3. Thomasnet.com lets you download files in the formats of many popular modeling programs.
You can also try doing a Google search for specific 3D models. Of all the sources I've mentioned, this one is my least recommended. It's not that you can't find what you're looking for, but when you find it you may be surprised by the cost and the formats that are offered. Not all 3D models available on the Internet are meant for product design. Most are for the entertainment or graphic arts arena, so what you're likely to find is digital clip art. Most sites make you pay for the models, and they aren't cheap. Those with the most detail are going to be the most expensive.
A site such as Digimation's Viewpoint Model Library has all kinds of very high-end models that are used for everything from advertising to moviemaking (figure 4). You even can find anatomy models, which are very handy in the medical product business. You need to show your product in use at some point. But Digimation's file formats generally are 3D Studio or OBJ. Some modeling programs can translate these files, but you won't get any parameters.
Figure 4. Digimation s Viewpoint Model Library has models ranging from anatomy and architecture to vehicles and machinery. Most are surface models.
If you haven't guessed by now, 3D model libraries are all about choices—and you have many. What you want to do with the model is perhaps your best guide. For more details on online parts catalogs and things to keep in mind about such models, go to www.cadalyst.com/daily110106.
Mike Hudspeth, IDSA, is an industrial designer, artist and author based in St. Louis, Missouri.
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