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CAD Menagerie

02/02/05

Transferring any files between different CAD packages is a much more complicated ordeal than the creators of those packages would have us believe. There are basically 2 types of modeling, surface and solid, but within each of those systems for creating forms, the software has myriad different ways of constructing, and defining the parts.
3 computers on a desk

I for example have had lots of problems exporting from SolidWorks, where solid bodies will break down into their component surfaces, which then shift, deform, or just get lost. You can get a peek at their inner workings of the program when a fillet on the corner of a box turns into a full torus (donut shape) sticking out of the corner. The software knows the math to display a torus in 3D, but somehow the part of the data telling it where to cut up the torus to make the fillet gets corrupted.

Getting the most usable and transferable data takes time. It seems as if the smaller the feature, or more complex the shape, the harder it is for a model to export correctly, and when you're doing something the size of a watch, you're putting a lot of detail onto something which is at about the lower limit of the software's size range. Sometimes little mistakes sneak through when you assume 2 surfaces are going to line up, and they don't, making a wedge shaped gap about a micron wide, and barely visible when zoomed in all the way. These “invisible” mistakes are probably the easiest user controllable way of having a quality model, you just have to know not to get yourself into a position where you have to use them, and also be able to look at your finished model and diagnose where there might be possible cracks.

Sometimes the manner in which you go about making a part can determine whether or not it will export without defects. For one project I was making a simple oval lozenge shaped part which was very easily made by making the whole part as a series of revolves, and then stretching the body to the correct proportions. This simple solution was well within the rules of SolidWorks, but either the feature in the program was too new, improperly implemented, or just didn't work right. The resulting ASIC and IGES files (exported to Cobalt on the Mac) had a weird dimple in the middle. I remade that simple part at least 2 more times in order to minimize the dimple, creating the body with several lofted surfaces, which I then mirrored. After lots of experimentation, I figured the right direction to loft, and that even mirroring things was too much of a shortcut, but the dimple was minimized to the point where it was within the error of the tooling. Hopefully.

IGES seems to be the standard for transferring 3D model data but this standard is hardly standard. Each software package puts it's own spin on it, just as each also has different schemes for reading and importing the file. Of course there are some user controllable import/ export options, but most of them are not very helpful, and some companies are removing these options all together. In 2004 SolidWorks decided to remove the resolution slider option for exporting IGES files, while Cobalt has controls over both imported and exported file resolution.

IGES is also limiting because it only gives you a body. This is fine if you're ready for tooling, or only need to measure parts, and possibly add a simple hole, boss or fillet, but sometimes even that is too much to ask and they just don't stick. Most of the time IGES leaves you with a useless chunk, with no history which describes how the shapes were created, and if the object has been engineered to the point where draft is added in, not much of it even makes sense. Drafted parts with no history tree are the hardest to work with. Any flat edges perpendicular edges can no longer be trusted, and you're never really sure where to make measurements from.

Ideally everyone would have the same software, and all 3D data hand offs would be in their native format, where if you wanted to change something, you'd just roll back the history tree, check the options of the feature, or just delete what feature. Right now, the state of the industry is that after the designers are done with the plastic shell, someone will either rebuild the part using their own CAD program, using the old part as a template, or they'll get a semi-legal copy of whatever program you used, and make their changes with that. There are some third party applications which attempt to capitalize on this whole file exporting mess, but most don't work well, and they are not cheap. Basically, all we can do is make the best parts we can, and work with the most compatible, powerful, easy to use software we can afford.

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CAD Menagerie

02/02/05

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