CAD Repair Programs for Unprintable Files

Using CAD repair programs to save unprintable files

You spend hours creating a custom piece in CAD for one of your favorite clients. The metal she loves, the colored stones she's been wanting, a unique expression of her style. Now the file is up on screen, the 3-D printer is ready to lay out the model for casting, and with great anticipation, you click to get things started. Which is when your printer looks at you and says, "Hey, about this file? We need to talk."

CAD designs can look absolutely beautiful on a screen, gorgeously rendered in living color and bigger than life. But underneath, it's all a complex weave of ones and zeroes, the yesses and nos of machine language. If the numbers don't add up at that level, your file will not—and cannot—successfully find its way from screen to model. That's when you need a good CAD repair program to save the day.

What's the Issue?

In many cases, the errors in a CAD design that prevent a successful prototyping come down to not paying close enough attention to the way selected parts intersect with one another. Just because you can put gallery A onto shank B and fudge it a little to make it look nice doesn't mean the underlying mathematics understand your intention. When A + B doesn't equal what the computer and the printer expect it to, the system's going to stop trying to work it out.

"For people who build models with sweeps in Rhino, for example, there are traps they can fall into, especially if the design has self-intersecting surfaces," says Matt Feliksa, owner of Golden Sun Mfg. in Plains, Montana. "We see rings with twisted shanks, and the piece intersects itself or the sweep will intersect itself like a knot. It can be problematic if you don't break the piece up into different parts and Boolean union it."

The Boolean union command is a common tool in CAD programs. According to Rhino's site, it "trims the shared areas of selected polysurfaces [two or more joined surfaces] or surfaces and creates a single polysurface from the unshared areas." In other words, down below the surface of the nice screen image, it helps to make the numbers work the way they should.

Sometimes, says J. Tyler Teague of JETT Research in Fairview, Tennessee, the issue is that designers do use Boolean union…or at least try. "If people are trying to do Boolean unions and don't understand them, they may not know why they can't get them to work. Usually it's because there are several solids intersecting in more than one place. If you have a square box and there are two or three cylinders intersecting the box and each other, you have shared surfaces between the solids and the Boolean function can't reconcile what it's supposed to union." Many customers try to select all the parts of an item and union them at once, he notes. "Often, if they selected every other prong and the ring body for a union first, then selected the newly unioned part and the rest of the prongs as a second step, it would work."

Whatever the cause, when the 3-D printer goes on strike because your files aren't right, it's time to call in a repair program. Like many types of software, they run the gamut from free to expensive, letting you find one that suits your needs and your budget.

The Rescue Squad

One of the most important things to remember, both about CAD programs and CAD repair programs, is that they don't know what you're making, nor do they care. Your designer bracelet might as well be a jet engine. When a file isn't working, the issues a repair program will look for aren't related to design, they're all about consistency in the numbers that are being output in your STL file.

"The programs will create a good and correct drawing. It will not create a good, technically correct design," says Linus Drogs, president and CEO of Au Enterprises Inc. in Berkley, Michigan. "It will do things like creating a watertight file so it's ready to 3-D print or machine. But it's not going to go in and correct mistakes it doesn't understand is a mistake."

"The program knows you need one solid shell that doesn't have any holes in it in order to produce it on these printers," says Kevin Mueller, director of operations with TechForm Advanced Casting in Portland, Oregon. "So it's going to do everything it can to knit triangles together or eliminate extra ones, things of that nature. If you have a really messy file, I have found that Netfabb doesn't make them perfect but it makes them good enough that the machine will print it and you wouldn't really notice the problem on the other end. Where you might notice it is when you go to process the file to print. It might take a few extra minutes, whereas if the file is really cleaned up and beautiful, it processes it quickly."

"The goal is to get to a mesh that's watertight," Feliksa says. "There are mesh repair tools that can fix small errors and places where there are holes or geometry problems like overlapping triangles—things that can cause the pre-printing software, the slice tools and things like that, to create errors."

In very extreme—and rare—cases, he adds, the "repairs" that the repair program makes to a particularly messy file could alter the CAD file. Mueller recalls one instance where "it removed portions of the ring because it was trying to stitch things together, and suddenly half the setting was gone." This is why after running a repair program, it's a good idea to look over your CAD file again to make sure it's intact.

When talking about CAD repair programs, the first name out of most people's mouths is Magics RP from Materialise BV (software.materialise.com/magics). This program, which repairs files automatically, "allows you to correct problems, as well as create watertight data…STL file repair includes fixing of flipped triangles, bad edges, holes and other defects," according to the company's website.

"It gets into the real nitty gritty of math behind the designs to create a fix," Feliksa says. Among its tools is a "shrink wrap" function that he has used in the past. "It takes the mesh and puts a wrap around it, like cellophane," he says. "Then it creates essentially a new mesh over the original design." In essence, it places a single external STL surface over the existing one, which, although not "fixing" the design errors per se, makes it printable.

However, one frequently cited hurdle regarding Magics is its considerable price tag—at least $10,000—that may put it out of the reach of small shops. It does have a free version, which Mueller took for a test drive at TechForm, but found it limited. "The only thing it gives you the ability to do is view the STL file. You can do cross-sections of rings so you can look into them and see if there are any overlapping shells and things of that nature. It doesn't give you the ability to repair, but it's a great tool to see if you have a problem."

Like the full version, the free version will highlight and list the errors it finds. The difference is that it will not then go ahead and make the repairs. That's still up to the user, and is dependent on their expertise level with CAD design.

Between its cost and its background as more of an industrial program, Feliksa says, he sometimes questions its applicability in smaller shops. "It's like the more expensive 3-D printers," he says. "It'll do things the smaller, less expensive ones won't, but you may not need it. If you were running three or four Solidscape machines and knocking out a bunch of models a week, I could see where you might have a dedicated server running Magics to handle that."

That's where lower cost and free programs enter the picture. Another well-regarded repair program is Netfabb (net fabb.com), which also has both free and paid versions.

"It essentially does a lot that Magics does [for less]," Mueller says. "It has an auto-fix button and three different levels for how in-depth you want it to go. You can put in a file that has overlapping shapes and bad edges and open shells, then click fix, and it will think away and then repair everything."

In a perfect world, it would always go just that smoothly. In the messier real world of math and logic and complex design, that might depend on your file. "Some of the really messy files that I have received I could not clean up on Netfabb because it would constantly crash the program," Mueller says. "But I had no issues [with those] in Magics. Some of the other functions that both programs do are also much faster in Magics, like hollowing a part or reducing the amount of triangles in the STL."

One feature that both Magics and Netfabb share is that when they check the design for flaws, they generate a list of issues that require fixing. The user then has the choice of going into each of these and effecting the repair, or basically hitting "Go" and letting the program do its job. And while the latter may seem like the obvious choice, it's not always the case.

"Sometimes the batch fix doesn't work because what it's fixing is out of sequence," Teague says. "You might need to do the repair steps in a particular order. Otherwise, it can become circular. It'll fix one thing, then show you ten problems that it fixed before."

You don't always need to go to an external program for your CAD repair. Some design programs come with that functionality already on board.

"In Matrix there's a tool called mesh repair. It's brilliant," says Feliksa. "It has revolutionized the workflow for service bureaus in particular, but for anyone that's doing 3-D printing, where purity of mesh really matters, it's a life-saver. It takes co-planar surfaces and anything that's intersected and unifies your geometry into one mesh."

The Composer software that comes included with Asiga printers sold by Jett Research runs its own auto-repair function when it imports an STL file, Teague says. "A lot of our customers just group elements rather than even trying to Boolean union. [Composer] doesn't care if they do that. If you group items, our software will ignore the overlapping surfaces automatically and build it anyway."

Whichever way you go, from fresh program to existing tool, there's going to be some learning involved.

Getting to Know You

On average, a repair program will take a few minutes to look through your file, find those naked edges or non-manifold troublemakers, and work them out. You will spend a bit longer than that learning how to get the best results from them—and it's time well spent.

"You can learn to use Asiga's Composer really well in a few days but you can be growing parts in just a couple of hours with its behind-the-scene auto-fix, combined with using the auto-support function," Teague says. "All it does is import the STL files you grew, helps you apply supports for growing, and prepares them for export to the Asiga printer. There's not that much to learn. If you're using Rhino, you could learn to fix files in a week or two if you really focused on it. With Netfabb, the learning curve is probably 10 to 30 days to use it really well. You could start fixing your files within an hour or so just messing around with it and using its Fix Wizard function."

Feliksa notes that one of the benefits of the rise of affordable 3-D printers is that it lets designers more cost-effectively do their own in-house R&D without breaking the bank, since the resins they use are less costly than wax. Plus, they're faster than, for example, a Solidscape machine, which is designed to print actual wax models where the support system is dissolved away. "This allows for unlimited geometry," Feliksa explains, "whereas in an SLA system, the supports, if practical, require more setup time and then labor to remove, and can denature or damage the piece." The comparative speed and ease of the less expensive printers enable designers to work on their CAD skills and also to get more adept at using the repair programs. "These printers are not as finicky about meshes," he says. "Like the B9 Creator—you can practically throw anything at it and print it. We didn't even Boolean anything [when we used it]. As long as something was intersecting, you could print it."

Whether you're getting your feet wet in CAD or actively using it in production, one suggestion kept coming up: Keep two files for any piece. One is the bare-bones printing and casting version; the other, more complex or finished version is the customer-facing one, the one meant to show the fully rendered piece. The repair programs will have an easier time going through the production version and making it printable, than it will the glossy version, which may contain design elements, such as millgrain, that you're better off doing by hand. The fewer highly technical details you're asking your printer to output, the more likely it is to produce a clean model. Depending on the printer you are using, the output resolution may not lend itself to an accurate reproduction of those elements.

Also, when in doubt, turn to the experts. "Working with your caster is the way to go when you're drawing something, even on the original design," Mueller says. "We ask people that even if they're in the middle of a CAD file and they have questions, just send it to us and give us some time to look at it."

Remember: With the number and quality of CAD repair programs available to designers now, help—and a printer-ready model—is literally just a click away.

By John Shanahan

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Originally published at MJSA‘s award-winning flagship publication, MJSA Journal.

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