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Tool-path generation makes no bones about making bones
By Design News Staff -- Design News, June 3, 2002
Chicago — In the past, when people at the Field Museum of Natural History were short a few bones, they would take what they had, perhaps a dinosaur's right humerus, and find somebody who would actually carve or sculpt its left humerus. The museum got its bone, but often it was not as accurate as they would like.
Today, with Hollywood leading the way in precise, high-tech creation of real and imaginary forms, anything seems possible… bionic men, dinosaurs, and extra-terrestrial beings. But the final machining of the prototypes, molds, and net shapes of these forms is still being done the old fashioned way, with lathes, mills, grinders, and routers.
In fact, Tom Corlett, of five axis CNC router-builder Quintax Inc. (Canton, OH), tells of a customer who recently used his equipment in conjunction with just such high-tech methods to recreate a smashed automobile hood. Since the damaged car couldn't be brought into the courtroom for a trial, Satellite Models was asked to recreate the smashed hood out of foam. They started with 3D scans of the hood, then developed a program that machined it out of high-density foam.
It was this rapid prototyping shop that a 3D laser-scanning house contacted about a job for the Field Museum. When the museum called for help, Kelly Hand of Satellite Models (Belmont, CA) answered. The problem: Sue was missing a few bones. (As you may recall, Sue, a 67-million-year-old fossil, is the largest, most complete, and best-preserved Tyrannosaurus Rex ever found.) The bones ranged from tiny four or five-inch forearm and toe bones to five-ft long ribs and leg bones.
The museum had had a 3D scanning service scan the bones. In turn, the scanning service contacted Satellite Models. Satellite does sculpture enlargement, museum exhibits, architectural patterns, and litigation models. Its technique is based on the use of a three-dimensional CAD program that allows Hand to create mirror images (or resizing) of the scans. Then a post processor program converts the coordinates to G-code. The Fagor 8055B control used this G-code to direct the five-axis Quintax CNC router to make a mirror image replica of Sue's bones. The bones were machined from Renshape—a Ceiba Geigy high tech modeling and prototyping board.
In replicating, each bone must be machined in four to six different directions to get in all its detail. Then the Field Museum makes molds of the prototypes and cast the bones in a resin used for museum dinosaur bones.
"The scan data for most jobs is an STL file," Hand explains. The data is basically made up of thousands of tiny triangles depending on the surface resolution. The number of scans depends on the geometry. If it is the human figure, they will take 20 or 30 individual scans to get patches of the surface so that they get into all the little nooks and crannies."
Then those patches are put together into one STL file. In the past they would have taken that data and built a NURBS (Non-Uniform Rational B-Spline) surface, according to Hand. That was the typical data required for machining. But now, there is software that allows you to generate tool path geometry from the STL file. This eliminates the expensive step of trying to build a NURBS surface onto these organic shapes.
"I can quickly go from the STL to generating tool paths," says Hand. "From this I generate a tool path for my five-axis machine."
One part he's working on has an 11.5 MB code file. "The speed at which the controller processes the data and the interface are everything," he explains. A 32-bit CPU with math coprocessor, and one MB of RAM on the Fagor control helps handle these large file sizes.
Satellite Models uses post processor programs to convert coordinates from its 3D CAD program to G-code. A post processor program generates a machine's G-code underneath the CAD.
Why did Quintax (the manufacturer of a Satellite Model's five-axis router) decide that Fagor controls could meet the needs of high tech prototyping shops as well as clients in aerospace, rotational molding, and vacuum thermoforming?
Tom Corlett explains, "We found that the upper end controls really didn't offer all that much more in terms of capability compared to what Fagor has. They were just much higher priced. Fagor gave us a fairly powerful control at a reasonable price. But what's more important, they have been willing to listen to us."
For more information about CNC controls from Fagor Automation: Enter 534
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