In the past, when the people at the Field Museum of Natural History in Chicago were short a few bones, they would take what they had, perhaps a dinosaur''s right humerus, and find somebody who could 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, extraterrestrial 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.
Make Me a Hood
In fact, Tom Corlett, Jr. whose company, Quintax Inc, of Canton, OH, makes 5-axis CNC routers, tells of a customer who recently used Quintax 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 of Belmont, CA was asked to recreate the smashed hood out of foam. They started with 3-D scans of the hood, then developed a program that machined it out of high density foam.
Satellite Models was also the rapid prototyping shop contacted by a 3-D, laser scanning house about a job for the Field Museum.
Good Old ''Sue''
The problem: Sue was missing a few bones. (As you may recall, Sue, a 67-million-year-old fossil, is the world''s largest, most complete, and best-preserved Tyrannosaurus Rex. She was acquired by the museum in 1997.) The bones, already scanned by a 3-D scanning service, ranged from tiny 4 or 5 in. forearm and toe bones to 5 ft long ribs and leg bones.
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 Satellite''s Kelly Hand to create mirror images (or resizing) of the scans. Then, a post processor program converts the coordinates to G-code, which his Fagor 8055B control uses to direct the 5-axis Quintax CNC router to make the mirror image replica of Sue''s bones. They were machined from Renshape -- a Ciba Geigy high tech modeling and prototyping board used for pattern making and prototyping. Each bone needed to be machined in as many as four to six different directions to get in all its detail. Then the Field Museum made molds of the prototypes and cast the bones in a resin that is typically used for museum dinosaur bones.
Rapid Prototyping -- From STL Files to CNC Control
"The scan data I received for this and other prototyping jobs is an STL file," explains Kelly Hand. "That''s the file format that most rapid prototyping technologies use. The data is basically made up of thousands of tiny triangles ranging in size from a thousandths inch to an inch or so. It just depends on the surface resolution of the part or the piece we''re trying to capture.
"Typically, it works like this," he continues. "Our client, whether a sculptor or museum, will get their parts or model laser digitized, i.e., scanned. First the laser scanner creates what is called point cloud data. That is millions of individual x, y, z coordinate points in space that describe the surface. 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 watertight STL file -- a file that has all of the surface data on it as little triangles.
"In the past they would have had to take that data and build a NURBS surface. That was the typical data that was 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 (polygonized, i.e., little triangles data) to generating tool paths. Then I can scale it, mirror image it, or do whatever my client wants. From this I generate a tool path for my 5-axis Quintax with the Fagor control on it. I mill these models out of whatever material is appropriate -- from lightweight urethane foam and EPS foam, up through some of the more rigid modeling boards that are out there."
Lots of Bytes
Hand describes the resultant files: "My file sizes are typically 2 to 3 megabytes up to 20 megabytes of G-code files. For instance, a part that I am working on now has an 11.5 megabyte code file. The speed at which the controller processes the data and the interface are everything."
He can handle these file sizes because the Fagor 8055 control has a 32-bit CPU with math co-processor, one MB of RAM, it''s equipped with 4 MB of memory for programs and PLC but can take two memory cards of up to 24 MB each. It also has a 2 gigabyte hard disk with Ethernet option.
Control Technology
George Fletcher, a Ph.D. in physics who taught controls at Clemson University, explains how the control can handle such rigorous assignments. (Fletcher''s machine building firm, GII Solutions, is in central South Carolina.) Though we''re talking about putting a CNC control to the test by downloading some high tech files, surprisingly, Fletcher feels that -- unlike the 8055 -- expensive CNC systems designed solely for such high tech files are too limiting.
"NERBS and STL are external technologies from which you process data into a form that the 8055 can use. Other machines on the market operate exclusively from NERBS or STL data. With the 8055, you have a choice," he says. "Not everyone needs the NERBS or STL level of sophistication. But they do need to have standard G-code and M-code technology. An operator can be trained at a local vocational school to do G-code and M-code programming. To use NERBS or STL, you have to have a degreed engineer just to understand the mathematics involved."
Fletcher notes that using NERBS and STL has another drawback. "It''s impossible to manually machine these shapes. Their mathematics for surface rendering creates little triangles and polygons. The CNC then machines these little polygons. Only a computer can direct this type of machining."
He continues, "This advanced technology definitely has its place, but it is hard to make money off it. Too many things can go wrong. The best way to make money with a CNC machine is by using current technology that you know works as opposed to advanced technology where you''re doing the beta testing. Most of the time, the more sophisticated you get, the more that can go wrong. But the Fagor 8055 is a good, reliable, work-every-single-day kind of control. Plus, it gives you options. It has an assisted programming mode that will talk you through the G-code programming. Then it has another mode that is even more conversational than that. And then it has the regular G-code and M-code. Underneath that, it has, built into the main programming capability, in addition to the G-code, high level programming. It has a lot of high level capability. Then, on top of that, at the OEM level, it has what they call ''User Programs.'' With this you can write a program that works in the ''edit'' mode and actually allows you to set up programs. For example, you can machine families of tap grinders that way. Since all the taps are specified the same way, you can tell the Fagor control the threads per inch, the diameter of the part, the thread length and depth. Then you just push the button and it will automatically generate the code for you. It can do that for very extensive programs. It has full-blown mathematics capability built in."
Flash Memory for That Sense of Security
Losing any part of a control''s default parameters is a user''s worst nightmare. It''s like losing your computer''s operating system. Fletcher explains how Fagor addresses the security issue: "The 8055 has some memory built in that allows you to store your PLC programs and much more. Then if you have a failure or a program is contaminated, you can recall it from flash memory. As an OEM, I use that all the time. I write an OEM program, simulate these OEM settings to make sure they work and then store them in the flash memory. The flash memory is not quite as fast as RAM, but it''s almost like an EPROM. It''s really kind of an E squared PROM. The default parameters, PLC program or OEM settings can be recalled out of this flash memory. The flash memory can store all the stuff that makes the machine work. If a problem arises, I''m able to help clients -- even over the telephone. I can say, ''Okay, do the following key strokes,'' and bingo, the programming miraculously returns. For more storage, you can even put hard disks in the controls."
GII designs and builds specialized grinders for wheel truing systems in addition to one-of-a-kind automation equipment. Fletcher depends on Fagor controls even though most of the work his customers do takes only 64K of memory.
The Post Processor Is Key
Satellite Models uses post processor programs to convert coordinates from its 3-D CAD program to G-code -- as do most medium to large manufacturers who use CAD programs. A post processor program generates a machine''s G-code underneath the CAD. Fletcher writes his own post processors, which allow GII''s customers to draw a 2-axis part on AutoCad with the post processor generating a program that can be downloaded to the Fagor controls.
A Case In Point
"We did that for a spark plug manufacturer that had a bit of a problem," says Fletcher. "Our customer, who presses the ceramic into a rough form, grinds a precision form into the greenware state, before sending it through the kiln. The spark plug always shrinks in the kiln. The problem is, it has to be exactly a certain size and you can''t change the spark plug''s shape once it is hardened in the kiln. So, what they do is calculate how much the shrinkage will be by cutting a single part, firing it and then measuring it to see if it came out the right size. This tells them if they have to go back and adjust the original form.
"We wrote a post processor program for the Fagor control that is on the machine we created for grinding the form. This post processor program allows them to draw that part on AutoCad and our program downloads a complete ready-to-use G-code program into the Fagor 8055 for grinding the spark plug form."
Of course, Satellite Model''s post processor file sizes are huge compared to the file Fletcher created for milling spark plugs. Satellite has them created by a vendor that specializes in post processors for STL files.
Control Versatility Arises From Customization
Why did Quintax (the manufacturer of Satellite Model''s 5-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. I''d have to say that Fagor gave us a fairly powerful control at a reasonable price. But what''s even more important, Fagor has been willing to listen to us. To listen to what we have to say about the development direction of their control, what features they should add. They''ve been very open in helping to develop the control along the lines that we would like to see it developed and they''ve accommodated us on features for high speed machining. They''ve also developed certain features like tool length sensing, probing and other proprietary features for our machines in addition to customizing the software."