STL Format Files Work For Machining Tool

Clinkenbeard & Associates, Rockford, Illinois is a pattern shop making the leap into the digital age. Its craftsmen still build meticulously detailed foundry patterns and core boxes and these tools are among the last vestiges of industrial artistry. Fragrances of cut and sanded wood permeate the shop. Next door, Clinkenbeard is computerizing this painstaking craftsmanship. Saws, sandpaper and woodworking skills have been replaced by laminated object manufacturing (LOM) and the Stereolithography Tessellation Language (STL). Blueprints and tracer mills have yielded to CAD, CAM, and CNC.

The 24-man company specializes in patterns for sand-cast and permanent-mold aluminum and magnesium castings for aerospace. Clinkenbeard also makes foundry core boxes, molds, and prototype and permanent mold tools. A typical job is the complexly cored housing for a turbine-driven auxiliary power transmission designed by a large aerospace firm in Rockford for use in military and civilian jets.

STL plays a vital role in Clinkenbeard's transition. It is the key to a foolproof means of converting customer files into CNC programs for six vertical machining centers. STL was developed for use in Rapid Prototyping machines including LOMs and Stereolithography apparatus (SLA). STL translates 3D solids and surfaced wireframes from CAD systems into data for these new processes cleanly and quickly. Exact replicas can be made in a few hours.

Until recently, the benefits of STL hadn't reached CNC machining. This shortcoming restricted the many benefits of Rapid Prototyping to plastic replicas of parts. These were fine for verifying form, fit and function, but not if the part was highly stressed.

If the part was highly stressed, prototypes had to be machined from steel or aluminum--and this was not rapid. Translating the CAD file and making sure the CAM system's tool path was perfect often took a week. Michael A. (Mickey) Barlow, Clinkenbeard vice president, was frustrated by this huge performance gap. Each of his two Helisys LOM machines turned out parts several times faster than any of the six CNC machines just a few feet away. Clinkenbeard began using LOM in 1993.

"I couldn't understand why STL files couldn't be used by a machining center as they were by the LOMs," said Mr. Barlow. Then he discovered PowerMILL CAM system (Delcam International, Birmingham, England). It could convert STL files into CNC tool paths within minutes. If the proper steel and cutting tools were on hand, Clinkenbeard could be making chips in a few hours.

STL capability soon opened new avenues of business for Clinkenbeard. "We do a lot of aerospace work and these customers do not trust, and will not use, IGES," Mr. Barlow explained. "They do, however, trust STL."

This was confirmed by Sundstrand Aerospace, Rockford, a major Clinkenbeard customer. "We use STL files for all Rapid Prototyping work," says Richard W. Gee, manager of design tool applications. "We don't like IGES because it is very unpredictable. Sometimes we get extra features on drawings and sometimes things are suddenly omitted. Our experience with geometry translation has not been very good.

"STL is especially helpful with Clinkenbeard which is sometimes called `Sundstrand West' because we do so much work with them," Mr. Gee continued. "We are in a partnership to evolve our tools and processes together and help each other. They really do a good job for us." Mr. Gee didn't know of any other vendor able to take STL files into machine tools as Clinkenbeard can.

Most solid modeling at Sundstrand Aerospace is done with EDS's Unigraphics, although a few seats of Pro/E and Catia are available, too. Mr. Gee said STL files are the general format for any rapid prototyping work whether it's SLA, LOM or selective laser sintering (SLS). "We get a lot of early stage parts made this way," he said.

Mr. Barlow himself runs PowerMILL. His "training" was little more than a demo and some guidance by Delcam's Rockford distributor, CadCam Systems Inc. (CCS).

Clinkenbeard had been slowly losing business because hand-made patterns were too costly or too time consuming for many jobs. But in 1995, things turned around. A major reason is that STL, combined with the CAM systems quick NC tool path generation, permits Clinkenbeard to respond to bid requests in a few hours. "Speed is the big benefit in quoting jobs," he continued. "Quotes that took a week are now turned around in hours." Clinkenbeard always quotes tools and prototypes based on delivery dates. "Price, especially on prototypes, is not the most important consideration," Mr. Barlow noted. "Sometimes we'll offer a 10 percent or 15 percent discount in exchange for a 30 percent longer delivery time. This would give us three more working days. They'll say no."

This newfound speed translates into a real need greater capacity. Clinkenbeard is installing a sixth CNC vertical machining center. The system also has the capability for two-stage machining at 45-degree angles. "This allows us to machine complex shapes that would ordinarily require a five-axis machine which we don't have.

In machining their molds with PowerMILL, Mr. Barlow says he never worries about violating surfaces. "Our third job was a permanent mold for an aluminum foundry in Israel," he recalled. "We started with a piece of H13 tool steel about 30 inches square and 6 inches thick. We made the program from a blueprint, converted it to STL, sent it to PowerMILL, and ran the tool path.

"Though no weld repair would have been allowed, there were no tryouts and no test cuts," he said. Both halves of the mold were complete in two weeks even though the job had more gates and runners than is usual."

None of the CAM system's operators program full-time, Mr. Barlow noted. "For `casual' users like us, the ease of use of an intuitive GUI is very important. To use the system effectively you have to understand NC programming. But you don't have to know much about patterns or core making."

Because Clinkenbeard doesn't use PowerMILL all day, every day, its payback period has been extended. Mr. Barlow estimated payback at 12 to 14 months. Had he been a full-time user, the investment would have been recovered in half that time--including the cost of the UNIX workstation. (After a recent CPU and RAM upgrade for the workstation, PowerMILL now calculates tool paths about 40 percent faster, Mr. Barlow said.)

Mr. Barlow feels STL has only one shortcoming. Unlike IGES it is not features based, so measuring things such as bosses or holes can be difficult. His work-around is straight-forward: create a plane through the vertices of three STL triangles, then measure the appropriate intersections. This capability has been added to the newest release of the software.

If STL maintained features rather than tessellating all surfaces into its characteristic tiny triangles, it would have all the problems IGES has. Those problems are caused by the need to accommodate every definition of every geometric primitive supported by every CAD system.

Meanwhile, Sundstrand Aerospace is developing new uses for STL. "We use STL files to verify the completeness of the geometry in our solid models," said Mr. Gee. "Some of our assemblies have 1200 parts and create files approaching 100 megabytes in size." Trying to find little problems in files that size can be nightmarish. "So we run STL files on them every day or two," he continued. "We look for gaps or disconnects between blended curves and for duplicated features. Duplicated features can be particularly troublesome because when you delete one to make room for something else, the first one is still there and bizarre things start to happen."

The ability to work with STL files in CAM has changed Clinkenbeard's business for the better. It provides tools to meet the digital CAD data needs of customers who watch the clocks, calendars, and quality with equal fervor. MMS