Companies often move into new areas of business because of changing technology or the introduction of new products by a key customer, who requires their support. Such a scenario unfolded for BPS Industries, Baltimore, Maryland, a firm that machines industrial plastics. The customer was Flowdata, Inc., a manufacturer of precision flow-metering systems. The product was a radically new gear-like impeller called the Vector.
Flowmeters measure liquid flows with paired impellers, and great accuracy is required in machining their complex geometries. To work properly, Flowdata's impellers have to mesh perfectly. The patented Vectors mesh so well and run so smoothly that Flowdata eliminated the synchronizing gearboxes, a system common among competing flowmeter designs. Based in Richardson, Texas, Flowdata designs, assembles, tests and calibrates but does not cut metal. It relies on BPS, 1400 miles away, for machining.
Vector impellers are true gears but contain only three or four oversized teeth known as lobes that twist around the impeller's core. Although elegant, Vector's lobes are generated like any other gear teeth, with a mathematical formula called an involute curve. Machining the curved surface of the product, however, is a challenge using standard approaches.
Cutting the teeth of the Vectors required a breakthrough in metal cutting. BPS bridged the chasm between gear cutting and CNC machining by mastering the mechanical design through manufacturing software, Pro/ENGINEER, from Parametric Technology Corp. (PTC) in Waltham, Massachusetts.
The most important aspect of any gear design is the profile of its teeth. These are machined with a tool called a hob. The hob itself is generated from mathematical and kinematic formulas unique to gear making. Though Flowdata's Vector is a standard gear profile - a double-helical similar to a herringbone - it could not be hobbed.
In its search for smoother-running and tighter-meshing gears, Flowdata modified the characteristic V of the herringbone tooth into a gracefully rounded U. "We softened the edges and added a gentle nose twist, which lets BPS machine the lobe in just one pass," said Pedro Fernandez, lead Vector designer.
The biggest challenge of the new involute design was machining the root surfaces between the Vector's lobes. BPS and Flowdata quickly realized that four- and five-axis CNC machining was their only alternative. However, CNC machining requires complete geometric surface data, not just the involute's mathematics.
BPS and Flowdata realized that machinable tooth profiles could not be generated simply by plugging the involute calculations into the 3D solid modeler into a CAM package. Flowdata painstakingly converted the mathematics of the involute into surface data points accurate to 0.00001 inch.
When BPS received Flowdata's impeller work, it invested several million dollars for new machine tools and a coordinate measuring machine (CMM). Seven new vertical machining centers with 10,000 rpm main spindles were bought. Two have five-axis capability and three have four-axis controls. The other two VMCs are three-axis machines with automatic pallet changers. For years, BPS had made nearly all of Flowdata's plastic impellers and gearing. But until the Vector, BPS had cut very few metal parts for Flowdata.
Having designed the Vectors using Pro/ENGINEER, Flowdata urged BPS to buy the system. "Having 3D solid modeling and parametric-design capability was the only way to design these impellers effectively," said Flowdata President Dave Foran. "We did in weeks, days and sometimes only hours would have taken us months with our old CAD system." After testing a few other CAD packages, BPS concurred. Using the same system "means more than just avoiding IGES transfers and eliminating geometry translations and cleanups," said BPS President Richard G. Scherr. "It lets us look at the model the same way the customer does. It's all about embedding design intent among critical features of the part."
"Using Pro/ENGINEER, I can modify designs from two lobes to ten in just a few minutes," explained Mr. Fernandez. "I can re-size the impellers anywhere in a one-to-ten range. I can change the helical twist, which determines the amount of lobe contact, from zero (that is a simple spur gear) to 180 degrees, which wraps the lobes halfway around the shaft. With each change, the 3D solid model of the gear is automatically reconstructed, and it is completely accurate."
At BPS, parametric capability plays a major role in keeping up with changes like these and dozens of Vector engineering iterations. Of particular importance is the fact that new surfaces can be substituted quickly and easily in the 3D solid model. Each time Flowdata makes changes, BPS obtains a Pro/ENGINEER file in self-extracting zipped (compressed) format on its electronic bulletin board system.
Lead BPS Programmer Jim Narimatsu explained how the engineering-change process works. "The first step is to replicate the lobes and assemble the new impeller model in the software along with the accompanying fixture using Pro/ENGINEER's assembly mode. This enables us to visualize potential part or cutting tool interference problems and collisions. We graphically compare the new model with the previous version, then develop new cutting strategies, redesign the fixtures and select new tools," he explained.
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