A machine tool manufacturer turns to laser light to capture the intricate geometry of a novel idea.
This article was prepared by staff writers in collaboration with outside contributors.

There's one thing certain about innovation: Nobody has done it before, so you're in new territory.

A case in point is a new idea for a ball nut, created by John Drake of Drake Manufacturing in Warren, Ohio. In an automobile, the ball nut uses a recirculating stream of ball bearings to transmit the rotation of the steering wheel to the steering mechanism. Conventional designs recirculate the balls through an external mechanism. Drake's idea was to recirculate the balls internally. It would simplify the structure, making it cheaper to build and more compact.

The only complication was that the very newness of the design presented a challenge in manufacturing it. Drake's internal recirculating ball nut has a unique design. For about two-thirds of a revolution, the thread is nearly identical to a conventional ball nut with ball bearings traveling along the grooves in the forward direction. When they reach the end of this section, the balls enter a reversing groove that transports them up and over the land of the screw. When the balls reach the opposite end of the reverse groove, they are directed down into the screw at the beginning of the forward groove to begin the cycle again.

Drake Manufacturing is not a supplier of automobile systems, but of manufacturing tools and machinery. The ball nut tech- nology, which John Drake has patented, has been licensed to a manufacturer of power steering systems, and Drake Manufacturing is designing the machinery that will produce it.

CAD file for a ball nut (above) started with the s
can of a prototype cut in half (below). 




Because of the novel design, however, engineers ran into a challenge in machining the area of the thread where its direction changes. Measures taken to overcome helical path interference produced such a complicated geometry that it was very difficult to define it mathematically.

A coordinate measuring machine, or CMM, would not have been able to capture enough points within such a small area to define the geometry to the required level of accuracy. So the company hired a laser-scanning service bureau, GKS Inspection Services in Plymouth, Mich., to execute a CAD model.

GKS solved the problem by applying a combination of a CMM and a laser scanner. GKS is a division of Laser Design Inc., a manufacturer of laser scanning equipment in Minneapolis.

"Laser scanning helped us get the new ball nut defined by accurately measuring the geometry of our prototype so we could establish dimensional limits," said James Vosmik, president of Drake Manufacturing.

Fewer Points of Failure

According to Drake Manufacturing, the new design offers considerable advantages over conventional ball nut designs. It eliminates the need for additional components, such as a crossover insert or external recirculation tubes.

The simplicity leaves fewer possible points of failure. The company says that elimination of the external recirculation system makes a more compact package and reduces friction losses. The simplicity of the new system also makes it less expensive to manufacture than conventional ball screw assemblies. The nut in the new design can be machined in a single setup on an internal grinder.

Drake Manufacturing's engineers defined the simple geometry of the internal threads on the ball screw nut on a solid modeling system. The thread form is executed by a milling cutter or grinding wheel that has a special form designed to produce the nut thread.

As with most internal thread grinding, the internal curvature of the nut interferes with the tool. Drake engineers were able to calculate this effect, called helical path interference, for the forward and reverse threads. They compensated for it by correcting the form of the grinding wheel, despite an off-axis spindle and a "live" articulating helix axis that is swinging the spindle as it grinds the passageway. They were unable to mathematically define the form produced by these components at the point where the wheel changes direction to cut the reversing passageway.

"We had to precisely define the inside dimensions of the nut so that we could define manufacturing limits," Vosmik said. "But the form was much too complex to measure using manual gauging techniques or model with our 3-D software."

Point Clouds

Laser scanning seemed like a solution. Laser scanning systems work by projecting a line of laser light onto surfaces while cameras continuously triangulate the changing distance and profile of the laser line as it sweeps along, enabling the object to be accurately replicated. A computer translates the data as a "point cloud."

The laser scanner picks up tens of thousands of points every second. This means that the scanning of the most complicated parts can often be accomplished in a few hours or less. Laser scanning can collect data on parts, like the Drake ball nut, that are too complex to digitize practically one point at a time.

Software can process the point cloud to generate a CAD model of the scanned part that faithfully duplicates the original.

"We spoke with Drake and first addressed the question of how to gain access to the critical internal thread forms," said Steve DeRemer, general manager of GKS. "We determined that the part should be cut lengthwise using a wire EDM machine." The wire electrical discharge machining process allowed the part to be cut without subjecting it to undue stresses that might alter its dimensions. To allow the two pieces to be aligned after cutting, the entire part was first measured on a CMM. The internal and external diameters, end planes, notch, and other features were recorded for later reference. After cutting, temporary reference spheres were adhered to each of the two halves of the nut. Then each half was digitized on one of GKS's laser digitizers. By laser digitizing the parts, hundreds of thousands of points were picked up on each half of the nut.

Display model of a new ball nut shows internal recirculation of ball bearings, a feature intended to make the device cheaper to build and more compact, according to Drake Manufacturing.

According to DeRemer, the point cloud data, which was gathered through laser scanning into the native Laser Design Surveyor Scan Control software, was used as a reference to describe the shape of the part. The point cloud data was saved to Geomagic Studio, where it was cut into cross-sections and then saved in a generic IGES format. The generic cross-section files were then read into SolidWorks to build a solid volumetric model, which is what Drake needed for the application.

The abundance of point data aided in modeling the complex grooves found on the inside of the part. After modeling, the surface data was compared to the point cloud data to ensure that it conformed to the digitized part. The finished solid model was sent to Drake Manufacturing's engineers, who compared the solid model to the physical part using both manual methods and a CMM.

"GKS estimated that they could achieve 0.002 inch accuracy, but our measurements show they have done better than that," Vosmik said.

According to Vosmik, Drake Manufacturing is refining the manufacture of the ball nut. The machinery is in limited use and the company is "still trying to define manufacturing tolerances," he said. Questions remain over how close tolerances need to be, and how much quieter the ball nut should be when it is used in a vehicle.