CNC hobbing machine raises quality by two AGMA classes.

Much has changed since a Scottish millwright named Frank Horsburgh founded the company in Cleveland, Ohio that 110 years later would be a leading manufacturer of industrial power transmission equipment. Horsburgh & Scott routinely custom designs and manufactures gears and drives weighing up to 50 tons or more for high torque, low speed and critical applications for the steel, mining, stamping and processing industries.

The same manufacturing expertise that Horsburgh & Scott uses on large gears and drives is at work in its GearTec division in Willoughby, Ohio, which makes precision gears and shaft assemblies in sizes up to 3 feet in diameter. According to Jim Weaver, GearTec Plant Manager, the average parts run is two or three per order. As might be expected in this job setting, reducing changeover time is a real challenge because of the small lots and the many variations involved. More than 200 different hobs are inventoried, making this a natural application for the versatility of CNC control and the flexibility of software programming.

That's exactly how the engineering and programming staff at Drake Manufacturing, Warren, Ohio, regarded the challenge to design a CNC gear hobber based on an old standby, manually geared and controlled, gear hobbing machine that had been a workhorse at GearTec for so many years.

The resulting machine, the Drake GS:H CNC Hobbing Machine, was a product of GearTec's experience in gear hobbing and Drake's understanding of the capability of CNC ma chining and programming. The result, one year and a half after taking delivery, has exceeded GearTec's expectations. That's going some, because while the GS:H was a work in progress, GearTec management would approach Drake with additional machine-design requirements.

"It got a little scary at times, because we would approach Drake with our latest request and they would say 'no problem' and they meant it. The amazing thing is that they designed everything in from fast hob changeover to complex progr amming for a myriad of different gears, shaft combinations and sizes," said Jim Weaver.

Mike Hughes, Drake Systems Engineer, adds, "At Drake, we believe that part of our service to the customer is parts programming on an ongoing basis. When new programs are required, we write them -- and we prove them."

The GS:H machine has impressed GearTec management with the quality of its machining, as well as the ease and speed of changeover and setup. The machine is used to cut helical gears from 10" to 24 " in diameter, typically from 4150 hardened steel with a Brinnell of 330 to 360. DP ranges are from 7 to 2-1/2; typical part cycle time is four to five hours. Hobs used are double A HSS TiN coated.

While the majority of its work is to AGMA Class 8, Mr. Weaver reports that the machine can easily produce Class 10 quality and has turned out Class 12 on occasion. According to the American Gear Manufacturers Association, Arlington, Virginia, the quality level of a gear and its Quality Number are determined by how closely the gear geometry of the finished piece matches the accuracy requirement of each rating. The AGMA Gear Handbook formalizes this concept of process control in gear manufacturing as a quality determinant by which gear accuracy is achieved and maintained through control of manufacturing equipment, methods and processes.

The key to hobbing quality, for example, is control of backlash and superior rigidity of both cutter and work drives. The Drake GS:H machine uses proven pre-loaded ball screws and an advanced Heidenhain table encoder for precise and consistent tooth spacing. "We can almost cut better with the Drake machine than we can grind," says Rob Zuchelli, GearTec's lead machine operator. He reports that the Drake GS:H has been a workhorse. "There hasn't been any unscheduled down time since it was installed," he adds. GearTec is planning to purchase a second Drake GS:H CNC hobbing machine.

System software and servo motors control all movable axes. No change gears are needed and both changeover and setup are simplified. The operator simply puts in the workpiece information and material characteristics and the control recommends feeds and speeds. Gone are the days of calculating the setting of gearing and gear differentials for each different workpiece. GE Fanuc control technology allows Drake engineers to tailor each GS:H machine's software to the user's specific applications, including hob SFM and feed IPR with no EIA-formatted programming.

The Drake GS:H Series includes six machines capable of precision hobbing of helical, spur, worm, internal, spline, rack and clutch gears and shaft workpieces. The different machines can accept diameters in 6" (150 mm) increments from 6" to 36" (900 mm).