Gear Production Technology on Display
Makers of gear generation equipment are incorporating machining concepts long employed on machining centers into their products
By James Koelsch
Gear generation has not been immune from the trends affecting general metalcutting. Like other manufacturers, gear producers are looking for ways to cut their products ever better, faster, and cheaper. They need flexible machines and tools designed to accommodate smaller lots, squeeze out idle time during changeovers, and process jobs quickly. They want machines that, in some ways, resemble general-purpose machining and turning centers.
The more than 30 exhibitors in this year's Gear Generation Pavilion have responded to this demand. They will be showing a variety of products that have adapted concepts that began appearing in machining and turning centers more than a decade ago. High-speed cutting and quick changeovers will be important themes.
In its booth, for example, Liebherr Gear Technology (Saline, MI) will show a hobber capable of cutting as fast as 7000 rpm and 1700 m/min. Using language common among machine tool builders, the company says that this machine can load a new workpiece in 4.4 sec chip-to-chip. It also relies on such things as quick-change grippers and fixtures to streamline changeovers.
Spindle speeds between 5000 and 10,000 rpm will be a recurring theme for cylindrical-gear production, because builders have been busy designing new machines that get the most from the latest generation of hobs. Like other cutting tool manufacturers, hob producers have learned to exploit the latest coatings and substrate materials, as they become available for an increasingly larger number of applications.
The most important development in this field has been the various chromium nitride coatings that will stick to high-speed steel and tungsten-carbide hobbing tools. Chromium nitride has a unique mix of heat resistance and lubricity that makes it the coating of choice for these tools. "The coatings are allowing people to run faster," says Kent Yunker, director of R&D and new product development at Gleason Works (Rochester, NY).
He also reports that he and his competitors have been experimenting with ceramic blends, because ceramic is a substrate that can maintain its strength and integrity at the very hot temperatures generated at even higher hobbing speeds. Consequently, it's possible that you will see some ceramic prototypes at the show. No one is promising to bring any, but hopefully a vendor or two will bring some to show off a bit, and to lay the groundwork for their introduction once saleable versions are developed.
Although chipping of these materials has been a concern in the past, manufacturers have found ways around this problem. "We avoid chipping in ceramics the same way that we do it on carbide," says Yunker. "We are rounding the edge a little." Others are developing manufacturing processes that boost the toughness of these historically brittle materials.
For the production of large gears with coarse pitches, the trend has been a marked increase in the popularity of hobs and gashing cutters using indexable carbide inserts, according to Scott Yoders at Liebherr. For this reason, many tool suppliers will be showing expansions to their indexable lines.
Although the initial investment is much higher than that needed for conventional tools, gear manufacturers using indexable tools can reap productivity gains in large-batch production similar to those that other manufacturers enjoy from using indexable drills and mills. Inserts allow the tool to cut at higher speeds and feeds.
The new coatings and substrates are freeing machine builders to design and build faster machines. Not only will this generation of machines be stiff, but it also will have precisely built spindles that will not vibrate at the higher speeds characteristic of this new equipment. "The runout usually has to be twice as good," says Gleason's Yunker. The alignments of the spindles and outboardsupport mechanisms also must be better in these machines.
This higher speed and greater accuracy translate into better finishes and accuracy, a result that is transforming hobbing into a finishing process. Historically, hobbing has been a semifinishing process that usually required shaving, and perhaps grinding or honing, to follow after heat treatment. With these new machines and tools, subsequent operations are often unnecessary. Although this performance has been true for specialty machines built for high production for a while now, builders are expecting it to become more mainstream.
Another way that gearcutting machines are eliminating subsequent operations is to adopt the multitasking concept that has become popular among other genres of machines. Now that the automation and control technology necessary to combine operations onto one platform has become sophisticated enough to apply to specialized processes like gear making, builders are incorporating a number of gearcutting technologies into their products. Gleason, for example, is developing a machine for cutting cylindrical gears that will be able to do some turning and milling before it goes into hobbing mode.
Liebherr has developed two ways to add chamfering and deburring to its hobbers. The first involves adding a parallel-processing unit, such as roll deburring for high-volume jobs, or an end-mill-type chamfering unit for flexible production. The second approach relies on a combination tool from LMT-Fette Inc. (Cleveland). Called ChamferCut, the tool contains a hobbing cutter on one end and a profile cutter for chamfering on the other. Software in the controller adjusts the feed of the tool to put the correct end in the cut at the appropriate time.
Although the tool system is expensive, because it must be made as a special for the application, it can more than pay for itself by eliminating a subsequent operation. "You don't have to remove the workpiece after hobbing," explains Yoders. "Chamfering is done in the same station."
Besides multitasking, another way that builders are boosting the flexibility of their products is to design gear grinders that can use a variety of wheels. Yoders points to the new line of gear grinders that his employer will be showing. These machines can do generating and profile grinding with both nondressable CBN and dressable aluminum oxide. Induction sensors and fast loading devices cut the idle times for loading and meshing in half, making series production with grinding much more practical.
Another important trend in gear generation has been greater interest among automotive manufacturers in making bevel gears at angles other than 90°. The interest began when European governments passed laws reducing the height of the front of cars in an attempt to improve pedestrian safety. "The drive shafts can't be in line because it would make the front of the vehicle too high," explains Yunker at Gleason. "So with the lowering of engines, there has come a need for a new angular gear set in the automotive industry." For this reason, he says that attendees will see a variety of new testing machines that will be able to inspect gear sets made at these angles. — James Koelsch
This article was first published in the August 2008 edition of Manufacturing Engineering magazine.