Business is booming, but that doesn’t mean it’s simple or easy
It’s a good time to be supplying the automotive powertrain sector,” said David Goodfellow, CEO of Star SU (Hoffman Estates, IL), reflecting the impact of record setting vehicle sales in 2015. Gears are Star SU’s business. The company provides cutting tools, machine tools, and servicing for practically any industry needing gears. “About 50% of our business today is supplying the automotive powertrain sector,” he said, not including off-highway customers.
While business is good, it does not mean it is easy. “The trends in the automotive sector are achieving higher performance and better fuel economy,” he explained. He has observed high horsepower engines in the 300- to 500-hp range mated to transmissions boasting up to 10 speeds. More speeds translate into better fuel economy by allowing the engine to operate at its most efficient load point more of the time. There is also an increasing demand for all-wheel drive, which means more transfer cases with even more gears. Competitive pressures require all these gears to be quiet and robust over a long time. That puts pressure on machine tool makers to deliver machines that cut gears faster to an almost unbelievable tolerance level.
A good example are automatic transmissions. These often use planetary gear sets, composed of a set of ring, sun, and pinion gears. “We’re producing some of these pinion gears in four or five seconds today,” he said. “Ten years ago, they were being produced in a minute or two.” That translates into obvious savings for the customer, requiring fewer machines. Faster cutting means a much more rigid machine tool combined with efficient automation and associated gaging. “Robust, noise-free gears mean cutting them to a very tight tolerance,” he said. Where even just a few years ago the industry spoke of tolerances in the thousandths of an inch, today it is expected to be 3–5 μm, according to Goodfellow. “That is a pretty dramatic change in the fundamentals of gearcutting,” he said.
Process Changes, Quality Gears Made Cleaner
Another dramatic change in the process itself is the move away from first hobbing a gear, then shaving it and then heat treating it. Hobbing is a multipoint cutting process that rotates both tool and workpiece in precise relations to each other. It is used to rough cut multiple gear teeth at once. “Today, automakers are hobbing gears, heat treating them and then grinding them post-heat treat. This provides a harder, more robust gear. Post-heat treat finishing of these gears eliminates distortion or changes of geometry in that gear set. The tolerance requirement after grinding is pretty critical,” he said.
The push for environmentally friendly factories is adding yet another challenge for machine tool builders—eliminating potentially toxic coolants. “Nowadays, everybody wants to machine dry to get rid of oil, contamination, smoke, and the potential harm to operators as well as simply keeping a clean shop floor,” Goodfellow said. Towards that end, Star SU partner Samputensili (Bentivoglio, Italy) introduced a new dry grinding machine in October, 2015, in response to the pressure to eliminate the mess and bother of coolants while grinding after heat treating. Called the SG160 Sky Grind, it features two spindles, one for skiving hobbing and the other for generating grinding. Generating grinding is becoming a common process for gears post heat treating. The grinding tool resembles a worm gear with a grinding coat. Like hobbing, both the tool and the workpiece rotate rapidly in precise relation to one another.
Speed and Cost
“Cost is always a concern with automakers,” said Loyd Koch, vice president and founder of Bourn and Koch (Rockford, IL), citing another challenge amidst the boom. Bourn and Koch supplies a wide range of machine tools under 25 different brands, including gear shaping machines. “Each time they add a speed to the transmission, that translates into them buying a lot more machine tools and may require setting up a new plant to make those transmissions,” he said. However, he stressed that even after a contract is awarded, there is usually a cost improvement rate written in requiring incremental year-over-year reductions. “They figure once you get out of the engineering stage and into production, you should be able to get better tools, better tool materials, better coatings, and faster machines. We try to accommodate them by building ever faster machine tools,” he said.
Star SU is a supplier of the cutting tools used by gear shapers like the Fellows 10-4 that Bourn and Koch supplies. “From our standpoint, the shaping end of it, they are looking for high speeds, but the tools have a big impact on that. It does not make any sense for the machine to go faster than the tool can tolerate. As tool life improves, then that pushes the speed of the machines,” explained Koch. To make advanced machines that can drive the improved tools, he explained, it is important to have linear motors replace ball screws. Using advanced CAE tools to provide the lightest yet strongest machine movements is just as vital. “It is important to not overdesign gear shaping machine tools. That is especially true with linear motors. Those devices are fast and they do not like having to move a lot of weight,” he said. He compares today’s machine tools to designing an airplane, with honeycombs and special shapes to reduce weight.
Lighter Gears, Better Fuel Economy
Most of the original processing methods and innovations for gear manufacturing date to the early 1900s (or before), according to Scott Yoders, vice president of sales for Liebherr (Saline, MI), including the invention of hobbing, shaping, and skiving. However incremental improvements and attention to detail can have profound impact on the quality of automotive transmissions. As noted above, improved tooling and grinding wheels have made some processes economical only recently. Other techniques seem truly revolutionary and Yoders believes now is the time for automotive to consider them.
One example is lightweighting. “In recent years, to reduce weight, manufacturers have reduced the face width of their transmission gears,” he said. This makes the chamfer on the teeth ends, that runs the length of the involute, especially critical as manufacturers reduce the width of the face. The common way of producing that chamfer was to plastically deform the sharp edge with a roll/press process. “A better way to engineer that chamfer is through cutting, but that can eat into cycle time without the right process. Lost seconds are real money in automotive,” he said. Starting in 2014, Liebherr began offering a method of both hobbing and precisely cutting the end relief chamfer simultaneously while roughing the gear tooth. Offered on the LC ChamferCut series of machines, Yoders describes this as a parallel operation. “We believe it is five times cheaper than the roll/press method when all tooling and expenses are included,” he said. He noted that the method was developed in conjunction with LMT-Fette, with its ChamferCut tooling solution, highlighting the need for a cutting tool that can keep up with machine speeds.
Fuel Economy with Better NVH
While engineered chamfers are important for weight, Liebherr has also developed a new gear grinding technology that improves fuel economy and NVH directly. Gear manufacturers are increasingly designing asymmetric gear teeth—different pressure angles from left-flank to right-flank. To the layman’s eye, these appear to be misshapen examples of gears, but in fact account for different loadings in power transmission on one flank of a tooth compared to the other; drive-flank compared to coast-flank. “This increases load carrying capacity while reducing NVH,” said Yoders.
The challenge, according to him, was developing a generating grinding process that specifically addressed dressing of the grinding worm and the complex machine movements to accurately grind asymmetrically. “We believe we are the first to market to do this for asymmetric gears,” he said. Available on the Liebherr LCS- and LGG- platforms, he also noted that these machines can also provide the completely new development called generated end-relief (GER). Until now, end relief in gear grinding was only available with the much slower profile grinding method, where each tooth is ground individually. The GER grinding has a similar effect—gears can now be designed with shorter face widths (i.e., lighter weight) but still carry the same loads.
Perhaps the most exciting new technology for addressing NVH is what Yoders describes as Noise Excitation Optimization, or NEO, developed in a partnership with the Technical University of Munich “This is similar to noise cancellation in that we grind into the teeth a specific wavelength and amplitude of the form error, both in lead and profile direction on the gear tooth,” he explained. This introduces, on purpose, a defined and engineered “waviness” in the profile form deviation (commonly defined as ffα) and lead form deviation (commonly defined as ffβ). Yoders presented audio data in an interview with SME that dramatically demonstrated a gear with significantly less gear whine after NEO correction.
Other gear manufacturing equipment suppliers have noticed the increasingly stringent technical requirements in automotive. “I think if you were to speak to an automotive engineer about transmissions, they would say the difference between the quality of the transmission [today] and one from seven or eight years ago is something like 500%,” said Scott Knoy, vice president of sales for German Machine Tools of America (GMTA; Ann Arbor, MI). A unique process from GMTA that helps provide that accuracy is their trademark scudding process.
What is scudding? “Think of it as a cross between hobbing and shaving,” explained Knoy: A continuous generating process using a multipoint tool that eliminates issues such as the spacing error known as drop tooth. “There are no idle strokes on the machine tool, as you get with the gear shaping process,” he said. He also noted that it probably will not completely replace shaving, since it requires a cross-axis angle that prevents the process from getting as close to a shoulder as shaving. “It is especially useful in increasing the quality of the green machining, so that after heat treating you do not have to do as much honing or grinding,” Knoy said. “Presenting a more accurate part after heat treat keeps costs down.” Knoy also hinted that hard scudding might be available in the near future, replacing a grinding or a honing operation after heat treating and further reducing costs while producing higher quality, lighter gears.
There are a number of applications where it could be the process of choice, including ring gears, sliding sleeves, and annulus gearing. Synchronizer parts and hubs are also ideal, according to Knoy. For internal ring gears, he notes that a special advantage of scudding is in making lead corrections along the face or flank of the tooth. “You can crown. You can taper. This is something that the industry hasn’t been able to do with broaching, which was the major way of making internal ring gears for 40 years. In some instances, we are replacing broaching,” he said.
The search for a quality finish is, in some cases, leading automotive manufacturers to look at honing after heat treating and grinding gears, or as an alternative to grinding. “We are seeing some desire for honing after gear grinding,” explained Rodney Soenen of Involute Gear and Machine (Chesterfield, MI), though he observes it is more popular in aerospace. Involute is a provider of gear hone tools and accessories, as well as a distributor of gear hobbing and gear inspection machines. “We have seen some companies experimenting with honing a shaved gear to increase quality, or in some cases to repair gears. They may have a thousand gears that perhaps for whatever reason were slightly below expectations” and honing brings them back into compliance, he said. He also noted that honing is popular for finishing pinions and sun gears, critical and numerous gear parts in automatic transmissions. “The honing process can be very fast, depending on the style, and improved surface finish correlates to increased durability,” he explained.
Machines that provide multitasking operations is another niche area of gear and spline cutting. “Multitasking is aimed at low- to medium-volume production,” explained Mike Finn development engineer at Mazak Corp. (Florence, KY). Multitasking machines that power skive gears and splines using the synchronized rotation of a multitasking machine’s milling spindle and C axis, as is done on some Mazak machines, offers a lot of flexibility for low-to medium-volume production runs. “This would be useful for building prototype parts and aftermarket applications, as well as off-road equipment,” said Finn. “We successfully did that with crankshaft prototypes many years ago and the same mind set could be made towards gearcutting.”
For its skiving process development, Mazak offers its Integrex Series Multi-Tasking machines, according to Finn. “Many machines can cut gears, depending on the gear type, but our most popular models are the Integrex series of machines, ranging from the Integrex 100, 200, 300, 400, and e420,” he remarked. “These cut gears and splines from 1 to 12″ [25.4–304.8 mm] in diameter.”
Mazak is currently developing cutting processes for power skived gears and splines of all sizes and types, both OD and ID in either straight and angled teeth patterns, said Finn. “These machines allow for easy part and tool changeover with the toolchanger the tilting B-axis milling spindle to accommodate different cross-axis angles,” he said. He noted that flexibility is the key advantage of a multitasking machine. The same multitasking machine that power skives a gear one day can machine completely different, nongeared components the next. The same machine can also cut the part’s mating surfaces on the same machine that cuts the gear teeth.
This article was first published in the May 2016 edition of Manufacturing Engineering magazine.