Machine manufacturers are working to streamline the gear-making process, to deliver a more highly finished gear in fewer steps
With demand for precision gears big and small growing across a wide swath of industries, along with the economy, new innovations in gear production technology are changing the way some manufacturing minds think about making gears in the first place, which hasn’t substantially changed in decades.
In fact, these new methods of manufacturing gears, which aim to streamline the processes of delivering high-quality gears using CNC programming and precision cutters, may even open up the possibility for creative new gear designs in the future.
The traditional method of making a large volume of gears, namely hobbing, gashing, scudding or skiving, requires machines and tools specific to gear production and in many cases even to the size of the gear itself—and very little else.
“A gear hobbing machine only does gears,” said Nitin Chaphalkar, manager, Advanced Solution Development, DMG MORI USA (Chicago, IL).
What’s more, depending on the design and material out of which the gear is cut, these traditional gear-making processes may require highly skilled operators and will always need additional pre and post operations.
At the same time, however, demand is also growing for higher quality gears with better surface quality and robust wear performance as a greater number of higher-efficiency gears are sought for automotive transmissions, heavy equipment, airplanes, marine vessels and wind turbines.
That means more manufacturers are trying to churn out more gears faster. While many large OEMs may be inclined to stick with their tried-and-true manufacturing methods, rather than make sweeping investments in new manufacturing approaches, Chaphalkar estimated that about 40% of gear production is done by job shops that are contract manufacturing a variety of parts and may not need to do high volume machining of gears all the time.
For those customers doing smaller to medium batch sizes, DMG MORI offers its gearMILL software solution that enables a standard lathe, mill or multitasking machine, outfitted with standard off-the-shelf tools, to deliver a highly finished gear that is almost done-in-one and ready for heat treat.
The software, according to DMG MORI, allows complete machining of diverse gear types in its diverse line of multitasking machines. The builder has 20–25 different models of machines that are suitable for machining various sizes and types of gears.
The obvious benefit to this approach over, say, a hobbing machine is that it offers flexibility for when a shop wants to machine other parts. Generally speaking, it can also offer a higher quality finished product in pre-heat treat stage.
Essentially, the gearMILL software develops the most ideal toolpath to create the gear with a standard machine and tooling. “A good gear geometry is the key to everything,” Chaphalkar explained.
Any Gear, Any Size
DMG MORI has been developing its gear solutions offering since 2006. In 2010, Sandvik Coromant (Fair Lawn, NJ) invented the process of InvoMilling, a unique approach to making spur and helical gears using indexable insert cutters by slot milling and turn-milling in a path that moves the cutter along an involute path. “It was a very new thing,” Chaphalkar said of the method. “Nobody was doing it this way.”
So the two powerhouse companies decided to partner on commercializing the new technology. DMG MORI developed software that develops the toolpath based on the gear shape, and now InvoMilling is added to hobbing and flank milling with an end-mill as the choices customers now have to make a gear on a multitasking machine with gearMILL.
“Now we have a broad portfolio,” Chaphalkar said. “Spur, helical, herringbone, bevel, spiral bevel, hypoid—whatever type of gear you have, it can be programmed and made.”
One of the main challenges in traditional gear production is the lead time for acquiring the right hob to make a part, which can take up to 8–10 weeks if you don’t have the right size in stock already. But using a standard machine and tools eliminates most of that waiting, Chaphalkar said. “You will already have the tools most of the time. There is no waiting in most of the cases.”
—Sarah A. Webster, Editor in Chief
A Deeper Look at InvoMilling
Sandvik Coromant developed its InvoMilling gearcutting program and CoroMill 171 and 172 indexable carbide disk cutters to provide maximum flexibility for its customers who wanted to manufacture gears in smaller lot sizes, typically for applications like heavy trucks, agricultural and construction equipment. Sandvik Coromant worked initially and proved the concept with DMG MORI, which integrated the software into its CNC controls. InvoMilling will soon be available through other major machine tool makers as well.
“We are seeing a shift in how gears are being made because of the availability of advanced technology, including multitask machines, even three-axis vertical machining centers with fourth axis rotary tables, and five-axis machining centers,” said Aaron Habeck, marketing project manager, Sandvik Coromant.
“High-volume gear manufacturing for automotive applications like transmissions is still going to be done on traditional hobbing machines using high speed steel. But InvoMilling and full form disk cutters allow our customers who want to produce gears on non-traditional gearcutting machines, the ability to cut splines or gears. With a multitask machine, they can machine the part in one setup using the main spindle and subspindle to do all the turning or the milling of flats and keyways and drill/tap holes,” said Habeck.
The advantage of InvoMilling is that a wide range of gear tooth profiles can be generated using a very small assortment of tools. “InvoMilling uses somewhat standard products to cut the involute of the gear tooth by using the movement of machine’s axis to generate the involute curve, not the tool itself. The tool typically has a straight cutting edge and, depending on the machine configuration, movement of the C axis and the Y axis, generates the involute curve,” said Habeck. “InvoMilling isn’t going to be the fastest overall cycle time, but for a customer who has invested in a small assortment of tools, a wide range of different gear profiles can be manufactured.”
Each gear cutting process has its own benefits in terms of quality and cycle times that can be achieved. “Hobbing will always be the quickest; InvoMilling, the most flexible. CoroMill 171 and 172 cutters can achieve DIN 7 or AGMA 10 quality class, for example, typically what our customers require. CoroMill 171 and 172 involve easy, straight line programming. There’s no need for any special synchronization of tool and part movements like that which is required for hobbing,” said Habeck.
—James Lorincz, Senior Editor
High Chamfer Quality, One-Cut Machining
Even among those companies that provide traditional gear-making technology, efforts are being made to streamline.
Liebherr Gear Technology (Saline, MI), for example, takes a fairly traditional approach, but is not afraid of new ideas. Its new hobbing machine for work up to 180 mm, the LC 180, includes an integrated Chamfer Cut unit for deburring and chamfering the face edges. After hobbing, the Chamfer Cut tool generates precise and reproducible chamfers that are increasingly demanded by the automakers for smoother gear engagement and quieter transmissions.
The new solution eliminates the former main disadvantage of chamfer cutting: prolonged machining time. In the past, hobbing and chamfering took too much time at the same setting. “We have solved this by integrating a complete second machining unit for chamfer cut tools—two machines in one, so to speak,” said Oliver Winkel, director of Application Technology and responsible for technological development of gearcutting at Liebherr-Verzahntechnik (Kempten, Germany).
Chamfering no longer prolongs machining time because it takes place in a separate unit within the same machine, while the next workpiece is hobbed. “We know from transmission design development that the subject of ‘chamfering’ is becoming more and more important. This innovation enables the machine to combine an already undisputed high chamfering quality, provided by the proven Chamfer Cut procedure, with cycle times that correspond to the demands of the automotive industry,” said Winkel.
This technology is not limited to the auto industry. It can be of benefit to any gearmaker whose current procedures are too time-consuming, whose tooling costs are too high, or those who need to take follow-up processes such as honing into consideration.
Compared to press deburring and chamfering with finger mills, the chamfer cut process has the lowest chamfering costs.
Anticipating further downsizing trends in the auto industry, the Liebherr chamfer cutting technology can also generate even smaller, more precise chamfers for transmission components. As the importance of a reproducibly generated chamfer increases, the smaller the gear will be.
In the industry, parts frequently do not conform to drawings due to imprecise chamfering procedures. “This is especially true the thinner the gear face width becomes,” said Winkel. “The importance of the transmission designer being able to calculate the limits of design feasibility is increasing, so the chamfer is becoming more and more of an engineering factor. Since its actual impact can now be calculated, its importance also has increased. The ever tighter design of transmissions is one reason why the importance of chamfer quality has increased. It makes a huge difference in the case of a gear from an automotive transmission that is only around 12-mm wide, whether the chamfer is 0.5 or 1 mm—and consistently throughout high-volume production,” said Winkel.
—James D. Sawyer, Senior Editor
Cutting Smaller Gears, Faster, Without Quality Loss
A Liebherr LC 1200 Gear Hobbing machine resides at Ingersoll Cutting Tools (Rockford, IL), where it demonstrates Ingersoll’s indexable carbide inserts for machining large gears, including hobs and gashing tools.
“Most of our customers can already make good quality gears with their current equipment—but they’re trying to increase productivity with no loss of quality,” said Frank Berardi, gear machining product manager at Ingersoll Cutting Tools. “Usually we’re taking first-time customers from high-speed steel to carbide, and that brings a big jump in productivity right there.”
A more recent design innovation is allowing Ingersoll’s clients to work more effectively with smaller gear sizes. “Most of our work in gearcutting has been focused on the larger gears used in the mining, power generation, and construction industries, to name a few—industries that generally use 8 Module and higher,” said Berardi. [‘Module’—the ratio of the pitch diameter in millimeters to the number of teeth—is a standard gear measurement unit.] “Basically customers adapted our products to their larger gears first, and then asked what we could do for these smaller gear sizes. With our radial insert design, we now have an answer for them.”
In recent years Ingersoll has concentrated in producing gearcutters for the smaller gear tooth sizes, particularly in the 4–8 Module range, where Berardi says there has been a void of indexable insert products. That size is used by customers in the medical-equipment and large-truck industries as well as other industries. “The challenge has always been how to produce cutters with secure insert retention in the smaller tooth forms,” Berardi explained. “To accomplish this we developed new concepts for indexable insert hobs, which utilize radial mounted inserts instead of our typical tangential inserts.”
Tangentials can get down to about 6 Module. Any smaller than that and it becomes difficult to make an insert small enough to locate in pocket, and clamp it down and be robust, according to Berardi. The radial mounted design can get down to 4 Module. The tradeoff is that the radial design allows a fewer number of indexes—however, the advantages of the radial design more than compensates.
The radial insert design allows for a larger, more secure insert pocket. The hobs can be made with screw-down or clamp style inserts. Also important, the radial insert hob has double the number of effective teeth as a tangential insert hob of the same size. “This results in much higher productivity,” said Berardi. The hobs can be produced in single and multiple-start versions.
—Michael C. Anderson, Senior Editor
Single-Minded Modular Machining Lines
EMAG LLC (Farmington Hills, MI) offers two machines, the VLC 250 WF and the VSC 400 WF, that can be used to turn and hob in combination. According to the company, there is a quality advantage to such combination machines.
“The advantage,” said Joerg Lohmann, Deputy Sales Director, Koepfer GmbH a member of the EMAG Group (Salach, Germany), “is that the workpiece doesn´t have to be unclamped between the different processes.”
Despite this advantage, EMAG is phasing out this product family, except, said Lohmann, “for [supporting] customers that already have these machines. Some of them, however, are even trending towards our new gear manufacturing systems, for instance the combination of a VL2, a VL2, a VLC200H and a VLC100D.”
In combination these machines become a modular multipurpose machining line. They are still, however, single-purpose machines. And the reason, according to Lohmann, is that time is money.
“EMAG has chosen to develop its gear manufacturing system with single technology machines,” he said, “to offer an alternative to [combination machines]. The hourly production cost with single technology machines is simply lower compared to multiprocess machines. And in a modular system of these machines one process follows immediately after the other so the value stream is optimized. Throughput times are reduced dramatically by means of avoiding workpiece transportation.”
EMAG has shown gear manufacturing systems that perform not just the basic metal removal operations but which include welding and workpiece hardening as well.
“We are developing our modular construction system further,” said Lohmann, “and we always implement the latest technologies on our vertical platform. The EMAG group offers the whole process chain for green and hard gear machining processes. We are convinced that the standardization of machines with different technologies on a modular platform will help our customers not only to reduce manufacturing cost, but also make a contribution to increased quality.”
—James D. Sawyer, Senior Editor
This article was first published in the June 2014 edition of Manufacturing Engineering magazine.