Shifting to High in Gearcutting
Today's gearcutting equipment is fast, performs more operations with a single setup than traditional machines, and meets strict process capabilities with tight tolerances
By Bruce Morey
Gears are made using techniques ranging from profile grinding to hobbing. Hobbing is generally quite productive, and more gears are made by hobbing than any other method.
Technical trends that make more efficient and accurate gear cutting possible today are:
- Tooling advances, particularly in coatings and substrates,
- The search for better tooling designs, and
- Gearcutting machines that are more flexible or perform more operations with a single setup.
As gear manufacturers strive to meet strict process capabilities with tighter tolerances, the question becomes: "How much more accurate and repeatable can future gearcutting machines be?"
Dry cutting as a trend continues to thrive in the gear community as tooling coatings and substrates advance, enabling faster cutting speeds and longer tool life without coolants and lubricants. Dry cutting is currently the standard for bevel-gear cutting, while parallel axis, cylindrical-gear production is also moving to dry cutting.
Bevel-gear cutters use inserted carbide stick blades coated with a thin, hard coating that dramatically increases tool life. Titanium aluminum nitride (TiAlN) remains popular, according to T. J. Maiuri of The Gleason Corp. (Rochester, NY), while titanium-free aluminum chromium nitride (AlCrN) is the current coating of choice. Provided by Oerlikon Balzers AG (Balzers, Lichtenstein) under the Balinit Alcrona trade name, AlCrN boasts a hardness as measured (HV 0.05) of 3200, a coefficient of friction of 0.35 and, most importantly, a max operating temperature of 1100°C.
Coating more surfaces of the tool increases its life as well. "Stick blades used for cutting bevel gears often have a coating only on the front face of the blade," explains Gleason's Maiuri. Applying a coating to the remaining surfaces of the stick blade—the tops and sides—can lead to an increase in tool life of 50%, without making any other changes to the blade or process."
Hobs used to cut parallel-axis gears are shaped differently than those used to cut bevel gears. They are typically cylindrical with rows of teeth that resemble a wormgear, use high-speed steels (HSS) as a substrate rather than carbide tools, and employ advanced coatings. Powder metal HSS is popular among many hob providers, while others stick to more conventional substrate material.
As an example of advancements in parallel hob-cutting tools, Mitsubishi Heavy Industries, Machine Tool Div. (Wixom, MI) has come out with a newer version of its Superdry product. The company's hobbing tools are a combination of an HSS substrate called Mitsubishi Mach 7 and a proprietary coating, Mitsubishi Superdry, reports Ian Shearing, VP for Mitsubishi Heavy Industries. The Mach 7 substrate is a simple HSS alloy, not a powder metal. "Mach 7 is malleable and able to accept high chip loads," says Shearing. "This is desirable when dry hobbing, because the larger chip carries the process heat away from the working zone. In addition, the use of Mach 7 as a substrate avoids the problem of chipping, which is sometimes encountered with highly alloyed powder-metal HSS steels. Mach 7 will crater before chipping, which is a more predictable condition."
There are two versions of the Superdry coating: "Superdry 1 can operate at cutting speeds to 200 m/min, while Superdry 2 allows the Mach 7 substrate to operate at speeds as high as 250 m/min for the same gears," says Shearing. The Superdry 2 coating is a basic AlCrN coating with a patent-pending Mitsubishi component added.
Optimizing the geometry of the cutting tool itself can also lead to longer life and more productive gear manufacturing. Manipulating tool geometry is a cheaper way to extend tool life and productivity than adding coatings or switching to a tougher substrate. Rather than attempting to make the tool stronger, Alexander Klein of the University of Aachen, Aachen, Germany attempted to make the chips less damaging to stick blades used in face milling of bevel gears. After studying how chips are formed, Klein created a cutting geometry that would flow the chip evenly away from the tool. By developing an experiment that mimicked the process of face-hobbing bevel gears, he was able to study various designs and observe their ability to make nondamaging chips. He demonstrated a 115% increase in tool life with novel designs of the stick insert blade, comparing old and new designs made with K10F carbide tools coated with TiAlN. Gleason has rights to the patents resulting from this work, and may be developing commercial versions of his designs soon, says Klein.
"The better wear behavior is not only caused by the better chip flow," explains Klein, "but at least as much by the fact that the tool corner—the area between tip and flank cutting edge—is less loaded thermally and mechanically."
Gearcutting equipment is also becoming more versatile by performing multiple operations in a single setup. For example, Liebherr (Kempten, Germany) and LMT-Fette (Auburn Hills, MI), working together, set out to revitalize finish-hobbing, a one setup process where a hob is used to both rough-cut and then finish-cut a gear.
Because current hobbing equipment does not allow automatic toolchanging, a single high-quality hobbing tool usually performs both cuts in finish hobbing, prematurely wearing out the tool. Creating separate roughing and finishing zones extends tool life by optimizing each zone's wear. The finishing tool now runs several times more parts, because it is not worn out from the roughing process, according to Darryl Witte, business unit manager, Gear Cutting Group, for LMT-Fette. Liebherr and LMT-Fette developed the concept together, with Liebherr providing the hobbing machine technology that can use the multizone hob.
The same team created a patented shape for the finish hob to eliminate convexity errors, known as twist, from helical gears with crowning. Typically, an additional finishing process, such as shaving or honing, removes twist. Twist-free finish hobbing means possibly eliminating these post-finishing processes.
"The twist-free technology enables a hob to overcome the drawback of typical finish-hobbing operations, which were not able to realize a requested flank twist," says Oliver Winkel of Liebherr. Adding a chamfering tool to such tool systems provides more value for the same setup, removing burrs before the part moves off the hobbing machine.
"In the last few years we are making quantum leaps in the capability of gearcutters. Tooling has become more durable—it's typical to see 8000 parts produced by a hob before it needs resharpening, whereas it was once necessary every 100–200 parts," says Tom Ware, product manager of gear tools for Star SU (Farmington Hills, MI) a company that represents gearcutting manufacturers Samputensili and Star Cutter. "Today it's possible for machines to produce speed rates of 450 m/min. We are hobbing a gear today with a quality that you would see off a grinder 5–10 years ago."
Ware reports that Samputensili now offers hobbing machines that provide multiple operations in a single setup, such as simple lathe operations that deburr grooves as well as cutting gears. This additional operation can be added to any of Samputensili's CNC hobbing machines. Star SU also provides a patented single tool that hobs and chamfers, says Ware.
While tooling continues to advance, the gearcutting machines that use the tools are also undergoing evolutionary change. The challenge of designing dry cutting machines for bevel gears is greater than the challenge of designing the tools, according to Hartmuth Müller, chief technical officer of Klingelnberg GmbH (Hückeswagen, Germany). He designates four strategic design points in the development of new machines:
- Optimizing chip flow as fat, thick chips are cut faster,
- Elimination of hydraulic units to reduce maintenance issues,
- Creating a smaller machine footprint to integrate better with autoloaders,
- Including automatic deburring, rather than deburring in a separate setup and station.
He advocates a vertical-spindle machine, rather than a horizontal arrangement. Chips will naturally flow away from the cutting head in a vertical spindle design, down specially designed inclines into a chip transporter. Klingelnberg's new C29 and C50 machines use a vertical-spindle design. The C29 can cut a maximum gear diam of 290 mm, and the C50 can cut a 500-mm-diam gear.
The vertical design allows for a smaller footprint and uses gravity to keep the workpiece in the fixture, according to Müller. The smaller footprint translates into an easier fit into autoloaders, while the gravity-assist means the loading device does not need to push the workpiece into the fixture while clamping it. Electromechanical clampers replace hydraulics. The C50's footprint is roughly half the size of that of the comparable Klingelnberg C60 machine. Pneumatic grippers are included to move parts weighing as much as 100 kg into and out of position with the cutterhead, providing for a clear movement of parts through the cutterhead. Klingelnberg asserts that the integrated workpiece loading with high-speed deburring incorporated on their C50 machine can reduce the nominal processing time per part from 3 to 5 min to approximately 15–18 sec, chip to chip. For the C29 machine, they claim 8-sec chip-to-chip processing time.
Gleason is also making their existing gearcutting machines more flexible by letting customers cut different gear designs on the same machine.
"There is still a considerable demand for straight bevel gears," observes Hermann J. Stadtfeld, vice president, bevel gear technology for Gleason, writing in a paper released in January 2007. "However, there was never a full CNC machine developed to satisfy the present demand. Manufacturers of straight bevel gears were relying on remanufactured mechanical machines." Straight bevel gears are processed in low quantities in a variety of designs, according to Stadtfeld, typically in shops that carry out many changeovers. Such shops might invest in a modern machine for straight bevel gears if they could also use it to cut spiral bevel gears as well, making the machine available for as many potential jobs as possible.
Gleason developed a process using a straight bevel Coniflex cutter for their CNC-controlled Phoenix line of free-form machines, so that they can now cut straight bevel gears in addition to spiral or hypoid gears. Cycle time improvements for cutting straight gears on Phoenix machines are anywhere from 100 to 120% over mechanical processes, according to Stadtfeld.
Gearcutting machines are multiaxis machines. They are frequently driven by direct-drive motors that replace one gear set in the table and another in the hob head drive.
Liebherr, which specializes in machines for producing parallel axis gears, provides CNC-controlled gearhobbing machines that use direct-drive, geardrive, or wormwheel-drive systems. Although many consider direct-drive motors to be state-of-the-art in gear hobbing, there are still valid reasons to consider a gear or wormgear drive in a hobbing machine, according to Christoph Bunsen, manager of development and design of gearcutting machines for Liebherr-Verzahntechnik GmbH (Kempten, Germany). Direct drives provide flexibility and typically require lower maintenance, while providing higher speeds and more precision than gear drives. On the other hand, according to Bunsen, gear and worm drives can provide higher torque at low speeds, in contrast to direct drives.
For example, comparing similar hobbing machines produced by Liebherr, one direct-drive unit rated for 23 kW with a hobbing head produces 130 Nm of torque at the spindle at 7000 rpm. A gear-driven unit with a similar hobbing head rated at 20 kW produces up to 270 Nm of torque at the spindle at 500 rpm.
"Sometimes it is a question of fashion," explains Bunsen. "There are customers who insist on getting direct-drive and we do not know why they want it. Sometimes, for the process, it could be better to use a gear drive."
Specifying Quality Requirements for Gearcutting Machines
Customers are demanding higher tolerances and machine processes that deliver ever-higher capability indices, as measured in Cp or Cpk. There is some concern voiced by suppliers of gearcutting machines over the proper specification of tolerances.
"Most customers require some type of capability study for machine acceptance," says T.J. Maiuri, manager of application engineering for Gleason. "Requirements for capability indices Cp and Cpk are generally 1.66 using a six-sigma analysis, though we have some customers asking for Cpk of 2. In some cases they were asking for an 8 or 10-sigma analysis, though today sixsigma is becoming more the norm."
He reports that customers are specifying tighter tolerances and requiring more gear parameters inspected before machine acceptance. As an example, a typical automotive planet pinion will have a size-over-pins tolerance of 50 µm or less. Also, more part designs (i.e. more part numbers) have to be evaluated before a machine is accepted. He reports that part runs for capability studies are normally 25–30 parts, though some requested runs have gone as high as 150 parts.
Oliver Winkel of Liebherr relates how a drawing tolerance of 60 µm can shrink to an effective tolerance of 25 µm for a specified CmK of 1.67 (CmK is a Cpk applied only to the machine, not the process under real production conditions). According to him, based on empirical evidence, an increase in CmK typically results in a linear proportional tightening of the tolerances (see the accompanying graph). "The request to be within 100% of the given tolerance is not identical to the request for a CmK of 1.0 for the same tolerance," he explains. The CmK request of 1.0 leads to a 30–40% reduction of the effective tolerance. Effective tolerance is determined by centering all data points between bands on an X-bar – R chart that are tighter than the upper and lower control limits, which are half of the drawing tolerance. A six-sigma analysis determines the bands, which are more stringent than the drawing tolerance. Although the results can change from situation to situation, the chart shows the general trend.
"We have a saying in German that a part should only be manufactured as good as necessary, not as good as possible," says Winkel. "Unnecessarily tight tolerances lead to a longer production time, which increases costs."
This article was first published in the September 2007 edition of Manufacturing Engineering magazine.