Gears make the world go ‘round, especially for automotive parts manufacturers. But when it comes to making gears, there are a wide range of methods, machines and considerations depending on the application, volume and quality requirements.

For high-volume makers serving OEM markets such as automotive and consumer durables, the engineering challenge has never been tougher. Gearboxes are now frequently sealed with factory lubricants, which means gears need meshing surfaces that are hard, tough and don’t rely on the fresh additive packages offered by lubricant changes. At the same time, automotive and consumer appliances are quieter—especially true as electric cars become popular.

Gear profiles are now highly complex, and noise is more important than ever. While these profiles are more sophisticated and more difficult to produce, the pressures to lower costs are higher than ever. For more complex parts and tighter margins, the solution is a higher level of gear making technology.

At which point should a production shop consider specialized machinery for gear production? What gear making process is best: shaping, skiving, broaching or hobbing? To answer these questions, engineering.com spoke with Jeff Moore, regional sales manager at EMAG.

When to Invest in a Dedicated Gear Making Machine

Specialized, dedicated machines and automation provide enough cycle time gains to make the investment in dedicated equipment a no-brainer for high-volume production. However, for smaller job shop operations it’s typically preferable to invest your money, training and square footage in more versatile equipment, such as five-axis machining or turn milling. The problem, according to Moore, is that there is no clear rule of thumb to identify where the line is between these two elements.

“The volume is typically what dictates what type of machine you're looking at,” said Moore. “Many smaller job shops cost their work out by the hour, so it's a very tough decision to make. When you start to get into more than 20,000 pieces per year, it's hard to justify having somebody stand at the machine and load by hand when you can automate the machines quite easily. But it's a very tough point to figure what that volume is.”

Along with volume, gear quality and technology is another factor which leads shops to invest in specialized gear making equipment. But ultimately, there’s no hard and fast rule for when smaller operations should make the investment. Frequently, the sales process drives the switch, and that “big score” with a major customer overwhelms the shop’s production capacity—and challenges margins.

Gear skiving technology can be a solution. For operations that are planning to invest in specialized equipment, what are the advantages of skiving over other techniques?

Overview of Specialized Gear Making Processes

Hobbing

In gear hobbing, teeth are progressively cut into the gear by a specialized tool called a hob (see the gold-colored part in the image below). A hobbing machine has two spindles, one for the hob and one for the workpiece. The two shafts are turned at different rates relative to one another to achieve different gear types, including spur and helical gears.

Hobbing can only cut on the outside diameter of the workpiece, which means internal gears must be cut using another process. Of all the processes for making gears, hobbing is the fastest, and also delivers high accuracy.

Shaping and Broaching

Gear shaping and broaching are similar processes to hobbing. On a gear shaper, a cutting tool moves in a reciprocating motion, cutting away material on one pass, then pulling back. Meanwhile, the workpiece rotates to index the next tooth position with the cutter. In this way, the shaper continues until all the teeth reach the correct depth. Compared to hobbing, shaping is slower and requires a precise indexing motion.

Gear broaching works on the same principle as any broaching process, in which a broach with the profile of the tooth is pressed into the workpiece to cut away material. Both these processes are hard on the tool, and tool life is shorter than for hobbing.

Power Skiving

In gear making, the term “skiving” refers to power skiving. Power skiving is a highly efficient method which essentially combines hobbing with shaping. Skiving can be three to five times faster than shaping and offers more flexibility than broaching.  For example, to skive an internal spur gear, the workpiece turns on one spindle, while the cutter spins at the same rate on a spindle parallel to the axis of the gear. The cutter then moves along the axis to cut material away. The cutter spindle can also rotate in the b-axis and shift in y-axis allowing different types of gears to be cut.

According to Moore, power skiving is typically used on internal gears.

“For example, on a part with an OD gear and an ID gear, if you are able to gear hob the OD gear, it's much more cost-effective to do that, then to skive it,” said Moore. “You can also look at broaching as well for the OD gear, if that’s feasible depending on volumes. Typically skiving on the OD gear is done only when high quality between the ID and the OD are required. Skiving can produce very good run outs between two gears, and so skiving may be used for the OD gear when the relationship has to be perfect between the two.”

Making Precision Gears for the Automotive Industry

According to Moore, technological advances in the automotive industry mean higher and higher demands for precision and quality of gears. Today’s drivetrains must transmit higher torque and horsepower, and electric vehicles produce less noise to drown out transmission whine. Quieter, harder gears that can last for the life of the vehicle are needed.

Of course, helical gears run quieter than spur gears (which is why the reverse gear on many cars makes an audible whine, while the others don’t). Other gear technology innovations also help with noise.

“On most of the automotive gears you see today, there is a crown on the gear face, as well as on the lead, of two to three microns. As the gears mesh together, the crowns help them fit together better, and make less noise,” explained Moore. “For some higher-end vehicles, for differential or transmission gears, they'll actually lap the gears together using a wet paste. They run the gears together in this wet paste until they fit, and then they're shipped in pairs.”

For quieter, longer-lasting gears, a better surface finish and slight innovations in the profile of the gear are what makes the difference, as well as tighter tolerances. So, in addition to production volume, these quality requirements could be a reason to invest in specialized gear making equipment.

Set Ups, Workholding and Automation

Another consideration when making precision gears is the number of operations required to make it, such as turning, drilling or milling. For any precision part, setting up for a second or third operation can be difficult and time consuming, especially for low to medium sized batches. These extra set-ups require skilled labor and use valuable production in order to achieve the needed quality.  Higher volume scenarios require tightening of in-process tolerances, very good fixtures and capable processes in order to maintain final quality requirements.

According to Moore, EMAG makes machines such as the VMC450-5MT, which does skiving, turning and drilling for external and internal gears for those low to medium sized batches. The EMAG VMC450-5MT has 5 axes, glass scales, direct drive spindles, a cast polymer granite Mineralit base and a tool changer that can hold up to 80 tools, just to name a few of the features.

“When customers can do turning and drilling and milling in the machine at the same time, maybe they don't have to buy three different pieces of equipment,” said Moore. “Maybe they can stick with just one piece of equipment and then get rid of their gear shaping, which is a very slow process, and improve their productivity. Therefore their cost gets reduced, and they can make more profit.”

Automation for machine loading and unloading also becomes a cost consideration when high volumes are involved. However, it may not make sense to dedicate a machine to a robot and fence off access when the next job that comes in does not have the volume to make programming the robot cost-effective. For these higher-mix situations, EMAG offers a machine which has two doors: one for a robot, and one that is always accessible to a human operator without entering or dismantling the robot enclosure. This allows automation to stay in place for those higher volume jobs, while keeping the option for a human operator to run the machine.

For higher annual volumes, EMAG offers VSC250 PS single spindle and VSC 250 DUO PS two-spindle power skiving machines.  Each of these machines has a 12-position turret which is offered with an option for driven live tooling, such as drills and mills. EMAG vertical pick-up machines have the ability to do complex ring gears requiring deburring, drilling, tapping and milling in a single clamping, helping to improve quality. These machines can be linked together with turning, hobbing, and hard turn and grinding operations using EMAG Trackmotion automation.

According to Moore, automotive parts suppliers can expect gear qualities to trend upwards in the years to come.

“Complexity is becoming deep. These are not simple transmissions anymore; they are pretty complex, and they have to require a lot of torque. We're talking, especially in a ten-speed transmission, up to five of those speeds being overdrive,” said Moore. “From the aspect of material science, I haven't seen changes in materials, but I have seen that the gear quality is getting better and better.”

For more information about skiving, hobbing or other machining equipment, contact EMAG.