How Skiving Can Solve Complex Gear Cutting Challenges in Manufacturing

Recent technological advances promise to turn what was once a niche operation into a high-volume process for manufacturers, particularly in the automotive sector.

EMAG has sponsored this post.

Gears make the world go round… but manufacturing them is a complicated and time-consuming endeavor. This is particularly true in high-volume industries such as auto manufacturing. Manufacturers have been searching for ways to more efficiently produce these vital components.

The VS 400 PS user interface. (Image: EMAG)

The VS 400 PS user interface. (Image: EMAG.)

Current conventional options for manufacturing gears include hobbing, broaching and shaping.

  • Hobbing: This method is well suited to producing outer gears. It involves using a specialized milling machine; the machine uses a helical cutting tool called a hob to progressively cut the teeth or splines of the gear into a metal cylinder.
  • Broaching: While hobbing is used to cut outer gears, broaching is used for inner ones. The method involves pulling a broach cutting tool through an object. The tool has teeth that gradually increase in size, carving the desired shape into the object.
  • Shaping: this method can be used to create gears that hobbing is unable to produce. With this method, a gear is created through continuous, same-plane cutting to create the gear’s teeth. The teeth are created one by one, as opposed to hobbing where all the teeth are created at the same time.

While each of these methods has its advantages, each also has its limitations. It is possible to create outer gears with hobbing and inner gears with broaching, but it’s less efficient to do so. And shaping doesn’t allow for material removal during the back stroke.

This is where skiving may be the best solution. Skiving, also known as scudding, is a machining process that combines gear hobbing with an axial feed. That axial feed allows for high cutting speeds and engages more teeth at a time than shaping does. This continuous machining sequence makes it a very efficient process. According to Daniel Nille, head of Technology Development at EMAG, a manufacturer of CNC machine tools, power skiving is around 50 percent faster than gear shaping and more flexible than gear hobbing.

“For example, if you use shaping, you need the tool to move after every cut to get back into position for another pass,” said Nille. “But when you use power skiving you need less movement from the end of the gear to the beginning of the gear.”

Skiving helps reduce the impact of interfering contours, such as workpiece shoulders. There is always a need for some space at the end of the gear when machining it; however, the smaller the space, the more risk of collision with the cutting tool. The power skiving process allows for that space to be smaller, enabling the tooling to get closer to the interfering contour while creating the gear.

The VSC 400 PS Skiving Machine.

Machines with these features can help a gear manufacturer find significant efficiencies in its production line. An example of this is EMAG’s VSC 400 PS machine, which holds up to four power skiving tools and six turning tools, combining a traditional turning machine with a skiving machine, which ensures highly effective combination machining.

EMAG has recently added a special B-axis component that allows for enhanced skiving. The machine’s mineralit base is eight times better at damping than the more commonly found cast iron bases, allowing for longer tool life and lower terminal sensitivity. The machine handles hard tuning with a spindle on the upper side.

“That means you have optimal chip flow because the work piece is arranged vertically at the top,” said Nille. “Another benefit from this machine is we use hydrostatic guiding principle for the set axis, which is also good for damping vibrations and stability. The machine has symmetrical design with controlled terminal linear expansion.”

In addition, the machine features two milling spindles for power skiving. “That means it can be used for different functions,” said Nille. “One spindle for roughing and the other for finishing—or it can produce two different gearings on one work piece or use two of the same tools to extend tool life.”

According to Nille, the EMAG machine may be the only one on the market with six turning tools and two spindles on one B-axis, significantly shortening the time needed for tool changes—reducing the time from tool to tool to less than one second.

EMAG’s technology also helps mitigate some of the challenges involved in power skiving. One particular challenge is the proper synchronization between the tool and the component being manufactured, which is essential in high-speed operations such as power skiving. “It has high-resolution incremental encoders on the main spindle and the meeting spindles,” Said Nille. “This allows for perfect synchronization.”

The machine is also programmable, helping the operator define the spindle speed, which can function at 150 meters-per-minute. That data, which includes not only the spindle speed but also feed rate and cutting depths, is normally supplied by the tool supplier, but EMAG can calculate it with its own simulation software. “We have placed a great deal of emphasis on massively simplifying the entire operation,” said Nille.

Programming a power skiver can be complex because it involves both cylindrical and conical tools. The conical tools have a clearance angle, while the cylindrical tools don’t. Here the tool must be positioned with a kappa angle to obtain a clearance angle. Compared to a turning process, the cutting tool needs to be set under the middle of the work piece to get that clearance angle, which requires precise positioning. In skiving, it is common to change the kappa angle in every cut, requiring that the work piece and the tooling be in position to fit perfectly into the existing gear for the next cut.

“It has an intelligent user interface created specifically for power skiving,” said Nille. “It allows you to intuitively program the process. You only have to fill in the data from the tool, the drawing and the work piece. Then our software creates the program for you and calculates all the correct positions for the components. That means it’s very easy for the worker to program the power skiving process.”

The software independently calculates each step of the power skiving process while allowing for corrections and adjustments through interactive images and automatic safety prompts.

Skiving has significant potential for wider use in manufacturing. “We expect growing market opportunities,” said Nille. “Thanks to the further development of power skiving tools in recent years, this process has become much more attractive and is being used more and more in industry.” Nille noted that power skiving can potentially be used to produce internal gears for the planetary gearboxes found in many electric vehicle motors. It can also enable more efficient production of complex component geometries.

While it is not a new method, skiving’s dynamic nature and use of high rpms has historically been difficult to execute on older machines; as a result, it was only used for niche applications in the past. However, advances in manufacturing technology, including new tool coatings, has made skiving more feasible for manufacturers. Machines such as EMAG’s VS 400 PS can help gear manufacturers to boost productivity and implement more flexible operations to respond to consumer needs.

Visit EMAG to learn more.