The Best of Both Worlds in Gear Manufacturing: High Flexibility with High Productivity

April 09, 2013

A gear-milling method in which the tool path, instead of the tool shape, determines the gear.


Sophisticated yet simple, providing full flexibility combined with high productivity, InvoMilling is the latest development when both efficiency and production versatility are needed for gear-wheel manufacturing.

This is how one gear-making company describes itself: “Many of the gear wheels we manufacture are done to specified customer requirements, where a customer may want a specific wheel, a batch of wheels, or a transmission solution. It is mostly small series that we handle—between one and 100 components—but we also have orders where we produce a couple of thousand gear wheels throughout a year. “Just-in-time” deliveries are the norm for some products, particularly when it comes to components for vehicle gearboxes. We are a specialist manufacturer of helical and spur gear wheels with our output being for a range of different industries.”

Until recently, this company was locked into using engineered and unique-for-the-application gear-making tools for every application. With a reverse-engineering capability, unknown gear wheels that are worn can be determined exactly as regards the design and then manufactured quickly; flexibility is part of a successful business idea. Figure 1

But being versatile in production has been hard to combine with high productivity. The necessary flexibility has meant re-rigging the gear-making machines with new tooling and programming every time a different gear wheel needs to be made.

A new approach for flexibility in gear machining is based on an innovative method of milling in the multi-axis mill-turn machine: InvoMilling. This is a unique series of operations developed by Sandvik Coromant (patent pending and a registered trademark) and presently being offered in collaboration with Mori Seiki. Mori Seiki wanted to offer an alternative to the specialized, relatively slow process of flexible gear wheel making dominating the market today. The creative multi-axis solution lies in a sophisticated yet simple cutting strategy in standard, multi-purpose machines with off-the-shelf cutting tools for spur- and helical-gear machining, as opposed to gear-hobbing methods and means.

The new multi-axis machining method of InvoMilling is dedicated to flexibly producing spur and helical gears using a standard, disc-shape milling cutter as well as a standard, dedicated facemilling cutter. With simultaneous motion of the x and b axes or y and b axes, the cutting tool follows an involute path. Consequently, variations in gear-shape are according to the path of the tool—not the shape of the tool. And for this purpose, the mill-turn type of machine is ideal.

With a DMG/Mori Seiki NT-model machine, an altogether new approach could be taken. This multi-axis mill-turn centre offers a combination for milling and turning machining. The flexible B-axis with a rotation range of plus/minus 120°, the high speed and precision due to the direct-drive motor, indexing capability of 0.0001°, had showed itself to siginficantly reduce machining time, especially when machining complex shapes with simultaneous 5-axis machining. Gear hobbing is one operation performed on the machine, as well as a number of other component types related to transmissions. But hobbing is not the optimal solution when many different types and small series of gear wheels are to be produced. Figure 2

Today, if a machine shop has, or is thinking about investing in, an NT-machine, the InvoMilling software is an easily-obtained option. The mill-turn machine can begin making gear wheels directly. Gear-making cutter kits, with three tools for several modules, are available for different module ranges with which a large range of gears can easily be produced, with great flexibility and to very high quality standards.

The InvoMilling method is a combination of slot-milling and turn-milling operations. It enables the machining of gears of any helix angle, both involute and non-involute profiles with the same tools.The number of cuts involved depends on the size of the gear and the size of the cutter being used. The method makes use of the modern machine control and the high accuracy of the NT-type machine, as well as on the accuracy of the cutting tool to make precision gears as well as semi-finished gears for finish-grinding. Applying the latest developments in insert grades has also enabled higher cutting data to be applied for optimum efficiency. Figure 3

Chamfers at the tooth's outer diameter can also be incorporated, as can different protuberance angles, profiles, and radii at the tooth base. The chip cross-section per cutting action is small, with no side-pressure to speak of and consequently minimized vibrations-tendency. 

In machining the tooth-space of an outer spur gear, for example, a groove is first milled in an axial direction of the workpiece. For the following cuts, the gear-wheel rotates slightly, coordinated with the simultaneous radial plunging of the cutter. This results in an involute curve-shape being milled by the facemill through turn milling. For larger modules, more cuts may be necessary. For wider gears, the cutter is off set in the axial direction and the procedure repeated. Figure 4

InvoMilling means lower tool costs for small to medium batches as each InvoMilling tool covers several module sizes, where, for example, 2–4, can be machined using the same cutter; the range 2–12 is covered by three cutters. To date, the largest gear wheel milled using the InvoMilling method has a module 18 and a diameter of 800 mm. This means advantageous options available for producing gears to attractive lead times and costs.

Modern indexable-insert technology is the means that makes the new method successful. With InvoMilling, gear teeth with quality level 6, DIN 3962, or better, can be achieved. Moreover, surface finish better than Rz 3 microns can be achieved through the right application of tools in sufficiently rigid machines. InvoMilling with precision tool-path motion and precision tools can produce some really high-quality gears very competitively. Various gear-grinding allowances can also be generated by the method. The low cutting forces of the method enable the high level of cutting data to be used, while ensuring accurate profiles for close tolerance ranges. Combined with the flexibility of this gear wheel-making method, this is a winning success-factor for the manufacturing in question.

Standard, cemented-carbide indexable inserts for the milling tools involved are easily indexed outside the machine and stored in the magazine.  The cycle time of InvoMilling for modules 4-10 is for number of teeth below 30 comparable to single start hobbing with HSS. Since many operations can be integrated in the multi-task, the gear can be finished in one machine set-up, resulting in short lead-times and change-overs, and consequently improved work-in-progress advantages for flexible production of one-off, small- and medium-sized batches of gears.

So even cases where the cycle time for manufacturing the gear itself is longer than when using hobbing, the total manufacturing time is shorter. Finally, since there is no waiting time for a dedicated tool to be made available, production can be started after the new set-up. By introducing InvoMilling, Sandvik Coromant has provided a technology shift, giving significant improvement in costs, flexibility, and lead times with the option of utilizing standard-purpose machine tools for manufacturing high-quality gears. 

About The Authors

Christer Richt

is a technical editor for Sandvik Coromant, based in Sandviken, Sweden. He has been writing about metal cutting and the industry for the past 40 years, including gear hobbing since 1980. For more information about the company’s InvoMilling process, call 1-800-SANDVIK, or log on to www.sandvik.com.

Aaron Habeck

is marketing project manager for Sandvik Coromant Col, Fair Lawn, N.J. For more information about the company’s InvoMilling process, call 1-800-SANDVIK, or log on to www.sandvik.coromant.com.