The best of both worlds in gear manufacturing: high flexibility with high productivity

“Many of the gear wheels…

…we manufacture are done to specified customer requirements, where a customer may want a specific wheel, or a batch of wheels or a transmission solution. It is mostly small series that we handle - between one and one hundred 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.”

This is how one gear making company describes itself, and up to recently it was locked into 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 design and then manufactured quickly. Flexibility is part of the business idea. 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…

…to have 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 face milling 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, a new approach altogether 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 significantly 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.

Today, if a machine shop has, or is in the process of - 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, as such, 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.

Chamfers at the tooth 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.

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 face mill through turn-milling. For larger modules more cuts may be necessary. For wider gears, the cutter is offset in the axial direction and the procedure repeated.

InvoMilling means lower tool costs for small to medium batches as each InvoMilling tool covers several module sizes. Where, for example, 2 to 4 can be machined using the same cutter; the range 2 to 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. Combine this 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 teeth numbers 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-offs, 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 start after the new set-up. By introducing InvoMilling, Sandvik Coromant has provided a technology shift, giving significant improvement in

costs, flexibility, lead times with the option of utilizing standard-purpose machine tools for manufacturing high-quality gears.

For more information please visit
http://www.sandvik.coromant.com/zh-cn/knowledge/milling/application_overview/gear_milling/pages/default.aspx

Sandvik Coromant

Sandvik Coromant is a world-leading supplier of cutting tools and tooling systems for the metalworking industry and is represented in 130 countries. 27 state-of -the art Productivity Centers located around the world provide customers and staff with continuous training in tooling solutions and methods to increase productivity. Sandvik Coromant is part of the Tooling business area of the Sandvik Group.

Contact details for editorial enquiries
Contact: Lianne Mills
Tel: 86-10- 65399902
Email: Catherine.li@sandvik.com

www.sandvik.coromant.com