PITTLER T&S – Power Skiving: Flexible and effective production of internal gears

The process with the somewhat cum­bersome title of “method for cut­ting gear wheels using a cutting tool similar to a gear wheel with cutting edges on the faces of the teeth” in the patent application more than 100 ye­ars ago has matured into one of the most economical gear cutting techno­logies around – power skiving.

The inventor, Julius Wilhelm von Pittler, intended to use it to make rings with in­ternal gearing. Yet it was only with the technological accomplishments of the 21st century that PITTLER T&S GmbH were able to realise gear cutting machines on the ba­sis of the PV series for the efficient produc­tion of ring gears, for example. All those years ago, Julius Wilhelm von Pittler was forced to link all machine movements me­chanically. It wasn‘t until intelligent soft­ware and a computer-controlled numeric control came along that it was possible to move the five axes extremely precisely and synchronously to one another in an inter­polating way. Developments in the tool segment such as more sturdy powder and carbide alloys as well as high-performance coating for tools also played their part. Due to a lack of efficient production tech­nology for internal gearing, spur gears and external gearing became establis­hed in drive engineering in the course of the last 100 years. In the field of internal gearing production, the methods shaping and broaching were the order of the day. Yet this has all changed thanks to the latest developments in the field of power skiving technology. This technology allows manufacturers to get round the disad­vantages of the competing processes and benefit from increased flexibility and higher output at the same time.

Production of internal and external gearing in one clamping

Unlike shaping, power skiving is a continu­al hobbing manufacturing process which avoids the unproductive stroke and remo­val movement. The advantage: machining time is reduced by at least the factor 3. In addition, power skiving is much more fle­xible than broaching, because the machi­ne operator can configure the dimensions such as the two-ball dimension using the NC control. Helix angle and the flank sha­pe can also be modified using the NC con­trol. The high adaptive capacity becomes clear when it is born in mind that gearing can be cut both internally and externally. In the case of external gearing, the tech­nologies of power skiving and hobbing are competing more and more often. Since significantly less space is required for the tool outlet for power skiving, the techno­logy is opening up new possibilities for de­signing cost-saving gearbox components. Even workpieces with constraining con­tours can be produced efficiently using the power skiving method.

Power skiving has obious advantages

In the past, critics claimed that power ski­ving was less productive than hobbing. This claim cannot be generalised, however. Because in an optimised process, the main times and tool costs are by all means com­parable. During broaching, component quality is determined mainly by the tool geometry of the broach, and can thus not be influ­enced during the machining process. The high costs of the tools, set-up, treatment and storage of the broaches is another dis­advantage.

Tool change saves costs

With the PV range, PITTLER T&S offers a whole series of machines using power skiving technology. This covers the who­le workpiece diameter range from 30 to 1250 mm. All the machines have 5 axes and a tool magazine for simple and fast tool change. Alongside the pure tool management of different skiving wheels, the tool change inclu­des tactile measuring sensors and other metal-cutting tools. This allows the cutting process to be divided into roughing and finishing. A roughing tool with standard indexable inserts removes more than 90% of the material. This goes decidedly easy on the finishing tool, which creates the involute contour and results in lower tool costs. A further advantage of the tool magazine of the PITTLER PV range is the complete machining option for workpieces. This me­ans procurement and transport to other machines is no longer required. Reclam­ping faults are avoided. Further advan­tages: processing and delivery times are speeded up, the complexity of workpiece handling is reduced, intermediate storage is not required and manufacturing structu­res are flexible rather than rigid.

Economic production of crowned tooth flanks

Unlike the methods already described, power skiving influences the straightness of the tooth flanks. If both flanks are cut offset, the result is crowned tooth flanks. This allows the contour to be corrected on the one hand, and new geometries to be created on the other. This cannot be done more efficiently by any other other manu­facturing method.

Simplified user interface

The power skiving machines from PITTLER T&S are all constructed with the very latest control technology on the ba­sis of Siemens 840D SL. This allows the machine to be used as a 5-axis machi­ning centre with ShopMill. In addition, a suitable post-processor makes free-form surfaces with milling possible. The com­pany, based in southern Hesse, equips the machines with a special user interface for power skiving. This allows an employee with basic knowledge to produce gea­ring. The NC program is generated auto­matically. The data is requested with the aid of a graphic interface. The data is also transferred to an automatic measuring program which checks the quality of the gearing. This means the measuring sen­sor can measure the result directly after machining. It does not replace a gearing measuring machine, but does provide a first quick check of the quality achieved and is thus an important aid during pro­cess run-in.

Clamping technology with DVS system

Concentricity is essential for component quality, particularly where thin-walled ring gears are being machined. This is why PITTLER T&S got together with SWS Spann­werkzeuge GmbH, an associated company within the DVS TECHNOLOGY GROUP, to develop a special diaphragm chuck as well as special chucks using the hydro expansi­on principle for this application. This DVS system solution permits sensitive clamping without warping the component.