Large gear milling - technology on the change

7 min read

The production of large gears is migrating away from dedicated gear producing equipment to standard machining centres. Steed Webzell investigates the growing momentum behind the trend (includes videos)

(Main video shows Hermle process - other videos, see links below) Over the course of the past 12 months, a number of machining centre manufacturers have entered the market for producing large gears. Far from mere coincidence, the development represents a paradigm shift in gear machining philosophy. The fact that one of the world's most prominent producers of dedicated gear machinery, Gleason (01752 724424), has formed a strategic alliance with machining centre manufacturer Heller (0121 275 3300) is a clear indication of shifting market patterns. "There's always been demand for large bevel gears, but it has increased in recent years, driven by construction, mining, shipbuilding and other heavy industry applications," states Alan Finegan, company director at Gleason Corporation. "Some of this growth is related to the infrastructure needs of developing countries. "In the past, dedicated gear machines were productive, but were expensive and often poorly utilised, due to relatively low batch quantities," he adds. "Now, 5-axis CNC technology and Gleason gear technology allows users to cut bevel gear teeth and perform other metal removal processes like turning, thus maximising return on investment." In terms of capacity, Heller's existing MCH series of machines is capable of producing bevel/conical gears up to 1,890 mm diameter. However, a new range is under development for gears up to some 3,000 mm diameter. The company's recently introduced uP-Gear Technology, developed in co-operation with German engineering group Voith and tooling specialist Sandvik Coromant (0121 504 5400), forms the cornerstone of the solution (video). MATHS CHALLENGE There's a commonly held belief that there is no comprehensive mathematical model for bevel gear tooth systems. Consequently, every CAD model used for gear-related CADCAM processes are approximated models. However, the approach taken by Heller differs from the traditional method. Although resembling a hobbing process, gear parameters specified on drawings can be input directly into the machine control. The control then uses the data to generate the 5-axis paths for both roughing and finishing, in real time. As a result, a mathematically accurate 3D geometry model isn't required – rather, pre-defined gear data from drawings is used. In addition to tooth space machining, Heller also provides a process for the pre-milling of blanks. The upshot is, the complete pre-machining of both external and internal contours is possible in the first set-up, with tooth milling taking place in the second set-up. Of course, there are other 5-axis machining centre manufacturers claiming that their machines are equally versatile, when programmed accordingly, and that the use of standard end milling cutters, for example, provides high flexibility. However, according to Eric Pollard, Heller UK sales manager, this method is not particularly efficient or productive. "We operate in fields with high chip removal rates and aim to achieve the same in gear milling. Therefore, we co-operated with Sandvik Coromant to develop a customised milling cutter with indexable inserts. This has played a major role in increasing productivity by a factor of three, compared to conventional methods. It means the wider the tooth space, the more productive we will be." Heller says that uP-Gear Technology is aimed at manufacturers in industries such as railway, shipbuilding, mining and power generation that are also looking to use a machining centre for a range of tasks. Voith, of course, is a benchmark case in point. With a number of new contracts, including a prestigious order for transmissions destined for the Chinese railway industry, Voith decided to build a new factory in Mergelstetten, Germany. According to Kenneth Sundberg, business development manager – gear milling, at Sandvik Coromant, the original project at Mergelstetten started with soft machining, looking to improve productivity against the use of solid carbide end mills. Here, Sandvik Coromant created a type of 'disc' cutter that opens the gear's tooth gap step by step to the desired shape, with the result being an improvement in throughput of 300%. Subsequently, attention turned to the finishing process (hard milling). Here, a premium disc type cutter with adjustable CBN-tipped inserts for micron-level precision is proving very effective, so much so that it may even allow elimination of final grinding operations. Michael Skarka, an operator at Mergelstetten, works on a Heller MCH-350-C milling machine, equipped with Sandvik Coromant tools. Behind the glass screen, the machine works the raw material – grinding edges, cutting out tooth gaps and grinding gear shafts. The process previously took 5.5 hours, using dedicated gear machinery, he says. Now it's down to 2.5 hours. Image: Reducing gear cutting time with Sandvik Coromant tooling Mr Sundberg says that a few machines with uP-Gear Technology have now been sold and that Sandvik Coromant, the exclusive tool partner in turnkey operations, has developed a semi-standard programme of tools needed for different modules. Tools for up to module 20 are currently available, but up to module 40 will be catered for by the end of 2012. STANDARD MACHINE, STANDARD TOOLSM Another machine tool supplier competing in this field is DMG (01582 570661). With a modular package comprising a standard machine, standard tools, a measuring module and gearMILL gear cutting software, DMG offers the entire process chain for the production of gear wheels, commencing with geometry calculation, followed by automated programming, including traverse range verification, through to precision soft and hard machining. Image: DMG's solution is a standard machine, standard tooling and its gearMILL software Image: With DMG's solution, milling herring bone gears without a central channel is possible According to Martin Adams, engineering services manager at DMG UK, customers "can use almost any DMG 5-axis machines for this function". He adds that many machines have been sold in Europe, but not, as yet, in the UK. Worm gear cutting with freely definable flank modifications is possible, due to the use of new mathematic algorithms in the software. Moreover, this type calculation method will, in the future, allow the free definition of new gear wheel geometries or will allow optimisation of existing gear cutting methods, with regard to their flank and profile design. The effects achievable by this can be shown, for example, by the calculation of herringbone gears, where gear wheels can now be produced without a centre groove. This, in turn, allows the transmission of substantially higher forces with the same installation dimensions, or reduced dimensions for the same transmission force. DMG can accommodate gear wheel machining on universal machines up to 5,000 mm diameter. Precise surfaces are created automatically from the tooth space geometry that maximises the quality of calculated milling paths. The milling paths, in turn, require hardly any specific machining knowledge from the gear cutting expert. All that is needed is a standardised work plan, assigned to the geometry, that carries out the entire programming for the gear wheel or pinion automatically. DMG can provide gear cutting technology for spur gears, bevel gears and worm gears, while the latest development steps are now focused on complementary software modules for further tooth types. Gear manufacturer C u W Keller GmbH & Co KG has been using DMG gearMILL software since 2010 to produce sets of bevel gears on a DMU 160 P duoBLOCK, equipped with standard tools. Here, the software also allows the manufacture of splined toothed wheels, if necessary, even without centre groove, as well as spur gears with distorted modules, meshing angles and modified flanks. Others to offer a solution in this area include Hermle, available in the UK via Geo Kingsbury Machine Tools (023 9258 0371), following a tie-up between the German machine tool builder and Dutch software firm HPG Nederland BV. The process is for producing small batches of gears up to 1,200 mm diameter (see main video). Hermle's 5-axis milling technology allows roughing and super-finish machining of gear teeth and other complex 3D contours, either via a C40U or C50U fitted with a Heidenhain iTNC 530 or Siemens 840D control. Using HPG software, a gear can be completely machined, using standard milling cutters. Via an online connection to an HPG-licensed service provider, gear geometry data, such as module, diameter, tooth width and height, contact area ratio and crowning, is transmitted. On the basis of this data, the software performs the mathematical calculation for the exact tooth shape, which is developed as a point cloud and translated into an NC program for milling the gear. Image: Hermle employs Dutch software to generate the NC paths for its machines Also available in the UK (from Lead Precision Machine Tools 01625 434990) is the Breton Ultrix 1000 RT HD 5-axis machining centre, which can execute milling, turning and grinding operations on workpieces up to 1,200 mm diameter. The machine is equipped with software for the manufacturing of Klingenberg and Gleason bevel gears (video). Further options are available from Mitsui Seiki via UK agent 2D CNC Machinery (0844 871 8584), where the HU80A-5X 5-axis HMC can be used for producing spiral bevel gears. One of the largest Mitsui Seiki 5-axis models, it can produce gears in diameters up to 900 mm. Here, the gear is mounted flat and then turned 90° (or other appropriate angle), so that an end mill can cut at the best attitude to the gear profile while roughing. Ballnose end mills are used to generate the final profile. It seems that the overall cost of producing gears in low volumes on a 5-axis machining centre is likely to compare well with traditional gear cutting machines when tooling economies, reduced part handling and improved process control are taken into account. Box item Plum job for a hob Sandvik Coromant's indexable insert solution for hob cutters is another new concept. Indexable carbide insert technology is well established for very large modules (10 upwards) used, for example, in the mining and wind power industries. Until now, it has not been available for still large, but slightly smaller gears typical of transmissions used in applications such as those found in heavy vehicles, agricultural machinery and trains (modules 5-8). The firm's Kenneth Sundberg (see main text) says the biggest challenge when developing such a solution at Voith was achieving the micron precision required to meet DIN standards. However, the special hob cutter presented – which featured 60 or so cutting teeth in spiral formation – increased productivity by 50% (saving 7,000 hours) on one particular job alone. The result of this success is the subsequent development of CoroMill 176, an indexable, full profile carbide insert hob for gear milling in roughing, semi-finishing and finishing operations (video with online feature). Currently, solutions are available for gear profile sizes in the module 4-6 range, although solutions for modules 3-9 are in development, largely with eyes on the truck market. (See video) Image:Sandvik Coromant's CoroMill 176 Video links Heller Machine Tools' uP-Gear technology in action Sandvik Coromant's CoroMill 176 carbide inserted gear hob Machining gears on Breton's Ultrix machining centre First published in Machinery, January 2012