Stepping up a gear

Gear cutting traditionally is something of a black art with few companies possessing neither the specialised equipment nor the expertise to be able to carry it out successfully. However,  

Since EMO 2009 a modular package comprising software, standard tools and a measurement module has been available from DMG which enables gear wheels to be manufactured – even from hardened materials – to tolerances smaller than DIN/ISO Q6. DMG says that it is now possible to create arbitrary tooth profiles on modern Deckel Maho 5-axis machines on lengths from 600mm up to 3,400mm and on universal milling centres from the monoBLOCK, duoBLOCK and Portal machine series'.

Therefore, the manufacture of larger, more precise conical gear wheel sets with helical tooth patterns is now possible. Complex tooth space forms can be produced simply, quickly and cost-effectively on Deckel Maho 5-axis machines in small to medium batch sizes. At the same time on mill/turn machines, toothed parts, such as pinion shafts can be manufactured completely on a single machine. 

Motivational factors

So what motivates a manufacturer of universal machine tools to become concerned with gear wheel production when specialised companies exist?

Well, as is often the case, demand was customer driven. Several of DMG's clients needed the capability to mill helical tooth conical gear wheels and so tests were carried out, mainly employing customers' own NC programs. Although successful, DMG recognised the need to produce its own software programs which were, in the first instance, used to prepare geometric information, obtained from third party software, for further processing in a dedicated CAD/CAM system. Along the way the soft and hard machining of spiral bevel gears was carried out for a number of interested parties around the world and, convinced by the high quality of the resulting products, many customers ordered corresponding machines.

In a second step Deckel Maho developed a software package which, on the basis of general gear wheel data (module, number of teeth and profile correction) calculated the complete tooth gap geometry and from it created direct 5-axis control programs. 

More accuracy needed During this development period, Deckel Maho opted not to use existing CAD/CAM systems despite using them extensively for the generation of programs for 5-axis simultaneous machining operations. The main reason for this is the fact that these software packages calculate milling paths on the basis of curved surfaces and orientate the tool axis on surface parameters. Intensive investigation – in particular into the processing of all round curved tooth flanks has shown that the available calculation algorithms do not produce the required accuracy. The requirement was therefore for new and more precise calculation routines. Software was already available which calculated the tooth gap geometries ready for further processing in CAD/CAM systems and generated complete NC control programs for 3 and 5-axis universal milling machines. But despite extensive research, Deckel Maho was unable to locate a partner prepared to adapt its software to suit the requirements of the machines concerned and offer the software as part of a competitively priced package. For this reason, the company decided to develop gear cutting software that incorporated the algorithms necessary to generate optimum tool paths. It collaborated for a number of years with a well-known gearbox manufacturer and methods for the manufacture of pre (unhardened) and final (hardened) conical gear wheels were developed. The gear tooth profiles were patented by the Klingelnberg company under the registered trademark Zyklo-Palloid and then extensively tested.

Total package

Ultimately however, Deckel Maho was compelled to find its own solution for the manufacture of gear wheels on universal milling machines. The solution, in the form of a complete package comprising the machine tool and software, would be required to enable the customer to manufacture gear wheels of the necessary quality from the component drawing and the gear tooth data (module, number of teeth, profile correction and contact pattern).

A prototype of the software package was presented to a number of interested gear manufacturers at a company exhibition in Pfronten in February 2009. The milling of the gear wheels was carried out on various universal milling machines including the DMU 40 monoBLOCK with XYZ travels of 450mm x 400mm x 480 mm respectively, with the machining of a helical tooth conical gear pair from 18 CrNiMoV5 with diameters of 90mm and 100mm.

As is the case with gear manufacture on all Deckel Maho machines, a fifth NC axis is necessary, acting as an NC-controlled swivelling milling head (B-axis). In addition to the automatic tool changer and Heidenhain iTNC 530 control, the machine equipment features an NC turntable.

Options available include a motor spindle with HSK-A 63 receptacle and 24,000 U/minute due to the small diameter of the standard tools. Test probe and tool measurement in the workshop, the product package with IKZ and 40 bar, the 3D quickSET – a package for 5-axis kinematic control – and the Application Tuning Cycle (ATC) with adaptive in feed control, all supplement the options package.

By equipping a configured DMU 40 monoBLOCK with options such as the 60 position tool magazine, an automation interface and an Erowa pallet store from System 3R, it is possible to mill small series gear wheels.

On show

Also shown at the in-house exhibition as a further application of Deckel Maho's gear tooth technology was a DMC 80 U duoBLOCK universal machining centre executing the soft machining of a 650mm diameter pinion with a spiral tooth pattern with module 25.

The machine has a 800mm x 800mm x 800 mm work area with fast tools and pallet changeover. In addition to the standard equipment with 60 magazine positions, NC rotary table with pallet carrier and two pallets, the production package and the 3D control Heidenhain iTNC 530, other options are available in the form of the NC controlled swivelling milling head as the B-axis with motor spindle, HSK-A 63 receptacle and 18,000 U/minute. The matching spur bevel gear matching this pinion manufactured in 18CrNiMoV5 on a 5-axis DMC 210 U machining centre.

The 1,800mm x 2,100mm x 1,250mm (XYZ) DMC 210 U portal machining centre presented at the in-house exhibition is configured similarly to the previously described machines with NC controlled swivel milling head as the B-axis, NC rotary table with pallet carrier, linear pallet changer with two pallets (1,400mm by 1,600mm), production package and 3D control Heidenhain iTNC 530. The 44kW heavy motor spindle at 10,000 U/min has an SK 50 receptacle and a rack magazine with 120 tool positions supplements the equipment.

In terms of functionality, the Deckel Maho tooth cutting software package currently encompasses: calculation of the tooth space geometry of spiral bevel gears in accordance with input data, including profile correction, contact pattern modelling and optimisation of the tooth root geometry; generation of 5-axis roughing and smoothing programs; generation of an alignment program for workpiece alignment on the machine after heat treatment; the production of measured data and: a post-processor for Deckel Maho 5-axis milling and mill/turn machines.

Software modules for other types of tooth layouts have also been developed at Pfronten and according to DMG, the two main benefits of its machine/technology packages are: competitive pricing, and the provision of a complete turnkey programmable system that includes tooth cutting software with design modules for calculation, the CAM module for the measuring machine and a training module for process support.

Long in the tooth

The turnkey package is also versatile because the machines can fundamentally be used for any arbitrary metal removal task but ultimately, a significant advantage is the fact that gear tooth cutting can now be carried out on standard monoBLOCK machines with duoBLOCK or portal machining centres being used for larger gear wheels of 600mm diameter and above.

Savings can consequently be made on jig and fixture costs through the use of pallet changers or automated modules such as pallet stores or robot units. The milling of hardened gearwheel blanks (HRC 60) can be accommodated and quality values of ≤ Q 6, through the use of permanent correction options and axis error compensation in combination with the 3D quickset option, are achievable. DMG also claims that through the excellent quality finished obtainable, additional processes such a grinding can be avoided.

DMG
www.gildemeister.com