CnC Face Milling of Gears

CnC milling is now an accepted practice in gear production. It allows cutting just about any type of gear using simple cutters such as Conical Side Milling Tool (i.e. CoSIMT), End Mill and Ball Mill tools.  When compared to face mill cutters - used for Zerol™/spiral-bevel/hypoid gears, and dish type cutters - used for straight-bevel gears, the CoSIMT, End Mill and Ball Mill cutters show significantly decreased production levels, the CoSIMT being the least affected and the Ball Mill showing the worst performance.

Therefore, only small production lots should be envisaged when using CoSIMT, End Mill and Ball Mill tools. This can easily be justified when cutting large bevel gears, spiral or straight, since face mill cutter price increases dramatically with size. In addition, large bevel gears call for large machines and tools, which are not always available. For example, cutting a crowned straight bevel gear where the outer transverse module mte = 18 mm, Z1 = 20, Z2 = 60, OD = 1.090 m, using the largest available Coniflex dish type cutter (15”) is not practical because of the circular tooth root line produced. Milling thus appears to be the only choice to produce such a gear set. Figs. 1 and 2 below show the Z2 = 60 gear member after the soft cut, done using End Mill Tools.

Fig. 1: Straight bevel gear – Z2 = 60 mte = 18 mm OD = 1.090 m

Fig. 2: CMM output - Straight bevel gear – Z2 = 60 mte = 18 mm

At the other end of the spectrum, small size spiral bevel gears of mte up to 5 mm can effectively be produced in quantities on 5Axis CnC machines since cutter diameter can be limited to 6” and therefore the power requirement can easily be met by currently available CnC machines on the market.

Likewise, small size straight bevel gears can be cut using dish type cutters, such as Gleason’s Coniflex cutters. The problem here is that standard dish type cutters come in only 3 sizes: 4.25”, 9” and 15”, and because of power requirements, only the 4.25” cutter can be used effectively on usual 5Axis CnC machine, which limits mte to ~1.5 mm. Using a made to order 6” dish type cutter would allow pushing maximum mte to ~2 mm while maintaining power requirement to an acceptable level.

There is a definite cost advantage in going 5Axis CnC manufacturing when cutting small size gears since the market abounds with precision machines to guarantee gear set quality. The large market for 5Axis CnC machines also generates competition on prices which clearly favor the buyer.

To the production engineer and manager, the main hurdle may appear to be the programming of the machine controllers to guarantee that the part cut on a 5Axis CnC machine has the same tooth flank topography as that cut on a conventional mechanical machine or a modern 6Axis machine from Gleason and Klingelnberg.

The HyGEARS™ software produces machine-ready code for virtually any 5Axis CnC machine and controller. Therefore, the manufacturer can use an existing in-house machine and put it to work without delay. The manufacturer is also free to shop around for the machine offering the best cost advantage. HyGEARS™ has been calibrated against Gleason’s GAGE and Klingelnberg’s KIMoS CMM nominals, producing the same tooth flank topography when using the same machine settings. And Closed Loop and Reverse Engineering being integral to HyGEARS™, strict adherence to the target tooth flank topography is thus guaranteed. HyGEARS™ supports all face milling cutting cycles and most CMMs found on the market.

Fig. 3 below shows a Z1 = 26, mte = 1.5 mm spiral bevel pinion cut on a 5Axis CnC machine using a 2” solid face mill cutter; the CMM results in Fig. 4 are obtained after the 1st Closed Loop iteration.

Fig. 5 below shows a Z1 = 32, mte  = 1.0 mm straight bevel pinion cut on a 5Axis CnC machine using a 4.25” dish type cutter; the CMM results in Fig. 6 are obtained after the 1st Closed Loop iteration. 

In both above examples, the machines and controllers are different and thus initially needed to be defined within HyGEARS™. Definition is required only once. To generate a part program, once the gear set geometry is known to HyGEARS™, the user simply needs select the tool from a toolbox, select the desired cutting cycle (single roll, double roll, plunge cut, etc.), save the selections as an Operation, and the machine-ready code  is generated on the fly to download to the machine’s controller.

CMM output used in HyGEARS’™ integral Closed Loop function generates a new set of machine settings that are fed to the Operation created for the initial cut. The exact same cutting selections are thus used, with the modified machine settings, in order to eliminate tool and machine errors, as shown in Fig. 4 and 6 above. Thus, HyGEARS™ does everything internally and does not need to access a cloud point, an external tooth flank generator, or an external CAM software to generate the part programs.

Cycle times of course vary, and depend on gear size and machine power. As a reference, the straight bevel gear of Fig. 5 requires on average 6 sec. per tooth flank, for a total of ~6.5 min./part.

Finally, Toe, Topland and Heel chamfering Operations can be combined to the face milling Operation to create a single file Process part program in which all tooth cutting steps are contained.

References

  1. Gosselin C., Thomas J., A Unified Approach to the Simulation of Gear Manufacturing and Operation, International Conference on Gears, Technical University of Munich, October 2013.
  2. Gosselin C., Thomas J., Integrated Closed Loop in 5Axis CnC Gear Manufacturing, International Conference on Gears, Technical University of Munich, September 2015. 

Authors of this article

Claude Gosselin, Ph.D., P.Eng.
Involute Simulation Softwares Inc., Québec, Canada

J. Thomas, Dr. Ing.
ZG Hypoid GmbH, Germany

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