New Chamfer Cutting Technologies for Cylindrical Gears

Gear chamfering and deburring delivers significant advantages for the processing and handling of gears, preventing damages and premature failure of transmissions. Two new chamfer cutting technologies have been developed to overcome limitations to existing processes: Chamfer Hobbing provides precise chamfers with very low tool cost-per-part during high volume gear production. Gears do not show measurable burrs, increasing the efficiency of hard finishing processes downstream. Fly Cutter Chamfering features a single milling tool with standard indexable carbide inserts. This universal process covers a range of different workpieces which makes it ideally suited for the production of larger cylindrical gears in lower volumes.

Why chamfering of cylindrical gears makes sense

Cylindrical gear chamfering and deburring processes deliver significant advantages downstream the gear manufacturing value chain: first, to improve the processing and handling of gears, particularly in advance of hard finishing; and second, to prevent damage and premature failure of gears operating in gearboxes. While not considered a ‘value adding’ process such as hobbing and fine finishing, chamfering and deburring is performed on most gears up to 400 mm in diameter and module 8 mm.

After gear cutting, like hobbing, a burr at the tool exit position remains (Figure 1). While the primary burr can be removed right after the cutting process (using deburring discs), an actual chamfer on the edge between the flank and the gear face is desirable for several reasons (Figure 2): Chamfers prevent damages and break-outs in gearboxes if acute edges become brittle during super-carburization in the heat treatment process. Under load, especially during edge load situations, material can break off and hardened particles can reduce transmission life or even cause premature transmission failure. This is of special interest with gears for electrical vehicle (EV) drive systems where significantly higher torque at lower rpm leads to higher gear deformation if compared to gears used in conventional combustion engine transmissions.

Fig. 1:   Hobbed gear with remaining burrs       //      Fig. 2: Gear chamfered

Several processes are available to remove burrs and to chamfer gears with an undefined or defined chamfer. With increased dry cutting and hard finishing of gears, demand for efficient and precise chamfer processes has grown. New chamfer cutting technologies like Chamfer Hobbing and Fly Cutter Chamfering are now available to manufacturers of both automotive and truck-size gears.

Chamfer Hobbing for medium and high volume production

Chamfer Hobbing is a chamfer cutting process for the medium and high volume production of automotive and light truck gears, which addresses most of the existing chamfer tolerances and those expected in the future. Chamfer Hobbing employs one dedicated Chamfer Hob for each tooth flank. The hob profile is specifically designed for the particular chamfer form required.

By cutting into the tooth gap, no secondary burr is created on the face side of the gear. Additional deburring is not necessary. Chamfer cutting works fine with regular chamfer angles of up to approximately 40 degree with no measurable burr remaining on the tooth flank (Figure 3). This is a great advantage, since remaining secondary burrs can greatly impact tool life in subsequent hard finishing processes, like gear honing.

Fig. 3 – No measurable burrs on the edge of the chamfer

Chamfer Hobs require materials and coatings similar to regular gear hobs and can be resharpened and recoated accordingly. Thanks to their tool shifting capability, tool wear on Chamfer Hobs is evenly distributed among teeth, resulting in very low tool-cost-per-chamfer, approx. 0.01 EUR for typical automotive gears.

Fly Cutter Chamfering for maximum flexibility

With larger truck-size gears, requirements are different from automotive gears. Smaller batches, larger variety of parts and long cycle times are typical for this type of gears. Likely, Chamfer Hobbing with its dedicated tools cannot live up to the required flexibility.

Chamfering with fly cutters offers this flexibility. It generates a chamfer with the desired characteristics along the gear edge by synchronizing fly cutter and workpiece rotation (Figure 4). The tool features a star-shaped body with four standard, indexable carbide inserts configured in a way that the straight cutting edge is used as much as possible to maximize tool life.

Fig. 4 – Fly Cutter Chamfering with standard, indexable carbide inserts

Since each edge of the tooth is cut separately and the chamfer size and angle depends on the machine’s movements, universal tools can be flexibly employed. During input of workpiece and tool data into the machine’s control, chamfer size and chamfer angle can be freely defined and do not depend on the tool design.

Tool cost-per-part can be less than 0.03 EUR for a truck-size gear with standard chamfer sizes. With a cycle time of roughly 1 minute when chamfering all four edges in parallel to the hobbing process, the target market is the manufacturing of medium size gears with generally longer cycle times and greater flexibility.

Summary

Two new chamfer cutting options meet different requirements of gear manufacturers:

Chamfer Hobbing features dedicated tools for automotive-size gears, and is ideally suited for the production of medium to high volumes with tool cost-per-chamfer of approx. 0.01 EUR.

Fly Cutter Chamfering with standard, indexable carbide inserts for truck-size gears, provides a highly flexible process, targeting low batch, small and medium volumes with cycle times of 1+ minutes and tool cost-per-part of less than 0.03 EUR. Depending on the specific requirements it is very likely that one of these two cutting chamfer technologies will address many of the varying challenges customers may have going forward.

Details and use cases will be presented on Sep 16th, 2021, during the 4th International Conference on Gear Production.

Author

Dipl.-Ing. Gottfried Klein studied Mechanical Engineering at Technische Universität München. After his employment at Iveco, he held sales and project management positions at Gleason-Hurth before he assumed the position of Director Product Management Hobbing, Chamfering, Shaving and Rack Solutions with Gleason Corporation.