TUC 3m Lab
m3 Gear Hobbing



 Gear Hobbing
 FEM Simulation

Gear hobbing, as any cutting process based on the rolling principle, is a signally multiparametric and complicated gear fabrication method. Although a variety of simulating methods has been proposed, each of them somehow reduces the actual three-dimensional (3D) process to planar models, primarily for simplification reasons. The paper describes an effective and factual simulation of gear hobbing, based on virtual kinematics of solid models representing the cutting tool and the work gear. The selected approach, in contrast to former modeling efforts, is primitively realistic, since the produced gear and chips geometry are normal results of successive penetrations and material removal of cutting teeth into a solid cutting piece. The algorithm has been developed and embedded in a commercial CAD environment, by exploiting its modeling and graphics capabilities.






 3D Simulation - Program HOB3D

To generate the produced chip and gear volumes, the hobbing kinematics is directly applied in one 3D gear gap. The cutting surface of each generating position (successive cutting teeth) formulates a 3D spatial surface, which bounds its penetrating volume into the workpiece. This surface is produced combining the relative rotations and displacements of the two engaged parts (hob and work gear). Such 3D surface “paths” are used to split the subjected volume, creating concurrently the chip and the remaining work gear solid geometries.


This algorithm is supported by a universal and modular code as well as by a user friendly graphical interface, for pre- and postprocessing user interactions. The resulting 3D data allow the effective utilization for further research such as prediction of the cutting forces course, tool stresses, and wear development as well as the optimization of the gear hobbing process.




 Gear gap verification




 Wear Deretmination

Gear hobbing is an efficient method of gear manufacturing. Due to the fact that during the cutting process every hob tooth always cuts in the same generating position, while in the various generating positions the formed chip has different geometry, the resulting tool wear is not uniform on any particular hob tooth. In order to overcome this problem, the hob is shifted tangentially after a certain number of cuts. Mathematical models to calculate the progress of hob wear in the individual generating positions, considering the existing process parameters, were presented. In order to calculate flank wear regarding the complicated chip geometry, equivalent chip dimensions, such as the cutting length l, the chip thickness hs and the characteristic chip form (chip group) were introduced. Based on these calculations, a computer algorithm for the determination of the hob flank wear, which depends on the shifting conditions, was presented.


Simulating the hobbing process with the aid of a computer program, it is possible to determine the length, thickness and group of every chip in the various cutting and generating positions. With the aid of these parameters, the progress of the flank wear on a hob tooth during cutting in the same generating position in all successive cutting positions along the gear width can be determined. This procedure is repeated for all generating positions.

To optimize the shift displacement and amount, the course of the flank wear versus the number of hobbed gears is calculated in every individual generating position as well as the wear distribution at the hob teeth. The calculated number of hobbed gears and the occurring width of the flank wear, using various shift conditions. The shift displacement is expressed as a multiple of the hob axial pitch ε. Using such diagrams the shift displacement and amount can be determined with respect to a prescribed maximum value for the flank wear.



 FEM Simulation

The wide, almost exclusive, application of gear hobbing, as a flexible manufacturing process for external gears has led to the thorough description of its kinematics, dynamics and tool wear mechanisms. However, in various cases, especially when cemented carbide or coated tools are utilized, the cutting tools experience critical stress components, which are able to cause premature tool failures. The complicated kinematics, as well as the particular tool geometry exclude analytical stress filed solutions, and require arithmetical ones conducted with the aid of the finite elements method. The computational results explain sufficiently the failure mechanisms, being in agreement with corresponding experimental data. The verified parametric FEM model was further applied for various cutting cases, indicating the most risky cutting teeth with respect to their failure danger. Herewith, the optimization of the cutting process is enabled, taking into account that a proper selection of cutting parameters can eliminate the failure danger of cutting tools, and achieve satisfactory cost effectiveness.


                                                                                             Technical University of Crete      School of Production Engineering & Management      e-mail