Gear Hobbing

High precision gears are components that can determine the quality, performance, service life, safety and reliability of high-end power transmissions. While there various methodsof manufacturing these gears such milling or grinding, hobbing is the most widely used method of gear manufacturing. Generally, when compared with the grinding process, the hobbing process has the advantage of high precise, efficient at lower cost. (1) Though the application of this process can be extremely limited by part geometry, hobbing is still by far the most productive form of gear tooth generation for external spur and helical gears. (2) 

Three important parameters are to be controlled in the process of gear hobbing indexing movement, feed rate and angle between the axis of gear blank and gear hobbing tool (gear hob). The schematic diagram of the set up of a gear hobbing machine can be seen in the figure.  The aims of the hob are set at an inclination equal to the helix angle of the hob with the vertical axis of the blank. If a helical gear is to be cut, the hob axis is set at an inclination equal to the sum of the helix angle of the hob and the helix angle of the helical gear. The operation of gear hobbing involves feeding the revolving hob until it reaches the required depth of the gear tooth. The process of the gear hobbing is types according to the directions of feeding the hob for gear cutting, but not to be confused with the two different strategies that can be used in gear cutting that will discussed in the next paragraph. The different types of gear hobbing are: hobbing with axial feed, hobbing with radial feed, or hobbing with tangential feed. Axial feed is when the hob is fed along the face of the blank and parallel to axis. This method is used in spur and helical gear manufacture. Radial Feed is when the gear blank and hob axises are set normal to each other and the hob is fed against the gear blank in a radial direction. This type of feed is used in the generation of worm wheels. Tangential feed, which is used for worms, is a case where the hob is held with its axis horizontal but at a right angle to the axis of the blank. The hob is set at the full depth of the tooth and then fed forward axially. The hob is fed tangential to the face of the gear blank. This method is also used to manufacture of worms. (11)

There are two different cutting strategies when gear hobbing, climb cutting and conventional cutting. The term “climb” and “conventional” cutting refer to the direction of hob feed into the workpiece with reference to the table or spindle nose. For maximum stability during cutting, it is recommended that conventional cutting be used when possible. In conventional hobbing, the hob is fed into the work, moving toward the table or spindle nose, parallel to the blank axis. In climb hobbing, the hob is fed moving away from the table or spindle nose, parallel to the blank axis. (1) To determine which hobbing strategy should be used in a particular application the direction and axial feed must be defined by the three distinct motions of the gear hob: the tool rotation about its axis, tool axial displacement, and the workpiece revolution about its axis. (5) A general rule of thumb is that climb hobbing yields better tool life and accuracy while conventional hobbing yields a better finish. (6)

In hobbing multiple teeth of the tool, the hob’s teeth, are in contact with the workpiece at the same time. Together with the continuous coupling of the tool and the workpiece rotational motion, these teeth create the involute tooth gaps in a generating way. (9) In hobbing one of the complexities is arranging the cutting configuration in a way that will produce the desired tooth profile whether standard or modified in some way as the cutting configuration corresponds to the selected design structure specifying the geometric and kinematic parameters of the tool’s tooth profile such that the tooth trough in the gear is produced in an orderly manner and the cut layer is of the specified shape.(8) The kinematics of this complex process is based off of three relative motion between the gear and the workgear due to the synchronization needed to produce gear teeth. In gear hobbing, the hob tool and the workgear move in a linked revolution ratio. These revolutions are synchronized with hob axial-feed and are dependent on the number of starts in the cutter and the number of teeth of the workpiece. (2)

The generation process is one of the most fundamental concepts in the gear hobbing. The cutting tool geometry can determine cycle times and the tool wear of the process among other factors of gear manufacturing. The hob itself is essentially a worm with gashes cut across it to produce the cutting edges. The hob could also be considered a series of racks positioned around the circumference of a cylindrical tool. Each successive rack is shifted axially to create a worm, typically a single thread. It is possible to design the shape of a cutting tool to produce modified tooth forms. The advantages of the different modification is shown in Table 1: (6)

Tool wear is also concept of gear manufacturing that is highly researched as it is very difficult to model and predict due to the generating-rolling principle that governs the hobbing kinematics. Referring to the wear influence of the tool geometry, there are critical gearing parameters that can influence the tool life time in a very positive or negative way. Modifications can be made to the tooth profile of the hob in order to optimize the production of a particular workpiece. These special profiles are widely used in industrial applications whereas only company internal knowledge currently exists. (9) The variant chip formation on each cutting tooth during hobbing provokes different wear laws and usually leads to an unequal wear distribution on the hob teeth. (7) The hob will also have certain defects that contribute to tool wear: for example, the simultaneous and intermittent participation of two or more cutting edges of the tool teeth in hobbing. consequently , pronounced local wear can appear at the tooth tip, with associated loss of tool life. These defects need to be taken into consideration on certain jobs in which a custom hob must be made with longer runs. (8)

There are many factors to be taken under consideration during the optimization of the gear hobbing process; the cutting forces involved in this process are one of the most important factors as the cutting forces will contribute to the chip formation and the tool wear of the hobbing process. The tool and machine wear caused by the forces can thus affect the pricing of the part in production. There are various models in which authors have created models to predict the cutting forces, but presently there is not a way to accurately measure the cutting forces, tool wear or chip formation process due to the complex nature of the kinematics of gear hobbing. Cutting forces are generally separated into using small cutting edge elements as shown in Figure 1. (4)

Besides the geometries of the gear workpiece and the hobbing tool, the material properties also affect the gear hobbing process. Common materials of the hob tools are high speed steel and carbide. The material used for the hob is dependent on the material of the gear blank and the speeds at which you would like to run the hobbing process. Steel gear workpieces, for example, are typically formed by the hobbing tools made from solid tooling material, such as tungsten carbide. (5) Studies have also shown that a major increase in the applied cutting speed is possible when matching the substrate with a convenient coating. (9)

New suggestions and methods to improve the precision and efficiency of hobbing have been introduced by researched by researchers such as modeling the kinematics of the hob. (10) Machine control and tool development intertwine and ensure the continuous improvement of gear manufacturing. (9)

1. Sun, Wang, Wang, Lim, and Yang. “Prediction and optimization of hobbing gear geometric deviations”. Mechanism and Machine Theory. (2018): 288-301. Web.
2. Endoy, R. (1990). Gear hobbing, shaping, and shaving: A guide to cycle time estimating and process planning (1st ed.). Dearborn, Mich: Society of Manufacturing Engineers, Publication Development Department.
3. Sabkhi, Pelaingre, Barlier, Moufki, and Nouari. “Characterization of the Cutting Forces Generated During the Gear Hobbing Process: Spur Gear.” Procedia CIRP 31.c (2015): 411-16. Web.
4. Sabkhi, Pelaingre, Barlier, Moufki, and Nouari.(2016). Prediction of the hobbing cutting gorces from a thermomechanical
5. Dong, X., Liao, C., Shin, Y. and Zhang, H. (2016). Machinability improvement of gear hobbing via process simulation and tool wear predictions. The International Journal of Advanced Manufacturing Technology, 86 (9), 2771-2779.
6. Gimpert, Dennis. (1994). The Gear Hobbing Process. Gear Technology. (Jan./Feb.): 38-44.
7. Bouzakis, K.; Kombogiannis, S.; Antoniadis, A.; Vidakis, N. (2002) Gear Hobbing Cutting Process Simulation and Tool Wear Prediction Models. Journal of Manufacturing Science and Engineering. Feb. Vol. 124.
8. Mitin, E.; Sul’din S. (2018). Hob Strength in Gear Cutting. Russian Engineering Research. Vol. 38, No. 8.: 24-27
9. Merklein, M., Franke, J., Hagenah, H., & WGP-Jahreskongress. (2014). WGP congress 2014: Progress in production engineering : Selected, peer reviewed papers from the 2014 WGP Congress, September 9-10, 2014, Erlangen, Germany (Advanced materials research ; Volume 1018).
10. Kaylani, K.; Amereshwari, C.; Rao, M.; Babu, S. (2017) Design and Performance Analysis of Hobbing Milling Cutter. International Journal of Research. November. Vol. 04 Iss. 14. 3670-3676.
11. Unit 6 gear manufacturing.