How Gear Hobbing Works Lead

back to basics How Gear Hobbing Works Lead

Hobbing is one of the most fundamen- piece (in blue). As can be seen from the Lead tal processes in gear manufacturing. Its figure, the relative motions of the cut- Lead productivity and versatility make hob- ting tool and workpiece are what allow Pitcha bing the gear manufacturing method of straight-sided tool—the hob—to generDiameter- choice for a majority of spur and helical ate an involute curve. gears. During this rotation, the hob is typi- Pitch One of the most important concepts cally fed axially with all the teeth being DiameterPitch Diameter to understand about gear hobbing is that gradually formed as the tool traverses it is a generating process. The term gen- the work face (Fig. 1b). erating refers to the fact that the shape of The hob itself is basically a worm with the gear tooth that results is not the con- gashes cut across it to produce the cut- jugate form of the cutting tool. Rather, ting edges (Fig. 2a). Each cutting tooth Figure 2a The hob is basically a worm with the shape of the tooth is generated by is also relieved radially to provide chip gashes cut across it to produce the cutting edges. the combined motions of workpiece and clearance behind the cutting edge (Fig. Relief cutting tool. During hobbing, both the 2b). This also allows the hob face to be ReliefRelief hob and the workpiece rotate in a con- sharpened and still maintain the originalTooth Face tinual, timed relationship. tooth shape. In its simplest form, the hob ToothTooth Face For a spur gear being cut with a single- tooth takes on the shape of a straight- start hob, the workpiece will advance sided rack tooth (Fig. 2c). The final pro- GashGashGash one tooth for each revolution of the file of the tooth is created by a number cutter. When hobbing a 20-tooth gear, of flats blending together. The number the hob will rotate 20 times, while the of flats corresponds to the number of workpiece will rotate once. The profile cutting gashes which pass the workpiece is formed by the equally spaced cutting tooth during a single rotation. Thus, the edges around the hob, each taking suc- greater the number of gashes in the hob, Figure 2b This side view of a hob shows cessive cuts on the workpiece, with the the greater the number of flats along the how each cutting tooth is relieved radially to provide chip clearance Pressure Angle workpiece in a slightly different posi- profile, which improves the “smooth- behindPressure the cutting Angle edge. tion for each cut. Several cutting edges of ness” of the tooth profile. the tool will be cutting at the same time. Pressure Angle Figure 1a demonstrates the path of the cutting tool (in red) relative to the work- Cutting DepthCutting Depth

Cutting Depth

Figure 2c In its simplest form, the hob tooth takes on the shape of a straight- sided rack tooth.

Where Can I Learn More? The material in this Back-to-Basics Brief was adapted primarily from “The Gear Figure 1a Hobbing is a generating process. Hobbing Process,” by Dennis Gimpert, The relative motions of the cutting which appeared in the January/February tool and workpiece allow a tool with straight-sided cutting edges— 1994 issue of Gear Technology. The the hob—to generate an involute original article goes into much greater curve. detail about cutter modifications, hobbing machine mechanics, different hob For Related Articles Search Figure 1b Hob and workpiece each rotate feed approaches, multi-start hobs and during the hobbing process. For a spur gear being cut with a hob thread-spacing errors. hob basics single-start hob, the workpiece will advance one tooth for each at www.geartechnology.com revolution of the cutter.

[www.geartechnology.com] 88 GEAR TECHNOLOGY | March/April 2013