A look at hobbing machines, hobs, and the hobbing process.
In our last issue, we introduced basic information about hobbing machines and the hobbing process. A hob is a worm-like cutter that makes successive generating cuts to produce gear teeth. This is achieved by a hobbing machine that ensures exact rotational synchrony between the work piece and the hob. In this issue we will complete our look at the hobbing process.
While the hob and work piece are rotating, the hob normally feeds axially across the gear face at the gear’s tooth depth to cut and produce the gear. In conventional hobbing, the direction of feed matches the direction of the cutting motion. Alternatively, in climb feeding, the feed is opposite to the direction of the cutting motion. Generally, conventional hobbing produces a better finish, whereas climb hobbing yields better tool life. For either method, the cutting forces of the hob should be directed towards the work spindle and not the tailstock.
Figure 1 – Climb hobbing (left) and conventional hobbing (right)
Hob shifting is a strategy to uniformly distribute wear on a hob by changing which hob teeth cut. This can be accomplished in several ways. The basic method of hob shifting simply uses a hob in one position while it cuts multiple gears until the cutting teeth become worn. At that point or at a predetermined number of parts, the hob is shifted tangentially to position sharp hob teeth for the next cycle. Alternatively, the hob is shifted in small amounts after each part. These processes are repeated over the length of the hob. When cutting large, coarse-pitch gears with large face widths, a hobbing machine can simultaneously feed the hob both axially and tangentially (this is in respect to the work piece axis of rotation). This provides continuous hob shifting while hobbing the entire gear.
Figure 2 – Combined axial and tangential feed
To cut a helical gear, a standard hob cutter can be used. A hob essentially is a worm that “mates” with a spur gear being cut, but that same hob can produce a helical gear. This is not accomplished by changing the angle of the hob relative to the gear. Rather, the hobbing machine provides an accurate differential mechanism that gradually offsets the rotation of the hob relative to the work piece with respect to the feed of the hob. This creates an angled gear tooth for a helical gear. Mechanical hobbing machines provide this same differential through a series of change gears that provide an exact ratio to create the gear tooth helix. Today, CNC hobbing machines electronically provide this necessary differential to produce helical gears.
To conclude, the hobbing process provides an efficient, flexible method for cutting gears because of the worm-like nature of the cutter. Hobbing can be performed conventionally or by climb hobbing, for either better finish or better tool life, respectively. Hob shifting is a strategy to distribute wear over the entire cutting tool by successively introducing new, sharp cutting teeth. Finally, helical gears are produced by using a differential mechanism of the hobbing machine to accurately and gradually offset the rotation of the hob relative to the work piece while feeding.