A gear hobbing machine contains two skew spindles. One of these spindles houses the hob, while the other houses the gear blank. The angle at which these spindles are placed relative to each other depends largely on the type of gear being manufactured.
Once the spindles are placed at the proper angle, the machine is set up to begin rotating the shafts at a speed ratio that suits the gear type. As the shafts rotate, the hob gradually cuts the teeth into the gear with the proper depth. To facilitate faster production, manufacturers can stack multiple blanks on the spindle together, allowing the hob to cut teeth into multiple gears at once.
Just like there are multiple different angles, speeds, and techniques manufacturers can use to customize the process, there is also a wide variety of hobbing machines available for use. Most hobbing machines specialize in distinct applications. Hobbing machines are built to handle gears of a particular size and come in two different varieties: single-threaded and multi-threaded hobs. Multi-threaded machines allow for increased production, but they aren’t as precise as single-threaded machines.
Hobbing is one of the most fundamental processes in gear manufacturing. Its productivity and versatility make hobbing the gear manufacturing method of choice for a majority of spur and helical gears.
One of the most important concepts to understand about gear hobbing is that it is a generating process. The term generating refers to the fact that the shape of the gear tooth that results is not the conjugate form of the cutting tool. Rather, the shape of the tooth is generated by the combined motions of workpiece and cutting tool. During hobbing, both the hob and the workpiece rotate in a continual, timed relationship.
For a spur gear being cut with a singlestart hob, the workpiece will advance one tooth for each revolution of the cutter. When hobbing a 20-tooth gear, the hob will rotate 20 times, while the workpiece will rotate once. The profile is formed by the equally spaced cutting edges around the hob, each taking successive cuts on the workpiece, with the workpiece in a slightly different position for each cut. Several cutting edges of the tool will be cutting at the same time.
During this rotation, the hob is typically fed axially with all the teeth being gradually formed as the tool traverses the work face.
The hob itself is basically a worm with gashes cut across it to produce the cutting edges. Each cutting tooth is also relieved radially to provide chip clearance behind the cutting edge. This also allows the hob face to be sharpened and still maintain the original tooth shape. In its simplest form, the hob tooth takes on the shape of a straightsided rack tooth. The final profile of the tooth is created by a number of flats blending together. The number of flats corresponds to the number of cutting gashes which pass the workpiece tooth during a single rotation. Thus, the greater the number of gashes in the hob, the greater the number of flats along the profile, which improves the “smoothness” of the tooth profile.
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