Hobbing is a milling variation that uses a cylindrical cutting tool called a hob to cut sprockets, gears, and splined parts. The hob has helical tooth rows and is fed into the workpiece on a gear hobber. The process is cost-effective and accurate, producing a range of toothed products.
A hobber is a variation of milling used to cut sprockets, gears, and splined parts using a specialized cutting tool known as a hob. The hob is a cylindrical cutting tool that has a series of helical tooth rows. The gear hobber has two spindles, one of which holds the workpiece and the other the hob. Both spindles rotate at a predetermined ratio as the hob is fed into the workpiece to cut the teeth. Cutting toothed parts on a hobbing machine is an economical yet accurate method of manufacturing a wide range of products including worm gears, ratchets, involute gears and helical gears.
Hobbing machines are similar in operation to conventional milling machines, but for the specialized cutting tools used. These tools are known as hobs and are used exclusively for the hobbing process. Cylindrical in section, the hob has progressive rows of helical cutting teeth. The hob is fluted or fluted from top to bottom at regular intervals, giving the utensil a distinctive hand grenade appearance. These flutes serve to reduce cutting friction and aid in the removal of chips during cutting. The pitch and profile of the hob teeth depend on the desired end result, although they all follow the same basic design.
The construction of a hobbing machine is similar to that of a conventional milling machine and has a pair of rotating spindles, one for the workpiece and one for the hob. These spindles are placed at opposite angles to each other depending on the type of part being produced. For example, if you are cutting a spur gear, the hob spindle angle will equal the gear’s helix angle, if any. The ratio between the rotation speeds of the two spindles is dictated by the pitch of the teeth of the finished product. For example, a gear with 40 teeth will be machined with a 40:1 rotation ratio.
The hobbing process involves feeding the hob through the rotating workpiece until the desired tooth depth is achieved. Up to five gears can be cut in one pass, and multiple separate gears can be stacked in the workpiece spindle and cut simultaneously. Very large pieces are usually cut before hobbing. Roughing involves cutting the part rough on a universal milling machine before it is finished on a hobbing machine. Hobbing is a very cost-effective method of accurately cutting a wide variety of toothed products, including helical gears, splines, worm gears, spur gears, and involute and cycloid gears.
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