Gear hobbing

Gear hobbing

Gear hobbing is a continuous process eliminating the unproductive return motion of the gear - shaping operation to understand the concept of gear hobbing, an analogy similar to that used in gear shaping will be given imagine an involute rack being pressed into the gear blank with a very soft rim up to the  point when the pitch circles of the rack and the gear blank meet.  During this process, the rack is moved lengthwise while the gear blank is rotated such that it rolls with the rack without slipping.  Theoretically, correct tooth profiles would be formed on the gear - blank rim as it is very soft.  The number of teeth formed depends upon the size of the gear blank used. 

    The above procedure may be difficult to implement practically.  It is, therefore, modified in the form of a gear cutter called hob, where the rack is wound round the cylinder in a helix, like a worm, as shown in Figure. Since the involute rack teeth are straight lines, the hob threads are easier to produce and inspect.  Gashes will be cut across the threads, as shown in Figure to provide the cutting edges.  These cutting edges are relieved behind for the clearance similar to a form - milling cutter.  Rotation of the hob causes the theoretical rack to move along a straight line.

Gear hob

     A hob may have one, two or more threads of cutting edges.  When a gear blank of N teeth is cut with a single thread hob, the blank tums a complete revolution when the hob completes N revolutions.  For a multi threaded hob, the gear blank rotates as many times more as the number of starts.  Thus, a multiple - threaded hob is much faster in production compared to a single - threaded hob.  However, single - threaded hobs are more accurate than to the multi threaded ones.

      The workpiece is mounted on a vertical axis and rotates about its axis.  The hob is mounted on an inclined axis whose inclination is equal to the helix angle of the hob as shown in Figure. This adjusts the gear blank teeth in the same plane as that of the gear - hob teeth, which is termed the generating plane.  The hob is rotated in synchronization with the rotation of the blank and the hob is slowly moved into the gear blank till the required tooth depth is reached in a plane above the gear blank.  Then the hob is fed slowly in the axial direction of the gear blank till the complete tooth - face width is achieved.

Gear hobbing operation

      For hobbing helical gears, the hob is swivelled by an additional angle of the helix angle of the gear to be made.  The hobbing process, though used for gears and splines more frequently, is also suitable for other shapes such as ratchets and sprockets.

      Since hobbing is a continuous process, it is fast, economical and the most productive gear - machining process.  It is also possible to mount more than one gear blank in the work axis to increase production rate.  However, this process cannot be used for machining internal gears or gears with shoulders and flanges because of the clearance needed for the hob.