Gear Hobbing
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Gear Hobbing
Gear Hobbing - The most accurate way of cutting spur and helical gears is by the hobbing process. The cutter used in this process is known as Hob, which is a rotary cutting tool with teeth arranged along a helical thread. A (gear) hob can be described as a cylinder on the surface of which (single) thread has been cut.
The thread has the shape of the involute gear tooth. Gashes or flutes are then cut across the spiral to create cutting edges at regular intervals. The hob is most frequently 75 to 150 mm in diameter. Double and triple thread hobs arc also made. They cut faster but are less accurate.
Hob
| A - Hole Diameter | C - Flute spacing | E - Outside diameter |
| B - Radial Cutting face | D - Outside diameter runout over tops of teeth from flute to flute |
| A - Tooth thickness | C - Tooth profile or pressure angle | E - Hub face |
| B - Lead | D - Gear S+F |
F - Thread |
Elements of Hob
| A - Tooth thickness | C - Start of approach | E - Gear addenda |
| B - Straight portiion | D - Tooth profile or Pressure angle | F - Whole depth of out (D+F) |
| G - Gear dedenda (S) |
The hob revolves and cuts like milling cutter. Its teeth lie on a helix like a worm. Lengthwise gashes expose the cutting faces that have a contour simulating a rack. The teeth are form relieved behind the cutting edges.
A hob is used as a cutter on the gear hobbing machine. This machine can cut spur gears, helical gears and worm wheels. The machine has three operative motions:
(i) Hob rotation,
(ii) Hob feed
(iii) Gear blank rotation.
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| Vertical Hobbing Machine | Horizontal Hobbing Machines |
Gear Hobbing Machines
| A - Additional Dive | D - Arm | G - Work Table with Arbor | J - Stanchion |
| B - Hob Slide | E - Work Arbor | H - Work Table | K - Main Drive |
| C - Hob | F - Gear Blank | I - Bed |
The construction of a hobbing machine is on the lines of that indicated at (a) and (b) vertical or horizontal, this referring to the direction of the travel of the hob slide. In both the machines, the hob is both rotating and feeding. The diagram gives details of a gear hobbing machine. Stanchion 2, work table 11, and work arbor support 8 are mounted on bed 1.
Main drive 3, mounted in stanchion 2, powers all the mechanisms of the hobbing machine. Additional drive 4, providing for rapid traverse of hob slide S, is mounted on the top of the stanchion. Hob 6, with its spindle and drive are mounted in the hob slide whose saddle travels vertically along the stanchion ways.
Increment of cuts as a Hob Generates a Gear Tooth
| A - Hob | B - Blank |
Generation of an Involute Profile
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Gear Hobbing Operation
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| Cutting of Spur Gears with a Hob | Cutting of Helical Gears with a Hob | Gear Hobbing Operation |
The circular work table with arbor 10 on which gear blank 9 is clamped can travel horizontally along the bed ways by means of a special mechanism. The upper end of the arbor is supported by arm 7. The hob and gear blank are fastened to shafts which are at right angles to each other.
The axis of the hob may be either vertical or horizontal. The hob must be tilted through its helix angle (or.) when machining straight spur gears so that its teeth are square with the blank and produce a true involute shape.
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