Re: single vs. multiple thread hobs

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Posted by Rajesh Poddar on November 23, 2004 at 07:39:23:

In Reply to: single vs. multiple thread hobs posted by Kurt Instigator on November 10, 2004 at 13:22:16:

Hobs are classified as single- or multi-thread hobs. The single-thread hob, when used in conjunction with a gear, is interconnected for rotation by means of a positive system. For example, when a single-thread hob is used on a 30-teeth gear, each revolution of the hob is the equivalent of a one tooth index of the gear being hobbed. Therefore, 30 revolutions of the hob would be required while the 30-teeth gear is making one revolution. A double-thread hob in one revolution would be in timed relation to a two-teeth index of the gear. A double-thread hob would be making 15 revolutions while the gear is making one revolution. As may be noted, there are some advantages of a multiple-thread hob in terms of production time. This production time is not proportionate to the increase in the number of threads and could not be so unless the number of flutes were increased in proportion to the number of threads. The gashes or flutes can be straight and parallel with the hob axis or can be of the spiral type on a lead, such that the spiral would be normal to the thread angle of the hob. The spiral gash is usually incorporated in hob design when the thread angle is in excess of 4 degree, as caused by a coarse pitch or multiple threads. The greatest accuracy is obtained by the single-thread design as opposed to the mutiple-thread design. This results from the "hunting-tooth" condition of a single-thread hob. It is very desirable to avoid the use of multiple-thread hobs when the number of threads in the hob is divisible into the number of teeth in the gear being hobbed. Inherent accuracy of the hobbing process is negated if this condition exists. Another consideration in the use of multiple-thread hobs is the fact that the speed of the indexing worm gear increases when multiple-thread hobs are used. In the case of low numbers of teeth, it is necessary to check the speed of the index worm gear to ascertain that it does not exceed the maximum speed recommended by the hobbing machine manufacturer. The selection of the number of flutes to be introduced into a hob depends on tooth depth and hob diameter. It is pretty well standardized with the cuting tool manufacturers in order to give maximum tool life for any particular size of hob. Increasing the number of flutes in a hob has the tendency of reducing the amount of available sharpeing life on the hob.

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