Thread Milling Using Hob

Thread Milling Using Hob

Thread Milling Using Hob - The hob is made in the same way as a machine relieved milling cutter and has the thread form to be cut machine relieved on its teeth. The threads on this hob are not helical as a screw but are in the form of a series of separate and parallel threads. The hob must be a few threads longer than the longest thread it has to cut, and it has to be designed so as to be held overhanging on the end of a stub arbor incorporated in the machine (this limit the process to relatively short thread).

During the operation the work and the hob are carried on parallel spindles, the hob rotating at milling cutting speed. The cut is commenced by feeding the bob up to the portion to be threaded and sinking it in for the full length and depth of the thread. Immediately this has taken place, the feed of the work is engaged and this consists of two motions:

(1) a slow rotation at feeding speed, to cause the threads being cut to encircle the diameter,

(2) an endwise feed relative to the hob of an amount such that, when the threads have encircled the diameter, each thread will have advanced one pitch.

The direction of this feed will determine whether the thread is RH or LH and, when the full circle has been cut, each thread will be in the next groove to that in which it started and will so produce a screw. A diagram showing the application of the hob and directions of feed for producing a RH screw is shown in fig.

The internal threads are produced in the same manner, except that the hob must be smaller in diameter to minimise the tendency for interference to take place and affect the thread shape. This can and does occur to a slight extent since the hob axis is not inclined to the thread angle, but with the large diameters and fairly fine pitches concerned the effect is negligible. If, however, an internal thread were cut with a hob only a little less than the thread diameter, there would be a considerable length of engagement and interference would take place.

To keep their diameter small (if possible this should not exceed one third of the hole diameter) internal hobs are often made solid with their shanks and not bored to fit on an arbor as in the case of the external types apper threads can be milled if the machine is equipped with a taper attachment. To attempt to produce such a thread with a tapered multiple cutter, without the attachment, results in a thread with a step because the feed is parallel to the work.

For multi start threads, an index mechanism is provided for indexing the work spindle.

Thread Milling Hob

Thread Milling

Thread Milling at End of Work