Posted by BillS on October 27, 2008 at 12:31:50:
In Reply to: Diffrence between a single start and multi start hob.. posted by Thiagarajan on October 23, 2008 at 14:12:51:
Imagine the hob cutter without gashes - the straight grooves ground to form cutter top lands. The hob would then be a thread, or worm. Just as a worm has starts, so does the hob cutter.
Your question may be more about why different start hobs are made. What is the reason for multiple start hobs, you ask?
A single start hob has a predetermined helix angle based on its pitch and its OD. This presents no problem for hobbing cylindrical gears with a few exceptions (e.g. dealing with machine index and availability of index gears).
If you were hobbing a throated worm gear whose mating worm is multi-start, then hob cutter specs become important. To get a good contact pattern, you would want to use a cutter with same hand and same number of starts as the worm. Also you would want a cutter with slightly larger OD. These cutter specs are all critical to a good contact pattern.
If you want to read up on hobbing throated worm gears, Ron V has found an excellent article:
Manufacturing Single Enveloping Worm Gears
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