Re: hobber - single or double threaded worm?

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Posted by BillS on April 11, 2008 at 12:13:03:

In Reply to: hobber - single or double threaded worm? posted by Tom P on April 10, 2008 at 14:59:34:

While a 2 or 3 lead worm should never be as accurate as a single lead worm, this could hardly be an issue on a gear hobber. There are so many other things in the machine to contribute to inaccuracy. If you need high accuracy, consider shaving or grinding.

Actually, the choice of multi-thread or single thread hob has more to do with the index constant in the typical machine design. If you want a low index constant to cut smaller numbers of teeth, without using more than two change gear pairs, pick a multithread hob or use multistart hob cutters.
Fo instance, some single index hobbers have 24 or 36 to 1 index constants. This means, to cut 10 teeth, you need an index ratio equal to 2.4 or 3.6 respectively. Now, if the machine had a triple index worm, the index constants would be 8 and 12 respectively. To cut 10 teeth, ratios would be .8 and 1.2 resp. which is much easier to find change gears for.

For large numbers of teeth (say 100-200T) the larger index constant makes ratios easier.

I like what G&E did, however. They give you an additional index idler shaft for another pair, which greatly extends the range.

I guess all I'm saying is, look for a tight machine that covers the range of gears you expect to cut (or two machines) and don't worry about the number of threads in the index as far as accuracy is concerned.

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