Posted by BillS on December 04, 2006 at 02:26:23:
In Reply to: Re: hobber recommendation posted by Bob M on December 03, 2006 at 20:33:16:
A hobber's work piece spindle is driven by a worm and worm gear. Machines are often built covering a range of index constants which are determined partly by the ratio of the index worm and worm gear. Say for instance, a machine with a 12:N index ratio uses a triple thread worm. If the manufacturer installs a double thread worm/worm gear, the index would be 18:N. If it were a single thread worm/worm gear, the index would be 36:N.
Index ratio is the ratio of change gears on a specific hobber required to cut N number of teeth. For example, to cut 50T on a 12:N machine, the change gear ratio must be exactly 12/50 or 0.24. Hobbers typically use two pairs of index change gears, so in this example an equivalent change gear set could be 24/50 x 40/80. As you can imagine, there are a lot of possible combinations that exactly equal 12/50.
One consideration with index ratio is that, if you plan to use the machine for small numbers of teeth, you want a triple (or even a quad) thread index. This will spin the index worm at a conservative speed, extending the life of components in the index worm/worm gear area. A single thread index will spin the worm at three times the rpm compared to a triple thread. And for multistart hob cutters,the index rpm increases by the number of hob starts.
The argument is different if you plan to cut large N's. Rpm of the index worm is not a problem when cutting a large number of teeth. But for a very large number of teeth the index change gear ratio can get so small that you can't find four gears to match it. For example, let's look at the triple thread index with 12:N ratio. What if you want to cut 300T? Assume your index change gear inventory ranges from 24T to 100T. You need a ratio of 0.04, but 24/100 x 24/100 (the lowest ratio possible) is only down to 0.0576. There is not a combination given these circumstances that will cut 300T (Thank heavens for G&E. Most G&E's can compound an additional pair in the index train). Okay, the savvy folks know that a multistart hob will effectively double the ratio to 0.08 and you can find a 4 gear index change gear set to match that (e.g. 25/100 x 32/100). Multistart hobs can produce larger tooth to tooth errors, but we won't get into that for now. The point here is that a 36:N machine could cut the 300T gear using a single start hob cutter. An example set of gears is 24/100 x 50/100.
Conclusion? A 12:N machine covers a lot of N values found in typical gear manufacturing, handling smaller N numbers with less wear and tear on the index worm/worm gear...