Posted by Bill S on April 24, 2012 at 14:02:50:
In Reply to: Re: first helical gear posted by Jim Pollock on April 24, 2012 at 12:26:08:
Jim, I emailed you a setup along with notes to explain some things you might encounter.
For anyone hobbing without a differential, it is important to have a complete gear inventory between 30 and 100T.
If your feed gear inventory differs in pitch, bore, etc. from your index gear inventory, then you will want both inventories to be 30 to 100. Remember, this is only important when you are cutting helical gears.
24 to 100 is better. Some missing sizes are OK, but it's almost required that missing sizes be distributed evenly. In other words, if you are missing all sizes from 60 to 100 it will be hard (I hesitate to say impossible) to find gear sets that can reach and mesh on the machine. CPC-HOB can quickly explore a wide range of table feeds to display many gear sets, but you will likely find that most index gear sets won't mesh on the machine especially when your index constant is large and your teeth to cut is small. That limits the possible index gear sets, and with missing larger sizes, the odds are against you.
CPC-HOB uses a measurement between fixed shafts and gear pitch to warn when gear sets won't mesh, but no matter what method you use, you will often arrive at the point where you need gears you don't have.
In this thread, we find that an index set could be found in the limited inventory, but it likely won't mesh (at least CPC-HOB doesn't think so). Mesh would likely not be a problem if a few larger gears were available. Which ones, you ask? well, that depends on the index and feed gear ratios. For instance, if you need a 30/37 in the index but they don't reach, then having a 74 would allow you to use 60/74. The next job might recommend 30/39, so the 74 won't help you there. See what I mean?
I think you get the idea. Complete sets of change gears is not only handy, but sometimes necessary.
Hey, you folks with differentials don't get a free ride either. You usually have missing sizes (comes that way with the machine, right?) So you often factor or multiply your way out, or trade some HA error for some differential change gears you have. Usually you will be able to make up index gears. By design, all the change gears are often from the same gear inventory (don't you love it when the hob manufacturer makes the gear trains interchangeable?).
Too many words?