Posted by BillS on January 22, 2010 at 14:05:44:
In Reply to: setting prime number gear ratio posted by Veneet Jain on January 22, 2010 at 06:03:54:
If the machine has a differential and feed change gears, you can do a combination setup. See the FAQ for an explanation how.
If your hobber has feed change gears but no differential, you can cut two helical change gears that will allow you to cut your 173T prime spur gear. The magic here is that you can cut a 173T helical gear without using a 173T change gear - Cool, eh?
In your case, cut a 173T RH gear and a matching 72T LH gear. You can cut the two gears with a smaller pitch if necessary to fit on the machine.
Then mount the change gears as :
40 x 72
----------
80 x 173
Note that the 40/80 combination is from your standard spur change gear inventory.
I doubled the 36 here so that smaller pitches can be used for the 72LH/173RH combination. That way, you can reuse the 72T LH gear with other large prime sizes (you would still need to make the other primes as RH gears in the smaller pitch).
Any helical angle will work, but I suggest something like 10 or 15deg to minimize end thrust.