Posted by Bill Simpson on March 29, 2013 at 12:23:46:
In Reply to: Hobbing Prime Spurs using differential posted by gearhead1 on March 29, 2013 at 09:42:09:
Hi Gearhead,
It's more of a procedure - assuming you have the formula for differential and the method
using a "Pickup Constant" for index/feed setup. Note: this only works on a machine with feed change gears.
First, select a helix angle, for instance 15 Deg. Any angle will likely work, but avoid too small or too large. 15 deg is an "easy" angle to work with.
Then find a set of differential gears that will cut 15 Deg.
Next, select index and feed gears to cut 15 Deg. Here is a link showing "pickup Constant" approach to find index and feed gear set:
Pickup Constant method to generate helical gear on non-diff machine
If your index/feed set cuts LH, set the machine differential to cut RH.
That's the picture - you set the machine to cut RH with one of the setups, and simultaneously cut LH using the other setup. Each setup cancels the effect of the other, so you get a spur gear - so be sure to set the hob table angle for spur gear!
Unlike cutting with differential only, you can't simply back the table up to recut. To recut, raise workpiece away from cutter to back the table up. Plus, with some extra effort, you will need to reestablish tooth and cutter alignment.
If anyone has a quick (or at least a sure) way to reestablish cutter and tooth alignment for 2nd cut, could you share please?
By the way, if you are using CPC-HOB to cut a large prime, it will automatically generate the diff, feed, and index gearsets for you. Just select "Dual" option in the final menu and follow instructions.