Posted by BillS on April 23, 2012 at 08:50:22:
In Reply to: first helical gear posted by Jim Pollock on April 20, 2012 at 22:12:17:
As you know, the helix angle must be accurately found. But part of the problem is where is the pitch line?
It's not uncommon for a pinion to have an enlarged OD (profile factor, shifted profile, etc), which results in a pitch line closer to root.
Also, special pitch cutters might have been used, so make sure your gear is 7 NDP. Roll your cutter in either gear (pick one with best teeth) to see if it rolls smoothly at angle normal to the teeth.
What is Teeth and OD of the mating gear? This can shed light on the pair's design and usually what the pinion specs must be.
Some quick calcs indicate to me that, for 12T 7 NDP and (32 to 35) Deg HA, the nominal OD = (2.307 to 2.378) is enlarged by (0.222 - 0.151).
This variation in possible HA means that you may be doing some trial and error cutting.
Once we establish the Helix Angle the machine setup will be simple.
Also, I need to know the index and feed constants of your No. 12 hobber. Then I can find you a set of change gears to cut your gear.
When you have cut the first gear, place it in mesh with its mating gear on a flat surface. This is a quick way to see if the angle is correct. If tight mesh "lifts" one of them, there is an error in angle.