Posted by BillS on April 23, 2012 at 09:13:33:
In Reply to: Re: first helical gear posted by Jim Pollock on April 22, 2012 at 21:54:32:
As Frank points out, the helix angle must be accurately found.
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 cutter rolls smoothly with cutter axis angle normal to the teeth.
What is Teeth and OD of the mating gear? This can shed much 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 (30 to 35) Deg HA, the nominal OD = (2.265 to 2.378) is enlarged by (0.265 to 0.151). You might expect the other gear to have a reduced OD by a similar amount.
This variation in possibilities means that you may be doing some trial and error cutting. So get the specs of the mating gear so we can minimize the "cut and try".
Once we establish the Helix Angle, the machine setup will be simple. But I will 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.
After you 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. An optical comparator or other means can be used to measure the angular error, if any.
Meshing RH and LH gears of same pitch together on a flat surface is a good way to check differences in HA among the other gears you are expecting to cut.
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