# Re: prime gear cutting without prime index gear

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Posted by BillS on November 25, 2009 at 15:34:05:

In Reply to: Re: prime gear cutting without prime index gear posted by Allan G on November 25, 2009 at 13:00:06:

Right you are, Allan. But if you can't double to get a practical or available size, check out:

Only problem is this requires a hobber with differential.

Another choice is to cut a helical change gear with 83T, then cut the opposite hand matching change gear for the first driver. You can easily cut the helical change gears on a non-differential hobber. Hint: you don't need an 83T change gear to cut an 83T helical gear.

Gee, who wants to cut two change gears in order to cut one workpiece? Depends on how desperate you are :o(

Actually, you could likely reuse the first driver with other helical prime change gears, where you only need to cut one change gear to cut a workpiece. If the index of the machine is 15:1 just cut a helical 30T change gear. In your example, cut the 83T helical change gear of opposite hand. Then mount a 1:2 set of standard spur change gears to bring the ratio back to correct ratio. Also for this example let's cut the 30T 15Deg LH and the 83T 15Deg RH. Here is an example change gear set:

30(L) x 50(S)
------------------
83(R) x 100(S)

(L) and (R) represent helicals, while (S) represents spur sizes. Of course, the 30(L) would always be reuseable if you need to cut another prime size. And, if you need a really large prime number, just reduce the pitch on the two helical sizes. Diameter doesn't matter - just tooth-to-tooth ratios.

Don't like using helical change gears? Okay, you're right - you can't rearrange them like you might want to sometimes. So use the helical prime to cut its equivalent spur prime change gear. That might be useful in the case of the 83T.

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