Gear cutting not centered

Hoping you experts can help with some advice.
I have a 10" Logan lathe that did not come with the original gears. Imperial threading works just fine with the gears it came with, even though they are not stock gears. However, it did not come with the change gears for metric threading. So I set out to make some. Bought a DP16 gear cutter set, made myself an arbor and 2 gear blanks and went to town.

Now the problem, I mis-aligned one of the gear blanks when I set it up on the mill. The gear blank was offset on the rotary by 10-20 thou. So after I was done cutting the whole gear, I noticed the lands between gear teeth on one side were smaller than the other. Dumb mistake.

I don't have any more stock for making another gear blank, I would have to go buy some. But I was wondering if it is worth it. Am I overthinking it? Would the gears run just fine? (it's only 1 of the 2) The bore and OD are concentric, I just cut one side deeper than the other.

Don't have much experience with gear making and appreciate any advice.
Jared

jroddds said:

Am I overthinking it? Would the gears run just fine?

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Put them on the lathe and see if you're satisfied. For the amount of time you'll spend cutting metric threads, I'm guessing yes, but up to you.

You would need a lot of backlash, and will cause a lazy tread, Pitch will change a little, so over-cut and it will work...Phil

Involute gears can be cut with some variation in the actual pitch diameter. This has even been used to make gears with +1 and -1 tooth, all with the same pitch diameter as the base number. An example would be a set of 47, 48, and 49 tooth gears all with the same pitch and outside diameters. And they will mesh with a properly cut gear, if not even with each other.

If you use a gear which was off center for threading then the thread pitch will vary + and - as that gear makes one revolution. This will repeat as the threading continues. They may work in some cases, but I would not want to be cutting threads like that.

If your gear was cut off center, you can just center it and cut each space again. Go just deep enough to have all of the teeth cut to an equal pitch diameter, no matter what that pitch diameter actually winds up being. With a 100 or 127 tooth gear, that difference will be slight and you should have an good, working gear which does not cut threads with a varying pitch.

As for backlash, if you cut it as I suggest you will have some additional backlash when you use that gear. But in cutting threads, the forces of the cutting will take up the backlash as first contact is made by the threading tool and the gear teeth will stay in contact for the duration of that threading pass. So the increased backlash is of no practical concern for this use.

As EPAIII says,

Put it back on the mill, centered properly and recut all the teeth to the deeper depth, then skim the diameter to get the correct depth.

Centre distance doesn't matter with changewheels, since you adjust it on the banjo.

3rd the recut, it will work good...Phil

Thanks for the advice, everyone. I'll chuck it back up on the mill and recut it. Centered this time, of course.

EPAIII said:

Involute gears can be cut with some variation in the actual pitch diameter. This has even been used to make gears with +1 and -1 tooth, all with the same pitch diameter as the base number.

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This is a common misconception but in the end, it only makes for more confusion.

A gear by itself does not have a pitch diameter. "Pitch diameter" in that case is an imaginary number.

Gears only gain a pitch diameter when they are placed in mesh with another gear. At that moment, the "pitch diameter" is determined by the center distance and the ratio between the numbers of teeth. Nothing else. There is no way to measure it. It is mathematically derived and imaginary.

In this case, I'd have tried the parts before getting upset, because space cutters are not correct anyhow and the "translating gears" go through several more reductions which will smooth out any unevenness in the motion, and you don't do metric on an inch lathe all that often anyhow. Why run the risk of a misindex in the dividing head and wrecking the part if it's not going to make any difference ?

At least try it first. Roll them together by hand and see how smooth they roll, that's all that counts. The teeth are not the correct shape even if they were cut on center, see what real-world difference this mistake made before getting too concerned.

Be sure the tightest has some clearance(.005 or so).