Goldstein, is that you that is saying the only way "to do it right is over wires"?
It's not the
only way, I've seen chucks and fixtures that locate off the teeth, but they are a bunch more expensive.
Fact is, thread title is "what is the
correct way to bore gears" ... the "correct" way is
not to locate on the od. OD is not necesarily concentric with the teeth.
For a South Bend yeah, who cares.
Well, some people do. You can hear runout. It goes rrrr-rrrrr-rrrr instead of whooooo. There are threads here complaining about that. I think it's silly but ... if you really care about that stuff, have to locate off the teeth themselves.
btw, 1.728 over the dp gives a good size for the wires but for something like this, they don't
have to be that size. Just all three the same and big enough to extend beyond the o.d. will work.
And what tolerances are you talking about with wires and rings, worrying that mess in a 4 jaw chuck?
For grinding, I'd habitually indicate to under a half thou runout over the wires. Try for two tenths but to be honest, lots of times after heat treat the parts aren't that round. And parts that aren't heat treated aren't always as great as they'd lead you to believe, either. Sometimes
can't get under a thou and you'll move the gear around two or three times to get the best spot. Stuff distorts. Then when you touch off on the bore, you see how the three-jaw that originally held the blank shows up in the part later on. The wheel almost always makes a triangle before it makes a circle. Keyways distort parts badly, too.
Just for shits and giggles ... for expensive parts we'd take forgings, rough them. Stress relieve. Finish turn. Cut teeth. Carburize. Quench on plugs. (Spiral beevils even have special quenching presses to control distortion during quench.) Grind the bores while holding over wires. Last, grind the teeth off the .0002" fit on a mandrel with ground centers. Check on a redliner for concentricity and an involute tester for tooth shape and a lead tester for straightness.
Was working at a place that said "Oh bullshit, we don't need to do all that" so they made 1,000 scrap parts. Made me snicker. When OP said "correct", well .... there is only one "correct". The better question is, "how good does it have to be ?"
I have the most accurate four jaw chucks, on the most accurate lathes possible, I reject your directive, and the troll jumps in and says.
Reject away, didn't mean to tell you you had to do anything
But if you really want concentric teeth on your gears, can't trust the o.d. Places like Boston make blanks as cheap as possible then throw them on a mandrel to cut teeth. Can easily have several thou runout between i.d. and o.d. and on top of that the teeth can be way off from both, if the hob has runout or the mandrel centers aren't so good.
Teeth is what counts. The o.d. fits air.