Tom,
At first I just wanted to say "but it still doesn't matter".
BUT, I went and found both the imperial and metric charts and looked at the again. Here is what I ignored in the passed and why I posted my comment. I hope someone else can benefit a bit from my slow comprehension.
For EITHER imperial OR METRIC threading the 'a' gear ALWAYS drives the 127 gear (the 120/127 set is always the same position with 120 outside and 127 inside)
But now for the difference, for imperial the b gear goes onto the shaft before the spacer and this means the b gear is driven by the 127 gear, while the 120 gear on the outside of the 127 is just there as a spaces and does nothing. But for metric threads the b gear is placed on the shaft AFTER the spacer. This means the b gear is driven by the 120 gear and the 120 gear turns because it is on the keyed bushing WITH the 127 gear which is driven by the a gear.
I ignored all of this until now because I was not cutting metric threads, but I did NOT completely understand it.
thanks for letting me sound this out and hijack the thread! Now you guys no how my wife feels when I come in from the shop scratching my head and I say to her, "Honey, I know you might not completely understand this but see if this makes sense..." This is when she puts her nose back down to her iPad.
ji