Thanks, I'm good on the chart. The math is easy and I have a limited range of needs; Automotive. No heavy equipment, no watchmaking. The math is number of teeth on the stud gear divided by the indicated thread pitch on the gearbox gives the metric thread pitch. Many pitches can be achieved by several combinations. The South Bend chart sticks to tumblers 1 and 3. I can get to some of the needed ones other ways and reduce the number of stud gears on hand.
The internet wisdom is that you remove the 64 gearbox gear, put a spacer on and reinstall the 64 to mesh with the outer 120 tooth idler. I noted a problem with this on my gearbox. The input shaft only has a keyway in the inner position. The outer gear is un-keyed. I see from your picture that they supplied a 64 tooth with an extended boss such that you just flip it over.
I first thought about keying, either with a dowel or a real key, the outer section of the gearbox shaft. But the potential for screwup dong it in-situ is extremely high, and I really don't want to strip the gearbox to do it in the mill. So I'm going to make a keyed hub and pin/bolt it to the existing 64 tooth. Or I may just make a new 64 and leave the original undisturbed.
Again, the internet wisdom was you just reposition the banjo to hold the bigger gear. This banjo is held by the bolt in a cross-slot at the top with limited range, and a grub screw at the gearbox boss end. If I had looked at it in the beginning it would have been obvious that the slot wasn't long enough (this I suspected) and would be inaccessible, totally covered by the 127 gear (didn't think of that). Right now it's being held by the grub screw only. But that is not a long term solution. I'm either going to extend the banjo and make a new slotted support point, or if the proper sized stock presents itself, duplicate the split clamp like in the picture.
And one more thing I see. There must be a longer banjo bolt and bushing not pictured. That 120 gear isn't hollowed out to match the original bolt/bushing length like I had to do. That would supply better support for the gear. But I'm not terribly worried about it. The only time there will be a cantilevered load from using the outer 120 gear would be low load low speed thread cutting.
But in the mean time, off to the next project. The wife needs a die to cut foam pads. Seems the embroidery machine supplier wants $6 (+S/H) each for a 70mm x 12mm circular foam pad... and there's 15 of them. So I get to whittle up some more 4140. But first, I'll need to make a tool post grinder......
