That particular machine that I had, I could have power feeds OR I could cut threads if I opened up the end panel and changed a series of gears. The threading charts showed the gear change in the end panel. The power feed charts showed a different set of gears in the end panel.
Yes that's what is in the Precision Matthews (PM1440HD) manual, the manual I got from Modern is pretty much useless. I haven't done all the math to determine what the unlisted gear combinations will do but one thing that annoys me is there is no 1.75mm thread listed for the GH lathe. I had to use a die to cut some M12 X 1.75 threads a few weeks ago.
I recently sold an older (1993?) 12x36 belt driven asian lathe that had the banjo style (I think they are also called Norton gearboxes?) and the only gear change outside of the gearbox was one gear to go from imperial to metric. All other thread combos were done with the gear box. I don't understand why they would not be able to keep that feature on every lathe. My little 10x22 lathe has knob selector gear box and the biggest range of threads I can get with any one gear combo change is three, everything else requires swapping out gears. I suspect there may be some other possibilities with my lathe but I have never been able to figure out the math to know for sure.I thought my banjo style gear box (or whatever you call it) was kind of outdated old school, but actually it offers quite a few in between sizes.
I think you're right, if I had a 28 tooth gear I could get the M1.75 thread. This the entire threading chart.Here is my gear chart. It uses the same 120/127 gears as yours to switch between metric and imperial. But I seem to have larger change gears.
Talking only about metric for now, yours are 30, 32, & 40.
Mine are 30, 56, 60, & 63.
Note that to get 1.75 pitch, I need 60 & 56.
The top of this column is 7.0. So that is your target top gear. It will yield 7, 3.5, & 1.75.
Now, I have no idea what the internal ratios are. But all these lathes including mine are designed to cut each primary gear by exactly 1/2 internally.
Just stabbing in the dark, here.... 20 is to 40 is to 60 just like 14 is to 28 is to 56.
So, if you put 40 on your bottom gear and 28 or 29 on your top gear, I'm guessing that you will get your 1.75. You would just need to make a 28 or 29 tooth gear which is pretty darn close to your 30 so it should fit and should work at your C-B-3 selected position.
It would be good to set up a spread sheet with all the internal and external gears on it so this could be done more rigorously than my intuition though.
On the other hand, if you like making gears......
View attachment 32084
Also note the engagement of the B Gear on my 3 charts. My B gear can engage either the 127 or the 120 gear depending on which side of the gear has the bushing. But the only position corresponding to the feed chart is the 127 gear, not the 120. So it's easy for me to see why @cuslog's feeds may have been wrong on his old lathe.
I think you're right, if I had a 28 tooth gear I could get the M1.75 thread. This the entire threading chart.
I noticed that they never calculated a feed chart for the metric threading change gear combinations for any of the lathes.