(do not fail to read the comments after this post! you will find that my thought process here was not quite right when I originally posted)
I came to the point in building my infill smoother that I need to fit an adjuster. The Norris style adjuster is one of the more famous features of a Norris infill, and the plane I'm making is a No. 51 reproduction. I was going to buy one, but then I thought about making one. I've never seen an actual Norris adjuster (two of my Lee Valley planes have scaled down versions). A quick search came up with a nice article on Handplane central, here. Keep that article and pictures in mind as I ramble on. (my scanner is down, so I can't really sketch anything until I figure that out)
How does the Norris adjuster work? Differential thread, duh! OK, so what is this differential thread thingy? Go back and look at the picture in the link. The larger shaft has what I will call the main thread, and the banjo that engages the back iron screw has what I'll call the secondary thread. In the original, the main thread is a right hand 35 TPI thread. The secondary is a left hand 40 TPI.
It's the combination of the coarse right handed thread and the finer left handed thread that makes this idea work. (you'll want to keep that picture up...)
Alrighty then, why does it make it work? If you turn a 35 TPI thread one full turn, it will move .0285", give or take. If four decimal points aren't good enough for you, I'll keep that in mind for next time. Anyway, using only the 35 pitch screw to adjust the blade would be very coarse. To adjust the blade for those super thin shavings we all like to get from our smoothers, you would have to turn the screw less than one tenth of a revolution.
"Come here you fine lefty" (I tried that line once, got a slap and a drink in my face). The finer left hand thread acts as a reducer. Gears and threads all work on the same principle, don't they? For every turn of a 40 pitch thread, you will move .0250". As an aside, if you ever wondered why micrometers were set up in divisions of .025", it's because they have a 40 pitch screw inside.
What Norris did, was to run the left handed thread into the shaft with the normal right handed thread. Here's where you put on your imagination cap and try to follow my nonsense. When the right hand thread advances one turn (.0285") the left hand thread moves the opposite way (.025"). The total travel of the banjo at the end is the difference between the two threads: .0035". Think about that...one full turn moves the blade a bit more than the thickness of a hair.
That is why the Norris is capable of such fine movements.
Now I just have to wait patiently for my left handed tap and die to show up so I can hit the machine shop and get this thing made. I'm pretty sure I can get a first time "go" on this one. I'm surprised I never took the time to think about it more before today. Once I get to working on the adjuster, I'll post some pictures. If you're not following the above nonsense, seeing the adjuster as it is made and assembled will help.
For you woodworking/plane making pranksters...how about a Norris adjuster with a left handed thread that's coarser than the main thread. That way, when your friends try to extend the blade...they'll actually retract it! That's some funny stuff, you'll all be in stitches and the party will be a RIOT. Actually, I should be in stitches just for saying that. But that's my life, too much time to think of sillyness.