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Tuesday, October 9, 2012

Norris Adjusters

(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 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 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.


  1. Good info on the Norris adjuster. I have wondered how it worked and now I know. Can't wait to see what your version looks like.

    1. Ralph-I'm glad someone took something away from that.My left hand tap and die won't be here until Monday. But I'm thinking I can get the rest of the pieces knocked out by then. I have one or two changes from the original in mind. Overall, this is a fairly simple task. The only part with added complexity is the banjo. Though there are ways to make that simple as well.

  2. Mark:

    You need to look at those photos of the adjuster again. If the two threads are of opposite direction (LH & RH), then their motions ADD (a clockwise turn of the RH adjuster pulls it downward through the threaded swivel and pushes the LH banjo farther OUT of the adjuster, so also downward). Norris’s 1913 patent makes no claim for fine adjustment, claiming “a screwed connection so that the differential action of the screw threads on the spindle may be employed for advancing and withdrawing the blade rapidly or slowly.” He further states “this arrangement may be varied according to requirements. For instance, the screw threads may be in the same direction but of different pitch, or in opposite directions and of the same or different pitch”. “This arrangement” referred to LH & RH screws used together.

    Using opposite screws will always add their motions in this arrangement (just like a turnbuckle with LH & RH threads). If the threads are both RH, then a CW turn of the adjuster pulls the adjuster through the swivel downward, but the banjo advances INTO the adjuster (upward), and the net motion is the difference. The nearer the two threads are to the same pitch (and the finer the pitch), then the smaller is the net motion. Note that if the threads are the same pitch, the adjuster advances through both, with NO motion of the banjo.

    Bottom line: no need for a LH tap. The thread combinations in the photos give adjustment rates equivalent to threads of about 18.7 and 17.8 threads per inch (not fine).

    Just remembered Jim Kingshott’s book. His comments on the LH/RH adjuster:

    “Added to this the two threads, one right-hand and the other left-hand, gave very rapid adjustment, and I thought that if the coarse adjustment could be modified so that it required more turns on the adjuster to move the iron, then all would be well.
    An adjuster was designed and made with two right-hand threads of differing pitch…… As one thread advanced the iron the other retracted it, the difference in the pitch of the threads being the amount that the iron was moved.”

    The LH/RH thing has been touted as a way to fine adjustment on many occasions, but that is wrong.


  3. Roger- Thank you for the detailed reply, I appreciate the input. After reading that, I made a quick and dirty mock up of an adjuster, and it worked just as you said it would. So, thanks for the correction!