Threading Small Diameter Wood - Part 3

Last time, I had some success threading 1/2" dowels and fitting them into 1/2" wooden nuts.  I had used a three sided box with dadoes that housed blocks.  One block holds a machine screw and nut with a specific pitch.  The end of the screw was modified to hold either a dowel ready to have outside threads cut, or a tap ready to cut inside threads in another block.  Another block simply gets an inside thread cut into it or it houses a cutter to cut an outside thread on a dowel that is fed into it.

Here's the box set up to cut outside threads on a dowel

After some experimentation, I made some improvements.  First, I wanted to angle one of the sets of dadoes so that the cutter mounted to it would cut outside threads at the proper pitch angle.  For the 1/2" - 8 threads and the 3/8" - 10 threads that I'm working with, this angles turns out to be around 5 degrees.  Without this angle, the V-shaped threads cut into a dowel would not be symmetrical: one side would be more vertical and the other side more angled.

This shows the angled block to account for the pitch angle.
The previous picture shows a straight-across block.

Second, I realized that my outside thread cutters were cutting a much wider thread than I want due to cutter angle and geometry.  So I modified the cutters to make a narrower cut (no pics of that).

Third, I made improvements to the methods of connecting the screw to a dowel.  Originally, I had used a piece of rubber hose and hose clamps, but these tended to slip.  Later, I made wooden couplers that used set screws to lock the screw and dowel in place.

Here's an early experiment with 5/16" tap cutting inside 3/8"-10 threads
in a block.  The machine screw has 3/4"-10 threads and the coupler is
made from plastic tubing, hose clamps and a split wooden ring to adapt the
5/16" dowel to the 3/4" ID plastic tube.

Now is where I get into 3/8" diameter screws with 10 threads per inch.  I use a 5/16" dowel for cutting inside threads.  In earlier experiments I just drilled a hole through a 5/16" dowel and stuffed a cutter in the hole to make a tap.

The little cutter was made from a steel hinge, rectangular in cross section
and fit tightly into the round hole drilled through the dowel.

That worked, but after the first use, the hole got wallowed out and the cutter wouldn't stay in place.  So I decided to try a different way to hold the cutter in place.  This method uses a set screw in the end of the dowel to lock the cutter in place.  But as you'll see, there were issues.

Drilled, tapped and put a set screw in the end of the 5/16" dowel.
Before tapping the hole, I fit the dowel into a 5/16" hole drilled in scrap.
This kept the dowel from breaking apart as I tapped.

Made a new cutter from an old 1/8" drill bit

After annealing, it shaped easily with hack saw and files

But after a few times tuning it into a (hopefully) new nut, it was destroyed.
This one was made from relatively soft red alder.

I made another 5/16" dowel from beech, hoping the harder wood would hold up better.  Then I ran it through another 5/16" pilot hole and it worked without getting destroyed.

Got the sample 3/8" screw through the nut

Some success!

The threads were very tight.  I'm pretty sure the problem lies in the profile of the external threads on the screw.  I'm using a 60 degree cutter, but I think the angle at which it is presented to the 3/8" dowel makes the effective angle greater than 90 deg.

After working on cutter geometry a bit, I got back to the screw to dowel connection.  The coupler is a block of wood with a 3/4" hole on one end and a 3/8" hole in the other.  Then I drilled a hole through the coupler and the 3/4" screw, as well as through the coupler and the dowel.  Both are pinned with wood pegs.

The new coupler has a 3/4" hole on one end ...

... and 3/8" hole on the other end.  After the screw and dowel are inserted,
a hole is drilled down through both and pinned with a small wooden peg.

First tried a very dry beech dowel.  It worked, but the beech didn't take threads well

I had soaked these two red alder dowels in mineral oil for a few days ...

... and this one took the threads far better than the dry beech.

Fits in the tapped threads in a block

But there's a problem: a flat spot with barely any threading

It turns out that the 3/4" screw is not sitting in it's block perpendicular to the block.  It's probably a few degrees off 90, so I shimmed the nut in the block and got it somewhat better.  The result of this problem is that the back end of the threaded piece wobbles as it is fed into the cutter and one side of the dowel gets much less threading than the rest.

Look at the difference in how these two took threads

Finally, I broke down and bought a couple things to help the situation.  The first was a 5/16" brass rod to make a new (practically indestructible) tap.  Similar to the above beech tap, it has a hole drilled and tapped down the center to take a 10-24 set screw.  The cutter for the tap is housed in a 1/8" hole drilled through the diameter of the rod.

Brass rod drilled and tapped - carefully to keep it centered

I also purchased a 12" length of 3/8" - 10 lead screw.  This was much easier to deal with than the large 3/4" screw.  And it also fit into tapped holes I had already made in blocks that fit the threading box.  And because the threaded blocks were aligned properly, the lead screw has very little runout.

Brass tap on left and 3/8"-10 lead screw on right.

The lead screw and brass rod have shallow drilled spots to allow
the set screws to engage them and hold them tight with no slippage

And this worked out great - got a nice thread in the block that fit the screw well

Well established 3/8"-10 inside threads

And here's where it ties back into the thing that started this rabbet hole.  The marking gauge that got me thinking about small diameter wooden threads is shown in the next picture.  The threads on the wooden screw fit perfectly in the tapped holes.

The wooden screw that started this rabbet hole fit perfectly!

With this success, I'm making a new marking gauge: one that holds the beam in the fence with a wooden screw.  And I'll share that next time.