Threading Small Diameter Wood - Part 2

Man, I had a lot of trouble writing this blog entry.  It has been tough trying to come up with a coherent story.  But here goes.

Last week I had taken a 1/2" dowel and, after careful layout, used a square file to file about an inch of threads into it at a pitch of 8 threads per inch.  Then I made a wooden nut and was able to screw the threaded wooden dowel into it.

A 1/2" x 8 tpi screw into a tapped hole

The method of making the nut involved a block with an angled kerf cut in it, into which a metal plate is inserted to engage the spiral slot sawn into a 7/16" dowel.  The dowel was fitted with a cutter which, when rotated in the block, can cut inside threads in a workpiece that is clamped to the block.

The tap without a workpiece

The tap with a workpiece ready to get an inside thread

A couple years ago, I made some larger diameter wooden screws and nuts.  To cut the external threads, I made a screw-box based the one in Roy Underhill's "The Woodwright's Workbook".  But this time I'm trying something different.  A guy named Paul Hamler has a YouTube video where he shows his method of cutting wooden threads for miniature plow planes that he makes.  I'm particularly interested in what he shows at about the 22 minute mark.  In this blog post, I'm borrowing some ideas from that video.

Like last post, I'm working with 1/2" diameter dowels for outside threads, 7/16" dowels for making inside threads, and I'm making 8 threads per inch.  This method takes advantage of a large 1" diameter metal bolt that happens to have 8 threads per inch.  By attaching a wooden dowel to the bolt and running the bolt through a captive 1" x 8 tpi nut, I can advance the dowel at the proper rate to cut 8 tpi threads.  And I hope to be able to cut both outside and inside threads!

This block helps capture the nut

First, I had to find a way to attach a dowel to the bolt.  I started by hacksawing, drilling and filing a 3/8" slot, about 5/8" deep, centered in the end of the bolt.  Holy crap - that was a lot of work!  Then I made a wood block with one end shaped to fit in the bolt's slot and a 7/16" hole in the other end to secure a 7/16" dowel.  Both connections were pinned with wooden pegs.

3/8" wide x 5/8" deep slot cut into the end of the 1" screw

Wood block shaped and later pinned to the bolt

I had to give this a try, so I clamped things to the workbench and gave a trial run.

A trial run at cutting inside threads

The dowel was fitted with a small cutter

And after a few passes, iteratively increasing the cutter's depth, it worked!
Here a 1/2" x 8 tpi wooden screw was turned through the new nut.

After that, I realized I could more simply attach the dowel and bolt by filing a couple of flats on the dowel and pinning that directly to the big bolt.  The two flats were made 180 degrees apart on the end of the dowel, so that it would fit into the bolt's slot.  A 3/16" hole was drilled through the bolt and dowel to hold the dowel in place.

Attaching a dowel to the massive 1" x 8 tpi bolt: note the
hole drilled down through bolt and dowel, pegged with 3/16" dowel

I also made a three-sided box with dadoes on opposing sides to house blocks that hold the different items needed for cutting threads.

Here's a box with captive 1" x 8 tpi nut.  One of the other slots in
the box will house a block with a cutter to cut the threads in a dowel.

The nut is recessed into the block and secured with a cover

The next thing I needed was a method to attach a V-cutter (for cutting outside threads) to a block that the dowel is fed into.

Here you can barely see a clamp for the cutter.  The clamp is just a small
piece of hardwood with two holes for bolts to tighten it against the cutter.
T-nuts on the back of the wood block provide purchase for the bolts.

Here's the cutter clamped to a wood block, starting to cut some threads

An action shot shows the shavings coming off the dowel

The resulting screw threads in some (very soft) redwood

But it fits the internal threads of the test block

Here you can tell I decreased the outside diameter of the dowel too much.
It made for a fairly loose fit in the nut.

On another test piece, made of harder wood, after cutting the threads
I chucked the dowel in a drill to file the threads for a better fit in the nut

Ready for a test fit ...

... and it fits great!

Top is the screw that I filed by hand.  The other three, from different
woods were cut with the rig shown above.

This is getting too long (again), so I'll add one final thing.  I made a nice improvement to the clamp that holds the cutter for outside threads.

A block holds the cutter and a set screw in the back can advance the cutter

Here's the cutter, the clamp block and you can see the hole for the set screw

In use, an Allan key turns the set screw ...

... advancing the cutter from not cutting ...

... to cutting deeper and deeper with each pass

End note: after all this, I found that you can purchase "lead screws" of a more reasonable diameter and the right thread pitch, lead screw nuts, and couplers that would handle the job of attaching the screw to the dowel much more efficiently.  Oh well, I guess I like to do things the hard way - with stuff I already have on hand.