Wooden Screws, wood threading, screw making etc has been a bane of my woodworking career for as long as I can remember.
Way way back at my beginning, I bought a couple of “ordinary” (cheaper) taps and screw boxes (the die of wood threading) expecting ordinary results. The threads broke off, or the cutters broke, or the tap and box cut so close that turning the screw in the threaded hole was difficult… I tried lots of things to make them work right. Oiling, soaking in oil, adjusting blade placement, geometry, sharpness, all to no avail.
So I put the tools away. For a very long time!
Last month I pulled them back out and bought yet another new cutter, and decided to try once more. Determined that if I couldn’t get a good cut and a good fit they would be someone else’s problem and I would purchase some of the expensive German made cast aluminum ones.
So I’m almost angry, while simultaneously elated, that the 1″ screw box made a nearly perfect screw on the second try.
The reason I want them working now is that I want to make 1) a “Moxon” vise, and 2) all the tools that book binders use that are wood, many of which incorporate screw mechanisms.
Wooden Screws have a very long history. A Greek, living in Ptolemaic Egypt, by the name of Hero or Heron, depending upon who wrote your modern reference, recorded many ingenious tools and devices that were known at the time. Much like Fabien, Roubo, Diderot, and Moxon many centuries later. Hero’s drawing of a wood screw cutting device is the oldest known illustration of any screw cutting tool.
So now that I can make a decent screw for the uprights of the stitching frame, or the screw in the plow, I need a couple of bigger screw sizes for the various presses. One size for the lying press, backing press, plow press, etc. And a bit bigger one for the book press. And maybe I want a “biggest” one for a wood bench vise.
I also want an even BIGGER big screw to build a “tensioning bench”. Which is a device for bending the sinew backed crossbow prods so that they may be strung or unstrung. Apparently they would remove the strings between uses (or battles) in order to maintain their draw strength. The screws on the tensioning benches were 3″ or 4″ in diameter. When you get this large and bigger (cider presses, Olive presses) my reading shows that both the screw and the nut can be carved by hand, custom sized to suit the material available.
To Recap: I now have a working 1″ x 6 tpi setup. I want a 1.5″ or 1.75″ x 2 or 3 tpi (maybe 4) for the presses. A 2.5″ x 2 or 2.5 tpi for a bench vise.
@ 25 years ago I made a 2.5″ x 2.5 tpi setup that worked ok, then I lent it to someone and never saw it again. I had only ever made one screw and nut. Sad face.
Lets make some screws!!
I’ll be making and showing a couple of different ways.
I’m thinking that if I had thought more to start with I would make the midsized ones first. Then the bench vise sized one. And make the tensioner last, or maybe first…
But I already started the 2.5″ bench vise sized one so we’ll go with that first! (and I’ve done it once before so I’ve got some confidence going for me there)
When making this sort of home made setup, most people (look up wood threading on you tube) make the tap first. The tap makes a “nut”. Then you incorporate one of your first nuts into your screw box (die).
Tooth geometry is usually 60 degrees for small screw and 90 degrees for larger screws. Really you can use many tooth shapes. The nut and the thread just need to match.
For this 2.5″diameter screw I’ll be using a 90 degree tooth.
It will help us to draw this with some accuracy, you can use a CAD or a drawing board.
Start with either your minimum or maximum diameter. And draw 2 parallel lines just that far apart.
Then draw the teeth along the “top” line (outside/on top if your lines are your minimum diameter, inside /under if you start with your major diameter). Then on the bottom line offset the drawing of the teeth by half a tooth. Now connect the upper points with the lower points and the upper valleys with the lower ones with lines that slope down to the right ( unless you want left hand threads). Make sure that you do not connect the tips on the top with the valleys below.
You will notice that the angle of the lines are NOT parallel! Which may seem strange, but it’s okay and correct.
Just like you have a major and a minor diameter, you now have a major and a minor pitch angle
You need the angle of the lines that connect the base of the teeth. AKA the minor pitch angle.
Now we are nearly ready.
Make or buy a dowel that is sized the same as your minor diameter.
Find some material that can flex around this dowel but will not mash by repeatedly running a pencil against it. But also thick enough to run a pencil along it.
You can mark the minor pitch angle along one edge then cut it. Wrap it squarely around the stock and tape it to itself. Or layout the pitch angle on the material and just wrap it around your stock and skew it until the edge describes the correct angle, the again tape it to itself
Now you can draw the pitch line around the stock (once), move the drawing guide along one tooth length and draw again and again etc. You need only a little more than the thickness of what will be your nut or your vise jaw. You also need an unmarked length of 4″ to 6″.
Now some tricky sawing… You are going to saw on this line, but only @ 1/4″ deep. 3/8″ is too deep, 3/16″ is not enough, but DO NOT DO IT ALL IN ONE GO. Remember your drawing? Remember how the root pitch angle (minor pitch angle) is steeper than the major pitch angle? The pitch angle gets steeper the deeper you go, AND it’s not a straight line anywhere! It twists. SO start by barely getting the teeth of the saw into the wood, have the stock arranged so that it can be rotated as you saw. Run your saw like this, barely half the depth of the saw teeth, the full length of your drawn line. Start over this time full depth of the teeth. Pause, put a piece of tape on your saw, or mark your saw with a marker to tell you how deep to stop at. Then proceed slowly like this until you reach your mark.
The bottom of the slot you have made is at a steeper angle than the top. It twists! If you were to saw to full depth in one spot, then move the dowel, and try to saw the next little bit to full depth, the saw will try to lay over at an angle to match the previous cut. By doing this a little (depth) at a time your saw kerf ends up wider, but you will have a perpendicular kerf at every point,
Now we need a hole through the shaft to hold the “cutter”. Near where the saw slot stops and the uncut dowel begins. We need to mark the position of the cutter. The width of the cutter wants to be perpendicular to the slot. The centerline of the front of the cutter wants to be right on the diameter of the dowel.
So pick the location of the cutter, draw a line perpendicular to the slot. The full width between one rotation of the slot and the next.
Here’s a cool trick! If you take a piece/length of angle iron and place it’s 2 edges are against the side of a cylinder the angle iron will run nice and straight along it!
So if we get a length of 1/2″x 1/2″ Aluminum angle we can make a line from the center of our line that is perpendicular to the kerf to the end of the dowel that is good and straight. Then we can use a centerfinder to mark that diameter that is also the centerline of the cutter.
Mark back from the diameter line a line showing the thickness of your cutter. Then mark back from that a last line at a bit of an angle for the wedge that holds the cutter in place.
Some terms: “screw pitch” is the angle that the screw spirals along the shaft. tpi= teeth per inch. Tooth geometry= the cross sectional shape of a tooth. Minimum diameter is the diameter at the base of the teeth, or the diameter of the shaft if you took the teeth off. Maximum diameter is the diameter at the points of the teeth