The Wooden Vise Screws
The wooden screws were as much a journey of discovery into the techniques of using wood to make machine parts as anything else. I wanted to make them using the same technique that was used in France in the eighteenth century. This turned out to be quite a challenge. Finding out how to make the female threads in the legs was actually rather easy. The construction of the "fales nut" that can pull or push a tap to cut the threads, and the construction of that tap are pretty easy to find. There must be half a dozen or more YouTube videos and Pinterest sites that cover it. But, for the screw itself, everything I could find before I started assumed using some sort of jig for a router, and obviously that was not how things were done three hundred years ago.
Now, it turns out that in one of Roy Underhill's books, he does briefly cover it, but I did not find that until long after I had made my three screws. Also, there is an intriguing plate in Roubo's book that shows a long chisel that is almost paring chisel or medium lathe gouge in length. So, my assumption was that somehow, they started by making a spiral cut the exact same way one makes the tap for the female threads, and then use a hand chisel to cut the taper.
Ernie Conover used to have a video up on YouTube on how to make the Barley Twist which provided the needed clue. So I made a big wooden handwheel as Ernie described. He called it a capstan. This enabled me to use my midi-lathe as a hand turning support, which is probably the way the 18th century craftsmen used their hand powered lathes (spring pole?). Whatever, for the barley twist, Ernie slowly turned the workpiece with his left hand while guiding a razor sharp chisel with his right. In this way he was able to carefully enlarge the spiral saw kerf to the barley twist shape. It was using the lathe a bit like a potter's wheel to make handwork perfectly round.
Furniture with barley twist details, such as legs and corner trim goes back at least into the 16th century, as it is common to Tudor style furniture, and may be a great deal older than that. So using a lathe this way is probably a quite ancient technique, so it seemed to me to be reasonable to conclude that wooden screws were made using a minor variation of this technique. After the fact, I discovered that Roy walks you through doing exactly the same thing in his book.
In my research on wooden screws, I noted that many suffer from chipped threads. I bought one on eBay that was salvaged out of an old bench so I could take a close look at it. Close inspection made two problems obvious. One is that if the threads are too sharp, work resting on them in the vise can cause damage, as can things being shoved around on the shelf under the bench. Also, every screw with damaged threads that I could find had an acute thread angle such that the pressure of the screw in use was on a line that would press right through the springwood (light softer growth) from one thread face to the other. Threads cut that way had to split just using them. A fellow named Curt Carter (Carter's Whittling) has a really nice discussion of this in his YouTube video on making wooden screws. He's right on with his analysis about threads, except of course he uses a router to make his screws. Below you can see pictures of the tools I made to produce my three wooden screws. Note that the one wooden block with its hole is merely a sample of the kind of gauge one needs when turning a large diameter dowel on the lathe to a perfectly consistent diameter along its length, when you are restricting yourself to techniques that were available to woodturners before the emergence of cast iron late beds with electric motors, and lead screws to guild a cutting head.
I will offer one opinion on the two options for making a tap for the female threads. A push tap uses a shorter shaft and applies a pushing force into the work by the fales nut by passing through the nut first, and then going into the pilot hole with its cutter. To make this work. the fales nut has to be securely clamped in place, and must not slip. One huge problem with this approach is that every time you want to remove it to check your progress, the clamps have to be pulled, and then you have to carefully get everything back into the exact same position. And, you have to do this on every second or third pass in order to adjust the depth of the cutting tool, and clean out the waste. This is just an awful waste of time in a process that is already super time consuming.
So, it is my opinion that making the tap's shaft twice as long, and mounting the cutter in its little mortise hole half the length of the shaft is a vastly superior way to go about this. This allows the fales nut to be clamped on the back side of the hole being tapped, and thus pulls the cutter toward itself. This way, the fales nut can stay clamped in place for the entire process. But, people do it both ways.
Oh, and one other thing, use at least 16 gauge steel for your fales nut, or laminate two or three pieces of a lighter gauge steel together. I used two pieces of 16 gauge, and it was just barely enough to keep it from flexing.
