Wooden Screws Part II: As part of my continued exploration of Leonardo Da Vinci mechanisms, I needed to produce several different kinds of lead screws and nuts for my projects. While commercial wood taps and dies are relatively inexpensive, I wanted to make my own, so I could get the right diameter and pitch. During my efforts I tread making several different kinds of taps, but wasn’t too successful making the old-fashioned screw boxes. Information on doing this can be found in Roy Underhill’s books on woodworking.
There is another technique for making wooden screws on a wood lathe called "chasing threads" but this takes special equipment and years of experience. Eventually, I built a couple of "screw duplicators" using a table-mounted router and salvaged lead screws, and these worked very well.
I used the lead screw borrowed from my bench vise, which had about 4.5 threads per inch, for screws made out of 1” poplar dowel. I used scraps of oak, mahogany, and some plywood to build a mechanism that can both feed and rotate the blank through the cutter. A drill press helps make sure the holes drilled are at right angles to the bearing blocks. A small 1/16” hole drilled through the end block and a short length of brass wire are used to pin the end block to the lead screw.
The whole jig was screwed onto the router table and the height of the router was adjusted for the proper depth of cut.
I used an inexpensive countersink bit in my router and this seemed to give good results. I am sure a carbide "V" cutter would work as well or better. This rig works remarkably well. I’ve built a second similar rig for producing 0.5 inch screws up to 1 foot in length.
The method for making taps is explained in detail in this reference(1). An approximate rule of thumb is that you need a piece of square stock 75% of the diameter of the screw you’re cutting the nut for. So for a 1” screw, you need a ¾” square steel rod. These taps work well up to a 1” diameter.
Briefly, a triangular strip of paper is used to transfer the thread to the stock. This is glued on to the square rod, and then a triangular file is used to notch the work piece. Then file away. It takes about two hours of filing to make a tap.
For nuts over 1” a tap box is needed. A tap box uses an interna
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