Threads in Wood

Chronicle of the Early American Industries Association, Inc., The, Jun 2003 by Packham, Jim

Holtzapffel's Construction, Action and Application of Cutting Tools, Volume II, contains a very thorough and detailed description of the design, construction and use of the screw box and tap for making threads in wood.10 These rather specialized tools probably were the main method for making wooden screws during the eighteenth and nineteenth centuries. Figure 31 shows the two main styles of tap for making internal threads in wood.

Figure 32 shows the parts of a typical screw box, as used for making external threads in wood. More detail on the design and construction of these tools can be found in the author's article, "The Screw Box and Tap," in the March 1987 issue of The Chronicle.11

Some screw box and tap sets were handmade by their users, but edge-tool manufacturers in Britain and continental Europe were offering sets of boxes and taps by the first quarter of the nineteenth century. A typical edge-tool catalog would include fifteen to twenty sizes, usually ranging from 3/8 to 3 inches in nominal thread diameter.

Although many edge-tool firms offered screw box and tap sets for sale, there is considerable evidence that only a few firms actually made these tools. The other firms arranged to have their brand name imprinted.

There is no record of any toolmaker in North America making screw boxes and taps in the eighteenth or nineteenth centuries. All were imported from overseas. A few sizes were made for hobbyists during the last quarter of the twentieth century by Conover Woodcraft of Parkman, Ohio.

Figures 33 a and b contrast the smallest and largest box and tap sets known to the author. The little set is for 1/4-inch threads and the large set is for 3 1/4-inch threads.

The box of the 1/4-inch size is 3 inches long, while the 3 1/4-inch size box is 18 inches long (including handles). The 1/4-inch tap weighs just two-thirds of an ounce, while the 3 1/4-inch tap weighs eight and three-quarters pounds-it is about two hundred times as heavy! The small tap can be turned easily with one hand, while the large tap requires two men using a steel bar about six feet long.

Despite the commonplace use of screw boxes and taps for over two hundred years, they also had some problems. The first problem came from the absence of size standards. Kven when a tap and box set was marked, say as 7/8-inch diameter, it could be as much as plus or minus 1/16 inch oft from its nominal size. In addition, there was considerable variation in thread pitch from one maker to another. For example, 5/8 inch diameter screw boxes exist with at least six different thread pitches from 5 1/2 to 9 threads per inch. One suspects that makers of screw boxes and taps were guided almost entirely by trial and error, plus personal preferences. The variations in sizes meant that if either of a matching box and tap were lost, it was almost impossible to find a suitable replacement. In turn, it is very difficult for collectors of antique tools to find a tap and box of the right size to repair say, an old plow plane.