Threads in Wood

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

The luthier's clamp (Figure 8) is just one of several screw clamps of unusual shape used in making string instruments. Up to a dozen of these particular clamps would be used when gluing up the body of a cello or a bass viol.

Most carpenters' benches of the eighteenth and nineteenth centuries were equipped with vises operated by a wooden screw (Figure 9). In addition, there were small supplementary vises (Figure 10) for mounting in the main bench vise. These were used for wood carving or for model making.

Traditional bookbinding included three tools that have threads in wood. On the stitching frame (Figure 11 ), sewing cords are tied from the cross bar to pins under the table, and then are stretched by turning the threaded nuts under the cross bar. Then each folded sheet of the book is sewn to the stretched cords. (A very similar application of screw threads was used in the stretchers for tambour and tapestry frames, except that the stretching was applied to both directions of the canvas yarns.)

After the spine of the hook is completely sewn, it is shaped and glued hefore cutting the pages to a consistent edge. The book is clamped in a laying press (Figure 12), and each of the three open edges is trimmed with a bookbinder's plow (Figure 13). One arm of the plow runs in the laying press guide. The two-sided blade cuts a few pages with each back and forth movement of the plow in the guide. After each pass, the blade is advanced slightly by turning the screw, ready to cut a few more pages.

Note the decorative finials on both the stitching frame and the plow. These tools were made about 1830 by Hampson & Bettridge of London. The laying press has a somewhat plainer design and was made about 1850 by W.O. Hickok of Harrisburg, Pennsylvania. This latter firm is still in business.

There are two tools used in making textiles by hand that include wooden screws. The spinning wheel of Figure 14 is much smaller than usual, but it has a typical tensioning mechanism. At the left end of the slanting table is the knob of the tension screw. It adjusts the position of the flyer/bobbin assembly relative to the drive wheel. Proper tension on the drive cord is necessary for good spinning. Figure 15 is a tension screw from a spinning wheel of more conventional sixe. It is included to show the standard shape of this part.

The click reel (Figure 16) is a measuring tool, used to wind amounts of yarn after spinning. This reel has a circumference of two yards and a forty-to-one worm gear reducer such that a click is heard after forty turns of the reel. This quantity of yarn-eighty yards-some-times is called a "knot." In the click reel, the wooden screw acts as a continuous wedge turning the worm gear. It is one of the most unusual applications of threads in wood.

Most wooden screws have a sixty-degree thread angle. This is a very good thread angle for most applications in tools. It also is suitable for the worm in a click reel. But if a worm gear is to transmit mechanical power, it should be a metal worm having a thread angle of about thirty degrees.