Saw an episode of "The Woodwright's Shop" a couple weeks ago in which Roy Underhill carved a twist in some table legs. Some tricks he used:
1. To lay out the spiral: Wrap a piece of paper tightly around the stick the length of the area to be carved, and trim the edges so it just fits around the shaft. Remove the paper and divide the length into (the number of twists wanted x the number of twist elements). Divide the width into the number of twist elements you want (ie, a 2 rope twist, 3 rope twist, etc).
Example: A two rope twist on 1 1/4" diameter that twists twice around it in ten inches of length. The paper will be 3.93" wide (pi x d) and 10 inches long. 2 twists x 2 ropes = 4, so divide 10 by 4 and the horizontal lines will be spaced 2.5 inches apart. The vertical line is the width divided by 2 ropes so it is in the center (1.965 " from edge or fold the paper in half to find center).
For a clockwise twist draw straight lines from upper right to lower left, ccw is opposite: Draw a straight line from the top edge at the center line diagonal to the edge of the paper where the first horizontal line meets it. Draw a second line from the corner to the 2nd (middle) horizontal line. Continue with parallel diagonals down the page.
When re-wrapped around the shaft and firmly taped, the diagonal lines are the layout lines for the grooves (They will form a spiral). Mark them by poking through with a sharp awl every 1/4" or so. Rotate the paper on the shaft 90 degrees (in this case with 2 ropes) and use a chalk bag or poke a pencil though the holes to mark the tops of the ropes. Remove the paper and connect the dots. (If using a pencil, you may want to use different colors for the top and groove)
2. When carving out the grooves (awl marked line), work down the grain and cut from the top (chalked line) down into the groove from each side (Same as you would turn a bead on a lathe?). This will prevent tearout from going against the grain.
Hope this makes sense. It sure looked simpler seeing it on TV!! It can be easily adapted to a single groove (eliminate the verticle line and just go diagonal edge to edge) or to multiples. It does eliminate trial and error for spacing and getting the desired number of twists in a given length.
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