All the descriptions I’ve ever read on these needed a good
picture, so I thought since I made another one recently, I’d
take a pic.
These mark square or rectangular stock into 8 sides, dividing
each face of the stock into three equal sections… the first step
in planing a cylinder from a square.
The nice feature of this
design is that it accommodates tapered stock, so is perfect for
spars and the oars I am making here.
Cut a block the diameter of the largest cross section of your
finished spar or loom…draw a circle in it so the circle
intersects the edges. Then use a 45 degree square to make an
octagon of it…keeping outside of your pencil lines.
Another block an inch wide and a couple inches longer than the
first block is laid atop…both the edges of the circle and the
corners of the octagon are transferred to the second block,
holes drilled for nails and pencil… and you have your gage.
Offset the holes for the guide nails to insure they are outboard
of the circle.
Then merely keep the guide nails in contact with the edges of
your stock, and you can mark any tapered piece into an octagon
for planing. In traditional construction, spars and looms are
thought to look best if the faces of the octagon have a ratio of
5-7-5 instead of 1-1-1, and many go to a lot of trouble making
gages that mark those offset lines. My approach is simpler…I
mark a perfect octagon and then plane either outside or inside
of my marked lines to get the look I want: