The bobbin is about 8" long with 1/2" shoulders of 2.1/2" diameter, the body being 1.3/4" diameter. Four holes are drilled in each end. These need to be accurately drilled so that they are evenly distanced from each other and the centre, (draw a circle with a compass and divide it into 4). They do not have to go all the way through, only through the shoulder with enough clearance to drill holes in from the side to meet up with them. Failure to do this accurately on the Mk I bodger's muddle created a small amount of vibration in use. The diameter of the bobbin body can be a matter of experiment. If you use a smaller diameter you will get less stroke per twist thus raising the rate that the "point of twist" travels to the extremities of the bows. The same effect can be achieved by using a thicker cord on the bows. Using the present dimensions with 6mm sash-cord instead of 3mm, gave me a useable travel of about a foot! The current dimensions give me all the travel I can use on my longest treadle.
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