Converting long square stock into a round cross-section does not need complicated woodworking skills. The square section is transformed into a hexagon, then to a 16 sided polygon and finally finished to a perfect round.
Either saw or plane the stock to a square section with sides equal to the diameter of the final round required. If a tapered mast is required produce a tapered square of the appropriate cross-section.
Mark out each face with lines to indicate the hexagon. This is either done with a marking gauge if the mast is parallel or using a baton if it is to be tapered. These lines should be 0.3 times the diameter of the finished mast from each edge. Also mark out lines on the faces to show one edge of the 16 sided polygon, these lines are 0.4 times the mast’s finished diameter from each edge. ( diag. 1). The hexagon cross section can now be planed.
The lines marking out the other edge of the 16 sided polygon are marked out along each finished hexagon face. (diag 1.) The width of these polygon faces is 0.2 times the diameter of the mast. The cross-section can now be planed to create the 16 sided polygon. The final ridges are removed by planing or sanding to produce the finished round section.
Sometimes it may be necessary to make the spar or mast it two parts. This could be to ease transportation or to facilitate manufacture in a small workshop. If this is the case the two parts need to be sleeved together with a metallic tube. The outside diameter of the tube (D) should equal the spar diameter where the join is to occur and the inside diameter (d) has to match a reduced round cross section. This ensures a smooth flush joint.
To produce a concentric round to fit inside the tube, mark out a line round the mast half the tube’s length from the end to be joined. This portion is divided around the circumference into eight chords. (diag.2). Tangents of length equal to the square root of D2-d2, are marked out from the eight lines which when sawn across the spar between these lines produce the exact depth to the new smaller inside diameter of the tube. This flat can be produced using a chisel, multiple saw cuts along the length will ease the task.
Once these eight flats have been created the sixteen sided polygon can be marked out (dig 3), and using the same technique as above, sixteen flats can be created.
The resulting polygonal cross section can now be dressed to create the final concentric round. The tube is now offered up and final adjustments are made to the mast to create a snug fit. The process is repeated on the other half of the mast, the tube can be glued to one half and various methods can be used to secure the other half when the mast is in use.
This cross-section of the square stock gives all the relevant dimensions for marking out to produce a hexagon and a 16 sided polygon encompassing the required mast diameter.
To produce a smaller concentric round section, the dimensions are shown for producing the hexagon, note each longitudinal hexagon line does not abut its neighbour.
This shows the length and position of the 16 sided polynomial.