To support Peter’s suggestion and with no malice directed to Matt, if you decide to remove the corner/arris of the beams, BRR states a maximum radius of 2mm. BRR 6.2 The main and rear beam shall be; a rectangular hollow section 50 mm plus or minus 1 mm by 39 mm plus or minus 1 mm by minimum 3 mm wall. Beam edges may be rounded to a maximum of 2 mm radius. From the Building Instructions: A 2mm radius arris can be removed from the cross beams. This is a restricted measurement! Make a radius guide by drilling a 4mm hole through a piece of sheetmetal. Remove the corner along the tangent lines. See Diag. 8.2. Remove the beam arris. Plane off the bulk with a woodwork hand plane, a No.4 or No.5 is ideal. Draw file with a mill 2nd cut file to final shape. Smooth with emery cloth.
I have heard of using a router with a radius cutter. This would also work but ensure the correct size cutter is selected and it is tungsten tipped as a HSS will dull before completion. Removing too much material from the corners will compromise the strength of the beam and it is a fact that mozzie beams are notoriously under engineered.
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Tortured ply is clearly beautiful.
Mozzie Aldebaran VI
1827