I'm sure none of you have slept well wondering how Paul will turn round stock into square...so I thought I should give you a brief update.
But first, I want to share my appreciation for the many good thoughts and suggestions (start with square stock / buy a mill / etc.). I will keep those excellent ideas in mind for a future build. Thanks, especially, to Dean who took the time to post his excellent tutorial.
I gave your technique a whirl Dean and it worked pretty well. The aspect that ultimately caused me to keep searching is that my mast blanks are hardwood and I wasn't getting a very prominent flat to use as a guide for the file work.
I ended up with a simple solution that I'll share in case anyone finds themselves in a similar spot.
You have seen my rough turned masts:
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I began by sanding one surface flat on a disc sander (shy of the final goal):
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I then turned the rod so the flat part was sitting on the guide table and sanded the next surface (perfect 90 degrees with no fuss):
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I then repeated those steps on the remaining surfaces and then snuck up on the final position of the square (I wanted the flat area to run perfectly into the round area at the apex of the curve):
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Confession: several times I overshot my goal and needed to take the part back to the lathe for refining.
Anyway, not exactly rocket science, but this worked nicely in my hands (and with the tools I have on hand).
One more thing. I mentioned that I adopted Kurt's (aka Mr. Destructo - if you follow his build log you know what I mean) mast tapering technique. Basically he uses opposing sanding blocks to keep the piece from breaking as the lathe spins. Just for fun - here is a proof of concept photo:
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The shorter piece is 120 mm of 4 mm stock tapered to 1.7 mm (still need to shape a ball on the top). The longer piece is nearly 200 mm long and tapers to 2.0 mm. Pretty cool.
As I mentioned earlier I am finding tapering these masts to be fantastically satisfying. Maybe there won't be a future ship build - I feel some woodturning in my future!
Thanks for stopping by! As always, I am humbled by your interest in my work.