Friday, July 8, 2011

Semroc IRIS Build Part 4 Fin Taper

This leading edge will be tapered down to almost a knife edge.

With balsa being a light color, it's hard to see how thick the leading edge is.
This idea was taken from how a luthier levels worn guitar frets. The frets are blackened with a permanent marker. When the top of the fret is filed down, the high spots are sanded off, the silver fret shows. The low worn areas remain black.

By blackening just the leading edge, you can clearly see how thick the knife edge is when sanding.

Don't go too slow when marking the balsa. You want just a light pass with the marker.
In the end, you'll want to sand off all of the black. Use too much ink and you will have to sand the leading edge too far, changing the shape of the fin.

I've sanded the taper using 220 grit on a block.
You can clearly see the thin black line left down the leading edge.

Look close, if you see a difference in the width of the remaining black line, sand that area to match the line width from end to end.

Before getting to the final width, switch over to 400 grit.

Here's the fin held up to a strong light. It's easy to see the taper with a back light. A little of the black line remains.


  1. Again, brilliant! I could never get the edge straight on a tapered fin. Now I know how! Thanks Chris.

  2. Hi Lonnie,
    This certainly isn't my idea, just an adaptation of what is done on a guitar fret re-crowning. It's not the only way to get a taper on a fin, this time I just felt like going this route. Thanks for the feedback!