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When I ripped the boards I took 1/4 inch off of one side, flipped the
board then ripped it to width. The blade is a WWII and the fence and
blade were just trued up. The 1x8s were pine (S4S) and reasonably
straight and unbowed. I can't figure out what in my setup or technique
would cause this "hourglassing".

To do this correctly you need to joint one edge, then rip the opposite edge.

Anyone have an idea why this is happening?

If you did the above and were still having a problem, I would say check the
face of the fence first, although it could be a combination of factors.

But, IMO, you are in effect chasing your tail by not having a known
reference edge to start with.

As far as the router issue goes, concentricity of the router bit with the
base is the likely culprit. Best to do this with a router mounted on a table
with a split fence so that the outfeed fence can be adjusted for the amount
of cut you take. This also takes possible lack of concentricity out of the
equation.

Suggestion: take one of your S4S boards and lay it on the same flat surface.
Note the amount of/lack of bow (your hourglass). Then using that edge
against your fence, rip and lay the freshly cut edge against the same
surface.

If the "reference" edge was flat and the second cut has the "hourglass", you
most likely have an alignment problem with the fence/saw/blade, or a
combination thereof.

If it's the same, you now have an excellent excuse to get yourself a
jointer, or a jointer plane.

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Last update: 12/13/05