noonenparticular wrote:
You might do better using a tablesaw. Screw the plywood strip so it
overhangs
the opposite edge, and bear against the TS fence with it. This way, the
impact
of small variations in the plywood will be reduced by the long fence.

I thought about this and it should work in theory. Here's the problem I ran
into in practice.
First, the fence would have to be over 3 1/2" high for the plywood to bear
against it. No problem, fasten a tall subfence to it, right? Well yes, but
now you have the problem of the mass of the timber pressing sideways against
the top of the subfence which is fastened to a short TS fence. The leverage
of all that mass will make the subfence flex away from the blade, I'm guess
substantially. Secondly, I only have a 10" blade for the table saw which
has a max depth of cut of 3 1/4" while the boards are a *strong* 3 1/2. So
the blade wouldn't even make a through cut, which would concern me from a
safety issue with all that mass against a comparatively flimsy fence.

I think you should be able to use the outside edge of the board against
the fence, if it's reasonably straight. It only needs to be straight
enough to keep the saw blade from binding -- since you're cutting both
boards at once, they'll match up.

Similarly, if you make half-through cuts on both sides, it doesn't
matter if they don't match each other perfectly, because the two halves
will still match.

Nonetheless, it still seems like a lot of weight and a pretty deep saw
cut, and I'd be a little concerned that holding up that much weight and
keeping it straight against the fence would be like trying to walk and
chew gum at the same time, and I'd end up not holding it quite straight
and cause the saw to bind. One could avoid that by using a handheld
circular saw (or possibly a router and making lots of passes) against a
plywood straightedge instead, I suppose.

....

One possible variant of this idea (specifically, to cut both sides with
the same guide at the same time, so that they match regardless of
whether the guide is perfectly straight) would be to mount the plywood
guide so that your router will just shave a tiny bit off one board, and
then clamp the other board a little less than the diameter of the router
bit away, so that the router also just shaves a tiny bit off it as well.

If you don't want to cut both at once (which would mean that one's
getting cut the "wrong" direction), and you have two router bits of
different diameters that would both work for cutting the faces, you can
mount the plywood fence on the first board such that the router with the
larger cutter will just shave it, and then position the second board so
that the router with the smaller cutter will just shave it. Then, take
a pass with the smaller cutter to surface the second board, remove the
second board, and take a pass with the larger cutter to surface the
first board. Or something like that -- if you're following the plywood
guide with a roller bearing rather than the router base, then you could
use different sizes of rollers for the two passes.

- Brooks


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