I see someone has done a bit of construction This does indeed work remarkably well and is the way I'd go as well given the material and use.
Somewhere not to far behind would be using a router to shave the material off. Just behind that would be to just use a large chisel to hog the material off.
+1 on either RAS with dado or circular saw and chisel.
Just to throw another tool into the mix, my track saw would make nice straight cuts 2 inches deep on each face. One of the many challenges of working with 4x4 PT is that it is often not square, however. So cutting from two sides will not necessarily result in a flat surface. Since a circular saw wouldn't do the whole cut, in any event, I would finish with a hand ripping saw or a jigsaw. If the cut quality is not an issue, my Bosch can and has done ripping duty on PT wood this thick before. Surface would be a bit rough, about like using a hand ripping saw, but for many things it would be good enough and it could be cleaned up. Jig saw could do both cuts, long one and short one, just fine.
Why don't you just cut the shoulders with a speed square and a circular saw set for the depth and finish up with a flat bottomed straight bit in router. Start at the end and work up to the cut.
Dan
I just realized that I never followed up on this. I actually went a really easy route. Set up a 2x4 on each side of the 4x4 with the router bit set to the desired depth. Then used a piece of plexiglass as the guide so it would extend out over both of the 2x4s.
Worked out pretty good.
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Never caught this the first time. I'm amazed nobody mentioned the bandsaw, its sort of built for this sort of thing. Minimal dust, very fast, very accurate, even a 14" saw can make the shoulder cuts with a quick adjustable stand or saw horse out rigger to hold the weight, then push them in up to the shoulders....I'm guessing less than 2 minutes per stick all in? I have done something similar for a neighbor doing some fence repairs. He was breaking his but with a sawzall trying to trim 8' posts. Too painful to watch, . Looks like you got it done, I for one would not like to route all that PT lumber. Nice thing about the bandsaw, set the fence once....all four sides...as many posts as you want...done.
"A good miter set up is like yoga pants: it makes everyone's butts look good." Prashun Patel
Looks like you needed a "Tenon", instead of a "Taper".
Never, under any circumstances, consume a laxative and sleeping pill, on the same night
There's nothing wrong with doing it that way, but by resting half of the router on the factory face and starting at the end, you could just remove a bit at a time and work back to the 11" mark. Always referencing off the factory dimensions, you will be reducing it by the same proportional amount per side. No need for the giant base or rails, just keep some weight on the back side of the router. Matt nailed it in the first post, too...
Dan
My band saw tracks straight. I would start with that and finish with my #7 hand plane.