Here is the methodI would use.
1. First draw sharp lines, even use a knife to define all cuts both
shoulder and cheek etc so you end up with a shoulder line around 4
sides of the piece, cheek lines on two opossing faces and lines across
the end grain showing locations of the end of the tennons.
2. On my router table I would install the smallest straight bit I
have. Then using a fence I would adjust for maybe 1/4" deep cuts
aligned to the correct faces of the cut lines setting up a stop block
so all cuts stop at 3 1/2 exactly at the shoulder mark. Do slots on
two opposing faces along the edges of the tennons.
3. With slots now cut on two faces defining the edges of the tennons,
use a bandsaw (if you are good with a bandsaw and can trust your
setup), or use a hand saw (I would do this) and saw along the correct
face of the slots to the shoulder. Take your time and think about
4. Hand cut the shoulder cuts on the two outside pieces. You could
router these first on the table if the 4x4 isn't two long or use a an
edge guide and free hand it. Use a backer to avoid blowout at the
5. Drill out, with a single appropriatly sized drill or fostner bit,
the inside drop pieces drilling about 1/8" bach from the shoulder
line. Go slow and with light pressure to avoid drift. Then clean up
those two interior shoulders with a chisel.
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