I am working on a project that requires that a through-hole be drilled lengthwise through a dowel. (The resulting tube will be used as a cam collar for a wooden camshaft I'm making.)
I had to make several of these, and the techniques that I found online didn't work well for me. Here's my Instructable that shows what did work for me, in three easy steps.
Step 1: Build a Jig
Take a piece of scrap board, and a few inches from one end drill a hole the size of your dowel. (3/4" in my case.) Use a drill press if available, to ensure the hole is drilled perpendicular to the board.
Saw a slot about 1/4" wide from one end of the board up to the hole. (I used a table saw to do so.)
Step 2: Insert Dowel in Jig
Mark the center-point of your dowel. (There are some good Instructables that show how to find the center-point of a dowel.) Use a punch to indent that center-point; this'll help guide the drill bit in the next step.
Insert the dowel into the jig. It will be a bit loose, so tap in some shims to tighten up the fit.
Step 3: Drill a Through-hole
Put the jig on top of a piece of scrap wood and clamp it to your drill press table, lining up the dowel's centerpoint with your bit. I'm using a 25/64ths bit, because I want the opening in the dowel to accept a 3/8" dowel snugly but loose enough for the dowel to rotate.
Now use another clamp to squeeze the jig's "jaws" -- the arms on either side of the slot. This serves to grip the dowel hard in the jig, so that it doesn't spin once you start drilling.
Drill the hole. The jig holds the dowel at 90 degrees, and keeps it from spinning.
You'll end up with a nicely centered hole through the length of the dowel. This works well for bushings, stop collars, and cam collars for animated or moving toys, and any other instance where you want a precise wooden "tube."