Marking
I am marking both halfes (on top of top half and bottom of bottom half) to know where to start and end routing. I will be routing the mouth hole, the SAC and the bore.
I am using the following marks/sizes north to south (see picture):
60mm mouth hole –> first mark A (start of the SAC)
+100mm SAC –> second mark B
+40mm wall size –> third mark C
> 650mm bore (I route it open on the right side of the blank)
–> Overall length > 850mm
I add two more marks:
The first additional mark is B+, 15mm right of mark B which is where I stop routing the SAC which is half the bore diameter. Let’s call this mark B+.
The second additional mark is C- which is 15mm left of mark C (again half bore diameter). This is where I start to route the bore.
The end of the bore is not marked because I route it open.
Router head and Centering
Now it’s time for the serious stuff 😉
I am targeting a flute in C4m. Following most recommendations, it will need a bore size of 1 1/4 inch or 31.75mm. For me it was easy to get a 30mm diameter round router bit (see tools section) and this is what I am always using for my Cm4 flutes to make them comparable.
First you have to center the router guide so that the head is EXACTLY in the center or our half blank, which is 42mm in width. So the router center should be as close as possible to 21mm. In my experience this will never work 100% so I am using a trick later on to make sure the wall size is at least the same on each side of the blank in order to have a perfect circular bore.
Routing
I will be routing to a depth of 15mm in order to have a perfectly round bore with a diameter of 30mm after putting the two blanks back together (see picture).
If you would route to this depth in one step, the likelihood to get in trouble is extremely high. This is why I route several rounds with increasing depth. The more the better but I am using an increase of 2.5mm which means that I route 5 times to reach my target depth of 15mm.
Step 1
I start by lowering the half blank on the router head starting with the left side of the head beeing flush with my first mark A- and move it slowly to the left until the right side of the router head is at mark B+. You can simply put some marks at A- and B+ on the guide behind the router head that tell you where the router head starts and ends (see picture). This creates the SAC.
Step 2
The next step is to route the bore. I start with the left side of the router head at mark C- and move it left until the end of the blank. This will create the open bore that I will later be used to center the flute on my lathe.
Repeat Step 1 and Step 2 after increasing the routing depth by 2.5mm until you reach a depth of 15mm (5 times). I use a digital depth meter (see tools page) to measure the router head height to increase it very precisely.
After 5 rounds I have a “perfectly” smooth SAC and bore.
The other half blank:
Just follow the above instructions (step 1 / 2 / repeat at different depth) but with a twist to make the walls perfectly align: I do everything reversed with the other half of the blank. I turn the second blank around by 180° to make sure it has the same wall thickness on the same side as the other half. Because I have to move the half blank to the left on the router, I start right of the router head and move it to the left to route the bore first until the right side of the head reaches mark C-. Then I lift it up and lower it with the left side of the head at mark B+ to move it left until I reach A- (this is the end of the SAC).
The Mouth Hole
I finally change the router head to a 8mm diameter round head to route the mouth hole. I do this in one step because 4mm height (half the diameter of the mouth hole) works pretty well. I start putting the blank on the table right to the router head and move the blank to the left until I feel that I have reached the SAC (no resistance anymore when reaching it).
The same trick regarding wall alignment applies here. Simply start lowering the other half with the empty SAC area over the router head and move it left until the mouth hole is routed.
Another way to make the walls identical in width could be to route everything in both directions (turning both blank-halfs 180° during the procedure) but I have made really bad experiences re-routing an already hollow SAC/bore because the router moves relatively freely and only catches sometimes which can lead to really bad routing marks and worst case a destroyed flute. Another drawback of this approach is that the SAC/bore will not end up 100% round in case you have a slightly decentered router head. With the described way the SAC/bore might be slightly off-center but it is a perfect circle. Because I will use the mouth hole and the bore end to live-center the flute later on my lathe, the SAC/bore will end up perfectly centered in the flute.
I hope that explanation made sense ….
