Introduction
Remember the markings from the “Routing” chapter? We had the marks A (end of mouth hole), B (end of SAC), C (start of bore).
The SAC exit will be a rectangle left of mark B seen from the mouth hole side and the TSH rectangle will be left of mark C (see picture). Simply draw a rectangle approx. the desired size.
Specification of the final rectangles:
SAC Exit (slow air chamber):
10mm length (direction of the flute)
14mm width
TSH (true sound hole):
7mm length (in direction of flute)
14mm width
Cutting the Rectangles
Various tools can be used to cut the rectangles, such as drilling tiny holes and then sawing and filing, but I prefer to use a small desktop cnc machine. This way, I can obtain nearly optimal rectangles that only need minor adjustments.
The specific cnc machine I use is listed on the tools page, but any other cnc should work. Those machine are super fun to me because you can use them for so many hobby projects.
I am using the following 3D model and tool path files. Feel free to modify them to your needs. The tool bit for this step is a 2mm flat end mill.
The software I use for 3D modelling is called FreeCAD (see software) and is free like the name suggests. It is very powerful but has a very steep learning curve. There are much easier programs out there but they are usually pretty expensive (e.g. Fusion 360).
3D model in FreeCAD format: download
GBRL Tool path for my Genmitsu 4030 cnc machine: download
Filing SAC Exit and TSH
I typically use a flat diamond file to slightly correct and smooth both rectangles. In the case of the SAC-Exit, the exact length is not crucial, so I usually widen the width to 14mm and extend the length to the desired 10mm, plus or minus 1mm.
The subsequent step involves creating a smooth ramp from the SAC-Exit towards the TSH, which ends approximately 10mm before the TSH entrance on the left side (refer to the picture). The last 10mm remain flat and leveled. I use a chisel to carefully form an approx. 30° ramp or simply smooth the milled ramp that I then slightly round at the start and end to enable smooth airflow without encountering any sharp edges. Afterward, I turn the blank around and smooth all edges inside the SAC, including where the mouth hole terminates into the SAC.
Next, I file the TSH to precisely 14mm in width by 7mm in length. I create an inner ramp to establish the cutting edge, which should be between 30° and 45°. I carefully file this ramp through the TSH cut-out from the top, and then turn the blank around to finish the ramp from the inside. I round all edges and slightly curve the inner end of the ramp into the bore (see profile picture on the right). The cutting edge should be sharp but not excessively so, with a thickness of approximately 0.5mm, similar to that of blunt butter knife when touched.
Sanding and Sealing Everything Inside
Next, I sand the entire inside of the flute EXCEPT for the walls that must remain untouched and perfectly flat for gluing. I sand the mouth hole, SAC, SAC-Exit, and bore using increasing grit sand paper, starting from 120 and ending with 400 grit.
To make sanding the SAC and bore easier, I have 3D printed a helper. You can print it yourself using the the following stl model: link
When sanding the inner 45° TSH ramp, be careful not to sand too hard, as this may increase the TSH length slightly when I am not careful et the TSH egde. The length can still increase by up to 0.5mm, but no more.
I also sand the other half of the blank in the same way (SAC and bore). Again, be careful not to damage the walls, as we want them to align as well as possible without any gaps when gluing the blanks together.
To create a very smooth and shiny bore surface, I seal the inside with Shellac Sanding Sealer and apply 2-3 coats of Wipe-On-Poly. I sand up to 400 grit between each application (sealer, poly).
I also apply Shellac Sanding Sealer to both sides of the SAC, but I do not use poly at this stage. This will be done later, when the flute is glued together, to completely seal the SAC and protect it from moisture.
