Double Rip Cut
Selected Bamboo culms are sliced into straight strips 25 mm wide 1.2 meters long. You would want to choose large diameter culms that are more or less straight to begin with. Not all parts or the culm can be utilized for our purpose. Only the middle section is usable. The lower part of the culm near the roots, while having a larger diameter and thicker wall, has nodes that are spaced too close together. The upper part, on the other hand, is too small in diameter.
To cut the bamboo into straight strips exactly 25 mm wide, I devised a special double rip saw using two circular blades. A pre-cut bamboo section, 1.2 meters long, is strapped firmly in place while a movable assembly carrying the double-rip saw and motor cuts through the entire length of the bamboo section. After each cut, the bamboo is turned a few degrees clockwise and the next pass is made, repeating until the entire diameter of the bamboo is consumed.
At this point, only the internal nodes will be holding the bamboo section together. Applying a moderate amount of pressure will easily crush what’s left of the internal nodes that are still holding the bamboo. We shave off what’s left of the nodes and any protrusions. This can be done initially with a sharp knife and later with a jointer, then cleaning up with a thickness planer.
The end result is a bunch of nice and straight bamboo strips. The strips are sun dried and air dried for 3 weeks. After drying, the strips are boiled in water for 30 minutes to eliminate its natural starches and sugars, making them unattractive to termites and other pests. This will also make the bamboo less prone to shifting, expanding and contracting due to changes in temperature and humidity. The strips are then dipped in a 60% borax-40% boric acid solution for further preservation. Both boric acid and borax are great as insecticide and anti-fungal preservatives. After treatment, the strips are sun dried and air dried again for a few days until they attain 10% to 15% moisture content. Now the strips are ready for lamination.
The strips are grouped into 25 mm and 50 mm bundles, vertically oriented with their narrow edges facing up. Vertical orientation has better structural strength compared to horizontal orientation (with their wider edges facing up). Laminating epoxy (the same formulation used for laminating carbon fiber) is used to glue the bamboo strips. 2 tonnes of pressure is applied while allowing the epoxy to cure using a small press constructed just for this purpose (picture below). The epoxy resin slow-cures in 24 hours (faster if moderate heat is applied).
The planks undergo another thickness planer session to even out any irregularities. After making sure that the planks are perfectly squared up, we are now about to prepare one end of the planks for a 14 degree scarf joint. This will later become the neck-thru’s headstock.
A jig is utilized to cut a smooth surface, exactly 14 degrees, using a router. The jig serves as a guide for the router, mounted on a movable acrylic base. The router base slides over two edge guides while the cutter bit cuts the surface material at the desired 14 degree head angle. The result is very accurate and easily repeatable. This jig can be found in Patrick Spielmans book “Router jigs and techniques”. A complete description of this technique can be found here.
Two outer 50 mm planks and one inner 25 mm plank make up the central thru-neck construction. These are laminated together with up to 6 layers of carbon fiber for optimum structural strength. The image below shows the layering.
Another 24 hours of curing. Our patience has paid off. We have a nice bamboo-carbon-fiber plank ready for routing and shaping! Yeah!