I made this as a present for my sister who gave me the original idea, using parts found at Walmart, Ace Hardware, and on Amazon, plus my CNC machine.
Step 1: Adding Second Tier
I divided up the surface into 1/6 sections using my calipers, a ruler, and a calculator to draw lines 60 degrees apart. The center of each support column is about 1.25 inches from the edge to give me a decent margin.
First I milled a 3/8 inch hole down through the center point of each support for the screw. Then I milled a 0.5 inch concentric hole 1/4 inch into the bamboo, keeping in mind which half was top and which was bottom so that the 0.5 inch side would face the other when stacked.
I cut 4.5 inch long segments of the 0.5 inch aluminum tube for each support using a small hack saw. I made sure to deburr the ends of the tubes with some rat tail files so as not to cut myself or anyone else while handling them.
Depending on the tools you use, be sure to wear safety goggles or shatterproof glasses if necessary and if there is a lot of dust from cutting or milling a dust mask would be appropriate. I have no idea what kind of chemicals they may have used to treat this bamboo so breathing it is inadvisable. I used a vacuum to keep my cutting bit clear while milling but microscopic particles can make it through the filter and irritate the lungs and cover your work area.
Step 2: Milling Holes for the Tubes
Each test tube is spaced 10 degrees apart so that there is 5 per 1/6th segment. the 10 degrees for each of the supports provide the extra space for the whole 360 degrees. I planned my spacing for each tube using the rubber stopper as a guide and placing it on the appropriate radial line.
I increased the margin of the hole for each tube slightly so that the glass would have some wiggle room if someone pulled on it at an angle.
Each tube has a corresponding cradle on the bottom layer for it to stay in once placed. This is the same width as the hole in the top layer but simply milled halfway through the material instead of all the way through. I thought about doing a concave spherical cradle but due to time constraints and increased CNC code complexity I went with the simplest solution.
Step 3: Warning Against Overheating
A problem I encountered, being new to the CNC milling world, was figuring out at what rate I could cut through this bamboo material without stressing my machine, the bit, or the bamboo itself. I wanted to cut as fast as possible because I had a lot of holes to mill and not a lot of time to do it. Cutting too fast though caused my first bit to overheat and become slightly dull.
I could tell it was getting hot by the steam escaping the bamboo and the color of the bit. If the bit changes color it could be because of either carbonation from the burning material or the steel itself losing its hardness because of the temperature. Heat was generated either because I was trying to mill too much at a time, or because I was milling too little, causing the bit to simply sit and rub against the surface without cutting.
Make sure to give the bit enough time to cool if it looks like it is being pushed too hard. Using a vacuum to clear the path helped to make sure the excess dust wasn't causing extra friction on the bit, and the extra airflow helped cool the bit as any fan cools a heat sink. Slowing down my cutting rate helped reduce the risk of stress to the bit and the machine and made sure my operations were identical cuts.
Step 4: Final Thoughts
I was told that the rubber stoppers were better for spices than the cork stoppers for some reason. I don't know if it's because it creates a better seal, or if it resists chemical attacks, or something else. I haven't had the time or experience to find the differences between them but went with what I thought would last the longest.