This is my first instructable. The reason I made this scale is that I am a photographer and I mix my own developers using household compounds like instant coffee ( Caffinol---a future instructable) and I need to measure quantities as small as .5 grams. The electronic kitchen scales are not accurate down to that level, so I decided to see if I could make a balance scale that would do the job.
Step 1: Materials and Tools
You will need a plastic ruler, 2 old scratched CDs (new would do but why waste them), 6 6" fishing leaders with snap ends, 2 small pieces of plywood, long finish nails and some wood screws. You will also need a drill with an assortment of bits, a pencil and a US Nickel.
Step 2: The Ruler:
The ruler is the beam for the scale and must be as symmetric as possible. The ruler I used was a plastic Wescott with a raised thumb hold ridge down the length of it. It has a hole at one end to hang on a hook. Pick a drill bit of the same diameter and drill an identical hole at the opposite end. then drill a small hole next to both of the large holes. The small holes are for hanging the discs. Next pick a drill bit that is slightly larger than the finish nail, find the mid point on the ruler, on this ruler it is right at the 6 inch mark. Drill a hole through the raised holder, as close to the flat surface of the ruler as possible.
Step 3: The CDs
Stack both CDs evenly and drill 3 evenly spaced holes near the edge of the discs and clip 3 fishing leaders to each disc. Take two paper clips and thread each set of leaders onto them ( I did not have any paper clips so I used a finish nail bent into the shape of an elongated "C") and hang them on from the holes at the ends of the ruler.