Introduction: Pretty Good Postal Scale From Old CDs
With four old CDs you can build a pretty good postal scale to read up to about 3 ounces (85 grams).
This is an adaptation of a design for a scale made from a postcard, paper clips, and a coin by Arvind Gupta. It can be viewed here.
You can view the finished scale in Step 7.
Step 1: Tape the CDs Together
Tape the four CDs together with three pieces of cellophane tape.
Step 2: Prepare Finish Nails
Two finish nails will be needed as axles. First they will be used as drill bits to make holes that fit them as axles very precisely. In order to make the drill chuck hold them, it will be necessary to remove the heads from the finish nails. You could cut them off. I am grinding them off with a power grinder while the finish nail is spun in a handheld electric drill.
Step 3: Drill the CDs With a Finish Nail
The head has been removed from this finish nail and it has been chucked up in a drill press to use as a bit. Drill two holes through the CDs about 1 1/2 inches from each other. You can see the holes in the photo.
Step 4: Insert Axles and Begin Wire Attachments
Both finish nails have been driven into the two holes.
Cut some steel wire (about #15 gage) 7 inches long. This piece will make the handle by which the scale is held. Bend and/or twist it to your liking. With a needle nose plier make a loop in each end of the wire. Try to make the loops on the same plane with each other so the axle rests in them while the axle is level. Slip the handle loops over the ends of the axles.
Step 5: Make the Wire Hanger for a Clothespin
A clothespin will hold the letters you want to weigh with the scale. It should move freely and its movement should not be restricted by the scale.
Cut a piece of #15 gage wire 16 inches long. Bend and insert it in the clothespin as shown. It may be twisted together with about 3 twists above the ends of the clothespin handles.
Step 6: Attach the Clothespin Hanger to the Other Axle
Make loops in the ends of the wires that support the clothespin just as you did with the handle. Attach the loops to the other axle. Make sure the wires do not rub on the CD, but can move freely.
Step 7: As It Looks Assembled
This is a photo of the scale after it is fully assembled. The handle is in the upper left of the photo. The clothespin and its hanger run to the lower right. The inherent weight of the CDs acts as a counterbalance weight to the weight of the letter being weighed.
All that remains is to do the calibration.
Step 8: Calibration
Calibration marks can be made with a piece of frosted tape and a fine point marking pen.
I did an Internet search for the weight of a US Quarter ($0.25 piece). Those made since 1967 with the copper core weigh 5.67 grams each. I used a metric to English conversion calculator. In ounces that is 0.20003 ounce each. That means five post-1967 US Quarters weigh exactly one ounce. (No one at the Post Office will worry much about anything after four decimal places.)
A plastic Baggie for a sandwich has a negligible weight. I attached a Baggie to the clothespin and put five US Quarters into it. When the scale came to rest, I made a mark for 1 ounce at the point where the hanger wire for the clothespin crossed the frosted tape. It is the leftmost mark of the three you see. Then I added five more Quarters for 2 ounces. It is the second mark. The third mark is for fifteen Quarters or 3 ounces. You can add "1", "2", and "3" to your marks to avoid confusion later about their meaning.
The marks appear to be a bit to the side of the wire hanger/indicator, but they really are not. That is because I did not take the photo straight on to the CD. I wanted to avoid reflections that would make the photo less useful.
The more carefully you make and read your calibration marks, the more accurate your scale will be.
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