Pretty Good Postal Scale From Old CDs





Introduction: Pretty Good Postal Scale From Old CDs

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

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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    35 Discussions

    why couldn't you just use a wooden ruler, lay it across a battery and put the 5 quarters in a stack at one end, like a see-saw / teeter-totter, and put the letter or object at the other end?

    1 reply

    You could do that, but then it would not be a new use for old CDs, nor would it be an adaptation of the design I linked in the Introduction. What you suggest could work in theory, but the ruler would not be fixed in a stable way, and that introduces a high risk of sudden error. Also, where do you balance or hang the mail while weighing it? I think what I have suggested is eminently more practical than a ruler on a battery. I hope you will soon publish your first Instructable.

    If you put a third hole on the other side of the support point, and hang a pointer wire, you can arrange it so that you have a much larger movement per unit. Note, the scale will be non-linear, and will be more accurate at the heavy end.

    10 replies

    Good observation. Thanks. Quite a few variations would be possible. For example, instead of wire for the handle and for the clothespin hanger, string or fish line could be used. Your suggestion would make the scale a closer facsimile to the original by Arvind Gupta that I linked in Step 1. One of my goals was to get double duty out of as many components as possible and keep the number of parts to an absolute minimum. As it is now, the range of movement between unit marks is about the same, perhaps a little more than what you would find on a commercial spring-operated postage scale.

    Sorry I was bitchy that day don't take it too offensively... Sorry.

    I took it as a compliment, as if you thought I might know something useful. I hope you are feeling fine now.

    yeah I'm ok now and agian I'm sorry. It's cool that you want to know about technical stuff, my dad is a Machanic so I grew up around this stuff all my life.

    My favorite postage scale is the one made of a single piece of steel kinda 'L' shaped (well, squashed backwards 'C' with a pointer hanging from the suspensionpoint and the scal reading right to left. There is another, similar design with a 'button' weight which is about as good for letters, but not for weighing anything else (anything else bumps into the weight and throws off the readings.)

    I used to see what you describe in several stores. I have not seen them for quite a while. I wanted something that could be used easily without the letter or parcel coming into contact with part of the scale and skewing the reading. In your mind's eye you can easily impose the "L" or backwards "C" you describe onto the CDs. Although the CDs are actually a circle, the effect is the same.

    Right. I did find someone a couple years ago that was selling them online, and I should have a couple around somewhere.... The cutout shape was designed to move the weight to the outer edge to get enough movement on a scale that's only 3-4" long.

    I also saw some on-line. The price was pretty good, but the shipping and handling was several times more than the purchase price.

    Sweet! I've been looking for something like this for awhile! Thank you! Great i'ble!

    Very nice....and more accurate then those spring steel things my wife bought. They didn't last very long....she ships a lot of stuff on eBay

    2 replies

    I originally wanted to calibrate this scale to weigh at least four ounces. It would do that quite well, but I did not have twenty quarters available. I expect your wife sometimes ships packages heavier than four ounces. If she wanted to make her own scale, a balance beam type might be appropriate with a weight she could move farther out on the beam for different weights. See my Instructable on a bicycle torque wrench. I used a spring fisherman's scale, but checked its calibration with water. A gallon weighs 8.33 pounds. With that knowledge, you can make up any test weight you need, with the assumption that a common plastic milk jug has negligible weight. Mark the location of the counterbalance weight on the arm so the scale balances at threshold weights used by the US Post Office to indicate more postage is required.

    Oh yes, this would not be for packages, but for the mail she sends. We have a scale (kitchen variety with ounces and pounds on it) that does well with things over a pound

    Balmuge means 5 grams U.S. Where 28.35 grams = 1 ounce.