Micrometer Caliper

1. Print or copy the micrometer dial template onto cardstock. Cut out the dial, and set it aside. You’ll need it again in Step 7. (Note: If your printer does not accommodate cardstock, just print onto regular printer paper and use a glue stick to attach the dial to the cardstock.)
2. Check the machine screw to be sure the end shows a flat surface that’s perpendicular to the screw itself. You may find a small raised ridge around the edge, or possibly some irregular burrs. If necessary, use the metal file or drill with grinding wheel to flatten the surface. To work best in the micrometer, the end of the machine screw will need to uniformly contact the fender washer.
3. Screw the two hex nuts onto the machine screw, about 1 inch (2.5 cm) apart. Lay the assembly along the top edge of the wood block, with the hex nut nearest the end of the screw, about 1/2 inch (1.5 cm) from the edge of the block. Then use two rubber bands to hold the hex-nut-and-machine-screw assembly in place (see photo below).
   
4. Loop the cable ties around the hex nuts and wood block to fasten the assembly to the block. Use the needle-nose pliers to tighten the cable ties so the whole assembly is held securely together, with the fastening heads of the cable ties located at least halfway up the side of the block. When the cable ties are in place, remove the rubber bands; they will no longer be needed (see photo in Step 6).
5. Unscrew the machine screw, and remove it. You’ll need it again in Step 8.
6. Hold the fender washer against the face of the wood block where the flat end of the machine screw was located. Leave approximately 1/2 inch (1.5 cm) of the washer sticking up above the top of the block (the flat end of the machine screw will sit against it when it’s set in place in Step 11). With a pencil or marker, mark the position of the center of the washer on the block. Drill a pilot hole, and then fasten the washer to the block with one of the sheet-metal screws (see photo below). When the washer is in place, set the assembly aside to prepare the micrometer dial.
7. Using the tip of a sharpened pencil, poke a small hole in the center of the micrometer dial and slide it onto the machine screw. Use the two wing nuts and the two small washers to hold the dial in place on the machine screw, positioning the dial about 1 inch (2.5 cm) from the head of the screw (see photo below).
   
8. Screw the machine-screw-and-dial assembly back through the two hex nuts on the top of the wood base until the end of the screw is fairly close to the fender washer, but not in contact with it. It should be about 1/4 inch (0.5 cm) away.
9. Attach the binder clip to the bottom of the wood base, and then remove the clip’s handles (see photo below). This will give the assembly a flat base so the micrometer can stand up by itself.
   
10. Using the needle-nose pliers, bend the jumbo paper clip to make a reference indicator for the dial (you can cut the paper clip if necessary). Drill a pilot hole and use the remaining sheet-metal screw to attach the bent paper clip (see photo below).
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11. Attach the clothespin to the head of the machine screw (see below). The clothespin will standardize the force of the end of the screw as it comes into contact with the washer. Gently push on the clothespin to turn the screw so it touches the washer, and stop when the clothespin starts to slip.
12. With the screw remaining gently in contact with the washer, lightly loosen the wing nuts holding the dial in place. Rotate the dial to line up the zero with the reference indicator, and then tighten the wing nuts to hold the dial in this position. Once again, use the clothespin to tighten the screw until it slips, and check the alignment of the dial. (Be aware that, as the clothespin turns, the soft wood of the block may respond by traveling along the screw toward the dial, so you may need to move the clothespin each time so it always sits next to the head of the screw.) Repeat this calibration adjustment as necessary until the reference indicator consistently reads zero on the dial. At that point, your micrometer caliper is ready to go (see photos below).
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(Note: As an alternative to calibrating by loosening the wing nuts to move the dial, you could also just initially make sure the zero on the dial is somewhere near the bottom, and then adjust the indicator by moving it or bending it until it lines up with the zero mark.)

Tighten the screw with the clothespin until the clothespin slips and the end of the screw is up against the fender washer. The zero mark on the dial should be lined up with the reference indicator. Loosen the screw a little less than one whole turn (until the zero on the dial is almost to the reference mark again) and insert a piece of paper between the end of the screw and the washer. Tighten the screw again, once more using the clothespin until it slips and the end of the screw is up against the paper. Then read the dial.

Each mark on the dial represents about one one-hundredth of a millimeter, so the mark that lines up with the reference indicator indicates the thickness of the paper in hundredths of a millimeter. Ordinary copy paper usually has a thickness of between ten to fifteen hundredths of a millimeter (10 to 15 on the dial).

Try measuring other objects. Some things to consider are an index card, a paper clip, a piece of thin spaghetti, regular or heavy duty aluminum foil, nylon fishing line, wires of different gauges, plastic bag material, or even a human hair.

Periodically check to make sure the reference indicator still reads zero when there is no object in place. If it doesn't, then recalibrate the dial.

If the object you’re measuring is larger than one millimeter, you’ll have to keep track of how many turns you have to loosen the screw (and therefore the dial) before measuring the object, and then add the appropriate amount to the dial reading. Each complete turn of the screw moves the end of the screw one millimeter.

You can increase the reliability of a measurement by doing multiple trials and finding their average, rather than relying on a single trial.

The 24 in the specification of the screw (#10-24) indicates that there are 24 threads per inch. This is known as the "pitch" of the screw. It means that it takes 24 complete turns of the screw to move it one inch.

One inch is equal to 2.54 centimeters, or 25.4 millimeters. So there are 24 threads in 25.4 millimeters. This is close to 1 thread per millimeter. And though it is off by 1 millimeter in about 25 millimeters, that works out to just 4 millimeters in 100, or 4%, which is not bad considering the cost of this instrument!

Since there is one thread per millimeter, then one complete turn of the screw moves the end of the screw 1 millimeter. The dial divides one turn of the screw into 100 parts, each of which represents 1/100 of a turn. Since a whole turn is a whole millimeter, each division on the dial represents one hundredth of a millimeter, or 0.01 millimeter.

The micron is a unit of measurement still in common use in some areas of science and technology, but it is no longer an official SI unit. How does a micron relate to a meter, a millimeter and a micrometer (the SI unit, not the instrument you built)? How many microns are represented by one division on the dial?

Familiarize yourself with the following terms: sensitivity, precision, accuracy, deviation. See if you can apply them to your micrometer caliper.

Making and/or using this micrometer caliper can provide unique hands-on insight and experience in the measurement of very small things, but with limitations on precision and accuracy affecting its use as an everyday working tool. Pre-digital mechanical micrometer calipers operated on the same principle, though with greater precision and accuracy, but were relatively expensive. Now, however, there are inexpensive digital calipers with exceptional capabilities that provide a useful working tool for many aspects of science education.

The inspiration for this micrometer came from the original PSSC Physics materials created by MIT’s Physical Science Study Committee in the late 1950s and early 1960s.