How to use a Micrometer

Transcription

1 1 Introduction How to use a Micrometer By Marc Yarnall ENGL 202C A micrometer is a measuring device that uses a calibrated screw to measure small distances with high precision. All micrometers can measure distances to at least three significant digits after the decimal and sometimes even four digits. The different components of a micrometer are labeled below. It is important to learn the names of these components before trying to use the micrometer. As the user spins the thimble or the ratchet, the distance between the anvil and the spindle slowly increases or decreases. Since one full rotation only changes this distance by.025, a micrometer can be used to obtain very precise measurements. Mechanical and industrial engineers use micrometers extensively when inspecting dimensions on parts with tight tolerances. While using a micrometer is not hard, it is also not intuitive. It may take around 15 minutes to use a micrometer the first time. Once users have more experience, they can usually take measurements in less than 3 minutes. Still, it is important to take measurements carefully to ensure accurate and precise results. There are two steps to the process: measuring a thickness and reading the micrometer. To take a measurement with a micrometer, you will need: A micrometer The part that needs to be measured Pen and Paper (optional) CAUTION: Always store the micrometer at room temperature with the spindle and anvil not touching. Temperature changes will affect the device s accuracy. If the anvil and spindle touch, stresses could develop in the device and once again change the device s accuracy.

2 2 Measuring a Thickness Step 1: Choose the correct micrometer based on the size of the part that will be measured Micrometers usually only have a range of about an inch. For example, some micrometers can measure distances from 1-2 inches while others can measure distances from 3-4 inches. For the example used in this guide, a 0-1 inch micrometer is used. Step 2: Spin the micrometer s thimble counterclockwise (see the arrow in the picture below) until the part can fit between the spindle and the anvil.

3 3 Step 3: Place the part against the anvil and slowly rotate the thimble clockwise until the spindle is nearly touching the part. Step 4: Rotate the ratchet until you hear three clicks. CAUTION: Do not continue to turn the ratchet or the thimble after hearing the three clicks. This could potentially damage the micrometer and leave an indentation in the part. It can also lead to inaccurate measurements. Tip: Make sure the anvil and spindle are flat against the part or touching the part evenly to ensure an accurate measurement.

4 4 Step 5: Lock the thimble by rotating the lock clockwise and remove the part from the device. Tip: If the part does not easily slide away from micrometer, the spindle was probably tightened too much. The spindle and the anvil should only touch the object, not be jammed into it.

5 5 Reading a Micrometer Step 1: Look at the last large number not hidden by the thimble to obtain the first digit after the decimal. In the picture below, the length would be 0.8. Tip: Remember to add the micrometer s range to this measurement if applicable. For example, if a 1-2 inch micrometer was used, then the measurement would be 1.8. Step 2: Count the number of marks between the last tenth marking and the beginning of the thimble. Multiply this number by 25 and either remember or write down the calculated number. In this example, there are two visible marks after the eight. Therefore, you should remember the number 50. Tip: Each one of these marks is separated by.025. Paying attention to the numbers on the rotating thimble can help clarify this reading. If the number on the thimble that matches with the measurement line is small (between 1 and 12), then the edge of the thimble should be very close to the last visible marking. If this number is large (between 13 and 25), then the edge of the thimble should be far away from the last visible marking. If this number is exactly zero, then the edge of the thimble should be exactly on a 25 thousandth mark and this marking should be included in the calculation explained above.

6 6 Step 3: Locate which number listed on the thimble lines up with the measurement line. If the measurement line is between two numbers, always take the lower number. Remember or write down this number. In this example, the measurement line is between 18 and 19. Therefore, 18 should be remembered. Tip: It can be difficult to read this number if one of the markings on the thimble almost perfectly matches with the measurement line. The fourth digit after the decimal, which is obtained in step 5, can help solve this problem. If the fourth digit is low (between 0 and 4), then the measurement line should be barely above the correct marking on the thimble. If the fourth digit is high (between 5 and 9), then the measurement line should be well above the correct marking on the thimble. Step 4: Add the number obtained from step 2 with the number obtained from step 3 to find the second and third digit after the decimal. For this example, the number 50 was found in step 2 and the number 18 was found in step 3. These numbers add up to 68. Therefore, the total measurement to three decimal places is.868.

7 7 Step 5: Find which of the ten lines above the measurement line matches with one of the markings on the thimble. The number associated with this matching line is the fourth digit after the decimal. For this example, the five matches perfectly with one of the markings on the thimble. Therefore, the complete measurement is Tip: For this step, choose the marking that matches up the best. Sometimes, it can be a judgement call. Conclusion Overall, a precise measurement of four digits after the decimal is obtained. While micrometers are mostly used to take measurements of external features as explained in this guide, special micrometers can be used to find the lengths of internal features such as hole diameters. Even with these other methods, the basics of how to take measurements with micrometers still apply. While it may not be intuitive, using a micrometer is relatively simple once the user understands the basic concepts.