The micrometer also called the micrometer screw gauge, is one of the most widely used measurement tools across all sciences because of its accuracy. Every mechanist or engineer is expected to know how to use it although it may seem like a weird medieval tool if you are handling it for the first time. There are three main types of micrometers namely the inside micrometer, depth micrometer, and the outside micrometer which is the most commonly used tool to make fine measurements of the thickness of objects.
People use micrometers when they need to narrow down their measurements to the lowest possible distance between points especially when the finest unit of distance on the object can affect the whole project.
The micrometer will give you an accurate reading with an error of just 0.005mm, nearly insignificant. Objects with a sensitive thickness or diameter are supposed to be measured with the highest possible precision which is why micrometers can really come in handy while working in the field or in the lab. A micrometer narrows down the distance to one-millionth of a meter which gives you exact distance measurable once the clamp locks in on the two points.
What is an Outside Micrometer?
This is the micrometer you can use to measure the outside surface/thickness of objects with high precision. It can be used to measure an object that can fit between its anvil and spindle, for example, the thickness of a ball bearing, the thickness of a screwdriver, or the thickness of a metal bar. The outside micrometer works by gripping the object being measured in a C-Shaped clamp between the anvil (it’s stationery measuring face) and the spindle (its moving measuring face). The exact measurement can be seen on two scales namely the thimble scale, found on the rotating cylinder that covers the spindle, and the sleeve scale found between the clamp and the thimble. These features give the accurate distance between the two ends of the object down to two decimal places.
Parts of an Outside Micrometer and Their Uses
1. C Frame
This is the main part of the outside micrometer that holds the anvil and the sleeve in place and allows the spindle to adjust in and out locking in on the object being measured. It is the part the user holds while taking measurements. It is designed to be rigid to keep the other parts of the micrometer safely in place. The size of this frame determines the size of objects you can measure with your micrometer which is why it is used to determine the size of a micrometer. An engineering micrometer will have a wider frame than a small lab micrometer to allow for the measurement of large objects such as building blocks.
This is the stationary end of the measuring faces of the outside micrometer. It holds the objects in place until the spindle locks them in place. It is designed to have zero adjustments to avoid interfering with the readings at the sleeve.
This is the moving measuring face of the micrometer and also the most important part of an outside micrometer. It is threaded allowing it to roll through the C frame locking in on the object while the accurate readings at the sleeve and the thimble are set. The actual measurement is achieved once the spindle locks in on the object. This means the measurement of the object being measured is simply the distance that remains in between the anvil face and the spindle face once the spindle reaches the second end of the object loaded in the C frame.
The sleeve is the cylindrical barrel around the spindle just behind the C frame. It is calibrated making it the primary scale of the outside micrometer. As the spindle rotates locking on the object, more of the sleeve is covered by the thimble towards the actual millimeter that depicts the base thickness of the object. The first reading on the sleeve scale is the first digit just left of the thimble.
This is the outside micrometer part that rotates against the sleeve as the spindle rotates covering the sleeve scale to the lowest value which is the first reading of the measurement. The thimble is also calibrated with a scale that gives the two last digits of measurement increasing the precision of the reading to two or three decimal places depending on the units of calibration. It also becomes tight as the spindle locks increasing its precision. The actual thimble scale reading is the figure that aligns itself to the index line when the spindle finally locks onto the object.
6. The micrometer index line
This is the straight-line on the sleeve with values extending right up to the base of the thimble. This is the indicator that points to the exact reading on the thimble scale. It runs along the sleeve scale with its calibration also marking the sleeve scale readings.
This is the part of the micrometer that turns the spindle as the object is measured and also prevents overtightening as well as slipping of the object being measured. Most modern digital outside micrometers come with a clutch on the ratchet that automatically disengages the spindle to stop rotating it once the correct measurement of the object is detected. Without a ratchet, the spindle would move too fast and continue slipping against the surface interfering with the readings.
8. Lock Nut
This is an important part coming in different forms on different outside micrometers, either as a simple nut or a switch. It is the tool used to prevent movement at the spindle once the measurement has been achieved. It keeps the spindle in place at the accurate measurement set preventing any movements that may increase or reduce the readings while the measurement is being recorded.
This is the display interface for the results of the measurement. Manual micrometers don`t have this forcing the user to add the thimble scale reading to the sleeve scale reading to get the right measurement.
How to Use an Outside Micrometer
The correct use of a micrometer starts by holding the micrometer in a position that allows you to rotate the thimble with the same arm that is holding the C frame. If you don`t get it right, you may feel like you need a third hand to rotate the thimble. Before you start measuring anything with the micrometer, you have to ensure that the zero lines on the thimble are aligned with the zero lines on the sleeve when you lock your micrometer with no object loaded. This way, you will not have an error in the reading. If the values don’t line up, you may have to clean the measuring faces thoroughly with a piece of cloth or paper or just get another micrometer.
Here are the steps to follow when using an outside micrometer.
- Place the frame against your dominant arm and put the thimble in between your thumb and your index finger. Ensure your thumb is free enough to rub against the thimble because you will need to use it to lock the spindle onto the object.
- Put your ring finger or both the ring finger and your pinky finger through the frame and hold it down tight to prevent any movement by the frame. You have to ensure that after inserting your fingers through the C frame, there is enough room for the object you are measuring.
- Adjust your palm and the third finger to ensure the sleeve scale and the index line are in your direct view for a simple glance at the measurements.
- Once you have a sturdy grip and a clear view of the scale, take the object you are measuring in your non-dominant arm and place it against the anvil.
- With your non-dominant arm securing the object against the anvil, turn the thimble with your thumb until it locks onto the object then extend your thumb and turn the lock nut to secure the reading. If your outside micrometer doesn`t have a clutch, you will have to stop adjusting the spindle immediately after the spindle achieves visible contact with the surface. Overdriving will give you a false reading.
- Once you have secured the lock nut, you can remove the object because You now have your outside micrometer set with the figures you need to record your measurements.
How to Read an Outside Micrometer
Once you have secured the right measurement, you still need to understand how to read outside micrometer scales and combine both figures to call it a perfect measurement. Reading is quite easy once you have secured the measurement you recorded with the lock nut because the position of the spindle doesn`t change once you deploy the nut. The reading just needs a little maths because you have to add the figure on the sleeve immediately on the left of the thimble to the figure that stands on the same position as the index line on the thimble. Some thimble scales may be calibrated in a thousandth of an inch or one-hundredth of a millimeter depending on the micrometer`s origin. Sleeve scales are generally marked in millimeters with each thimble rotation measuring half a millimeter. In this example, we will use millimeters, you can always convert your readings on the scale of 1 inch equals 25.4 millimeters in case yous is calibrated in inches.
Sleeve Scale Reading
The sleeve scale has two calibrations on both sides of the index line. The lower readings are measurements in millimeters (1mm from one mark to another) while the upper marks come in between each millimeter calibration which means half of a millimeter (0.5mm). There are two main rules that a mark has to meet to be counted in the reading.
- The zero line on the thimble scale must make a complete rotation to settle on the mark before it makes a count, meaning the thimble scale`s zero mark has to make one complete rotation. Otherwise, the mark before it will be counted as the correct measurement even if the mark is very close to or exactly below the thimble. This doesn’t mean you will lose a reading, the thimble scale will account for the difference in the distance of the incomplete thimble rotation.
- If no mark is directly in front of the thimble after a rotation, the last mark, whether half a millimeter of a full millimeter that is visible makes the sleeve scale reading.
Say you are measuring the thickness of a ball bearing and your thimble rotates stopping just after covering the upper mark (half a millimeter mark) after 6 but doesn`t touch the 6-millimeter mark. Your sleeve scale reading will be 6mm.
If the rotation stops against or just before the 6mm mark before the zero line on the thimble scale comes around, the 6mm mark cannot be counted even if it is the one immediately visible which is why your reading will now be 5.5mm because the half-millimeter mark after the five mark on the sleeve scale will be the next reading after the complete rotation of the thimble.
Thimble Scale Reading
This is the scale that gives micrometer readings their ultimate precision. On a millimeter calibrated micrometer, the thimble is set in one-hundredth of a millimeter which means the readings will be added just after the decimal point on the right-hand side of the sleeve scale reading.
With the thimble scale, there are also two main rules that make a reading viable.
- The reading should be directly in line with the index line of the sleeve scale. Whichever this reading is, it is added directly to the sleeve scale reading. For example, if the number aligned to the sleeve scale is 12 after making the 6mm reading on your sleeve scale reading, the thickness of your ball bearing will be 6.12mm. If you had a reading of 5.5mm from your sleeve scale reading, a direct addition will give you 5.65mm as the correct thickness of your ball bearing.
- Sometimes, there is no reading directly in line with the index line. In this case, you will take the lower reading, the lower mark just below the index line. Say for example while measuring the thickness of your ball bearing, the thimble scale`s 12 mark just slides past the index line and the thimble stops before the 11 mark falls in line. You will take 11 as the reading instead of 12 for higher accuracy which means your thickness will now be 6.11mm in case your sleeve scale reading was 6mm or 5.61mm in case 5.5mm was your sleeve scale reading.
Micrometer Care and Maintenance
It is important to keep the outside micrometer parts clean and smooth to ensure zero errors as the spindle moves through the frame. It is also necessary to clean the anvil and spindle faces keeping them clean from dust and rust to keep the readings accurate because the slightest change in thickness may interfere with your readings.
Here are some tips on how to always achieve the best results from your outside micrometer.
- Store your outside micrometer in a cool dry environment all the time, away from water, ice, and dust which may interfere with the metallic parts. The best way to store it is in a case, most outside micrometers come in a storage case that makes this easy.
- Before use, ensure that your thimble and sleeve scale both read zero before using, if they don’t, you can lock the spindle on one clean sheet of hard paper and pull it out abruptly to expel any foreign material.
- Always use a dry piece of cloth when cleaning the micrometer to keep it safe from dust and rust.
- Avoid dropping your micrometer because it may cause a permanent shift that interferes with readings. If you dropped it accidentally, always get it checked for errors.
- If the zero reading cannot be reset by cleaning, you can recalibrate by rotating the sleeve using a compatible micrometer spanner.
- Apply multipurpose oil to the micrometer`s external surface if it is being stored for a long time without use to prevent dust and rust. You should also oil the spindle threads if stored for a long period or when using the micrometer frequently.
- Don’t keep the anvil and the spindle in contact all the time even when the micrometer is not in use. This may interfere with the accuracy of your readings once the object Is introduced. This is however not a problem for most modern outside micrometers because they come with an extension that can be stored in between the two faces.
Precise measurements are necessary for all fields of science which is why every engineer, student, or anyone in a mechanical field finds themselves in need of an outside micrometer at one point or another. Outside micrometers are limited by size which is why every package comes in different sizes and calibrations. It is still important to understand how to use an outside micrometer to improve your efficiency when working in the field or in the lab to avoid making mistakes. It only takes one small mistake to get everything wrong with an outside micrometer which is why this guide is a lifesaver for anyone that uses or expects to use an outside micrometer in the future.