By Allen Bentley 2 comments

How To Reduce Error When Using An External Micrometer

Whether you are measuring the thickness of paper or an engine part, you cannot afford to risk any error. More so, by getting the measurement wrong, even by a fraction of a millimeter could upset the whole design process of your application. 

In this article, I will show you how to avoid the main causes of error when using your micrometer for maximum accuracy.

How To Reduce Error When Using A Micrometer? The best way to reduce error when using your micrometer is to first of all, make sure you’re using the right type of micrometer, have the micrometer set up correctly and make sure you use the right techniques when handling such a device.

Micrometers can set you back a lot of money, so it is essential that you get the most out of it, and by not using it correctly, you defeat the object of using such an accurate device. But don’t worry, I’ll go through the basics to provide you with all the information you need to get the results you’re looking for.

1. What Is An External Micrometer

Firstly, let us just clarify what an external micrometer is. An external micrometer is a sophisticated device that serves the purpose of measuring an external dimension with high accuracy. Put simply, to measure the outside of an object. Whereby, the object to be measured is placed between the measuring anvils of the Micrometer.

To note: They are called external micrometers because they measure the outside of an object. For example, the length or thickness of objects, whilst internal micrometers are designed to measure the internal dimensions of such an object.

It is also worth noting that external micrometers are also different from calipers even though they serve the same purpose. Micrometers can normally measure to a higher resolution with better accuracy. However, calipers do have their uses because they are more versatile and often much quicker to operate.

2. Analogue VS. Digital Micrometers

Before, you jump into measuring you need to choose the right micrometer for you. Micrometers are available with a digital display or a mechanical scale (sometimes both), and realistically it's personal preference as to which one you decide to use. Older micrometers tend to be mechanical with an analogue scale, as digital technology is a relatively modern development and digital micrometers weren’t commonly used until the early 80s. However, most operators agree a digital display is much easier to read than an analogue scale, and this is even more relevant when the operator is not experienced reading analogue scales. This is definitely something you want to consider.

This leads onto my first point of reducing error. A digital display eliminates any human error you may create when reading an analogue scale.

2.1 What Is An Analogue Micrometer

A micrometers analogue scale is made up of two scale types. The first is a sleeve scale and the second is the thimble scale. These work together for increased accuracy. 

The sleeve and thimble scale is where you will read your measurement to two decimal places (0.01mm) if working in metric or to three decimal places (0.001”) if working in imperial.

 

There may also be a third scale that is normally located on the sleeve. This is used to read the final decimal place and achieve a reading down to 1 micron (0.001mm) if working in metric or 0.0001” in imperial. If this scale is not present, you may wish to estimate the final decimal place.

2.2 What Is A Digital Micrometer

A digital micrometer operates similar to an analogue version, but they have a digital display. A digital display eliminates any error you may produce when reading an analogue scale and is also much quicker to read.

 

 

3. Choosing The Right Anvil Configuration for your External Micrometer

There are many different types of micrometers available which normally have different anvil configurations. Before you come to measure the object in question you need to first understand the types of micrometer available and their purpose.  Choosing the right anvil configuration for your application can affect the accuracy of the reading. This is because each anvil configuration is designed specifically to measure a specific feature, such as a tube or thickness of a wall.

This means you need to observe the object in question and match the micrometer anvil configuration respectively.

For example;

      • What’s the shape of the object?
      • What’s the expected dimension?
      • What is the material?

Below, I discuss anvil types in more detail;

3.1 2 Flat Anvil External Micrometer

The most common type of micrometer has 2 flat anvils. One anvil located on the frame and the other anvil is located on the spindle (see image). These normally have a diameter of 6-8mm.

 

Examples include the DM3025, and DM1025

 

Now, although this is the most common type, it may not be suitable for your application. Take for example the application of measuring the wall thickness of a tube. The anvil on the frame must be spherical where as the anvil on the spindle could still be flat. If you were to use a standard flat anvil micrometer, the anvil on the inside of the tube wouldn’t contact properly and you would produce readings that were inaccurate.

3.2 Multi Anvil (Uni) Micrometer

Multi anvil (uni) micrometers allow bespoke anvils to be used as a frame anvil. This opens up more options with your measuring because, not only can the anvil be changed, but it can be changed to a home-built anvil to better suit your needs. This means that you can make any type of anvil, as use this in your measuring.

 

Examples include  DM4025SA 

Consider all options before choosing your micrometer, and if you still don’t think you’ve found what’s right, then this is most likely the probable option.

3.3 Standard Micrometer

Standard micrometers often have a rotating spindle, as you operate the micrometer and the spindle moves in and out, it will rotate. This is fine for most applications but if you are measuring a delicate material, it may not be wise to use a micrometer without this feature because when the spindle rotates, you may find the material will tear or stretch.

 

Examples include DM3025, and DM1025

 

3.4 Standard Micrometer With None-Rotating Spindle

Luckily many micrometers feature a non rotating spindle, such as snap micrometers.  These are operated by a lever as opposed to turning a thimble which allows for very quick measurements.

As you can see each micrometer is designed to be compatible with certain applications or materials. If you are struggling with this then do not worry! Feel free to contact us and we will be happy to discuss your needs so you can purchase the best micrometer for your application.

 

Example include: DM5025

 

4. Cleanliness

As with all measuring equipment, cleanliness is very important.  You cannot expect to accurately measure to microns if the anvils of the measuring device are not clean. You can use a piece of paper to clean flat anvils. Place it between the anvils and proceed as if you were measuring the thickness of this paper. You would then want to pull the paper from between the anvils removing any unwanted dirt or grease. It’s also a good idea to check the material you are measuring is clean.  

5.  Ratchet drive

Most modern micrometers feature a ratchet drive on the thimble which is designed to give the same measuring force each time you take a measurement. It’s important to utilise this as it will prevent over winding the micrometer, generating inaccurate readings and potentially damaging the micrometer.

Often ratchets are designed so that they produce a clicking sound when they are preventing too much force being applied to the test piece. It is wise to listen for the clicks and to settle on a reading after the same amount of clicks each time. If the ratchet doesn’t make a noise then try to always turn it the same amount each time it kicks in and prevents over winding.

6. Good Practices For Holding Your Micrometer When Taking A Reading

If you find it difficult to hold the micrometer when taking readings or if your work piece makes it awkward to accurately take a measurement while holding the micrometer, you may want to consider a micrometer stand. Stands are normally low cost and generally improve the measuring process all round, especially if you don’t have experience using a micrometer.

 

 

7. Calibration

Most Quality Systems demand that Measuring Instruments are calibrated periodically. The period is determined by the frequency of use. Typically, an instrument that is used daily/weekly should be calibrated at least annually. The calibration provides a check to prove the instrument is measuring accurately when checked against standards that are traceable to National Standards.

8. Testing

Finally, it is always good practice to check the accuracy of the micrometer that the manufacturer states, it must be suitable for your application and the tolerances you are working to.

An accepted method of checking suitability of your instrument to your application, is to carry out an ‘R and R’ study (Repeatability and Reproducibility). This method tests the ability of the instrument to record accurate readings repeatedly, relevant to your dimension and tolerance.

R and R studies also tests the ability of the instrument to give repeatable readings when used by different operators.

Conclusion

As discussed in this article, there are eight considerations you must be mindful of to reduce human error in your readings.

      • The first is to make sure a micrometer is the most suitable measuring device for your application
      • The second is choosing between analogue and digital and ensuring you have the experience to read an analogue scale accurately if you choose this type.
      • The third point is observing the object you are measuring and to choose the correct type of micrometer, specifically the correct anvil configuration.
      • The fourth point is ensuring that both the micrometer and test piece are clean
      • The fifth is to utilise the ratchet drive
      • Sixth, consider if a stand would make the process easier
      • The second is to ensure it is calibrated
      • Finally, you can conduct an R and R study to highlight any error

I hope you have enjoyed this article and remember if you have any questions then please leave comments or contact us directly and we will be more than happy to assist you and provide you with professional advice to meet the needs of the application you are measuring.

2 comments

  • Allen E Bentley

    Hello Mark,
    If you are measuring outside diameters then 2 flat anvils are indeed the best option to achieve the best result.
    Obviously technique plays a part but you are probably well experienced in that regard. Indeed I recently used a DM3050 whilst rebuilding my Triumph Stag heads.

  • Mark

    Fabulous article, well done.

    I came across this article after purchasing your DM 3050 measuring my classic bikes crankshafts, valves etc.

    Whilst being mesmerised on anvil configurations, I was waiting for the magic words of “This configuration is perfect for engine parts”
    Please put me out of my misery and say those magic words.

    Regards Mark

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