Frank Conci
Technical Services Committee Member
A.C. Motor Electric, Ltd.
Editor’s note: This is a 2-part article that appeared published in June/July 2021.
The focus of this article is on outside micrometers. The ideas and information are applicable to other mechanical measuring devices such as inside micrometers, calipers, protractors, levels, depth gauges and such.
The importance of precise and accurate measurements is more critical today than ever. Our customers are increasingly demanding of best possible practices and outcomes when having their machinery serviced and repaired. EASA is at the forefront of ensuring that this service level is available through its Accreditation program, technical standards and engineering support. Our customers expect EASA members to use appropriate instruments professionally and dependably to repair and assess their equipment.
The Micrometer
A quality micrometer (mic) is one that, if properly cared for, can repeatedly deliver precise measurements for many years. It should have a resolution of (is capable of reading in units a small as) 1/10 of a mil; or 0.0001” (0.002 mm). Typically, micrometers with this resolution should have an accuracy deviation not greater than ± 0.00005” (0.001 mm). Each mic (Figure 1) should be equipped with a lock nut or lock lever, ratchet, thermal insulator on the frame, adjusting tools and a test standard with thermal insulator.
Storage & Handling
Micrometers typically come in a case which is suitable for storage. They can also be stored in a cushioned drawer of a tool chest. As long as mics are kept clean and dry and are not subject to harmful vibration, many storage options are possible. It is important to minimize storage temperature extremes to avoid condensation and to shorten the time required to match the temperature of the mic to the object being measured. Always ensure that there is visible gap between the measuring faces of your stored mics.
Micrometers can be harmed by abrasive dust and dirt, causing threads to wear and surfaces to become scratched and worn. Cleaning solvents and caustic fluids can damage markings, surfaces and threads. Moisture will produce rust on surfaces and threads. Careful handling of mics ensures that they do not wear prematurely or get damaged during use. Always return the cleaned instrument to storage as soon as possible after use.
Calibration
The importance of calibration cannot be overemphasized. It is your assurance that the reading you take from your mic is true. It allows you to confidently assess and report the fits and clearances of your customer’s apparatus.
At least once per year, each mic should be calibrated using a certified gauge block set (Figure 2) that is traceable to National Institute of Standards and Technology (NIST) or your national standard. The gauge blocks should be re-certified or replaced every five years. Calibration should be done monthly if mics are subject to frequent and/or rough use.
Calibration begins with the micrometers to be tested and the gauge blocks reaching ambient or room temperature. Allow up to four hours and check the larger pieces with a non-contact thermometer to be sure that temperatures are uniform. A temperature differential of 10°C can produce a 2.5 micron [0.0001” (0.002 mm)] error in a 6” (150 mm) mic.
When calibrating, it is important, especially with mics over 6” (150 mm), to support the mics in the manner and orientation that you use them, to avoid measurement errors. Clean the gauge blocks, standard and the anvil and spindle of the micrometer with lint free paper or cloth before each measurement.
To calibrate a micrometer, check it against a gauge block equal to the smaller end of its range and then again in 0.2500” (6.35 mm) increments up to the larger end of its range. For a 5-6” (125-150 mm) micrometer, you would start with a 5” (125 mm) block and repeat adding a 0.2500” (6.35 mm) block, a 0.5000” (12.7 mm), a 0.7500” (19.1 mm) and finally a 1.0000” (25.4 mm) block or switch to a 6.0000” (150 mm) block if your set has one. Make each reading at least twice to verify its accuracy. The five readings should be recorded in a dedicated log in an “as found” row (Figure 3).
If the results are within the stated accuracy [i.e., 0.00005” (0.001 mm)], the instrument can be considered calibrated. If the results are out of accuracy uniformly over the range of the mic, then using the provided tools, adjust the micrometer to match the gauge blocks and record the new measurements in the “as left” row in the log. If the results are out of accuracy inconsistently over the range of the mic, then it is likely the internal threads are worn. That micrometer must be replaced.
Finally, measure the micrometer's standard and repeat to determine that the standard is accurate. Record that result in the log. Replace the standard if it is not accurate. If a micrometer is dropped or subjected to any unusual and significant force, there is a possibility that the anvil and spindle may be out of parallel with each other. It can be tested by using an optical parallel or flat between the faces. This method using optical flats, while being the most effective, can be costly. Often, replacing a suspected damaged micrometer is cheaper.
You can achieve a workable estimation of the condition of parallelism between faces by viewing the faces against an appropriate gauge block with back-lighting and examining each contact area for light passage. Another method is to do a gauge block test on each quarter (90-degree rotation) of the spindle face and compare readings. If they are within the accuracy of the mic, then it is likely still suitable for use. If the micrometer in question is expensive, then sending it to a lab for calibration and testing may be worthwhile.
Measuring With a Micrometer
Before using the micrometer to measure, it is worth repeating that both mic and work piece must be the same temperature and all surfaces must be clean and dry. Before use, check the mic against its standard to verifiy its calibration. Be careful to hold the mic and the standard by their insulators since heat from your hand can expand the standard and, in some cases, the micrometer itself. Wearing gloves is an option if you must handle the instrument for more than a couple of minutes, but this does impair your ability to feel the micrometer. Use the ratchet torque limiter to apply the required force during the measurement. Gaining experience in measuring by doing calibrations and working with the mics on highly accurate dimensions, such as new bearings, helps develop the feel needed to achieve accurate and precise readings.
EASA Technical Support Specialist Mike Howell authored an excellent two-part technical paper titled, “A Closer Look at Accuracy of Measuring and Test Equipment” focusing on micrometers. Published in Currents in November and December 2014, Mike goes into depth on this topic, providing valuable insight and helpful guidance for achieving repeatable precise measurements.
When assessing journals, it is necessary to take measurements at various points to check for eccentricity to be sure they are not tapered or oval. That will require measurements at multiple clock points on a shaft at each end of the length of the journal as well as the middle. Check at least three positions 60 degrees apart.
Once you have obtained a measurement, set the lock lever or lock nut to secure the mic for reading in better light or viewing angle. Use care in removing the micrometer from the measured piece to avoid damaging the anvil and spindle faces.
When reading a mic, always view the fiducial (index or zero) line, thimble and vernier lines from directly overhead. When viewed from an angle, the correct alignment position of the division lines cannot be accurately read due to parallax error.
To correctly read the measurement, begin by noting the highest ordinal number on the sleeve scale. That is your first decimal place. Next note the number of divisions after that number that are visible up to the thimble. On most inch mics, there are four divisions between each ordinal of 0.0250” (0.635 mm). For each visible line add 0.025” (0.635 mm) to your measurement. If any line on the thimble scale lines up exactly with the fiducial line on the sleeve, add that number from the thimble scale to the others to achieve a final reading with zero in the fourth decimal place. If no alignment exists between the sleeve and thimble, record the lowest number on the thimble below the fiducial line and add that, then read the vernier scale for the number that aligns exactly with a line on the thimble. Add that number to your total as the fourth decimal place (see Figure 4).
When readings are near the limits of the range of a bearing fit, take additional care to ensure that the measurement is accurate. That may involve re-measuring with another micrometer and/or colleague’s assistance. Also, keep in mind that measuring round or point faces may produce a result 0.0001” (0.002 mm) smaller than those from flat surfaces for the same face pressure. Closing the spindle at high rates of speed can produce an incorrect result, so take your time.
Final Points
It is a good practice after a journal is rebuilt to take the time to measure the finished product prior to installation. Everyone who follows this practice likely has a story of the improper shaft fit that nearly got away. You don’t need to have a premature failure and all the consequences that go with it for the sake of a few minutes spent confirming a journal dimension and integrity.
With bearing fits having a min/max range from 0.0003” (0.008 mm) for small bore bearings to 0.0009” (0.023 mm) for larger ones, there is not much room for measurement error, whether due to poor calibration, dirt, temperature variation, faulty technique or any combination of them. Eliminating these conditions is critical to reaching and maintaining the standards of ANSI/EASA Standard AR100: Recommended Practice for the Repair of Rotating Electrical Apparatus and meeting the expectations of our clients.
AVAILABLE IN SPANISH
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
Related Reference and Training Materials
Print