A 2 micron discrepancy could be caused by temperature (as David says) a minute speck of dust, a slight difference in pressure from the ratchet or almost anything. I am surprised that your micrometer reads down to that level on most the third decimal place is usually only 0 or 5.

John

When using a digital calliper, I always check the Zero before taking the measurement.

2 microns is not a large error, but you may be finding the limit of resolution of the instrument.

It could be the clearance between Thimble and Barrel threads!

Fuel injection equipment was made with those sorts of clearances, for 500 – 600 bar pressures, probably even less with current high injection pressures. of 1,500 bar.

It could even be inconsistency in the ratchet, or its use.

Was the micrometer at the same temperature each time, or was it affected by the heat of your hand, or even your breath?

A Standards Room or Calibration Room will not consider measuring anything until it has "soaked" to the ambient temperature of the room for at least 24 hours.

Did you turn the ratchet and thimble at the same rate both times? So that the momentum was the same, as far as possible.

Taking an extreme example, you won't drive a nail home by pressing the hammer on its head. But you will when it comes flying down at speed!

Bear in mind that a human hair is about 125 microns (Varies between 75 and 175 depending on colour )

Howard

Edited By Howard Lewis on 16/03/2021 12:11:29

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From a previous discussion:

Here’s a useful ‘benchmark’ image, courtesy of Wikipedia:

…. a 50 micron human hair, and a 6 micron carbon filament in the same frame:

**LINK**

https://en.wikipedia.org/wiki/Micrometre#/media/File:Cfaser_haarrp.jpg

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MichaelG.

0.002 mm is nothing to worry about.

Tony

Did you clean the micrometer faces before zeroing? I use a piece of clean paper gripped lightly between the faces and then drawn gently out of the faces. Be sure to not grip too tightly and produce paper dust. With the anvil faces clean, close and zero. If you can indicate thousandths of a millimetre then your technique is going to be very good to get repeatable readings. If you have access to slip gauges then measure some and see if your technique with the ratchet or thimble agrees with the slip gauge, you will be doing well if you get repeatable readings and if the reading agrees perfectly with the slips then you have a fantastic mic.

Mike

Edited By Mike Poole on 16/03/2021 13:16:46

Edited By Mike Poole on 16/03/2021 13:18:13

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Unfortunately, it is entirely plausible that a two micron dust particle landed on one anvil during the time you were conducting the test … unless, of course, you have full HEPA filtration throughout your working environment.

MichaelG.

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Edit, for context:

https://www.ristenbatt.com/xcart/Vacuum-Cleaner-Filtration-Efficiency.html

Edited By Michael Gilligan on 16/03/2021 14:20:31

What do you make MC?

You could do a lot worse than have a look at Stuart Models, they start small and go upt a good size. Noel.

A 5 deg change in temp on a 0-25 micrometer will result in a reading error of 2um to 3 um . You can do that test yourself. Needed is a thermometer and take a reading at the temp it is at. Then hold in your hands until it has warmed another 5 dec warmer. Re zero and you will see the difference in the readings. As most things are relative, and not absolute, if you are measuring a steel part , and the total environment is around the same, so same temp part and same temp measuring instrument, the dimensions will still be correct if the mic was zero at that temp. Making parts to less than 5um always becomes a challenge, especially when they are larger than 30mm diameter and length.