How to read a micrometer

Nice demonstration, Bill

… it reminded me of this, from one of my favourite books :

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MichaelG.

We stand on the shoulders of giants.

My suggestion of self enlightenment could be easier than relying on machining or emery - just measure and record all digits, then warm the item by a few degrees and record the new digits. Done at the same time with objects multiples in of the smallest item would clearly provide increases of multiples of that smallest increase - if done as a carefully controlled experiment.

A 1” inch by 2” block would likely be sufficient if the operator is able to measure both dimensions without causing a change of conditions. Results could easily be confirmed, as accurate, by comparison with the documented coefficient of expansion for that material.

We (should) all know that machined items should be allowed to completely cool before taking measurements - particularly important for close-tolerance fits.

Bill and NDIY make a good point about needing to account for temperature when taking high-accuracy measurements, but it's just one of several factors that reduce the accuracy of very fine measurements.

My feeling is good condition thou/0.02mm micrometers are on safe ground for practical purposes because their limitations (about ±0.01mm), hide a raft of errors that become significant when attempting to read tenths (0.002mm). Measuring tenths and better takes one across the border into difficult territory, where the operator has to get everything right. Owning a suitable micrometer in good order is just the first step.

Failing to account for sensitive error sources such as temperature means the measurements are unreliable. There's much more to measuring accurately below 0.001" than buying a micrometer, even a good one.

I suggest there are two good reasons for buying un-calibrated knocked about second-hand tenths micrometers:

• Collecting for interest. (I collect old slide-rules if I see them in charity shops, but rarely use them in anger.)
• As a way of doing a bit better than thou, but not expecting more than halving or quartering the difference. Even halving the difference requires the instrument to be checked against a gauge block.

The acid test is turn a selection of rods to various random tenths accurate diameters and have them measured by independently by someone who doesn't know what to expect. Both parties have to get the same answers. You can't mark your own homework!

Dave

Bill and NDIY make a good point about needing to account for temperature when taking high-accuracy measurements, but it's just one of several factors that reduce the accuracy of very fine measurements.

Dave,

My post was to reinforce that the OP could help himself by making careful measurements on items with slightly different dimensions, to determine how to read his particular instrument. Nothing particularly about absolute accuracy.

Understood ta. I hijacked your 'just measure and record all digits, then warm the item by a few degrees and record the new digits.'

We're both saying temperature alters dimensions, except I went a step further by pointing out accuracy requires temperature to be taken into account. I think I'm correct in saying modern micrometers are calibrated at 20°C whereas older ones were done at 64°F, causing a tiny error. Measuring tenths in an unheated workshop without recording the temperature is dubious.

I'm concerned new boys might waste money buying tenths micrometers due to misunderstanding what they're for. However, far worse things happen in my workshop!

Dave