micrometer / caliper storage: jaws open or closed?

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dr wogz

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Feb 5, 2009
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Land of Poutine!
Dial caliper, digital caliper, that device you use to measure material thicknesses to 0.001" (or hole dia or depths..)

I was once told, to never store your caliper with the jaws fully closed. While the documentation on why was never located, it's a practice I've been doing for years, leaving them open a slight amount.. To me, it make sense to help reduce stress on the parts, and that the parts are free to move (oh so ever so slightly!)

Does anyone have an argument as to storing them (the jaws) fully closed or partially open?
I was told by an old time machinist that the faces would rust if you left them touching. Most calipers are SS these days so I don't think that it is much of a problem. I always wipe the faces before I use mine.
Here is what I've heard...

1. Even stainless will show some signs of corrosion over time. I don't think you would have a problem with carbide faces though.
2. Temperature changes can cause wear on the thimble threads if they have a preload.
3. Pressure on the thimble threads over time can also displace the lubrication on the threads and lead to premature wear.
I learned as you did, that the jaws should be left slightly open. Leaving them open allows air to circulate to the faces and keep them dry, while closing them tightly would allow moisture to condense and be trapped between them which may, over time cause corrosion and reduce the accuracy of the instrument.
In my navy calibration days, we always left the jaws open, and all of our training material sited a variety of reasons why, all of which have been mentioned above.
Me too, I will be opening jaws when I get home. Interestingly though my Starrett case fits pretty tight, so it will just be a smidge!
I was never taught caliper storage etiquette. Pretty sure I always "intended" to close them when done, but, due to forgetfulness, have probably left them open 95% of the time.
Now I regret nothing!
For me, I close calipers and leave micrometers open. I leave micrometers open to whatever the calibration rod is for it so that I can verify it before use.
considering I got my calipers from harbor freight for $6, I don't think it matters much.
For me, I close calipers and leave micrometers open.

I always was taught to keep micrometers open, but never heard the same thing for calipers. I guess the difference is the amount of load on the frame. Zero for calipers.

Good point about the condensation and corrosion, but I have never seen it on any of my stainless calipers.
I was taught they should be left partially open to allow for thermal expansion/contraction in addition to the reasons listed above.
This is from Mitutoyo's quick guide on measuring instruments:

10. Notes on storage:
Avoid storing the micrometer in direct sunlight.
Store the micrometer in a ventilated place with low humidity.
Store the micrometer in a place with little dust.
Store the micrometer in a case or other container, which should not be kepton the floor.
When storing the micrometer, always leave a gap of 0.1 to 1 mm betweenthe measuring faces.
Do not store the micrometer in a clamped state.

no reasons given
Thermal expansion/contraction is a big issue. Think about how accurate your calipers are. Now, think about what would happen to the calipers/micrometers while you handle them. Holding your instrument for too long will affect the accuracy and even measurement.

If you close your mics at a cooler temp, but the temp in your shop rises between work sessions, they will expand and could potentially be difficult to open. After all, the spindle rotates while the anvil remains stationary.

Calipers aren't as big of a deal because they slide open/closed. The measuring faces do not rotate against each other so you can open/close them easily. However, that's not to say there aren't other reasons (as mentioned above) you should store them in an open position.
However, my Mitutoyo mic wears out the battery when it's left open.

If your calipers use the LR44 batteries (as most do), switch to the SR44 (silver oxide) variety. The LR44 and similar batteries, especially the cheap ones, are alkaline, and due to their lower open circuit voltage and steeper discharge curve they don't last long in calipers. The silver oxide variants are usually more expensive but I will pay it every time so I am not forever replacing batteries in the middle of a job. Their higher o/c voltage and flatter discharge curve work wonders...
The problem with mine is that is comes on by itself with any vibration, like a loud car driving by the house, if the jaw is open. So the battery dies about every month due to the mic turning on by itself too often. The tech guy told me that the sensor could detect movement up to one-millionth of an inch and that's why the mic turns on with the slightest vibration. I found it hard to believe it was that sensitive and asked him if he really meant one-millionth and he said 'yes'. Hmm.
I use calipers pretty frequently at work and mics for some fine measurements are not unusual. They get put in their case after use fully closed with the turn lock loose so that they are free to move. Micrometers are a different story. Before they are put in the case, they are turned fully closed and then backed off to relieve any pressure on the bearing surfaces. All of our measuring tools are checked for calibration annually and the only time we have had an issue is if the tool was handled roughly or dropped.

Technically, calipers are self calibrating. If the read 0 when fully closed they should be in spec; however, damage to the measuring face can affect accuracy in some circumstances.
Most micrometers, at least the better quality ones, are carbide tips. If you look at the shaft there is usually a slight color difference between the steel and the measuring surface. I have always left both instruments open for the above reasons too. Also, if you have more of an acidic base to your sweat, you should wipe down with a cloth and WD-40. Used to work with a guy that could rust stainless steel overnight....lol...