[Metrology] How to develop a feel for measuring round rods with a micrometer?

Just look, is easier said than done. That being said, it's getting easier and better, although you can see from my table below that it's still a work in progress. Locking the anvil helped quite a bit.
TSHS
11.93971.9391
21.93981.9391
31.93971.9391
41.939551.9394
51.93951.9392
61.93971.9391
71.93961.9392
81.93961.9392
91.93961.9391
101.93961.9392
mean1.9396351.939170
st dev0.0000870.000095

Much better in terms of standard deviation. Only a 1000 more measurements of practice before I can trust myself to get away with only 1 measurement!
 
Bob Korves said:
When working to tight tolerances, EVERYTHING matters. For instance, by just holding on to the micrometer with your hands while running a set of tests, it will grow, or warp, or both, and maybe more while you are doing the tests. To do fussy work, we must be truly fussy about everything that can change repeatability. I gave one example, there are MANY more subtle things that can change the measured results when everything else was done consistently from test to test. Beyond that, we hobbyists also have a penchant for trying to be far more fussy than what is needed for the project at hand (I plead guilty...). The first thing to determine is the needed accuracy for the measurement, and work to that standard as best as we can with the resources we have on hand.
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@Bob Korves, what I am trying to do is trust my measurements enough to align my lathe. My measurements initially showed a wide variance. This pretty much stopped my adjustments. So now that I'm getting a better handle on this, I can continue with alignment. No point of aligning a lathe to a random error!

I'm aware that handling of the mic, and temperature and temperature gradients can wreak havoc on measurements. What was bugging me was the lack of consistency. Found one of the bad things I was doing, and stopped it. Now the measurements are a little better. More practice wouldn't hurt either.
 
Aukai said:
I never trust my self with 1 measurement :grin:
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I don't trust a single measurement yet. Someday, when I'm all grown up, I will get cocky enough to trust 3.
 
WobblyHand said:
Only a 1000 more measurements of practice before I can trust myself to get away with only 1 measurement!
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This made me smile. When I was trying to learn to use a mic I remember spending WEEKS reading an entire gauge block set until I figured out how the mic worked and what it took to be consistent. Bob is right - everything matters. The good thing is that now I can do it once and know that as long as I paid attention to what I was doing the reading will be as true as my mic can read.

For what its worth, when I am working to really tight tolerances, I practice with my mics and gauges before critical cuts. I've learned not to get cocky because I know I'm only human. I assume nothing because when I do I end up having to start all over again. Being fussy about using a mic isn't just for new guys; its also for anyone who is looking to do fine work.
 
Every time I go to the shop it takes time to settle down enough to do serious measuring. It takes me quite a few times before it seems I'm doing it right. More practice and the eventual muscle memory might help.

Didn't quite write that quote quite correctly. I meant to say, "only 1000's more measurements (of practice) before I can trust myself to get away with a single measurement". Like any talent or skill, it takes a lot of practice to get it right.
 
Keep in mind that temperature and such matter, enough to throw off tenths measurements. Let alone hundredths. I doubt many people here could accurately use a hundred thousandths mic even if it were calibrated and in a clean room.

You're aligning a lathe that you expect to, at least for learning, be able to get +/-0.001 on. When you can hit that every time, then start worrying about getting tenths. You have to align them periodically as well, so that will be a good time to use your new skills to chase tenths on your 2 collar test.

I get that this is the metrology forum, but it feels a bit like using a sledgehammer to kill a fly. I'm pretty new at this stuff, so feel free to disregard if you really do need hundredths precision and have a mic that likely cost more than my car to measure it with. :)
 
ttabbal said:
Keep in mind that temperature and such matter, enough to throw off tenths measurements. Let alone hundredths. I doubt many people here could accurately use a hundred thousandths mic even if it were calibrated and in a clean room.

You're aligning a lathe that you expect to, at least for learning, be able to get +/-0.001 on. When you can hit that every time, then start worrying about getting tenths. You have to align them periodically as well, so that will be a good time to use your new skills to chase tenths on your 2 collar test.

I get that this is the metrology forum, but it feels a bit like using a sledgehammer to kill a fly. I'm pretty new at this stuff, so feel free to disregard if you really do need hundredths precision and have a mic that likely cost more than my car to measure it with. :)
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My crummy pure mechanical mic reads to tenths. I was only doing multiple measurements to see if my measurements are consistent. If they weren't consistent, how does one know what the measurement is? The crazy number of digits are only due to averaging or calculating standard deviation. No intention of measuring to hundred thousandths at this time.

That being said, I was burned by having taper. Have some parts that only go in part way - instead of all the way. I want to be able to reduce my scrap rate. There's only so many door stops I can have in my house!
 
Rock crushers have some pretty tight tolerances in them too...... :rolleyes:

Some times you go after the ten thous just cuz you can, not because it is needed.
 
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