Many thanks for your reply, the mic's I have belong to my Father and I believe now from what you say it was a crude way of checking the calibration your self and then zeroing, before QA QC got so hot,all his mic cases contain an old penny and I was just trying to remember the procedure
The way I know to verify a micrometer is with a standard block.
It is possible your father used a standard block, then measured a penny and made that "his standard" so he did not have to buy and carry around a standard block with him.
Have never heard about this before, and the guys I learned from became machinists during the WWII era, so going back a few years. A penny, or any other object made of a soft metal, is probably the worst possible material to rely upon for any kind of gauging or calibration. A simple dowel pin is a far better alternative if you are checking for wear or OOT throughout its range. A 0-1 mic is self-evident to field check zero. Any shop that is making anything of importance or consequence will have at least a few gauge blocks, even nearly a century ago.
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
Let's not forget that it's not ANY penny, it's a very specific penny; to that end, it's a very repeatable go/no-go test of the instrument, assuming that the testing is done exactly the same way each time.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!
Seems to me, you'd get a considerable variation depending on where you hit on the penny, they DO have designs on both sides. I would think using a specific washer would be considerably better in that regard. Or measuring the penny diameter might be more consistent than the thickness.
This one has had me scratching my head for a couple of days. I just can't imagine someone who makes his living off of measuring things doing this for no reason. So I decided that this needs a theory that fits the situation.
My thinking is that zeroing a micrometer would be the obvious answer for absolutely every time one could zero a micrometer. But is there a situation when you cannot zero it? Is there a type of micrometer that cannot be zeroed? Also, are we sure that the penny was used in the thickness dimension?
Obviously, you can zero a 0-1" micrometer (yes, we're using imperial measurements, since they're vastly superior to all others). But you cannot do that with a 1"-2" micrometer, or 3"-4", etc. But a penny is only about .75" diameter, so it wouldn't span the anvils, even on the smallest non-zeroing micrometer, so I'm assuming that these are not plain micrometers with flat-faced anvils.
However... What about a tri-flute micrometer? This does not zero.
What about a set of calipers, when using the tips of the ID measuring jaws? One can measure the ID of rim that goes around the outside of a penny and verify that the tips that are touching the work are calibrated if they have worn down somewhat. Average calipers are often less precise than the garden-variety micrometers, so a penny *might* be suitable down to 0.001", as opposed to 0.0001".
What about an ID micrometer? A few that I saw googling around looked like when they zero, the pins are not touching. Could the known OD of a penny be used to verify the inside distance between the pins of an ID micrometer?
@SCHUE: Can you post a photo of the micrometers in question? Admittedly, if they're just plain micrometers, then I can't figure out what the penny is for.
Engineering is not the science behind building. It is the science behind not building.
Depending on criticality of the measurement, decent machinists often will check their measuring tools at 3-4 points just as any cal shop would. Mic threads wear over time at commonly used measurement points as do the anvils. An ordinary penny might not be too repeatable, but a lightly ground or turned penny may be a good cheap gage for checking mics, personally I always used gage blocks for that purpose.
If you've never seen a machinist check the anvils on a mic for flatness and perpendicularity using an optical flat and monochromatic light source, there's some interesting youtube videos on the subject.
Today I thought I was about to solve the mystery. I was visiting a potential vendor (whose long career in industry was quite apparent) and when he pulled something out of one of his desk drawers to show me, I noticed a mic with a coin in its jaws. You can bet I was on pins and needles to wait for that part of the conversation to complete so I could ask him what the heck the penny is actually for...
Finally, it was time to explain to the grizzled veteran my level of nerdiness and beg to be enlightened of the significance and relative function of the two devices, monetary and metrological.
He said, "Well, the other day I was on a phone call here and I picked up this mic and a nickel that was in the corner of the drawer..."
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