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The micrometer <link> is a device used to measure the outside
dimensions of objects, at best they can be accurate to 1
micrometer. This is pretty impressive. They're available in a wide
range of sizes and usually made of high-quality steel in a C-shape.
Because they're compact & portable they're
used in a wide range
of environments. This is a problem, because micrometers are
made of stuff, and stuff expands & contracts with heating and
cooling. That means that the steel C-shaped backbone of the
micrometer will expand, by roughly 10 micrometres per metre
per degree C. Or 1-5 um/C for a typical instrument. Worryingly,
that error is right in the range of what you're trying to measure.
That being micrometres.
What you're supposed to do about that, is perform your
measurements in a consistent temperature range. That's not
always possible. Maybe you want to measure a component in a
hot/cold environment to see if it's out of spec. If you take a cold
micrometer into a hot environment and make 5 measurements of
the same object, the readings will gradually decrease as the
instrument warms up. This is not ideal. What can we do about
this?
So, we change the shape of the tool from a C to a steel H. The
measurements are between the lower two vertical parts. The
cross piece of the H will be steel and subject to standard
expansion and contraction. Between the top two vertical sections
will be a solid cross piece of another metal. A good example is
brass, which expands at ~ 2x the rate of the steel. That means
that as the middle steel cross piece expands 1um, the top cross
piece expands 2um and, with the central piece acting as a
fulcrum, pushes the measuring surfaces between the two lower
vertical sections together by 2um. Thus, offsetting the 1um
expansion in the fulcrum. There may need to be some
experimentation with the two metals, lengths of each section and
mitigation of bending etc, but the principle is sound.
Now, you have a relatively temperature insensitive measurement
tool that should be more precise in the field, even if that field is
Antarctica, or space.
Micrometer
https://en.wikipedia.org/wiki/Micrometer
[bs0u0155, Mar 04 2021]
Negative thermal expansion
https://en.wikipedi...ansion#Applications
Maybe you could use a negative thermal expansion material [xaviergisz, Mar 04 2021]
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Problem is, now you have to also ensure that every component you are measuring is also made of the same bimetallic compound otherwise your measurements are going to be fluctuating in the opposite direction. |
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Perhaps the solution is to have a whole set of micrometers, each with a different bimetallic layering to give a different co-efficient of expansion. Then all you have to do is to choose the appropriate instrument to match the material of the object you are measuring. |
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Or a micrometer with a built in blow-torch to heat up the sample, that could work too. |
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//your measurements are going to be fluctuating in the opposite direction// |
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But there's nothing wrong with *that* fluctuation. *That* fluctuation implies that the things measured are getting larger or smaller with temperature. And they are. Unlike the fluctuation which this idea seeks to correct, it's not an artefact, it's just a facty fact. |
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Ok let's try another line of criticism |
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Your geometry is only going to give correct results at the maximum width of the opening. Because the screw also expands, this micrometer will be out by 1um when measuring very thin things, while a standard one will be correct (because the screw fully extended across the gap will expand just as much as the C) |
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//Or a micrometer with a built in blow-torch to heat up
the sample,// |
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I'm specifically not correcting the sample. It's the sample
in it's actual environment I want to measure. There's no
use disassembling a machine and measuring a specific
component when it's not under any load, and it's in
perfect lab conditions. The information we want is: "Shaft
c was subject to excessive wear because, due to high
operating temperatures and a compressive axial load it's
diameter exceeded spec." |
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You could put a small restive heater in the micrometer
and use that to compensate. With heat you can only
compensate up, so the instrument should always be at
something like a toasty 100C, which should discourage
theft and rust. |
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//Because the screw also expands,// |
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It does. I wonder if they take this into account, and
maybe
calibrate the things at half max opening to build in a
fudge
factor? I don't know, but I strongly suspect that a lot of
knowledge about how this sort of thing works was gained
in
the early days and now companies that make them, just,
keep making them the same way without knowing why. |
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OK well that makes sense now that you have specified more precisely what you are trying to measure. |
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Now I like the idea of maintaining the micrometer at a certain temperature. Make it 1000°C and you have just invented the all-in-one micrometer toaster. You can now enjoy freshly toasted toast which you know the thickness of to the nearest 1um. You could even measure the thermal expansion coefficient of toast, as it cools. |
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//fudge factor// Now that's the kind of thinking that got us into the pickle that you are valiantly trying to solve. |
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//Digital micrometer + built-in electronic thermometer// |
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The way to do this is probably through electronics, but that's
way less fun. |
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[bs0u0155]; that ^ I like. Even Lego Technic is going the way
of "just put motors everywhere and control it all with
software" instead of cool mechanisms and gearboxes and
stuff. Mechanical solutions are just BETTER (& easier to
understand & fix...). Although I might be biased... |
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I assume you're planning to design the brass and steel parts
so they change temperature at the same rate? Thermal
conductivity of brass is about twice that of steel (or up to
10x for stainless steel depending on the alloys used). Maybe
by carefully adjusting the thickness and adding fins to the
steel parts, you can get the metals to change size
simultaneously. Be sure to wear insulating gloves to avoid
heating part of the device with your hand. |
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// Even Lego Technic is going the way of "just put motors everywhere and control it all with software"// |
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Ugh... I know. It all started going wrong when they started with specific panels to increase realism... Great, My Lego Porsche does look a bit more like a Porsche, but 50% of the kit is useless for anything else. If I wanted something that looked exactly like a Porsche, I'd have bought a Tamiya kit or something. |
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//brass and steel parts so they change temperature at the same rate?// Thermal conductivity isn't a big factor here I shouldn't think. They're linked together and the main limiting factor will be the air-metal interface which will be similar. It's possible some compensation might be needed, but a thin coat of paint will probably be all that's needed. |
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Thinking about getting a 3d printer so I can print me
off some custom lego pieces. |
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If you're going to be 3D printing, you can make any shape, why
bother with the intermediate Lego step? Besides, there's a lot
more fiddling about with 3D printing than I imagined. A resin
printer could get you Lego blocks that fit right more or less
every time, but there's clean up involved. With FDM, you're
likely going to have to mess around with and adjust the final
part to get it to fit into other Lego blocks right. |
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When I was a child I had my grandfather's Meccano set (the earliest parts of it were his older brother's from before WW1 I think), and I always had this idea that there were Technical Lego people, and Meccano people. And of course I considered Technical Lego to be a kind of strange inferior plastic Meccano-substitute. |
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//kind of strange inferior plastic Meccano-substitute.// |
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I'm ok with both, although a look at the Meccano website
reveals they've gone the same way as Lego Technic. A big
difference for me was that the things you made with Lego
could be played with for a reasonable amount of time, with
Meccano, the nuts and bolts seemed to rattle loose all the
time, the key was to add two nuts and back the first into
the second. but that made all the joints clumsy. |
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// all started going wrong when they started with specific panels// Agreed, the barrel on the technic truck 42112 is just lazy or penny pinching. Or, on second thoughts, a possible puzzle challenge. |
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No lock nuts in Meccano? tiny bit of plumbers tape? |
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//tiny bit of plumbers tape?// |
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Something, something DET CORD! |
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After a half century or so of poor storage in an old cigar box, all the surfaces get slightly corroded and then everything grips very nicely. |
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I like it, but (without first reading any of the other
comments or actually going to check) I'd be really surprised
if they didn't already exist (or hadn't already been tried &
found wanting). |
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Used to do quality control work for a small engineering
firm turning out car parts & the like on lathes in my teens,
ashamed to say we didn't use to worry about this issue
overly much. |
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I imagine for important work the simplest & most
commonly applied method is just to use a temperature
controlled room. |
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& for anything really important a laser measure
used at both extremes of the parts
expected operating temperatures. |
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//commonly applied method is just to use a temperature
controlled room.// |
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This is essentially the definition of a lab. But You can't
always take a component into a lab. My grandfather used to
make & refit parts for ships for Cammell-Laird back in WW2.
There are situations where you might want to measure
something like a propeller shaft. This is a component that is
subject to temperature changes, heating due to a bad
bearing in one spot while the far end is in winter sea water.
All the while, you need to make measurements in an engine
room which can be extremely hot, or cold. It would be ideal
if we can measure something like that prop shaft without
disassembling the ship, AND if the measurement tool weren't
thrown off by how hot the room was at the time. |
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Have a suitcase of micrometers calibrated for different temperatures in 1° increments |
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Oops sorry, deleted that comment, it wasn't right, really
doesn't matter that different materials being measured
expand at different rates, that would be what you want to
know after all, what the parts maximum & minimum width is
at different temperature extremes, so you only need to
correct for the expansion & contraction of the micrometres
materials. |
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Wouldn't the lab be smart enough to have one end in the cold and one end at engine room normal for baseline measurements? Normal operating conditions not just perfect fit together measurements. |
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So these measurements are for prediction of wear, catching wear? The relative data set should still show change. |
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All that aside I'm not entirely sure an ordinary room with a
working
thermostat really qualifies as a lab ;) |
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There's a lot of DIY garage/bedroom sound labs now with vibration affected thermostats. |
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// it's not an artefact, it's just a facty fact// |
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//I'm not entirely sure an ordinary room with a working
thermostat really qualifies as a lab ;)// |
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It's a start. You need a few other things but most are
specific to the experiment you're running. The idea is a
controlled environment, a thermostat is pretty snazzy,
historically speaking. |
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I once changed the water in my brother's aquarium. I took
the fish and put them in a big glass cup, and then decided
that the water might be too cold for them so I put the
"thermostat" in the cup and plugged it in. The thing was that
the heating element was in the cup, but the metal
thermostat arm was hanging in the air not testing the heat. |
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I was able to get the aquarium real clean ridding it from all
that green algae (or however its spelled) when I looked up at
his beautiful fish which where all floating at the top of the
bubbling and boiling cup. |
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//I once changed the water in my brother's aquarium. I
took the fish and put them in a big glass cup// |
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Oh god, I could feel where this was going immediately... |
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You change SOME of the water, the fish can stay. If you
really need to move the fish, just use one of you're spare
tanks that you have already set up as a
quarantine/hospital tank... we all have those don't we
class? If we need cleaning products we use
peroxide/bleach/vinegar as these can all be neutralized
in some way. Soaps/detergents shouldn't be allowed in
the same area code. |
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Maybe what is needed is a thermal response that slightly twists and untwists the threaded rod to shift the pitch of the threads with temperature. I imagine that a wire spring wrapped around a core might be able to act as a threaded rod that changed length but did not change pitch. The read could not be via the standard collett but by a worm gear possibly? |
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Right, this is a better expressed version of what I was trying to say. So it is the screw which really needs to be compensated. And [bs] agreed and suggested that "fudge factor" be employed instead. |
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//The actual problem is with the ruler itself// |
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I largely agree with your (& poc's point), but I thought up
the mechanism first and because I liked it, started to
think about an application that it could be shoe-horned
into. This is a half-baked idea after all. |
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Also, the problem IS with the ruler, and everything else.
The ruler is subject to thermal expansion, but, the C/G
shaped support structure is much longer and subject to
some level of relatively increased temperature-induced
distortion. |
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I suppose what you could use this mechanism for is to
create temperature-independent length standards, but
that's not as fun. |
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//a wire spring wrapped around a core might be able to act as a
threaded rod// |
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I love that idea, [WcW], but wouldn't you have a problem with the
compressibility of the spring? |
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You would have a bimetallic screw, a copper screw and a steel screw, perhaps concentric and running in seperate threads, and then at their ends they are attached by a lever and cam assembly (a bit like the initial idea frame), disposed such that the length from thread point to anvil point is maintained constant through a range of temperatures. |
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In other words, miniaturise this idea and fit it inside the screw |
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