A Leeuwenhoek microscope project

[John HaineParticipant @johnhaine32865]

Right. Matthys suggests making one-piece suspension springs starting with PB strip, say 3mm thick rather like SK's bar, and milling down a region to a suitable thickness. His method is to clamp the strip to a vertical surface and use the side of the milling cutter to remove a portion to form one side of the spring; then turn the half-finished piece over and re-clamp, fill the void with plaster of Paris, and machine the other side. Feels risky to me…

But there's also an interesting graph in his book that shows that it is only the very top of a spring that bends under tension, so springs don't need to be nearly as long as is often thought. That's why my new spring is only 6mm long. I was thinking that maybe one could make one-piece springs using SK's method with a ball-nose mill from each side.

[Michael GilliganParticipant @michaelgilligan61133]

Fair enough, John … and due respect to Matthys

My reasoning is that, like a good palette knife, a thinned spring would [should?] be contrived to bend gradually along some reasonable distance.

… I am not enamoured of ‘Stress Raisers’ and using a spring [supporting a heavy pendulum] which only bends ‘at the very top’ sounds to me like a ‘Stress Raiser’ in every sense of the term.

MichaelG.

[S KParticipant @sk20060]

The zero is about 0.5mm high. Note that the field of view is more constrained in the photo than what you can see with your eye.

I tried milling the shape of a hinge as a test. Expecting failure, I used brass rather than the phosphor bronze, because I only have a little of the latter and it was quite expensive.

I had a strip clamped horizontally and milled it using the ball-end mill across the surface, just as I made the stage seen above. I went about half-way in and flipped it over to mill the other side. I just left one end of the hinge free while clamping the other, rather than trying to keep both ends clamped. This resulted in some chatter as it thinned out, but it went surprisingly well. Clamping it to a plate at both ends is a much better idea, though.

As expected, brass is a terrible material for a hinge. It has no spring to it and will just permanently deform or break rather than bend. But the main problem was that, despite efforts to maintain parallelism, I wound up with a hinge that was slightly thicker at one end than the other. Again, clamping it to a fixture plate rather than holding it in a vise (as I did) would be better.

My feeling is that this approach is more difficult, and provides dubious added value, vs. using a thin strip of spring steel, etc. Almost the only advantage I can see in this approach is that there would be less or no need for stiffening chops at the ends.

 

Edited By S K on 03/06/2023 19:33:50

[Michael GilliganParticipant @michaelgilligan61133]

Posted by S K on 03/06/2023 19:26:35:

The zero is about 0.5mm high. Note that the field of view is more constrained in the photo than what you can see with your eye.

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Thanks for that

MichaelG.

[S KParticipant @sk20060]

BTW, I lean towards agreement with Matthys: A spring should be as short as practical. A long spring will only cause trouble due to it allowing the top of the pendulum too much freedom to wobble about.

For a typical 0.004" thick spring, I'd think that 0.25" long should be about right.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by S K on 03/06/2023 19:41:11:

BTW, I lean towards agreement with Matthys: A spring should be as short as practical. […]

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Agreed … which is one very good reason for thinning it.

… especially if you can also avoid those pesky ‘Stress Raisers’

MichaelG.

[S KParticipant @sk20060]

Another thought from my little trial: I don't think it's especially "risky" to cut this sort of hinge using a ball-end mill, as long as you don't try to make a longer flat hinge section. That is, two half-round channels, back-to-back, seemed easy enough to make "safely." You want to make sure both ends of the spring are clamped properly, and flippable in place, but I don't think more, such as packing, is needed.

That limits the spring length to basically a line rather than a strip. You might have to make it a little thinner than typical for a flat spring. The end result is about as short a spring as can be made. And for this, I think a ball-end mill is a better choice than side-milling or plunging from the edge. To reduce stress in the hinge, you may want to use a larger ball-end for a wider cut.

I think I'll try again with brass, this time clamped to a table rather than held in a vise. If I get a more even thickness across it, I'll try it in bronze.

But will it be better than a strip of spring steel, etc? I remain in doubt.

Q: What are these "stress risers" being discussed?

 

Edited By S K on 04/06/2023 02:51:45

[Michael GilliganParticipant @michaelgilligan61133]

**LINK**

https://en.wikipedia.org/wiki/Stress_concentration#:~:text=In%20solid%20mechanics%2C%20a%20stress,greater%20than%20the%20surrounding%20region.

MichaelG.

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P.S.

Predictably enough, I have failed to find a failure-mode-analysis for a clock suspension spring … but this, about knife blades, illustrates the underlying concept quite well:

https://knifesteelnerds.com/2019/04/15/how-stress-risers-lead-to-broken-blades/ 

Edited By Michael Gilligan on 04/06/2023 05:15:55

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 04/06/2023 04:36:02:

**LINK**

https://en.wikipedia.org/wiki/Stress_concentration#:~:text=In%20solid%20mechanics%2C%20a%20stress,greater%20than%20the%20surrounding%20region.

MichaelG.

.

P.S.

Predictably enough, I have failed to find a failure-mode-analysis for a clock suspension spring … but this, about knife blades, illustrates the underlying concept quite well:

**LINK**

Edited By Michael Gilligan on 04/06/2023 05:15:55

Michael's link opened a box of breakfast delights for me. 'What controls toughness' leads via 'Effect of Retained Austenite', to a forum post on Cryogenic Heat treatments.

When heat treating steel, I guess we all understand the importance of of heating the metal to the correct temperature for sufficient time for the internal structure to change before catching it by cooling rapidly. Never occurred to me that cooling below room temperature might result in a further improvement.

In most workshops heat treatment ends naturally at room temperature, but there are steels that benefit from going much lower: 'For example, a fully austenitized 1020 steel with 0.5% Mn and 0.4% Si would have 1.1% retained austenite at room temperature, 0.3% at dry ice, and 0.1% at liquid nitrogen temperature.

More info on Wikipedia

Now I want a few gallons of liquid Nitrogen to play with…

Dave

[S KParticipant @sk20060]

I milled a hinge in phosphor bronze.

The strip is 1/8" thick and about 0.5" wide. It was cut using a 3/8" ball-end mill, and the tops of the troughs are about 1/4" across. The minimum thickness seems to be about 0.003".

This time I had it clamped flat against a table, and such that it could be flipped precisely in the same position. After milling, I sanded the troughs lightly to remove most of a tiny central groove that the ball-end mill tends to leave behind.

This material is a lot springier than brass, but it will still deform or eventually break if bent more than some modest number of degrees. This happens without warning. The amount of bending that is tolerable is fine for a pendulum, and across that range it acts as a decent spring hinge. Whether better than a strip of spring steel, etc., is unclear.

The big down-side of this sort of hinge is fragility – they are very vulnerable to even slight fumbling. Some experimentation with different thicknesses should be done. But I think typical strip of spring steel or beryllium copper, etc., should be much more robust.

 

Edited By S K on 05/06/2023 23:58:25

[Michael GilliganParticipant @michaelgilligan61133]

Impressive that you got the minimum thickness so small

But it does, I think, serve to demonstrate the problem that I was suggesting we need to avoid.

… not only is the thickness transition quite steep, but you will inevitably have machining marks [i.e. additional Stress Raisers] running in the most unwelcome direction.

As a “thought experiment” … Compare that to a spring-steel strip, thinned from [say] 0.004” to 0.002” by the method I proposed.

MichaelG.

[S KParticipant @sk20060]

I don't view the transition to be steep where it matters – it's rather gentle, really. You can use a larger ball-end mill to make it more gradual if desired. You could round off the shoulders, too, but I don't see that as being a big source of problems.

But yes, you would want to polish out any machining marks in the troughs. I should have polished the edges of the strip before the milling, too, but it was just a test.

Even what I did so far was decent (light sanding with 400 grit using paper wrapped around a rod). But with hand-sanding it's almost impossible not to thin the edges of the hinge more than the center. Maybe a jig could help, but it still wouldn't be perfect.

That idea about thinning an already thin strip – have you tried it? I wouldn't have confidence that it would work out well for similar reasons.

The big take-away for me is that this metal, cold-worked 510 phosphor bronze in "spring temper," thinned to ~0.003", still does not have the right sort of spring to it, and is certainly too fragile, to be used in a practical clock pendulum.

Because I'm a romantic, I might still try to build it into a pendulum. But I'd want to compare it to an alternative, such as my knife-edge pivots, e.g. to see which results in the best Q.

[John HaineParticipant @johnhaine32865]

I guess the other point to remember is that there are thousands of pendulum suspension springs flexing 86400 times a day for decades clamped between square cornered chops, and how often do they actually fail?

[S KParticipant @sk20060]

Build everything else and save the thinning for absolute last, then gingerly hang it. It should be good then, and possibly better – I don't know.

At least one world-beating clock used this technique, so it's worth a look.

Edited By S K on 06/06/2023 18:34:03

[Michael GilliganParticipant @michaelgilligan61133]

Posted by S K on 06/06/2023 17:46:21:

.

That idea about thinning an already thin strip – have you tried it?

.

For this particular application, no … but the general method, yes

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

In a bid to get this thread back closer to microscopy … here’s an interesting freebie from the RMS:

**LINK**

https://www.rms.org.uk/resource/nature-s-smallest-glasshouses-visible-from-space.html

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Fig.18 might encourage amateur constructors … although I am rather concerned by its description as “a simple, but remarkably effective, miniature, compound microscope” since, to the best of my knowledge it is not a compound microscope 

MichaelG.

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Edit: __ See also https://microscope-antiques.com/algen.html

Edited By Michael Gilligan on 07/06/2023 08:57:34

[Michael GilliganParticipant @michaelgilligan61133]

… and a little more detail here: **LINK**

http://microscopist.net/ThumE.html

MichaelG.

[S KParticipant @sk20060]

Diatoms are fascinating!

I'm trying to decide on a next project. I could make a Mk II version of this, or else maybe a compound microscope.

As with my previous gravity pendulum project, I have an interest in replicating classic scientific experiments. A few ideas might include be the Millikan oil drop experiment or Young's double-slit experiment.

There's an interesting variation of the double-slit experiment called a Mach-Zehnder interferometer. Instead of using two slits, photons are split across two paths by a half-mirror. It's important to not fall into the presumption that individual photons are traveling through one path or the other. In fact, they are traveling through both paths simultaneously in a "superposition" of the two paths, even though the two paths may be very far apart.

Other mirrors are then used to redirect the two paths back together, where they encounter another half-mirror. If light was considered to be particles, you should see half the light exiting the second half-mirror one way, and half the other. But if the photons are waves, interference between the single photon traveling in superposition down the two paths should occur. If set up right, a detector at one exit of the second half-mirror should show all photons exiting with probability 1 (due to constructive interference), while the other should show none (due to destructive interference). Indeed, that is what is seen.

But what if you destroyed the superposition of two paths by blocking one path (equivalent to blocking one slit in the double-slit experiment)? Then, there is only one path left, and hence no possibility of interference anymore. Both exits of the second half-mirror should now show photons exiting, with probability 1/2 for both. This experiment thus demonstrates the wave/particle duality in the same way as the double-slit experiment, but by using two discrete paths rather than a field of paths.

The next step is called the "quantum eraser" experiment. In this, you can apparently measure the path that a photon takes, thereby destroying its ability to interfere with itself, but then erase the measurement and allow it to interfere with itself again. This experiment is difficult for an amateur to do (properly, though a simpler version is more accessible) since it requires entanglement, not just spatial superposition. It's normally done via a special "down-conversion" crystal – one that produces two entangled photons of lower wavelength from one input photon. These crystals are available, but are quite expensive. Particularly sensitive and expensive avalanche photodetectors, etc., would also be needed, since you want to detect individual photons and not just "light."

The step after that is the "delayed choice quantum eraser" experiment. In this, you attempt to change the apparatus after a photon is already traveling through it (such as if you measure a state or block a path, etc.). The photon must then – seemingly, but not really – go back in time to change its state or path in response. This experiment is endlessly debated by youtubers and authors who are into woo-woo physics. Unfortunately, because light travels so fast, this experiment is extremely difficult to do.

Any other ideas?

Edited By S K on 07/06/2023 16:30:30

[S KParticipant @sk20060]

I am making plans to build an interferometer (e.g. the Mach-Zehnder type), but I think that's outside the scope of this forum. A Mk-II or compound microscope should also be coming, eventually. But for the immediate here and now, I'll probably return to pendulums, i.e. for a Arduinome-type clock.

As a coda to this thread, though: I had a couple of spare lenses and turned them into a pair of pretty awesome magnifying glasses. They are antireflective-coated two-element achromats. The larger one is 50mm diameter with a 200mm focal length, and the smaller one is 30mm diameter with a 150mm focal length. They are a little higher power than the nominal 250mm of a standard magnifying glass. They are super clear and sharp, and so much better than my old scratched up plastic one! I've found them to be very handy in my hobby room. 🙂

 

Edited By S K on 10/06/2023 18:42:39

Edited By S K on 10/06/2023 18:43:27

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