A Leeuwenhoek microscope project

My memory was wrong: It is probably about 0.04", or about 1mm, maybe slightly less. That looks about right. Unfortunately, I can't measure it properly without taking it apart again.

So if I was off by a factor of 10, maybe 0.004" would make an OK hinge after all. I'll have to try making one with it to see.

What is "PB"?

Edited By S K on 29/05/2023 19:30:52

Phosphor bronze

Yes, looking again at your photo about 1mm looks reasonable.  The technique looks worth trying though.

Edited By John Haine on 29/05/2023 21:28:36

John …

Given your starting-point [thickness of material] … might I suggest that you consider the approach that I suggested elsewhere:

30/03/2023 16:03:27

**LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=184730&p=14

… It has the great advantage of imparting a gradual change in thickness.

MichaelG.

Per Wikipedia: "Phosphor bronze is a member of the family of copper alloys. It is composed of copper that is alloyed with 0.5–11% of tin and 0.01–0.35% phosphorous..."

It was designated by the supplier as "cold worked 510 bronze" in an "H08 spring temper." Here's the composition:

So while it wasn't called "phosphor bronze" by the supplier (those sorts of names often don't mean very much), it looks to be in that family. It was more than twice the cost of brass or pure copper.

I did follow the machining step by some sanding with paper wrapped around a suitably-sized rod, but that was mostly to remove a tiny machining mark in the bottom of the hollow left by the very tip of the ball-end mill. I was worried that it would be a source of stress if not removed. As it gets thinner and thinner, I'd feel more worried about introducing non-uniform thickness through hand sanding.

SK, would it be possible to take a photo through your microscope? I would very much like to see what you see, iykwIm!

Regards,
Hans

Here's a picture of the "slides": glass in printed carriers that drop into the stage. One is designed with a longer lens to subject distance than the other.

And here's a picture of some micro-print. It's a pretty bad photo, really, as it was quite difficult to take. It's very hard to position the camera lens. It looks much better when viewed directly, but at least it's something.

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Give us a clue, please … approximately what actual height is the number 0

Thanks

MichaelG.

Thanks for this Michael, and apologies for this slow response. Presumably this method is mostly applicable to thin material, which would readily wrap round the former? It would be difficult with say 3mm PB strip, especially to unwrap it and do the other side to get a symmetrical "neck". Or do I misunderstand?

Yes indeed, John … I was presuming you would be starting with shim-stock

… as per conventional suspension springs

MichaelG.

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.

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.

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

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Thanks for that

MichaelG.

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.

.

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

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

MichaelG.

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

**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

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

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

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.