A Leeuwenhoek microscope project

In my drawer I had a smaller micrometer head that's found its way on to the opto sensor platform on the latest clock.

It adjusts the lateral sensor position. It came from my dad's come in handy collection of bits he salvaged when the electron microscope company he worked for closed down.

I printed a primitive flexure as a test. It wasn't printed to proper scale or with any real thought.

It certainly is flexy, but I'm not very happy with how it moves. It's actually too flexible and I think it would have to be stiffened a fair amount. It's printed in PETG, which I use for pretty much everything. Maybe PLA would be better.

It's also becoming clear that miniaturizing an entire table that moves properly on flexures will not be easy. I might abandon this approach and go with a much simpler hinge-type mount. That has the disadvantage of tilting forward or backward out of the plane of the lens as it's focused (as Leeuwenhouk's did), but it may suffice.

I also ordered a few 10mm diameter, 1.1mm thick borosilicate glass windows (uncoated) to use as subject slides. When I was a kid, I built crazy things out of bent nails and scraps of wood, and proper materials were simply unavailable. It's so awesome to now be able to order exotic materials with a click of a button!

Edited By S K on 17/05/2023 16:38:32

I printed out a hinged stage to see how well that might work, and it works pretty well. It's just clamped in place so I could test its behavior.

It's a simple technique, though it has the disadvantage of losing parallelism to the lens while focusing (the original had the same problem).

The translation ratio (stage movement / micrometer movement) is a little below 1:1 at the lens, so a small reduction, which is fine - I wanted to avoid amplifying the micrometer's motion.

I think the right strategy would be to position the stage tilted slightly up (higher) than the nominal focus point when no micrometer pressure is applied, but such that the stage is flat when the micrometer moves it to the nominal focus point.

One of the goals of this project is to gain some experience with machining, so I'd like to do this in brass, too. But the prints are very helpful to test ideas and work things out, including the important task of calibrating the point of focus.

Edited By S K on 17/05/2023 22:16:49

A tidy little hinge, which I suppose can be categorised as a ‘bell crank lever’

Incidentally: I must admit to being surprised by your 3D printed interpretation of the flexure mechanism … because what I had in mind was two pieces of shim-stock separated by a spacer at each end.

MichaelG.

.

Edited By Michael Gilligan on 18/05/2023 07:13:12

Yes, it's similar to a bell crank in function. Haha, haven't used the term "bell crank" since I was a kid making control-line model airplanes.

The effective focal length of the lens is 4.5mm. But because the lens is 3mm thick, the back focal length is only 3mm (about 4.5mm - 3mm/2). So a flexure-supported table should be about 3mm away from the base. I had imagined using 3 or 4 of the flexures I printed (or something like them - it was just a generic test structure) to support a stage at its edges. But it would have to be scaled down a lot further still, which looked to be a big problem - the arms were already only two extrusions wide. Maybe photo-etching could make parts small enough.

Anyway, the hinge it will be.

Referring to the hinge area of the printed part, I wonder how thin I have to make brass before it will bend appropriately over a few degrees? Or is it going to fatigue quickly and just break? I suppose I should test that before going much further.

Edited By S K on 18/05/2023 13:10:24

Good practice for machining suspension springs from solid? Matthys recommends phosphor bronze IIRC.

A traditional spring can be made from a length of Brass strip, which is usually annealed soft when new.

Gently hammering the strip down to about ⅓rd thickness should work-harden it into a reasonable spring. Not like a proper steel spring, but good enough for this application.

Now I want to make one, and I'm already far too busy!

Dave

Bronze ... for when brass just isn't expensive enough! 😄

I'll order some. 🙂

S K, it may be too far from the original concept, but have you considered a mechanism for moving the lens for focusing, rather than moving the specimen?

dave8

Edited By david bennett 8 on 18/05/2023 23:51:45

Not really. I wanted to maintain at least some resemblance to the original, and a steady flat face is a virtual necessity due to the shallow eye relief. He got that part right.

I ordered a sheet of bronze this morning, and it came this afternoon. McMaster is amazing.

It's cold-worked 510 bronze, and it definitely has a lively, springy feel compared to the dead feeling of brass.

Yes, I can understand that. I was thinking of your lens set in a threaded bush through the main plate (threaded ), which need not mar the flatness too much.

dave8

Yes, I can understand that. I was thinking of your lens set in a threaded bush through the main plate (threaded ), which need not mar the flatness too much.

dave8

double post

Edited By david bennett 8 on 19/05/2023 02:01:43

Oh, yes, I've thought about interchangeable lenses! The lenses are so small that they could easily be mounted in threaded receivers, to be screwed into the main body. It would be easy since I already have the materials and small taps and dies that would be needed.

But that might be gilding the lily? I am happy with it so far. Maybe for the Mk. II version. 🙂

Edited By S K on 19/05/2023 02:33:45

A little more progress: I've completed the stage, which takes the place of the plastic version seen above. It's made from 510 bronze, with a brass ball on which the micrometer pushes. I gingerly tested the flexibility of the hinge, and it will move, but I may have to thin it a little more - not sure yet.

I've decided to fit the "slides" (10mm glass windows) into little 3-D printed holders, which then drop into the 12mm hole. In the event that I get the lens-to-slide distance badly wrong, I can reprint the holders to adjust the slide's position.

Still left to do is create a few mm thick shim with a very slight angle to set the distance between the body of the microscope and the stage, and drill the microscope's body to accept the stage. However, I'll soon need to mount the lens, at least temporarily, along with a slide, in order to judge how thick the shim needs to be, and perhaps what kind of angle to set. Then I'll be nearly done.

Edited By S K on 20/05/2023 21:03:08

I'm calling this one finished. It's a Leeuwenhoek homage microscope rather than a replica.

The lens is a 3mm diameter coated achromatic doublet with a 4.5mm effective focal length and a numerical aperture of 0.3. This gives it about a 55-56x magnification, which is on the low end of Leeuwenhoek's surviving microscopes.

Mine deviated quite a bit from his approach, though I tried to keep it recognizable as following in his footsteps. Some differences beyond the lens:

• The body of his microscopes were two thin metal plates sandwiching the lens, and then riveted together. Mine is a solid piece of brass into which the lens is held in a reamed hole.
• His "handle" was a screw that adjusted the vertical position of the subject. Mine is the micrometer head, which adjusts the focus.
• His had a crude means of adjusting the subject's horizontal position, whereas mine has none.
• His used a needle-like point to hold the subject, whereas mine uses a stage with a 12mm hole, into which glass "slides" can be placed.

It's sharp, with quite little distortion or chromatic aberration except at the far edges. The contrast is seems lower than I expected, but that's probably because I haven't figured out how to properly illuminate the subject yet. The field of view is decent, and is wide enough that you can "look around" the image a little.

I looked at a slide of a tiny smear of my own blood. At about 55x, you can clearly see the red blood cells, but they are only pale tiny roundish dots. You can imagine that they have a particular shape, but it's not clear.

If I could go back in time and give this to Leeuwenhoek, what would he think? Certainly the achromatic lens would have amazed him. But otherwise, his microscopes revealed as much or more than mine could given their generally higher power.

It was a fun little project. I already have a lot of ideas for a Mark II version. 🙂

Looks very nice! How did you machine the circular neck in the bronze please? Maybe useful technique for making suspension springs.

Nice, mental 'note-to-self' filed away... ..thanks for posting..

I used a 1/4" center-cutting ball-end mill to cut the neck, just running it ball-end-down horizontally across the metal.

I think it wound up being 0.004" at its thinnest. I'd try for thinner next time, as it was still a little on the stiff side, but fine for this purpose.

To make a pendulum hinge, it would have to be considerably thinner still: 0.001" max, and likely less. Accurate work holding and precise tramming, to something like 0.0001" or better across the material would be needed for sure, lest one end of the cut be thicker than the other. Also, you would have to reverse the material and cut both sides equally. A lot could go wrong.

This bronze is reasonably hard and has a good ring to it. While it's much livelier than brass, I'm not convinced it's well suited for a spring as flexible as would be needed for a pendulum. I may try to make something as thin as I can to see.

IIRC the BeCu shim I have is 0.1mm thick = .004in? Did you use PB?