Product to magnify and observe surfaces of small objects

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

Moderators: Pau, rjlittlefield, ChrisR, Chris S.

sludgemover
Posts: 1
Joined: Wed Mar 14, 2012 4:17 pm

Product to magnify and observe surfaces of small objects

Post by sludgemover »

I have a technical problem that requires an optics solution. I request input from this community to help solve the problem.

The problem: I have polished the shaft of a small nail (3/32 inch diameter, 1 inch long) to a very smooth finish using progressively finer sandpaper (from 400 to 12,000 grit), then diamond polishing paste (2, 1, 0.5, and 0.25 micron), and finally 0.05 micron diamond lapping film. After this polishing, the surface is extremely smooth and shines like glass. I want to experiment with different polishes to see if I can get a smoother surface on the nail (smoother than 0.05 micron). In order to know if a polish will give a smoother surface, I need to be able to see the surface of the nail shaft in great detail. What kind of equipment or product can I purchase (hopefully under $100) that will allow me to observe the surface of the nail in sufficient magnification to determine if my experimental product is polishing a smoother surface on the nail? The nail is used as an axle in a Cub Scout pinewood derby car.

I have considered jewelers loupes (20x to 60X), digital microscopes, and even a toy called an Eyeclops. I do not have the optics expertise to evaluate what product will provide sufficient resolution and optics quality at reasonable cost. I have also be cautioned that a lot of reasonably priced magnification products made in China have poor quality and may not provide a sufficient solution.

Any help would be appreciated.

Sludgemover

rjlittlefield
Site Admin
Posts: 21277
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Post by rjlittlefield »

Sludgemover, welcome aboard!

I am not an expert in abrasives or polishing. However, for what it's worth...

0.05 microns is about 1/10 the wavelength of green light. The scratches made by such a polish will be even smaller. You won't be able to resolve those with any optical method. Whatever you can see, if anything, will be larger defects left over from coarser abrasives. Those could be at any scale, so it's hard to say exactly what sort of observation equipment would be helpful.

I did a quick troll of eBay, searching for "microscope", and I see some astonishingly cheap new loupes.

I think it's safe to say that even though the lens is glass, $2.46 plus free shipping is not going to get you great quality in a Mini Pocket 45x LED Jeweler Magnifier Microscope Loupe. On the other hand, inexpensive lenses can be effective for visual inspection because you can use just the center of the field where the image is good. (Remember that van Leeuwenhoek did his work using just a spherical bead of glass.) Given the low price, I'd be inclined to give one of those things a try.

Moving up from there, it's a sizable jump to some sort of compound microscope. On eBay you can find quite a few used scopes for under $100, but the quality and condition is always suspect. Given your lack of experience, I'd recommend against going that route.

I wonder if you've considered making friends with the science teachers at your local high school? Most schools have decent scopes, and I'd think they would be happy to support a student project.

One last idea is to use an indirect measurement. While you won't be able to resolve scratches made by 0.05 micron abrasives, you can certainly detect light scattering off masses of them. That will appear like haze on the surface of a mirror. As you know, it's easy to detect haze on a mirror by going into a dark room and shining light off the mirror at such an angle that the direct reflection gets lost while the scattered reflection stays visible. I imagine it would not be too difficult to do the same thing with a small region of the surface of the nail, probably observing with a high power magnifier like the previously mentioned loupe. An idea, anyway.

I hope this is helpful. Interesting problem!

--Rik

g4lab
Posts: 1434
Joined: Fri May 23, 2008 11:07 am

Post by g4lab »

http://www.waynesthisandthat.com/awana.htm

http://www.pinewoodphysics.com/

It is not possible to evaluate the kinds of polishes you are talking about for less than $100.

The problem is similar to evaluating the finishes and wear patterns on hip and knee replacements. Working on a hip prosthetic ball made of cobalt chromium it was not possible to focus a scanning electron microscope very easily on the surface because it was so uniform. The investigator I was working for later purchased an interferometric device, from Wyko I think. There is no way to do what you want to do on the cheap.

The reference above is the best posting I have ever seen on the subject.
When my kids were in the pinewood derby some 23 years ago such treatments were against the rules.
Last edited by g4lab on Sun Mar 18, 2012 3:58 pm, edited 1 time in total.

ChrisR
Site Admin
Posts: 8589
Joined: Sat Mar 14, 2009 3:58 am
Location: Near London, UK

Post by ChrisR »

There's always some old crone saying "you don't want to be doing that" :roll:
Feel free to ignore this one!

Nails are soft steel. Low carbon stuff, not very good for axles, and also not made with any great need to be round. Is a surface finish (say) 20 times better than the ovality, any use to you? You could do a test with an HSS metal drill - the shaft is accurately circular, and pretty smooth, and they're hard - brittle though.
"Piano wire" is also more accurately made than nails, and harder. Model aeroplane people use it - I remember winding undercarriage springs with it :).
Why do you want a smooth surface ? Axles work best when lubricated. The thickness of the lubricant film will fill the gap between the axle and the bearing, so the surface finish isn't all that matters. The metal will ride on a film of lubricant, more or less.
Any newly abraded steel will rust, very fast, even within seconds, so if you look close enough, that's what you'll see.
Try running the stylus of a record-player over it, then you'll hear how rough it is!
Last edited by ChrisR on Mon Mar 19, 2012 7:11 am, edited 1 time in total.

AndrewC
Posts: 1436
Joined: Thu Feb 14, 2008 10:05 am
Location: Belgium
Contact:

Post by AndrewC »

Inspecting what you are doing is one thing and is interesting in itself - on the other hand is it really worth trying to find a better axle prep ? I think you could have more fun somewhere else. As it happens I spent the weekend polishing axles and wheel bores and buffing on a graphite lubrication layer

Building on what Chris said - true the nail, file and polish off stamping flaws, add a lubrication layer, reduce the contact area of your wheels and put your cog about a cm in front of the rear axle.

First heats for us on Wednesday :wink:
rgds, Andrew

"Is that an accurate dictionary ? Charlie Eppes

johnsankey
Posts: 20
Joined: Fri Mar 02, 2012 6:54 am

Post by johnsankey »

rjlittlefield wrote:One last idea is to use an indirect measurement. While you won't be able to resolve scratches made by 0.05 micron abrasives, you can certainly detect light scattering off masses of them. That will appear like haze on the surface of a mirror. As you know, it's easy to detect haze on a mirror by going into a dark room and shining light off the mirror at such an angle that the direct reflection gets lost while the scattered reflection stays visible. I imagine it would not be too difficult to do the same thing with a small region of the surface of the nail, probably observing with a high power magnifier like the previously mentioned loupe.
Given the small size of most scratches, this will be the only reliable way of measurement other than an electron microscope. Measure the angle of incident light (45 degrees from the surface is usual), also the angle of viewing (also 45 degrees from surface and 90 degrees from the incident light) then rotate your surface through a full circle, measure the average scattered light, compare with the direct reflected light, record the nature of the light (white colour temperature or whatever; pros use HeNe for things like this).

This will give you a measurement you can report as well as one you can repeat with different abrasives.

But as noted, keep everything bone dry to avoid corrosion overpowering your polishing!

DQE
Posts: 1653
Joined: Tue Jul 08, 2008 1:33 pm
Location: near Portland, Maine, USA

Post by DQE »

Just an ad-hoc thought, based on distant memories:

Aren't there specific standards and amateur-accessible methods for assessing the surface and optical quality of amateur astronomy telescope mirrors? Perhaps I'm thinking of the optical quality and not digs and scratches? The interferometry-based assessment of shape quality may be unrelated to surface smoothness quality. Can't quite remember how all this fits together.
-Phil

"Diffraction never sleeps"

Post Reply Previous topicNext topic