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TOPIC: What grit/micron should I have mirror polish?

Re: What grit/micron should I have mirror polish? 2 years 3 weeks ago #4345

  • PhilipPasteur
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I suppose that I am beating a real dead horse here, but
Shouldn't we have some kind of definition of "mirror edge"
before we try tell people how to get one?

I just finished a Ken Onion Foresite, suposedly an initial production blade in (depending where you read) Acuto 440 or Acuto +.

I went through the diamonds form 100 grit. I reprofiled to 18 degrees, just to see how this steel would do.

After the 1000 diamonds the blade was plenty sharp, but pretty much a matte finish.
I sued the Chosera 800 grit for 100 strokes, still matte finish. I went to the 1000 grit Chosera stones for 100 strokes. Now I was getting something that was pretty reflective, but still not what I call a mirror edge.

I progressed through the 2K and 3K Chosera stones, now I could see detail in the reflection form the blade. At this point I would call it real darn shiny, but not a mirror. I think at this point well taken photos on a macro level would show clear text reflected.

I went to the 5K Chosera stones for 150 strokes. Now it was real shiny. Good enough given the angle of the reflection to make real impressive pictures with text reflecting off of the edge. Still if a light was shined pretty close to along the edge and the eye close to parallel to the blade, there was a bit of fog.

I went to the 10K choseras fro 150 strokes. Now we are getting close to a mirror. It is gleaming with incident light, but at at angle close to 180 degrees, still a bit foggy.

I went to a Naniwa 12K Superstone for 150 strokes. Now it is getting close to your bathroom mirror. The overhead light quite detailed in the edge, without picking the best angle.

I have a 15K Shapton set still to try, but I skipped that and went to strops with 6 micron, 3 micron (DMT paste) and then 1 micron and 0.5 micron diamond on leather. I don't think that this changed the reflectivity much at all. I only did 100 strokes per grit. Just enough to refine the edge a bit and give a bit of convex grind to it. The knife is really scary sharp and glitters like freshly polished chrome.

So what is a mirror edge? Obviously to me, that is relative to the observer, the kind of steel and how much the edge is polished. The constant in the conversation is, finer grits make for a more polished/reflective edge. The more reflective the edge is, the more mirror like it is.

If you aren't getting what you consider to be a mirror edge, use finer grits !!

As a direct answer to the OP, for my definition of a mirror edge, you need to get to the 5K Chosera level (or Shaptons, but according to Tom, who loves the Shaptons, they don't polish as well). That is about 2.8 microns (2.94 for the Shaptons). You can definitely get there with pastes or 3M lapping films at the similar grit levels, but it will take much longer. There is just not as much abrasive involved in the pastes, sprays or films as the better stones.

Phil
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Re: What grit/micron should I have mirror polish? 2 years 3 weeks ago #4346

  • AnthonyYan
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I like to use the optics standard for mirror finish, which means that a surface is smooth enough to do high quality imaging, such as for a camera, telescope, or microscope.

Amateur telescope makers grind their own mirrors, and to test the quality of their polished mirrors, they use a laser-pointer test.
stellafane.org/tm/atm/polish/polish.html#Polished_Out


If the mirror is sufficiently polished, then all the reflected light obeys angle-of-incidence is equal to angle-of-reflection. (A fraction of the light is adsorbed of course, but that is irrelevant to our discussion.) This means that you should not be able to see any laser spot at all. This is because _all_ of the laser light reflects away from your eye (or camera), which means, you can't see it!

Unless of course, you are looking at it from the angle-of-reflection, in which case you are damaging your eye (or camera) with direct laser light.

In the picture above, the mirror is not polished out because you can see the front-surface reflection. This means that some light is scattering off in "random" directions from tiny residual scratches. Because light is scattered "randomly" by residual scratches, some of it is scattered into the camera, which is why we see it in the photo.

Once your scratches start going below half of an optical wavelength, you will be at or close to an optically smooth finish (ie: mirror surface). Loosely speaking, light cannot notice features which are much smaller than its wavelength. This is why astronomical telescopes are typically accurate to 1/10th wavelength. More accurate mirrors exist, but that is at the point of diminishing returns in terms of image quality. For astronomy a mirror that is accurate to 1/4th wavelength is considered to be the lowest quality that is still (somewhat) usable.

The optical standard(s) of mirror-finish are very high. Someday, I would like to attempt an optical quality mirror-finish on a knife edge for no reason other than my own amusement. :)

In practice, being able to read fine-print text in the reflection of your knife bevel is, in my opinion, a good enough definition of mirror-finish. Say, your eye 6 inches away from the knife and the text 6 inches away from the knife. One might also require that the surface have no noticeable "haze".

By the way, please remember:
(1) Do NOT look at a laser directly!
(2) Do NOT look at a directly reflected laser from a shiny surface. For example: windows, polished metal, etc.
(3) Do NOT look at laser light through any optics. Optical lenses and mirrors can focus laser light to the point where it will cause eye damage.

Doing any of the above can cause eye and/or camera damage. Even if the laser is a low-powered laser-pointer.

Sincerely,
--Lagrangian

P.S. The adsorbed light is not really relevant for our discussion in the following sense: Black obsidian (basically black glass) adsorbs a very large fraction of incident light (that's why it is black). But black obsidian has been used as mirrors in ancient times because it can be polished to a shiny flat surface. Flat and shiny enough, that you can clearly see yourself in the reflection.
en.wikipedia.org/wiki/Mirror#History

Just as black glass can have a reflection, even materials with relatively high adsorption can be polished to a mirror finish. Steel is more than reflective enough to make a mirror. For example, many high-precision steel ball bearings have a mirror finish. Here is a company that does optical-polishing of steel to create steel mirrors and optics:
www.precision-metal-optics.com/stainless-steel-mirrors.htm
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Re: What grit/micron should I have mirror polish? 2 years 3 weeks ago #4352

  • PhilipPasteur
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AnthonyYan wrote:
I like to use the optics standard for mirror finish, which means that a surface is smooth enough to do high quality imaging, such as for a camera, telescope, or microscope.

Once your scratches start going below half of an optical wavelength, you will be at or close to an optically smooth finish (ie: mirror surface). Loosely speaking, light cannot notice features which are much smaller than its wavelength. This is why astronomical telescopes are typically accurate to 1/10th wavelength. More accurate mirrors exist, but that is at the point of diminishing returns in terms of image quality. For astronomy a mirror that is accurate to 1/4th wavelength is considered to be the lowest quality that is still (somewhat) usable.

In practice, being able to read fine-print text in the reflection of your knife bevel is, in my opinion, a good enough definition of mirror-finish. Say, your eye 6 inches away from the knife and the text 6 inches away from the knife. One might also require that the surface have no noticeable "haze".

"Visible light is approximately in the range of 0.4 to 0.7 microns (okay, if you are really picky, the 0.38 to 0.74 microns). If you want, that's 400 to 700 nanometers, or 4000 to 7000 angstroms."

Is a quote that is helpful from one of Anthony's previous posts.
If we go with the half wavelength figure as indicating optical mirror quality we would have to polish to about 0.025 microns to get to this optical quality level. This assumes that the depth of the scratches we get are equal to the grit size, which is not always the case. Of course there is the additional polishing as the grit on the stones fractures and becomes smaller. We may get there with 0.1 micron abrasives as they break down... and with more time.

This site has an interesting chart:
www.warleypolishing.co.uk/technical/surf...nish-table-guide.php

They have two grades at the high end. Both claim a surface roughness figure of 0.5 microns. The difference is in the amount of reflectivity. It would seem from this that there is more to polishing than surface finish alone. There is the part that I call brightness. Maybe I can see a reflection enough to see print at 3 micron grit, but it is much brighter and better refined at 0.5 microns.

I like the 6 inches to the subject and 6 inches to the eye definition, because at least it is some kind of definition. Maybe we could refine it and state the font size that represents "fine-print". Perhaps a number 4 point font? The one thing that is not there, and without sophisticated measuring equipment, is very subjective, is brightness of the image. I am not sure how we cover that one.

One thing for sure though, we never can get an optical quality mirror surface on our edges. The max reflectivity of typical stainless steels is about 65%. This does not come close to the 90%+ (some as high as 99.9% in dielectrically coated flat mirrors as in the case of a couple of the star diagonals that I use)in an optical mirror.

Phil
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Re: What grit/micron should I have mirror polish? 2 years 3 weeks ago #4354

  • AnthonyYan
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PhilipPasteur wrote:
This site has an interesting chart:
www.warleypolishing.co.uk/technical/surf...nish-table-guide.php

They have two grades at the high end. Both claim a surface roughness figure of 0.5 microns. The difference is in the amount of reflectivity. It would seem from this that there is more to polishing than surface finish alone. There is the part that I call brightness. Maybe I can see a reflection enough to see print at 3 micron grit, but it is much brighter and better refined at 0.5 microns.

The science of surfaces is extraordinarily complex and I'm no expert in it. It can involve a huge number of different fiends, such as surface physics, tribology, friction, wear, optics, etc. In the chart from the Warley Polishing website, they're using R_a as a measure of surface roughness. You can find the formula at the link below. It's basically an average of absolute height of peaks and valleys.
en.wikipedia.org/wiki/Surface_roughness#Amplitude_parameters

Unfortunately, R_a is only a single number, and is only a measure of vertical deviations, so it's not clear how wide the surface scratches are. Perhaps it is reasonable to assume the width of scratches is about the same size as their depth? If not, then there are many different surface finishes with the same R_a, just as there are many distributions with the same average. This might account for two different surfaces having the same R_a but different levels of reflectivity.
PhilipPasteur wrote:
One thing for sure though, we never can get an optical quality mirror surface on our edges. The max reflectivity of typical stainless steels is about 65%. This does not come close to the 90%+ (some as high as 99.9% in dielectrically coated flat mirrors as in the case of a couple of the star diagonals that I use)in an optical mirror.

I think I would say this a little differently. In astronomy, high reflectivity is necessary because the light they are collecting is very dim. So, as you mention, they require reflectivity of 90% and higher.

But not all mirrors are used in astronomy. And so there are companies that manufacture optical steel mirrors (see link below). My understanding is that these are optical grade mirrors, just not designed for astronomy.
www.precision-metal-optics.com/stainless-steel-mirrors.htm

It may be my own personal terminology, but I would say glass has a mirror finish, even though, generally, glass reflects only a small fraction of incident light (because most of the light is transmitted). If you put a black screen behind a sheet of glass (to block transmitted light), then you could take a very high quality photo using the reflection alone. It just might be a bit dim (so you could increase the exposure time, etc.). I would say the same for the surface of still water.

I like to think of "mirror finish" as mostly about smoothness and somewhat independent of "reflectivity." But that's just my preference, and it is reasonable to me that "mirror finish" could mean "mirror like" and therefore have high reflectivity as well as being optically smooth.

Sincerely,
--Lagrangian
Last Edit: 2 years 3 weeks ago by AnthonyYan.
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Re: What grit/micron should I have mirror polish? 2 years 3 weeks ago #4355

  • AnthonyYan
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Here is another company that manufactures stainless steel mirrors. Since steel does not shatter, these are considered to be a type of safety mirror.

www.mirroredstainlesssolutions.com/


Not as bright as a conventional mirror due to the lower reflectivity of steel, but I think it would be difficult to notice unless you had a steel mirror and a regular mirror side-by-side.

Similarly, many people consider chrome plating to have a mirror finish. Yet chromium has a reflectivity of less than 60% for light which is perpendicular to the surface. In the link below, you can enter the wavelength of light in microns and get a plot of how reflectivity varies with the angle of incidence.

refractiveindex.info/?group=METALS&material=Chromium


Sincerely,
--Lagrangian
Last Edit: 2 years 3 weeks ago by AnthonyYan.
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Re: What grit/micron should I have mirror polish? 2 years 3 weeks ago #4356

  • PhilipPasteur
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You make valid points. They don't answer the question of how we define a mirror edge on a knife so that it is understandable to all. My original thought was that it is hard to tell a person such as the OP what grit they need to use to get a mirror edge (which was the original question) if we don't have a definition for a mirror edge. This came about specifically because I have had people show me knives that they thought had a mirror edge, but didn't look at all like the edges that I think have a mirror edge. All of our discussion about the minutia has not got us closer to getting a working definition.

Answering this question may be impossible as there seem to be many different types of mirrors with widely divergent properties in common use. Personally I would be quite happy to get my edges to the point where they look like the picture that you posted... Other than the picture, did you find out how they polish the surface. Maybe we can tell people that they have to match that process to get a mirror edge. At least that allows us to give some kind of answer based on a specific outcome.

:)
Phil

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