Getting started

[grepper]

Herman,

I did that many years ago and I don't remember the actual grits.  If memory serves, I started with the Tormek and ended with a belt grinder. 

Looking at it, I think you are correct that it required more intermediate grit to achieve a perfectly smooth bevel.  I know you are familiar with how, and have seen it yourself on blades, if you use a coarse grit and then quickly go to a finer grit, what you end up with is polished, smooth areas between grooves created by the coarser abrasive. 

So, to get a perfectly smooth bevel you either need to progress from a coarse through finer and finer grits, or spend a very long time trying to removing a lot of metal with the fine grit.  A lot quicker to do some intermediate grit grinding.

[grepper]

Jan,

You might also start dropping some hints about an early Christmas!

[Jan]

My early Christmas present arrived. 

I have got an off line working microscope with LCD display which can store the images on SD memory card. Optical magnifications are 50X, 125X and 500X.

Grepper and Herman thanks for your guidance! 

I am attaching two of my first images. The first is the edge of a new Morakniv. The magnification was 100X, so the length of the shown edge segment's is circa 0.7 mm (0.03").

The other image shows a thin layer of Tormek honing compound at 500X magnification. The size of the grains is circa 3 μm.

Jan

[grepper]

Very cool Jan!  Much better images than I am able to get with my cheapo USB scope.

Amazing how scratchy bevels and edges look up close, eh?

Exactly what scope did you get?

[Jan]

Thank you Grepper, you're too modest, your nice pictures of the burrs and edges inspired me to buy the microscope. Your guidance helped me significantly! 

I have bought a School LCD Microscope with 3.5" display and 5 MP CMOS sensor manufactured by Bresser GmbH. (Germany). http://www.bresser.de/en/Microscopes-Magnifiers/BRESSER-LCD-Microscope-8-9cm-3-5.html
Comfortable is that it includes a mechanical desk with vernier. http://www.bresser.de/en/Microscopes-Magnifiers/BRESSER-Biolux-Mechanical-Desk.html

I have bought it via Amazon.de for some 150 EUR (165 USD). 

What concerns the Morakniv blade I was also surprised that so many scratches rich to the edge where they weaken the edge and can act as a stress risers. 

Jan

[grepper]

Awe shucks, Jan.  Twern't noth'n.

What concerns the Morakniv blade I was also surprised that so many scratches rich to the edge where they weaken the edge and can act as a stress risers. 
Jan

When you get some time to play with your cool new toy, methinks you will find that a perfectly smooth, mirror bevel is an elusive beast indeed!  The problem presents well with magnification 100X and greater with good side lighting to show shadows. 

Also, the wavelength of light produced by the little LED's on my USB scope shows the grain in steel as different colors that can be mistaken for grooves.  Really a pain!  So I balance flashlights off to the side to get good side lighting that does not reflect directly back into the scope.  Don't know if that kind of problem will manifest its ugly head with your scope or not.  Then the sensor in my scope is sensitive to the PWM of my LED flashlights and can make bars in the images.  Another pain!  I have very few incandescent light sources left in my home, but that would work much better.  Anyway, I'm rambling...

Back to the elusive, perfectly smooth bevel:  Here is what I think is a 200X image of a blade that I sharpened on a belt sander with at least 5 levels of abrasives starting at, if memory serves correctly, 150 grit thru a 9 micron 3M Microfinishing film belt, then finished on a 3 micron (8000 grit) MicroMesh belt.  The MicroMesh belt feels about like grinding with blue jean material or a t-shirt.  Then I honed the blade with Tormek Honing Compound using a leather belt and finished with a felt belt and Mother's Mag and Aluminum Polish.

Now... You would think that would tame the elusive beast, eh?.   But, noooo!  Somewhere in the progression I didn't spend enough time and left some scratches from a coarser grit.   For all intents and purposes it looks like a mirror, but with a good little close up look-see, it's still far from perfection.  Geeeze!  What does it take!

In some future life when I have more time I might try it again, but it is illustrative of what is really happening during sharpening.

The original 5mp BMP image below was 14+MB and more detailed, which was then converted to JPG.  If you are looking at the cutting edge, please take into consideration distortion due to resizing and JPG artifacting distorting appearance the smoothness of the edge.  That blade was extremely sharp with a very smooth edge.

I'm looking FW to your images and thoughts on edges, burrs and sharpening!

[Jan]

Mark (Grepper), I fully agree with you that the "mirror bevel is an elusive beast". 

Vanity of vanities, says the Preacher,
vanity of vanities! All is vanity.
What does man gain by all the toil
at which he toils under the sun?  (Ecclesiastes 1)

My microscope may be a little bit more advanced than your USB scope but still far from being suitable for metallographic purposes. I do not have a polarisation set.

You are correct, metallurgical microscopes do not have LED illumination but use high intensity halogen light source. I am wondering why the halogen light is better than LED. LED light is coherent and as such may be more prone to interferometric effects which then obscure the observation.

I think we can consider the semi-regular grooves and scratches of a blade as a diffraction gratings causing the rainbow effect in the reflected light. Similar to ordinary pressed CD media.

Jan

[grepper]

I posted an image of the blade with a link pointing to an image in Photobucket.  Looks like Photobucket is down for maintenance and the image is not displaying in the forum, so I have attached it to this post.

Jan- I think you defined correctly the cause of how some of the image appear as a rainbow effect due to diffraction.  That is exactly how it looks sometimes.

[Jan]

Grepper, thanks for posting the image. If it is really 200X magnification than the edge is perfect. 

I am wondering what is the dark horizontal "valley" in the middle of the image. 

Jan

[grepper]

Yeah, that line looks odd as though there is some valley, but there is really nothing there.  I still have the knife and even though it has been sharpened since the image it is still the same.  The bevel is a smooth transition from side of the blade. 

However, there is a very thin, extremely highly polished area at the top edge of the bevel where it transitions to the side of the blade.  That happens because a malleable belt creates a convex bevel as belt flexes around the edge of the blade as it is sharpened and its first point of contact is the top edge of the bevel.  Exactly the opposite of the concave grind produced by a grinding wheel.

The extremely fine abrasive belts I used at the end of the process are very soft, flexible, cloth backed belts that very smoothly wrap the bevel.  The attached image shows just how soft and flexible they are.

This super polished, highly reflective mirror like area apparently was at such an angle as to not reflect light into the scope and appears to be cavernous, like looking at darkness in a mirror. Like I mentioned, the little USB scope has challenging illumination issues  to overcome.

[grepper]

If it is really 200X magnification than the edge is perfect. 
Jan

I wish I knew exactly what magnification my USB scope is.  It is rated @ 200X max optical.  It has a dial on the side with a super informative "+" on one side and a "-" on the other.  I have it turned all the way to "+".

I recently sharpened a knife and was playing around whittling human hair.  Below is an image of the hair I split with the knife and the other image is of the millimeter delineations on a steel ruler. 

I don't know what those images really mean.  Maybe one of you guys with a microscope with fixed, known magnification levels can make a comparison and form an educated guess as to what magnification I'm really looking at.

[Jan]

Thank you Grepper for your prompt and detailed explanation, appreciated. The absence of a death valley between bevel and side of the blade is for me really interesting. 

To estimate the magnification and the hair diameter from your images is not difficult, provided the scope setting was the same for both.* 

The hair thickness measured on my monitor is circa 5 mm. The distance between the mm marks of the ruler is circa 85 mm. So the hair diameter D = 5 / 85 = 0.06 mm = 60 μm.

The image of the ruler taken by your USB scope was displayed on my monitor with a magnification 85X, because 1 mm of the ruler corresponded to 85 mm on the monitor.



Jan

*P.S.: Also the processing and displaying of the images should be the same, not distorting the length relations. Most accurate is to work with the native scope format.

[grepper]

I'm feeling rather foggy minded at the moment, and I'm having difficulty correlating monitor display with real world measurement. 

Not considering the can of worms of comparing two different images, but just when you said:

"The image of the ruler taken by your USB scope was displayed on my monitor with a magnification 85X, because 1 mm of the ruler corresponded to 85 mm on the monitor."

Image files simply contain data representing RGB color values of pixels when displayed on the screen.  Wouldn't you need to take into account the PPI of the monitor, the image dimensions, and be sure that the image is being displayed without any scaling (even if aspect ratio was constant during scaling), and only use images that were not scaled to begin with?

It can't be as simple as just holding a ruler up to the screen and measuring the distance of one displayed mm in an image to one mm on a physical ruler.

I'm guessing I'm just having a DUH moment... 

[Jan]

It was not my intention to shock you by the simplicity of magnification calculation. Sorry for that.    The determination of the hair diameter shows that in principle this approach works well. 

It is similar to the situation when you have a map with a scale bar. You measure the distance in the map in mm and using the scale you get the distance between objects of the real world.

In our situation the magnification corresponds to the map scale, but is unknown. To determine the magnification we need to know two distances, one is the true size and the other is the size in the image.

Wikipedia says: "Optical magnification is the ratio between the apparent size of an object (or its size in an image) and its true size, and thus it is a dimensionless number."

Jan

[Herman Trivilino]

It can't be as simple as just holding a ruler up to the screen and measuring the distance of one displayed mm in an image to one mm on a physical ruler.

It is if, as Jan said, it's the magnification on your monitor. That tells you nothing about the magnification of your scope.

To see this take a photograph of some object with a camera and then carefully scale it so that when you print it the image is the same size as the object. Then post that image on a web site. When others look at that image they will see it with different magnifications. Some will see it reduced in size because they're viewing the web page on their phone. Others will see it enlarged because they're viewing it on a projector screen.