The three things that matter in photography of edges

[Cyrano]

Mike,

Here's what I have so far for photography of your rolled-edge samples.

I labelled your samples A, B, and C, with five inspection sites per sample:



Here are shallow-depth-of-field side views of all sites, with focus on the edge apex. The top row = sample A, middle row = sample B, and bottom row = sample C. In each row, sites 1 to 5 are presented in order, with site 1 at the left and site 5 at the right: 



I found sample B to be of the most interest, as the rough texture everywhere on samples A and C made edge details less conspicuous. 

For sample B, here are focus-stacked images of the sites seen from the side and seen from above:

Sample B, site 1:



Sample B, site 2:



Sample B, site 3:



Sample B, site 4:



Sample B, site 5:

[grepper]

Those are cool Mr. Cyrano. Thanks for posting and the effort. What is the black line running the length of and in the middle of Sample B, Site 5? Is that a very thick roll that is not reflecting light? Some of those are some gnarly rolls.

Mr. EOU, what were the sharpness readings of the sites and how many roll sets for each sits?

[EOU]

Very interesting Cyrano and thank you! The story behind Sample B is this Grepper. It was a factory edge but we subjected it to further deburring with a Sharp Pad and took 150 points off the edge in the process so that at point "2" the measurement was 238 and at point "4" 235. Then we rolled a 1.5" section centered at point 2 (5) cycles and point 4 (1) cycle. With post roll readings being "2" = 598 and "4" = 487. 

These are the best depictions of rolled edges we've seen and we're going to have to think a bit about what they mean. In some cases they seem to depict what we would expect and in others not. We'd really like to understand better just exactly what we are measuring with these rolled edges and it looks as if this photographic technique will help. These knives cost $4.87 each at Target Cyrano so please don't contemplate returning them to us.

[Cyrano]

In the two years since I last posted in this thread, I've added a metallurgical microscope to my toolbox.

This is blade B, site 4:

• The upper portion of the image shows an undamaged region of the edge apex. The edge apex is convex (perhaps from honing?), with what appears to be residual burr.
  
• The lower portion of the image shows where the edge has been rolled over. The rolled region has been pushed towards the viewer.
  

[grepper]

Great image!  Thanks for posting it and reviving this thread.  Once you know what to look for and understand what the image shows a rolled edge is obvious.  That is a excellent example.  

I know many folks who are not sharpeners think that edges dull due to abrasion, like the edge is worn away.  Unless the edge is used to cut abrasive material such as sandpaper, we know that edges dull because they roll.  Your image shows shows that rolling clearly.

It makes sense to think that harder steel like HRC 60+ would be more resistant to rolling, but EOU SET testing demonstrated that hard steel rolls just as much.  Additionally hard steel has the downsides of being harder to sharpen and more prone to chipping.   There is a reason why the major knife manufactures like Henckel, etc., generally use steel in the HRC 56 - 58 range.

From everything I've seen, our super sharp edges are so thin they roll regardless of steel hardness as show in that image.  I like to explain it as the bulldozer and egg syndrome.  A bulldozer lowered onto an egg will crush it.  Same for a dozen eggs even they should be able to support 12X more weight.  Slightly harder steel just can't compensate for the forces involved with everyday knife use. 

Want to see something cool?  Use the corner of  a razor blade or Exacto blade and slide it up the side of the bevel and push the bottom of the roll up and over the edge.  Then take a microscope image of it.  It will show very clearly how that roll is bent over metal.  It's strange, but you can actually push that roll up.  It's actually pretty wimpy and thin. 

Mr. Cyrano, that's a nice image.  What microscope did you get?  What magnification?  Bright-field or dark-field lighting?  I notice there are not any rainbow reflections so often seen with microscopy of steel.  Did you use a polarizer?

[KnifeGrinders]

Thank you Cyrano for taking time and effort to take this micrograph of a single roll on the Structural Edge Tester - it is spectacular.
I've added it to our last research What Edge Angle is good for Supersteels  as a demonstration of the rolling the SET does to the edge.

[Edgepal]

Old sharpening knowledge

When you look straight on the edge when you have the cutting edge pointing at you - you should se a black straight line, if you se this line, your edge are sharp.

A dull edge are white when you look at it in the same way.

[Cyrano]

@grepper and @KnifeGrinders: Thanks for the kind words.

My metallurgical microscope is an AmScope ME580T-PZ-2L. I'm quite pleased with its quality, optically and mechanically.

For these images, I used diffused external side lighting only. The composite images are the result of focus stacking approximately 150 micrographs using Helicon Focus. Final sharpening was done using Topaz Sharpen AI.

Here's the same region (blade B, site 4) at maximum magnification:

[KnifeGrinders]

Your last micrograph makes me climax - so beautiful.

[grepper]

That's beautiful Mr. Cyrano!  Great job with the stacking, exposure and post processing too!  Thanks for taking the time to do that tedious work and for sharing your results.

For anyone who doesn't know what focus stacking is:  Begin by focusing starting at the highest or lowest depth of the specimen.  We'll just say starting at the highest.  Take a picture.  Then focus slightly lower.  Take an image.  Repeat....  A tedious and time consuming process.  In this case Mr. Cyrano did that 150 times to produce that image.

Then take the resulting images into focus stacking software that analyzes and aligns them perfectly, combines them and  produces a final image.  

Why go to all the trouble?  Because at those high levels of magnification, in this case 400X, depth of field is extremely shallow.  EXTREMELY shallow.  Had he not focused at different depths and stacked the images only one very thin slice, like just the very top of the roll would be in focus.  Anything lower would be blurry.  

Here is an example.  On the left a single image.  Only a very shallow area of the subject is in focus.  On the right a focus stacked image.

   

Your image Cyrano so very clearly shows why it's called a "rolled edge" and why a rolled edge is dull.  

I had been looking at the ME300TZ-2L-10M but couldn't justify the cost and didn't really know what to expect in the image quality.  Now I do.  Thanks. And, now we know what true 400X optical means for edge microscopy.  Looks like true 400X is enough for knife  edges.

BTW, most excellent job with the lighting too.  Shiny surfaces like steel can be a real hair pulling freak show to photograph.  I know from experience not only how important that is but also that it takes time to get it just right.

I tried the best I could and posted images from my cheesy little USB scope, but your AmScope and effort blows them away. (Cry).  A real step up from the Dino Scope too. Your image Cyrano is the best I've seen on the Exchange.  Please do more!  (It's easy to ask somebody else to do work.)

Excellent image, great work and thanks again for sharing!