EDGE RETENTION/ROLLING PART II

[Larrin]

If the edges were losing sharpness due to deformation wouldn’t they be much, much more sensitive to hardness?

[grepper]

I think the original thinking was that if a blade was hardened and then sharpened, the very act of sharpening could cause plastic deformation of the edge resulting in the edge being de-tempered. 

However, that does not appear to be the case.  Total roll is actually less on blades sharpened after hardening than the blades that were not post-sharpened.

The thing I notice most that there is little significant difference between the hardened and rolled, and the hardened, sharpened rolled blade sets.  Actually the blades that were sharpened after hardening rolled slightly less indicating that sharpening does not negatively impact roll resistance.  (Mr. Reich told us that a long time ago).

I didn't calculate the the initial sharpness/sharpness after roll percentage differences between the two sets of blades or all of the other stats the could possibly be gleaned, however there does not appear to be enough significant difference for real-world use between the two sets  of blades to indicate anything noteworthy.  

I'm getting the idea that, even though it's really tempting to think that it is, the hardness difference between HRC 50 and HRC 62 blades is just not enough to make a significant roll resistance difference in real-world use of knives.  Furthermore, HRC =>60 blades are harder to sharpen and more prone to chipping even if they may be easier to deburr.

[Bud]

If I understand the question correctly Mr. Larrin, I think that's whats being said here with these tests, none of these edge apexes are hard in terms of how we might think about them. They aren't stiff, they are flimsy and it doesn't seem to make much difference whether they are Rockwell 50 or Rockwell 62. Maybe a different test with lighter weight would make some differences show up but apparently not with 150 grams.

[grepper]

"Maybe a different test with lighter weight would make some differences show up but apparently not with 150 grams."

I think you summed it up nicely Mr. Bud. Additionally, it is worth noting that slicing a 3" round potato with a sharp blade requires 3-5 POUNDS (1362g-2270g) of force.  Testing for roll resistance with less than 150g would indeed be a curiosity and interesting but the resulting data set would have little real-world use applicability unless the goal was to slicing single sheets of paper, etc.

For me the key consideration is what does all this mean for real-world use of my knives.  I'm sure that I couldn't really tell the difference in 10%-20% difference in roll resistance when slicing my evening's broccoli, zucchini, kohlrabi or even acorn squash.

[Larrin]

I believe the differences would show up with more weight not less.

[EOU]

Interesting way to look at the problem Larrin and we would propose that both could be true; more weight and less weight. If enough force were applied to begin bringing in the underlying and thicker metal into the calculation then one would suppose that a more brittle metal would have a greater resistance to rolling. It would seem to be more prone to chipping, but less to rolling. However, the design goal of the SET unit was to reproduce the sort of common rolls that are produced through ordinary and typical usage. 150 grams seems to have satisfied that need. In other applications, perhaps a greater application force would be appropriate. 

With regard to less force, we would say that it might boil down to this; if some physical effect on the order of "foil effect" renders HRC50 and HRC62 edges, essentially,  indistinguishable from each other then a lesser force is not going to affect the outcome comparatively speaking. If there is some difference between the two then a lesser force might help measure that difference. 

Speaking to our general audience here, we would like to remind everyone that while these tests, to date, might not seem to indicate a lot of difference between HRC50 and HRC62 steels with regard to edge rolling that there does seem to be a very large difference between hardened steels (HRC50+) in general and unhardened steels. Our tests on unhardened steels indicated roll rates far in excess of hardened steels.

[Jan]

EOU, I appreciate the exchange of posts concerning the results of the section two of your experiment. I think everyone has their own piece of truth.  

When the weight of 150 gf was sufficient to roll the edges of all four samples (HRC 50, 54, 58 and 62), we have to use smaller weight to find the difference between various HRC numbers.

I expect that using this smaller SET weight we will see substantial drop of sharpness for samples with smaller HRC numbers and significantly smaller drop of sharpness for high HRC numbers.

EOU, would it be digestible for you to imagine that the sharpness drop of a rolled edge is caused by the fact that the applied force has to be resolved in two perpendicular forces, where the vertical force, which actually cuts, is always smaller than the applied force? 

   

Jan

[Larrin]

Maybe if you used zero grams then the difference would be found. Deform the edge with air only.

[Jan]

Larrin, zero grams would not be helpful, but air has some weight. I think three cubic feet of air would be sufficient. What do you think?

Jan

[EOU]

Well there you go again Larrin, another interesting approach. We're fairly convinced that there should be some measurable difference between HRC50 and HRC62 (if for no other reason than that the supporting metal is more, or less, brittle) but confess that the only evidence we have of that is based on some pretty loose indicators. First and foremost of those indicators is the fact that non-hardened steels seem to produce the mother-of-all rolls. Even with 150 grams, perhaps we could readily see the difference between, say, HRC35 and HRC54. Anyway, use of a stream of air or a lighter weight might enable us to see these less significant differences, assuming those differences exist. 

Jan your proposal is not only digestible but exactly in line with how we envision the measurement process unfolding. We don't know that for certain but it makes sense until we learn otherwise.