How sharp is sharp?

[SharpOp]

I'm glad others are also finding the possibilities of edge measurement interesting.

As for metal memory, I've been experimenting recently with a bunch of sets of cheap, stainless kitchen knives that were bought years ago for practice.  I'd almost swear that some of them have "recurring burr" syndrome.

Of course, I could be confused.  ;^)

[Herman Trivilino]

Likely it's a brittle edge breaking off. Try a more blunt angle.

I bought a couple of extremely cheap pocket knives a couple years ago. Impossible to sharpen. If I made the edge angle extremely blunt I could prevent the edge from breaking off. Of course with an angle that blunt they aren't good for much other than loaning to neighbors.

[Ken S]

I started this topic by describing how I had tested the edges of my two paring knives with the KN100 edge tester. The less sharp edge tested 625 grams to break the testing media. The manufacturer rates that in the range of needing to be sharpened. Not terrible, but not good.

I have been experimenting with using valve grinding compound with the leather honing wheel. It does not leave as polished a surface as the Tormek compound, however, it cuts faster. I did some VGC honing with the paring knife. No work with the grinding wheel or steel. Just VGC honing.

I have a premeasured 375 gram weight. (It is a peanut butter jar partially filled with BBs turned upside down so that the flat lid sits on the KN100.) 375 grams is the manufacturer's top range (375-475 grams) for "factory sharp". A good sharpener can easily best that, however, it is a good starting point. The knife cut the media at that weight. I did not do any more precise testing.

I should state that I have two leather honing wheels. I do not think it is a good idea to use different grits with the same leather wheel. I can interchange the two wheels in seconds.

Using just the VGC leather wheel as a quick and dirty touch up seems of limited value to me. However, I do think it has potential when used with the SG grinding wheel in its coarse state. This could be useful for anyone doing a lot of knife sharpening, such as at farmer's markets. If the technique works well, it would eliminate the time needed to regrade the stone to fine. The finished edge is not as highly polished as when the knife is sharpened with both grits of wheel and the Tormek honing compound. For slicing knives this might actually be an advantage, as we like to add tooth to the edge.

I would welcome comments from other members who work more withknives than I do.

Ken

[SharpOp]

Likely it's a brittle edge breaking off. Try a more blunt angle.

It may be, but it really seemed as if a burr were reappearing on a honed edge.  Maybe the coffee is too strong.

Around here, I couldn't even give these knives away (no recognized and desirable brand names!) and, if I could, it would ruin my reputation forever. They're really terrible. Maybe they could be re-purposed for icing cakes and other jobs where edges aren't really required.

[Ken S]

Doug, be nice.....Herman never said that he liked the neighbors..........

Ken

[stevebot]

Here is a video I made about knife sharpness testing.
https://youtu.be/rMi9TyOXUcg

[SharpOp]

Doug, be nice.....Herman never said that he liked the neighbors........

;^)

[Jan]

As for metal memory, I've been experimenting recently with a bunch of sets of cheap, stainless kitchen knives that were bought years ago for practice.  I'd almost swear that some of them have "recurring burr" syndrome.

In my thinking, the physical reason for the "metal memory" or "recurring burr" syndrome may be the residual stress, which remains in the steel near the cutting edge after the sharpening process.

It is not fully clear to me, why the residual stress occurs, but a small edge asymmetry may be one of possible mechanisms.

The residual stress can be relieved by suitable thermal or mechanical trigger. Because the stropped cutting edge is often in a state of fragile equilibrium a tiny trigger may be sufficient.

Jan

[Magnus Sundqvist]

What about testers that measure the ability to stay sharp?

There are several ways to test a knife for sharpness but I'm more interested in ways to measure the steels ability to withstand wear.
If the knife is sharp or not is more dependent of the sharpener, the ability to keep the sharpness over time is more up to the manufacturer of the steel and the blade.

There is a British organisation called Catra that has a quite nice machine that measures how many paper strips the knife can cut over a series of strokes. The paper is pushed with a pre determined force against the edge.

I'm looking for something like that but a bit more simple, so I can have it in my shop or bring it when I lecture.

Have you seen something that can do the job?

[Ken S]

Magnus, you have asked the  right question at the right time. Edge On Up (www.edgeonup.com) has just introduced a lineup of edge testers. I just purchased the HT-50 model. The new HT-50 model is substantially easier to use than the KN-100 model I have used for a year. The website has a series of excellent short videos explaning setup and operation.

The HT-50 is quite portable and quick to use in the field. I think it would be an excellent choice for you when traveling.

In one of the videos Mike Brubacher begins with the test reading for a brand new  double edge razor blade fresh from the factory wrapper. He then makes only one small cut cutting through only half of the razor blade wrapping paper only one time. The difference in sharpness measurements was impressive.

All of the Edge On Up edge testers utilize the BESS standard. (Brubacher Edge Sharpness Scale) This is a scientific but simple standard which is consistent and repeatable. Questions you might receive from knife customers in Australia, the Czech Republic, the UK, the US or even Sweden, would be in a common "language" and repeatable.

Check out www.edgeonup.com. You will find Mike very helpful.

Ken

[stevebot]

I visited Mike Brubacher of Edge On Up a few weeks ago and one of the things we discussed was an edge retention tester. I hope that between the two of us we will have a prototype to show at the 2017 Sharpeners Report convention. Mike has experience with mechanics and load cells.  I built a prototype tester that slices and records the force vs the position on the blade. The retention tester will be somewhere in between - cutting at only one point on the blade but recording the force as a function of the amount of material cut.

There are a lot of questions to be answered:
What media to cut?
What to do about side friction?
When is a knife dull beyond use?
Which is better? Blade A starts at 200 and dulls to 400 in 10 ft. Blade B that starts at 300 and dulls to 400 in 15 ft.  Blade C starts at 400 but cuts 20 ft before it dulls to 425.

[Jan]

Steve, I think you have to find some formula for the edge retention. For example, it can be defined by the ratio of the length of cut vs drop in sharpness.

Then:
edge retention for blade A = 10 / (400 – 200) = 0.05
edge retention for blade B = 15 / (400 – 300) = 0.15
edge retention for blade C = 20 / (425 – 400) = 0.80

Following this definition the edge retention of the blade C is the best. 

The proposed formula, however, favors blades with low initial sharpness (blade C). This should be corrected by suitable additional term or formula modification. 

Jan

[Jan]

Steve, the modified formula for edge retention could read:

edge retention = length of cut * initial sharpness / final sharpness

For your example we get:
Blade A = 10 * 200 / 400 =  5.0
Blade B = 15 * 300 / 400 = 11.3
Blade C = 20 * 400 / 425 = 18.8

So, also based on the modified formula, the edge retention is the best for the blade C. The modified formula does not favor blades with low initial sharpness as the previous one. 

Jan

[Magnus Sundqvist]

The fact that you measure the sharpness on one single point makes me doubt this method a bit.
You can take a sharp knife and bounce it on your fingers without cutting your self but if you would move the knife just a bit you would immediately cut yourself quite badly. Do you see what I mean?

Anago in New Zeeland has a really nice tester as well that tests the sharpness over the whole edge in one sweep, sadly it's not very portable.
http://www.anago.co.nz/knife-sharpness-tester/

But neither this tester measures ability to withstand wear, unless you repeat the test over and over.
Perhaps it's back to paper cutting again...

//magnus

[wootz]

ANAGO tester is in the CATRA price range, both 100 times over the Brubacher's.