Polished edge retainsion

[Broos]

Knowing whether the blades tested are the same thickness and the same edge geometry is an important consideration for me when reviewing claims of one steel's superiority over another.

With a test using rope cutting to induce wear, and then cutting thread to measure sharpness, is it possible to have one knife requiring more force to cut the rope, yet test on thread as the sharpest? Vice versa could a duller/thinner blade cut the rope with less force than a sharper/thicker blade?

And if you answer yes to the above, wouldn't it follow that a given steel will have varying performance and edge holding at varying edge thicknesses and angles? If true, doesn't that prove that you need equal blades to make a equal comparison of steels?

Conversely, when testing with unequal edge thicknesses or geometry, you should qualify your conclusions by stating the differences between the edges/blades tested.

If a thin blade & edge knife of steel "X" has better edge retention than a thick blade & edge knife of steel "Y", then can you accurately state without qualifications that steel X has better edge retention than steel Y?

Don't know what my point was, but if you have one of Phil's knives, it is easy to see that his testing was important in how he has selected the geometry and heat treat for his knives.

We are talking about different tests. One is what I am doing - collecting numbers, building tables etc... And another - which is expert opinion, when experienced specialist looks at product and report his opinion. This is like testing wines. But in your case you have your educated thumb instead of wine expert tongue or aroma expert nose etc...

Phil's testing has quantified results in the form of numbers on a table, except his numbers are the force required to actually cut the rope, not a thread. Besides being an experienced knifemaker and knife tester, he is also a mechanical engineer who had a career in a technically demanding field. The moral of the story is when you consider your own testing akin to rocket science while comparing someone elses testing to wine tasting, you should always first make sure the other tester is not a rocket scientist!

 

[Phil Wilson]

Vassili, Thanks for trading information. I agree that the steels we work with all have different applications. I have used a lot of 420HC in the past for Chef's knives. Max hardness is only about 57RC but it is easy to sharpen and will roll and wrinkle on a hard cut instead of chip. Important since Chef's do not want steel chips in the food. An interesting thing happened today. I decided to make some cuts on the wood base I use for back up on cutting rope. This was too see if I could detect any edge loss just slicing on the board without the rope. The knife I had handy was a CPM S125V Meadows I had just finished up for the BACKA show here in Calif. After 100 slices 6 inches long, with the grain, at about 15. lbs on the scale there was no loss in sharpness. In fact the blade got sharper. I was surprised also. Seems like the alder wood acts almost like a loaded strop and refines the edge. If you have any soft wood available try this your self with one of your blades. I intend to try it with other steels as well. Phil

 

[hardheart]

With a test using rope cutting to induce wear, and then cutting thread to measure sharpness, is it possible to have one knife requiring more force to cut the rope, yet test on thread as the sharpest? Vice versa could a duller/thinner blade cut the rope with less force than a sharper/thicker blade?

And if you answer yes to the above, wouldn't it follow that a given steel will have varying performance and edge holding at varying edge thicknesses and angles? If true, doesn't that prove that you need equal blades to make a equal comparison of steels?

Conversely, when testing with unequal edge thicknesses or geometry, you should qualify your conclusions by stating the differences between the edges/blades tested.

If a thin blade & edge knife of steel "X" has better edge retention than a thick blade & edge knife of steel "Y", then can you accurately state without qualifications that steel X has better edge retention than steel Y?

Vassili does sharpen at the same angle, recall that he uses an angled block to hold his stones. And since they are at the same angle, they are also the same thickness up to the diameter of the thread, at the very least (I think you'd need something like a straight razor or thinner before the thickness of the thread would be greater than the steel at the top of the bevel)

For cutting the rope, one end is left loose, so there isn't bunching at the cut. The strands can fall away as they are severed, and I think that the tested knives are at least close enough in primary grind angle (5-7 degrees, maybe?) that it wouldn't make any difference. The rope is what, 1/2"? How much difference in thickness is there amongst these blades at 1/2" above the edge?

I think the main difference aside from edge finish is the slice vs push cut nature of the tests. They are using different media when measuring force, they are also using different cuts. Microchipping as one possible result of cutting is going to have a possibly more or less favorable impact, depending on the kind of cutting it is.

What I like most about Phil Wilson's testing is that he looked for an analog of a real world task that seemed repeatable and within constraints feasible for something done by hand in the middle of a field in varying weather and got enough precision to satisfy his needs in evaluating while staying relevant.

 

[nozh2002]

...This was too see if I could detect any edge loss just slicing on the board without the rope. The knife I had handy was a CPM S125V Meadows I had just finished up for the BACKA show here in Calif. After 100 slices 6 inches long, with the grain, at about 15. lbs on the scale there was no loss in sharpness. In fact the blade got sharper. I was surprised also. Seems like the alder wood acts almost like a loaded strop and refines the edge. If you have any soft wood available try this your self with one of your blades. I intend to try it with other steels as well. Phil

I will try this. I guess this is only right way to resolve this, instead of arguing...

Thanks, Vassili.

 

[Shaughnessy]

Hardheart,
Left you a visitor message. Thanks,

 

[Broos]

My recollection of Noz's video is that he is using maybe a 1" or so slice, not a pure push cut (I'll look for this again). What is the best definition of edge retention, loss of push cutting sharpness versus number of rope cuts using a 1" slice, or max cutting force versus number of rope cuts using a 3" slice. I do agree that these tests are different, though they are both trying to get the same place. Having one test using less slice is not a major factor for me. What is a factor in my opinion is that one test uses a direct measurement of the wear inducing cut force, and that method strikes me as a more pure measurement for edge retention.

For cutting the rope, one end is left loose, so there isn't bunching at the cut. The strands can fall away as they are severed, and I think that the tested knives are at least close enough in primary grind angle (5-7 degrees, maybe?) that it wouldn't make any difference. The rope is what, 1/2"? How much difference in thickness is there amongst these blades at 1/2" above the edge?

I understand your logic (I may have raised this point myself in past threads). So if we take a thin bladed dedicated cutter against a standard thick production edge thinned to 15 degrees. Both sharpened at 15 degrees/side. One being 0.008 at X and one being 0.03 at X (diagram below). Will both knives cut a 5/8" manila rope using equal force? The way I read you, you are thinking the answer is yes. If I had to bet which would cut with the least force, I’d put my money on the thinner edge/blade.

A thinner blade will cut longer just because it starts out with less friction and bind in the cut.

If a blade ground to .008 behind the edge and sharpened to an 20 degree angle to the stone does not roll or chip cutting rope up to about 25 lbs on the scale then the heat treat is right on.

Wayne & Phil will often specify the edge thicknesses of the knives tested up front, their testing is geared towards designing knives that will cut the rope using the least amount of force, and their test will get you towards this goal. Other testing methods may not tell you which knife cut the rope the longest requiring the least effort. The thread test requires you to assume that the force to slice 5/8" manila rope is directly relatable to the force required to push cut a thread. I still think this could be a bad assumption with varying blades and edges. We can agree to disagree.

 

[Vivi]

The thread test requires you to assume that the force to slice 5/8" manila rope is directly relatable to the force required to push cut a thread.

I don't think this is what Vassili is implying. Isn't the thread test simply being used as some sort of standard to measure loss of sharpness after cutting a specific media? As in a freshly sharpened edge will average a 20g thread cut, an edge that's cut rope 200 times will average 50g thread cuts, an edge that's cut rope 800 times will require 120g thread cut and so on.

 

[annr]

Is is possible to open that .bmp file on a Mac? My computer says 'no'.

 

[Broos]

I don't think this is what Vassili is implying. Isn't the thread test simply being used as some sort of standard to measure loss of sharpness after cutting a specific media? As in a freshly sharpened edge will average a 20g thread cut, an edge that's cut rope 200 times will average 50g thread cuts, an edge that's cut rope 800 times will require 120g thread cut and so on.

I understand, and didn't mention what he is implying. I was commenting about the differences in these two different test methods, and speculating about the issues that can arise when you are testing blades with varying geometry. I don't think it is a simple question, and could change my mind again.

 

[Phil Wilson]

Broos, I think you made my case better than I could. I am trying to predict the real world performance of a particular knife. Optimize for a specific task. We focus on edge holding but many other factors are tested as well. Durability of a thin fine edge. Over all ergonomics, balance and handle feel, no hard spots or sharp edges to raise a blister or accidentally cut. If we cut 200 or 800 cuts as the case may be on abrasive rope you are going to find out a lot about a knife. Cutting up to a force of 25 lbs is going to determine whether the edge is going to chip or roll. In real life say skinning an animal I would be surprised if the force was much more than 5 lbs. Filleting a salmon would be much less that that. We get into trouble comparing knives of different manufactures with different steels at unknown hardness and try to say one will hold an edge better than the other. The tests that I have put up here were with knives of known hardness and very close geometry and sharpening basically illustrated a few large differences between a few steels but also showed that another group with about the same hardness pretty much performed the same. Interesting stuff and I enjoyed the discussion. Phil