CBN Rods

[jackknifeh]

Do they have a break in period like diamond stones or do they keep the same aggressiveness as right out of the box?

[jackknifeh]

Thanks everyone for clarifying the 3-finger test. Surprisingly, I hadn't heard of it before. Guess I learn something every day. I suppose the name comes from how many fingers I would have left if I do the test wrong! :D

If I'm not mistaken (probably am) Mr. Carter explains this test as if he can get a good idea of how sharp a knife edge is using only the 3 finger test. I haven't used the test enough to be able to tell very much about an edge regarding sharpness. I can tell if it is toothy or smooth. But, I guess a dull edge feels smooth but so does a very sharp edge finished up to 8k or higher. So if an edge feels smooth that isn't the whole story as far as I can tell. OTOH, I can tell how sharp an edge is by slicing paper (news print of phone book) but I can't tell if the edge is toothy or smooth. It's amazing how well a sharp, really toothy edge will slice or shave. So for me to get a more complete idea of the condition of an edge I need to do both. Of course Mr. Carter has tons and tons more experience that I do. My understanding is probably first grade to his master's degree. He does say to become good at this test you need to perform it on a lot of knives with edges in different condition. Maybe after you perform the test then cut something enough times you start remembering a connection between the two.

Jack

[Cliff Stamp]

Do they have a break in period like diamond stones or do they keep the same aggressiveness as right out of the box?

I have not used the Spyderco diamonds, DMT diamond stones are very uneven as-boxed and the cutting aggression will not as much slow down as it will even out as they start cutting extremely unevenly. The Atoma stones do not have the same extremely rapid initial stabilization as they are deposited in a much more even layer.

Note in general, all abrasives will of course wear, and the rate of all wear has the same general behavior (fast first then slow).

If I'm not mistaken (probably am) Mr. Carter explains this test as if he can get a good idea of how sharp a knife edge is using only the 3 finger test.

Carter advocates using it with a simple shaving test, a sharp knife has to pass both.

I haven't used the test enough to be able to tell very much about an edge regarding sharpness. I can tell if it is toothy or smooth. But, I guess a dull edge feels smooth but so does a very sharp edge finished up to 8k or higher.

It doesn't sound like you are doing the test.

The method is to see if you can draw your fingers down the length of a knife, if you can then the knife isn't sharp. If you can not move your fingers at all (because you would be cut) then it is sharp. Again it sounds extremely silly to the point of being absurd, and yes it does take some care and a little sense.

[Surfingringo]

The question I always have with the three finger test is how much pressure to apply?

[jackknifeh]

It doesn't sound like you are doing the test.

The method is to see if you can draw your fingers down the length of a knife, if you can then the knife isn't sharp. If you can not move your fingers at all (because you would be cut) then it is sharp. Again it sounds extremely silly to the point of being absurd, and yes it does take some care and a little sense.

Maybe I am doing it wrong. Or looking for the wrong results. I place my thumb on the spine and 3 fingers on the edge very lightly. I then slide my fingers along the edge from heel to tip. Normally I only move my fingers 1/2" at most. If I feel resistance it's due to a toothy edge (like a hacksaw blade). It feels like the "teeth" are grabbing my skin. If I can slide my fingers very easily and the edge feels smooth (no teeth) I have thought it was a smoother edge after using 8k or so. Finishing with a 2k or below leaves the edge with a toothly feeling. These results (smooth or toothy) are both on edges that slice phone book paper very easily and shave arm hair in one pass. This is all I know to look or feel for or all I have done. Maybe I should check the directions again on Mr. Carter's site.

Jack

With this I test toothiness with the 3 finger test and sharpness by slicing thin paper or shaving arm hair if there is any that has grown back. :)

[Cliff Stamp]

The question I always have with the three finger test is how much pressure to apply?

This is an interesting question and it is what troubles most people because they imagine that Carter is telling them to push on the blade until their fingers are dented and then try to pull the knife which would obviously open your fingers up readily. However the force should be just enough to keep the edge in contact without your skin.

Now if you think about the physics, there is some minimum force which is necessary to exceed the rupture pressure of your skin (so it would be cut), even with a very sharp edge this isn't going to be zero. All this means of course is that the lighter you can press and still pass the test the sharper the blade scores.

At a basic level all the test is doing is measuring sharpness by the force required to cut a material, the sharper the edge the lower the force. With Carter's experience, most people can not apply a light enough force if the knife is as sharp as he makes it and thus he can judge the test pass/fail. But as you noted, if you can apply a very light force you could likely even "fail" one of his knives.

However like most tests/checks it isn't perfect, but I would argue that doing it (or something like it) is very necessary and to not just say shave=sharp because an edge can shave and be so rounded that it is almost harmless and you can press it so hard against your skin it just dents it with no ability to cut. The same edge has no ability to slice ropes, cut foods like tomato, etc. .

I place my thumb on the spine and 3 fingers on the edge very lightly. I then slide my fingers along the edge from heel to tip. Normally I only move my fingers 1/2" at most. If I feel resistance it's due to a toothy edge (like a hacksaw blade). It feels like the "teeth" are grabbing my skin. If I can slide my fingers very easily and the edge feels smooth (no teeth)

That is a fail on both counts, a pass is when you can't move your fingers at all without them being cut.

[Fred Sanford]

Thanks for the vid. Been sitting here trying to figure out what the heck CBN is. When using an acronym it's always helpful to list the real name at least once so people know what you're talking about.

[jackknifeh]

http://m.youtube.com/watch?inf_contact_ ... k1o70tMHYM



Above is the link to Murray Carter's 3 finger test. After watching it again I believe he is saying after doing this test on a bunch of knives you will learn how much pressure is required to cut your skin and not to go past that amount of pressure of course.

So, if I understand this right I might not REALLY KNOW how much pressure I can put on an edge that will easily shave my arm until I have reached the point of cutting myself. Like most tests to know where the fail or limit is you need to reach it. I guess. Out of curiousity I'm going to do this test on every knife I pick up to use. As of now I've only done it once in a while while sharpening. Obviously I'm not familiar enough with the test to understand it. I'm finally curious enough to email him for his opinion of what I'm doing. I've been happy with being able to determine toothy or smooth but it seems there is more to it. If someone with his experience emphasizes something this much maybe it shows more than I realize. If that is true Iwant to know what I'm missing.

Jack
PS
I think we are getting the original subject.

[Cliff Stamp]

When he talks about increasing the force, the blade has already failed and then you increase the force to see how much it fails.

Note there is nothing complicated about this check, it, like every test of sharpness which is meaningful is just a measure of sharpness as per how sharpness is defined which is :

-the force required (at the apex) to start a cut

A blade which is sharper just requires less force at the apex to make a cut.

If a blade is thus dull it will take much force at the apex to start the cut into your fingers and if it is extremely sharp the force required to start the cut is so low that you likely have difficulty even applying it and this is why he says you can not even move your fingers at all if the blade is very sharp because even very light forces are enough to cut your skin.

For those people who are a bit concerned about slicing their fingers off, you can do something very similar with a tomato. Just put the edge of the knife on the tomato, and with the lightest force you can apply, draw the knife back - the shorter the draw before the cut (and the lower the force you can apply) the sharper the knife. You will find that an extremely sharp knife will be very difficult to not cut the tomato immediately as at some level of sharpness they will even start to push cut and won't even require a draw and thus it becomes a matter of trying to apply tiny amounts of force.

There are of course ways to just measure the sharpness empirically which are more sensible but not everyone is willing to engage in numerical measurement. And again, the finger test doesn't require any devices, just your hand and a bit of sense.

[D1omedes]

Cliff, thank you very much for taking the time to make a video about the CBN rods. Have you had any experience with the diamond rods?

I'm not sure if the CBN rods are worth investing into if one must jump from those to the medium rods (as you explained).

[Bill1170]

Thanks for the vid. Been sitting here trying to figure out what the heck CBN is. When using an acronym it's always helpful to list the real name at least once so people know what you're talking about.

Cubic Boron Nitride

[Cliff Stamp]

Have you had any experience with the diamond rods?

I ordered a pair, I should have them in about a week.

I'm not sure if the CBN rods are worth investing into if one must jump from those to the medium rods (as you explained).

I am not sure what you mean by this exactly.

[gull wing]

Any suppliers have them yet? (CBN Rods)

[phillipsted]

Personally, instead of drawing the edge along my fingertips, I use my thumbnail. It is extremely sensitive and you can feel unevenness in the edge - and you can tell if the edge is biting into your nail without the risk of shedding blood.

To each, his own, I guess...

TedP

[Blerv]

Any suppliers have them yet? (CBN Rods)

I recall Cliff is the first outsider to get a set for some testing. Prob not a bad guy to run them by :)

[D1omedes]

From your video, and please correct me if I am wrong, you explained that there is a big jump from the CBN rods to the medium rods. I wonder if there is a less of a "jump" from the diamond rods to the medium rods. This would make sharpening a bit quicker, no?

[Cliff Stamp]

I will have a set of diamond rods in a week or so, I will update this then.

Yes, any grit in between will accelerate the sharpening process, ideally you want to split grits in half for high efficiency. If you do 500, then pick something close to 2000 if you want to finish at 4000 grit, try to keep the jumps uniform.

[sok]

Thanks Cliff for your impressions and video. Lots of good stuff. When I first saw that Sal was sending you a pair of these to critique, I figured he was confident in his product.

From just the sound of you using the CBN rods in the video, they seem more aggressive than the diamond rods.

I am a big big fan of the Sharpmaker and will most certainly buy these. I am still curious where these will fit in in relation to the diamond rods and also if these are more aggressive than the diamonds, why go with CBN as opposed to coarser diamonds. When it comes to Spyderco though, my default mindset is "Trust in Sal".

[Cliff Stamp]

... why go with CBN as opposed to coarser diamonds

In industry, diamonds are not used (or should not be used) to grind steel because of issues with heat build up and the fact that you can get diffusion based wear. However in manual honing I have not seen any data to show this is a significant concern.

There are marketing reasons to use CBN :

-it is being popular / branded
-it is different/new

it could also simply be easier to make a coarse CBN.

There are also some issues with diamond in how or what you do exactly. For example monocrystalline diamonds wear slow and have a steady loss of cutting. Polycrystalline diamonds fracture and maintain a much higher cutting rate/ability -but- the ultimate lifetime is shorter.

It might also be that the CBN is more durable in regards to abuse, and to be frank the hardness is so high that the advantage of diamond may not outweigh the issues. Note that CBN is so hard that on solid CBN wheels they will "stick" the wheel with a piece of aluminum oxide to grind away the bond and expose the CBN.

This is mainly conjecture though, I will have some information once I have the diamond rods.

[wrdwrght]

[video=youtube;xRQQgwRMDfU]http://www.youtube.com/watch?v=xRQQgwRMDfU[/video]

Thanks, Cliff, for the caution and reasons to proceed ever so lightly. I don't have the CBN rods but do have the diamond ones and would assume the same advice applies.