The Sebenza grind...

[Starfish]

Here's another question from a newbie to knives. What kind of a grind does my small Classic Sebenza have? I have seen the Sebenza's grind described as a hollow grind and as a convex grind. I would say I have seen it described as a hollow grind more than a convex grind, though.

If it is indeed a hollow grind, then here is another question. It seems to be an accepted "fact" that a hollow grind excels at shallow cuts, but is the weakest compared to a flat or convex grind. If that is the case, then why was a hollow grind chosen for a knife whose name means "work"? And since it obviously does work, why does a hollow grind on this knife defy the "fact" stated above?

Please note, as in my other thread, that this is not flame-bait. I'm just a newbie trying to stir things up...er....I mean learn something

 

[stjames]

Hollow (concave) grind, convex edge bevel. The superior blade steels available have allowed many makers to use a hollow grind instead of a flat one and retain sufficient strength.

 

[Starfish]

Thanks stjames, for the clarification. Although I am now a bit more confused about the whole hollow vs. flat vs. convex grind thing.

Let me ask these questions then:
(1) What advantage (or disadvantage) does the convex edge bevel impart to the user?
(2) What advantage (or disadvantage) does the hollow grind impart to the user?
(3) What advantage (or disadvantage) comes from using these two features together?

Thanks!

 

[Cliff Stamp]

Starfish :

What advantage (or disadvantage) does the convex edge bevel impart to the user?

It allows a more obtuse edge angle than a flat grind with the same level of cutting ability, thus the very edge is more durable however the edge as a whole is more prone to rippling. On the negative, a convex bevel is much more difficult to sharpen with benchstones than a flat bevel. It is usually sharpened with a medium with some give like an abrasive on leather, or a slack belt sander.

What advantage (or disadvantage) does the hollow grind impart to the user?

It provides relief for the edge thus raising the shollow cutting ability. It is also a very stable profile and does not get altered significantly with repeated sharpenings. On the negative it is much weaker and far less durable in regard to impacts than a flat grind, and even more so with a convex grind. The latter are not really much of an issue for a folder as the handle/lock will usually get damaged before the blade suffers a gross fault.

-Cliff

 

[Starfish]

thus the very edge is more durable however the edge as a whole is more prone to rippling.

Cliff, I think that this has happened to my small Classic Sebenza. There is a very small section near the curved portion of the blade that appears to have turned away from the true edge.

Is this what is meant by rippling?

Why is the convex edge more prone to rippling than the other formats?

Thanks,

Matthew

 

[Cliff Stamp]

Matthew :

There is a very small section near the curved portion of the blade that appears to have turned away from the true edge.

Is this what is meant by rippling?

Yes, it just means that the blade gets distorted. The very edge can ripple, the whole edge can ripple, or the whole blade can ripple.

Why is the convex edge more prone to rippling than the other formats?

First off it is easier to understand if you think of a convex edge bevel as two flat bevels, one that forms the very edge and one that forms the shoulders. This is the dual flat ground edge that Joe Talmadge has described generating with the Sharpmaker. To illustrate :

http://www.physics.mun.ca/~sstamp/images/double_flat_edge.gif

The first image is simply a flat ground profile or about 22 degrees per side. It is how many production blades are ground. The second image is of a dual ground edge. There is one bevel that forms the actual cutting edge which is of a more obtuse angle, and there is another bevel that forms the shoulders which provides "relief" to the cutting edge. If you just smoothed out this profile you would have a convex bevel.

Now since the actual edge angle is more obtuse in the double ground profile, the very edge is more durable. However because of the additional or secondary edge bevel which is very acute, a lot of material in the upper part of the edge has been removed which weakens it. Thus the entire edge can be distorted under a lower force / impact.

There is a similar problem with impaction for the two bevels. The flat one will take damage more readily under light loads, however as the impact energy increases then the convex bevel will take greater damage because it doesn't have the support from the shoulders.

There is also a secondary consideration which is that a convex profile has a lower drag profile than a flat ground edge because it has no sharp angular transitions and thus has a higher relative cutting ability. The effect of this however is rather low and it is swamped out by even a small change in the angle at which the edge is ground.

What is critical in the above is that both bevels are being compared when they are ground to have similar levels of cutting ability. If the convex bevel is simply ground over the same length as the flat bevel this isn't true . The following illustrates such a case :

http://www.physics.mun.ca/~sstamp/images/grinds.gif

The convex bevel in this case is thicker throughout than the flat bevel and thus it will be stronger and more durable, however it will have a reduced cutting ability.

-Cliff

 

[Starfish]

Cliff - your informative replies are well-appreciated, as always. Thank you!

By the way, have you ever reviewed the Sebenza?

Matthew

 

[Cliff Stamp]

Thanks, and no, I have not used a Sebenza extensively. It is an attractive design. It is pretty much the optimal profile for a light use knife, thin convex edge, strong relief with the high hollow grind, and high edge retention from the high hardness, high alloy steel. I would prefer CPM-420V with a full cryo treatement for that geometry, and maybe go up a point or so in RC. The handle ergonomics and security are not that high, though this can vary from person to person.

I personally have some problems with the lock for heavy use, I could jam the one I handled by "white knuckling". This is not much more than a annoyance, except if it causes premature wear by grating across the two mating faces. Integrals as a class also have similar problems as liner locks and can release due to a combination of torque and vertial loads. They can also be released by "white knuckling" if you use them in the wrong hand.

-Cliff