Semi-Convex edge.

B

Buck268

Joined
Nov 5, 2006
Messages
1,864
Well, I desides to fore-go the true shaving edge this time around on the good old 1095 Ka-BAR USMC knife (got some abuse over christmas was time to resharpen), so I went for a cross between a convex and microbevel.

I basically dropped the basic grind angle to just above flat to the stone and ground to a good edge (couldnt raise a bur, couldnt get one last time when I went to razor sharp either). Then I flipped the stone over to the ine side and smoothed out that grind. Bumped the angle up ever so slightly and continued on the fine grit until have the visible bevel was at the new angle. At this point I raised the angle ever so slightly again and reduced pressure and continued. After a short time the edge looked great and easily sliced into and splintered off pieces of 2x4.

I'm curious to see how well this holds, bu t it probably wont get any hard use for a while, so I'll have to wait.

Has anybody else done this and had better results than a simple micro bevel? Or how well does this replicate a true convex bevel's performance?
 
Buck268 said:
Has anybody else done this ...
Click to expand...

That is basically the sharpening technique that most people use to hand sharpen convex bevels. Johnston described this about 10 years ago on rec.knives. Grind a relief, raise the edge and grind another bevel, using natural hand slop to produce some combination of a flat/convex grind. I was taught a different way a few years ago by a native knifemaker who used one smooth arc to create one continuous bevel, but as of late I prefer dual bevels on most knives as it is simply way faster. Comparing the two is fairly difficult because the cross secton tends to be the most dominate factor.

-Cliff
 
Very close. What you can also to is apply a few different microbevels and finally blend the bevels into each other by rolling the bevel on the stone (you vary the angle continuously through the stroke. A convex grind is simply an infinite number of microbevels (approximation of a curve). You get the benefits of a multibevel geometry which is strong at the edge and thin behind it with the added benefits of low binding (which is achieved by any grind that minimizes contact area) and no sharp bevel transitions. I come more and more around to the realization that the friction generated by the shoulder is actually substantial enough to aim for the smoothest transitions possible. But if you apply a multibevel by hand, the slight variation in the angle that you hold will blend the bevels automatically. If you smooth out the bevles with some intentional "blending strokes" as described above you get as good a convex as possible. Finally you can finish of with a little bit of sandpaper on a soft background such as a mousepad to achive further blending if still necessary. This is essentially how the convex grind is traditionally achieved on a katana: Rolling strokes and many many microbevels which create essentially facets and finally finish with a stone that is cleaved into ultra thin pieces which then get mounted on ricepaper, which essentially creates a very basic sandpaper.

What you have to realize though is that convex is not equal convex. If you approximate the curve with microbevels you will get a very different geometry depending on the angles you choose for the microbevels. You can make a very sturdy and obtuse convex or a very slender and acute convex. If you ground the blade essentially to zero edge before starting the beveling, you are likely going to end up with a very accute and slender convex which will make a great slicer, but which might be a bit to thin for a good chopper. You can adjust these parameters depending on the intended use of the knife.

P.S. Not all steels form prounounced burrs. I find the ones that don't usually much easier to sharpen.
 
Well I'm working with 1095. Only knife I have in 1095 and I have yet to see it raise a distictive bur (at least nothing like AUS8). Compared to AUS8 or 1055 it doesnt have any bur at all.

Anyways, I started with a very acute angle for my standards (which are changing, growing my acute lately). Maybe in the range of 15 degrees per side. I just barely raised it for the second bevel, with my goal being to stay notably under what I perceived as 20* (which of course is far from exact). For the final angle I put a little bit for obtuseness into the grind to try and hold the final edge longer, what I would guess to be 20-25 per side, but I only did maybe 20-30 strokes per side and quite lightly.

It does bother me that its not a true razor edge, but it seems to slice very well. We'll see when I get the chance to put it through some paces.
 
That is actually quite an obtuse profile. If you take a BRKT convex edge for example (they vary somewhat depending on the intended use of the knife), they are about 12-13 at the edge and pull relatively quickly back to 7-8 deg. per side.
 
Well, remember the ka-bars are hollow ground with a long, thick, wide spine, so you can only go so acute. Its still a lot more acute than the last profile it had, and that cut very well so I'm not worried... Hell, my angle estimation may be a little off too.
 
I am sorry Buck, I didn't mean to be judgemental in any way, I simply wanted to give you a reference point. Hey, in any case it must be better than the prybar you started out with :).
 
Oh its all good, I figured you were doing that just illustrating my situation.

On of these days I'm going to spring for a real nice fixed blade and realy put one helluva edge on it... But those days aren't these days :)
 
I find a simple dual-bevel edge (which is a semi-convex edge) just as effective as a fully convexed edge. There is nothing 'magical' about a fully-convexed edge.

Most of my blades have main bevels of 10 degrees per side with micro-bevels of 15 degrees per side (up to 20 degrees for softer steels). The main bevels provide good slicing efficiency and the very narrow micro-bevels simply reinforce the cutting edge.

Edge maintenance is a breeze, since it only involves honing the very narrow micro-bevels with a simple hone such as the Spyderco DoubleStuff.
 
gud4u said:
There is nothing 'magical' about a fully-convexed edge.
Click to expand...

Unfortunate though, for each statement of this perspective, there are dozens of ranting nonsense about "matter separation" or similar which confuse the issue away from the critical points. I have come to believe that a lot of people are actually doing this intentionally to keep users in the dark because no one who actually used knives significantly could have such a viewpoint.

-Cliff
 
A burr is actually defective behavior, properly optomized steels, i.e., those made for cutting tools and hardened accordingly, don't actually produce burrs.

-Cliff
 
Good to know, I was getting worried because I can't raise a good shaving edge on any of my pocket knives with my hand stones.. Of course, this is most likely because they are all too coarse! Edge looks beautiful and slices like a dream but wont push cut... I need to invest in some fine and UF ceramics I think.
 
You must log in or register to reply here.
Back
Top