Why put a convex grind on a BK2?

O

oldmanron

Joined
Jun 19, 2012
Messages
195
Hi guys,

Quick question … why do some people choose to modify the cutting-edge of the blade of their KABAR BK2 knives from a flat grind to a convex grind?

I’ve had my BK2 for a couple of years now. It arrived with what I understand to be a sabre grind or perhaps a flat grind (as KABAR calls it) … a large flat primary bevel and then a very small (maybe one millimetre) 20-degree secondary bevel (sometimes called a micro bevel) with a cutting edge that was quite sharp and consistent but completely unpolished and showing all the fine grinding marks. An hour of work on a large two-sided Norton water stone removed those marks and gave the edge a nice polished look.

Unfortunately, in the two years since that time ... using state-of-art modern technology and my own ham-fisted technique … I have now created a convex grind on the secondary/micro bevel. It was totally my own fault, mostly using (freehand) a Worksharp Knife Sharpener and a small handheld pen-style diamond sharper. A convex grind … and I could never ever get that small bevel's cutting-edge feeling at-all sharp again.

But then a good friend, to whom I had once given the gift of a BK9, offered to take the BK2 (and a couple of other knives) to a professional/industrial sharpening shop in Lethbridge. My buddy is an arborist and has everything (and he has a lot of stuff) maintained/sharpened at this family-owned shop, and he had nothing but good things to say about their work … which they’ve been doing since 1978.

He brought the BK2 back on Thursday, and I could not believe the transformation. The all-important small secondary bevel is again a perfectly flat plane … and has gone from a shiny “flat” that was originally about 1 mm to a mirror-like “flat” that is now about 2.5 mm.

And … geometry bein’ what it is … the longer sides naturally mean that the angle at the edge is slightly more acute … going from the original 20 degrees down to maybe 17 degrees.

And the blade is now finally (and I never thought I would ever say this about that sharpened KABAR pry-bar) hair-poppin’ sharp. The original two bevels were beefy to begin with (the blade is a full quarter-inch thick, as y'all know), so the slightly thinner profile of the new secondary/micro bevel (now a slightly-more acute isosceles triangle) is still by-no-means fragile, especially with that fine original steel. And it now has a razory edge that would be nigh-on impossible (for most mere mortals) to achieve on a convex bevel.

Trivia question from southern Alberta: Why is it so darned important to " ... always keep an edge on yer knife ..." ?
 
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From what I understand the convex edge is stronger since it is thicker behind the edge and the shape also allows it to cut through material easier because there is less resistance.
I saw a really good illustration one time (on History Channel?). It showed how the hollow edge on European swords, while sharp, created a lot of drag/resistance when cutting while the convex edged Japanese swords could cut through material much easier.
 
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Comparing your "ham fisted technique" to that of a "professional/industrial sharpening shop"? Not exactly apples-to-apples, eh?

Would be interesting to see how long that new thinner 'hair-popping' edge holds up after a good session of chopping and batoning.

Each edge geometry has it's advantages and disadvantages. The intrawebs are full of technical explanations of each. Simply select the one that best matches the intended usage.
 
Short answer......because they like it.

I personally prefer a v grind.

Either one will cut stuff and there is no clear benefit to either, it all boils down to simple preference.

Moose
 
As a semi-recent convert from convex to the v-grind (purely for the sake of simplicity) the convex edge is an interesting animal. At the best I had ever managed, I have made it stupid, ridiculously, hair popping, falling cigarette paper-slicing sharp. It took a considerable amount of time and effort to get it to this point though, and with a v-grind I was able to get it almost as sharp in a ludicrously small fraction of time. Like 10 minutes versus hours and hours of stropping.

There is something very personal about the convex. The time that goes in, and the personal connection with the blade that is made in doing so. But ultimately, for anything but a dedicated chopper, a v-grind edge is more than sufficient.
 
I go back and forth on it. Have both and like both. A well done convex is a pleasure to use for woodcraft and easy to keep sharp in the field. Just strop it on your pants. Depends on your equipment too. If you have access to a belt grinder you can convex it in five minutes. A sharp V grind seems to do better for me at slicing and food prep. This old Cam BK2 has been convexed for twenty years or so, and remains one of my all time favorite knives. Just a matter of taste and blade geometry. Some knives benefit from it and some do better with the V grind. Thin knives or full flat grinds are lasers with a V grind. I personally like thicker knives convexed. I also like the edge Jerry Fisk puts on knives like the BK5. It's convex but gets a few licks on an India stone to finish it up. Just a personal preference.

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Now that I see exactly why you are asking I thought I would add this.

I've never sharpened a blade to a convex edge (if I have it wasn't intentional!). I would rather have a really good V edge than a rough convex edge.

That being said, I'm sure if you tested a convex edge done by a professional you would find it just as impressive as the edge on your newly sharpened BK2. A convex edge you created accidentally certainly shouldn't be your reference/standard when comparing the two!
 
The Becker line comes with fine edge as is. And gets even better when you properly sharpen it that way. I regret taking my BK2 through a strop session. The edge doesn't dull as fast from my sheath or chopping. But I miss the fine edge & micro serrations for slicing. Luckily I put some real serrations on my BK2 before I changing the edge. Though I admit I would love a full convex or full V wedge edge on it. Sadly I don't have proper tools or money for that.

 
oldmanron said:
Wow ... a lot of well-considered and sensible/informative responses here ... thank you.
Click to expand...

Your one-stop shop for Becker questions. It's a smart group unless you're talking chest shaving, up-kilt gazing, or head wounds. Then, all bets are off.

I'll add that I like a good convex. Cuts nicely and easy to maintain. But I'm not particularly adept at going from V-grind to convex.
As such, I usually stick to V grind. My KME sharpener takes out all the guesswork.

KME_zpsee3d5e66.jpg


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Love me some convex. Convex can be just as sharp as as a V, you can have toothy or polished convex or V, and I think either can be maintained with the same amount of ease, it all just depends on how you sharpen. It really all boils down to personal preference. Either way the BK2 IMO really benefits from a thinner edge. It takes the performance of the knife up quite a bit, in fact all of my Beckers (if not most of my knives) have had their edges thinned out. Just knocking the shoulders off a V grind gives a slicing boost IMO. Just have to find out what works best for you, and that specific knife.

MMMM CONVEX...

 
Good points here.

Then, of course, there is the Jerry Busse combined edge which is convex on one side and flat on the other side. Truth be told, I've had one, the other, both and variations of them.

This all gets too confusing after a while! :D:yawn::confused::eek:
 
Because I'm freehand sharpening, my secondary bevel isn't always perfectly flat, but I do attempt a flat grind. When I convex ground my BK2, it did it on the primary bevel, taking it from saber-ground to something with less of a step. My unscientific testing showed the blade going through wood even better once that was done.

For something as thick as the BK2, I don't think a convexed secondary bevel helps at all. Maybe it would for something really thin.

PhotoOct18221127_zps8f4b832f.jpg~original
 
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