Convexed Edge

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Feb 15, 2003
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My customized SAK - Victorinox Scientist that I have EDC'd since Feb/1993

Scientist2.jpg



As I was touching up the blade I thought I'd share what I did.

Since it is the knife that I have on me everyday - I have tried to make it as practically sharp as possible.

That does not necessarily a more actute edge bevel angle - since it is well known that the Victorinox SAK (Inox) steel is not exactly Hi-tech or "premium" and hardened only to about 55-56Rc - too acute an angle will mean the knife probably will need re-sharpening, or at minimum touch-ups, often.

What I figured was to use the general purpose tried and tested final edge bevel angle of about 22.5deg per side for an inclusive angle of about 45deg.

However just having this bevel will do nothing to improve the standard SAK since that is already the bevel from the factory!

To improve the cutting ability withOUT going to a more actute angle means I have to try to get rid of, or at least minimize, the sudden hard transition from the edge bevel to the face of the blade.

There are two basic ways to do this - to cut in a secondary bevel to at least make a transition angle - or to smooth out that bevel transition "corner" - ie: give it a convex edge.

This is honing/grinding away the transition corner of the edge bevel and using a rolling/rocking action to make that transition convex, ie: minimizing any corners to make as close to a zero bevel as possible.

EdgeBevel.jpg


I still use crock-sticks to finish off the final edge but the convexed zero bevel is there as shown by these photos.

One can test this by pinching the blade face and pulling toward the edge - one should not feel any transition corner/discontinuity going from the blade's face to the edge.

Scientist_Convex2.jpg
Scientist_Convex.jpg


Scientist_ConvexDtl2.jpg
Scientist_ConvexDtl1.jpg


--
Vincent

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Thanks.

I have had this kind of an edge probably for over 10 years now - and although shaving hair and scoring paper (ie: shallow cuts) are no better than factory - cutting into and through things definitely benefits from having a zero-bevel convexed edge.

My usage is not that heavy so I can't say for sure if the edge holding is any better - but in theory one can see how the convexed shape should support the edge better than a plain bevel and even one with a secondary transition bevel.

Just a note of interest you can see how long I've had been doing this the tip point of the blade is somewhat rounded - this is typical of sharpening touch-ups on V-hone crock-sticks.

My normal touch ups are stropping on cardboard which removes minimal if any material - if I see shiney flats on the edge I will lightly use crock-sticks until I see the flats gone - then finish with stropping rouge laden leather strop then strop on cardboard. I have only had to re-touch the convexing recently lightly freehand on a fine EZElap diamond hone.

I initially cut in the convex using slicing strokes and dragging strokes - ie: both ways for speed - but for my recent touch up I only used pulling-up dragging strokes going from face toward the edge - to save the finish of the blade I taped the face leaving about 3/16" exposed that way I can lay the blade flat on the hone with confidence to start the stroke and to ensure that the convexing is as "zero-beveled" as I can get within my limited skills.....

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Vincent

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Here's an ad-hoc unscientific test that shows the benefits of convexing either just the edge or the whole grind....

I simply took a typical unopened junk mail (ie: envelope and contents) and took slices with various blades -
CutTestKshawChiHnx.jpg
CutTestSAKOpinelBM.jpg


From the markings on the envelope the Opinel #8 stainless was the "winner" with the easiest and deepest cut - followed by my convexed edge custom SAK - compare this with a sharp but standard SAK.

Another surprising result was the Benchmade 710 Axis - this came pretty obtuse toward the tip from the factory - it annoyed me enough to re-sharpen/re-profile the edge to my home made convex - now this knife has a noticably much thicker blade, yet the depth of cut was impressive - of course some of this may have been the sheer length of the blade itself........

The most inexplicable was my most used kitchen knife the Chicago Cutlery Walnut 62S 5" utility/boning knife - this is a real cutter for kitchen stuff - but was quite an under-achiever here (and proud of it? :D )

Opening the envelope to count the thickness of paper - revealed another surprise for me....
CutTestInKshawChiHnx.jpg
CutTestInSAKOpinelBM.jpg


The contents shows that the Convexed SAK out cut the Opinel! The Opinel's tip must be sharper or pointier and sliced through the envelope at the end of its stroke without cutting through the rest of the contents......

Another interesting point is that there is variable thickness of papers - there is a tri-folded piece of paper shown in front of an enclosed business reply envelope - this discontinuity was enough to stop some blades or cause enough difficulties to retard the cut......

The most disappointing was the Kershaw Vapor - which I know works well as a general purpose cutter, and great value for an integral frame-lock knife - it seemed to be stopped almost before it began to cut..... could be the typical wedge edge bevel and distinct shoulder/corner plus the blade is bead-blast finished that may cause some resistance/binding for cutting through. Nevertheless this is surprisingly disapponting as I know in everyday use this is fine.......

So what does this prove if anything?

Well, strictly speaking nothing much - since this test, though I tried to make things as equal as possible, really only applies to slicing through 11 to 14 layers of paper.

However I am really pleased with how a simple modification to SAK of a convexed edge shows that it can out-perform even the already well-respected standard SAK.

But draw your own conclusions :)

--
Vincent

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Vincent, you continue to impress me with your ever informative posts, and your insight into all things edged! You are definitely a Big asset to BladeForums! Keep up the good work! :)
 
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How much work was it to get the initial convex edge? An hour? Several? I've never had a convex ground knife, although I like the idea, and may try it on an SAK if it doesn't take too long.
 
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Julian Elliot said:
How much work was it to get the initial convex edge? An hour? Several? I've never had a convex ground knife, although I like the idea, and may try it on an SAK if it doesn't take too long.

Not too long (caveat - if you have patience :D)

For me it was maximum of a couple of hours - but I was going very slowly and carefully since I didn't want to marr my custom SAK - and that was probably 10 years ago when I was less experienced - and first time I attempted to convert from conventional factory plain bevel to zero-bevel edge.

The big thing that helped was to tape the blade face exposing only about 1/4 - 3/16" so one can work with fair confidence without too much fear of messing up the finish of the knife.

Today I probably could do it in less than an hour - since the SAK steel is not exactly premium/exotic/hi-tech - just some workable good general purpose knife steel and not too hard about 55-56Rc.

Note I use a Fine grit diamond flat hone (EZElap). Initially work with a rolling motion to remove the shoulder/corner disconinuity between the factory bevel and blade basically to merge the two as smoothly as possible.

In this age of honing guides and mechanial sharpening aids to get precise angles - most people don't seem to realize that a rolling/rocking motion in free-hand sharpening is quite easy and actually natural!

I actually avoid sharpening all the way to the final edge initially - to prevent too much material removal.

Only when one gets close to a good convexed shape - sharpen all the way to the final edge to get a burr - I then finish with V-hone crock sticks to get a precise final edge angle - I then strop as described previously with leather then cardboard....

I examine the edge and convex using a 30x illuminated pocket microscope/magnifier - so I can see the honing marks and to confirm the edge burr - I also obviously feel for it as a progress check as I'm honing.

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Vincent

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Some more ad-hoc/informal cardboard cutting.

In spite the fact I tried to make it as "equal" as possible - it is not that consistent (*see later) -
but still a reasonable indicator (of sorts )

Cutting into regular corrugated cardboard across the corrugation -
CardboardCuts.jpg


CardboardCuts2.jpg


Where it says "stopped" in the Victorinox 3.25" Parer, Opinel #8 stainless and the convexed Victorinox SAK - they would have cut all the way through - so I just stopped when I thought it was enough - so those are not meant to be compared in absolute terms merely to show that they did easily out cut the thicker knives despite some very good profiles/geometry.
[note: the Victorinox Parer is as-is - ie: flat ground with regular plain bevel edge - it cuts so well mainly because it is a very thin blade]

* inconsistency - Notice that I re-did the BlackJack stuff and in one the Condor 79-Z cut less than the Convexed AWAC and in the other it's reversed - but they are still in roughly the same ballpark.........

These were the knives -
CardBCutKnives.jpg

TrailG_AWAC_Condor.jpg



--
Vincent

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Vincent,

I'm glad you made this post. After reading through it, I realized that I've done the same thing to my Vic Soldier, except I didn't mean to! I also looked at some of my other knives, and it seems that I've somehow formed convex edges on them as well. I never really thought about it until now. Thanks for all the informative posts.
 
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Chris Mapp said:
I'm glad you made this post. After reading through it, I realized that I've done the same thing to my Vic Soldier, except I didn't mean to! I also looked at some of my other knives, and it seems that I've somehow formed convex edges on them as well. I never really thought about it until now. Thanks for all the informative posts.

Chris,

You might not think it - but -
I am even gladder you responded ;)

You have just demonstrated that a convexed edge is actually a more natural/instinctive sharpening/honing motion.

In fact, I will go as far as to say in the old days that's how most people sharpened their knives - evidenced by the old used stones/hones that show distinctive scooped/hollowed out wear.

So what I've done is really nothing special - it's the way most people used to sharpen knives - before sharpening guides and systems......

Thanks for your input.

--
Vincent

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Vincent,

I sharpen all of my knives freehand, so it would seem that you are correct in saying it is a more natural motion. Thinking back, all of my oilstones are just as you described, dished out in the center.

And just for the heck of it, here's a pic of my Recon Tanto, with a convexed edge. Sorry for the horrible pic quality, but all I have is a cheap webcam at the moment.
 

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