Convex edges - pros & cons?

Starfish

Starfish

Joined
Jun 14, 2001
Messages
1,256
Hi all,

I finally tried sharpening with a mousepad and sandpaper last week, and obtained a nice, sharp edge on my Calypso Jr. for a minimum of effort. I started out with 60 grit paper backed by plate glass to set the bevels, and then moved to the mousepad to make a convex edge. I went from 60 to 2000 grit, using all of the grits I could find in between (100, 150, 220, 320, 400, 800, 1000, 1500).

So now that I have discovered for myself just how easy this method is, I want to know how a convex edge compares to traditional v-grinds. Specifically, I would be interested in hearing your opinions on areas where convex grinds excel as compared to a traditional v-edge, and conversely where convex grinds are a poor choice as compared to a traditional v-edge.

Or will I even notice in day-to-day usage?

Thanks!

Matthew
 
I've been sharpening knives for 25 years, making them for 12, and I can't feel a significant difference between a convex microbevel and a flat microbevel. I get my edges sharp faster when I use the convex (less metal removal). When my belt grinder is set up for it, I use a flat microbevel...if the platen is worn down I use a convex.

It would be very tricky to design an experiment to test the convex edge against the flat edge. If you find a noticeable difference I'd like to know about it.

The most significant factors are edge angle, edge thickness, and edge finish.
 
buzz was supposed to be testing a dozier convex & multi-bevel (as made by Bob) - but I haven't seen the results yet. Anybody?

I put convex edges on nearly all of my knives. Easy to sharpen (as wnease said), maintain the finish, etc.

I think the differences in performance (every thing else being equal) are minimal. You also have to remember to separate a polished convex edge from a "toothy" one.
 
When most people compare the 'convex' edge to the more conventional 'flat edge' they make the mistake of not considering the 'micro bevel' that actually does the cutting.

If you compare a convex sharpened edge with a 'two angle' conventional sharpened edge ( where one of the two edges is the micro bevel edge ) I contend that there is absolutely no difference as far as 'real world' goes.

Both will cut equally.

However a multi angled edge, applied with something like an Edge Pro, con look beautiful and will enhance any blade as far as appearance is concerned.
 
im waiting on a convex zero edge knife corkum first strike won on ebay and im wondering if it uses the same type of convex edge?

on the review thread about the blade, it seemed as if most had a huge problem getting the knife sharp

wernt traditional japanese tantos using convex edge?
 
I will agree with your "most people" comment - it is indeed true.

IMHO, a full convex grind (spine to edge) is the most beautiful of grinds. No interruption, or break in the grind. Smooth and solid. Not for everybody, true.....but works for me. ;)

If done properly, is very easy to maintain sharp.
 
yes, drakoon. The convex edge is the grand-daddy of edges. Harkens back to prehistoric times. Only with the "recent" advent of "super steels" have we turned away from convex edges.

It's only hard if you don't know what you're doing. If you're willing to learn, it's very simple.
 
"It's only hard if you don't know what you're doing. If you're willing to learn, it's very simple."

very wise words my friend
 
pendentive said:
[a full convex grind (spine to edge)]


If done properly, is very easy to maintain sharp.
Click to expand...


That is the most inefficient grind to sharpen, you have to surface polish the entire blade every time you sharpen it to the same grit you want to leave the edge.

This is like having to take a shower every time you want to wash your hands. If just your hands got dirty, its obvious its a massive waste of time to hop in the shower.

But if you have the time, and have the water, by all means, but you can't argue logically that it is "easier" than just washing your hands.

[note this has nothing to do with convex vs flat grinds, just the lack of an edge bevel]

And yes I have blades with exactly this grind, sharpening is very inefficient due to the grind, however since the steel is *very* easy to machine, and it only blunts lightly in use (vegetation), it isn't a problem.

However put the same grind, on a different steel, and then use it where you need to sharpen a lot - and micro-bevels and relief grinds start to become desireable very quickly.


Nosmo said:
If you compare a convex sharpened edge with a 'two angle' conventional sharpened edge ( where one of the two edges is the micro bevel edge ) I contend that there is absolutely no difference as far as 'real world' goes.
Click to expand...

Assuming the gross geometry is equal, yes, exactly. The only reason that people see performance increases in cutting ability when applying convex bevels is because they are applying a relief grind to the edge, the same exact thing can be achieved with a suitable flat or hollow grind. It is nice to finally see see this myth start to die.

-Cliff
 
Cliff Stamp said:
That is the most inefficient grind to sharpen, you have to surface polish the entire blade every time you sharpen it to the same grit you want to leave the edge.

This is like having to take a shower every time you want to wash your hands. If just your hands got dirty, its obvious its a massive waste of time to hop in the shower.
Click to expand...

You couldn't be more wrong, Cliff. And your analogy, though cute, is way off.

You still only sharpen the last bit of the bevel. :rolleyes:

There is still a small area similar to a microbevel, but where a microbevel has a sharp turn in angle, the convex edge is smooth = only difference.

All you have to do is remember to polish up to the same finish as the rest of the blade. If you strop with green CrOx compound (like everybody else does) you'll get a mirror finish on the bevel.




Still trying to imagine how all those weaponmakers throughout history have all been wrong according to Cliff?

You are probably right, though.....I'm sure none of them ever showered!

:p
 
Drakoon said:
well im a knife noob so i dont know if this site is legit but try looking here: http://www.mhcable.com/~yocraft/sosak/convex.htm
Click to expand...

That link is a tad old. It resides on the webserver of a former ISP, who has not seen fit to delete my old website. The latest version of this FAQ can be found at http://home.nycap.rr.com/sosak/convex.htm . The new version has some minor updates.

As far as convex ground edges go, I’ve found them pretty hard to pin down. I really can’t say that a convex grind is always good for this situation or that a standard grind is always better for something else. Much of it is simply a matter of execution and how polished the edge is.

Obviously, the big difference is in the sharpening. Some people find a convex grind easier to sharpen. Other people find a standard grind easy to sharpen. I will say that I personally believe that the convex edge is easier to maintain. Most of my convex grinds get stropped with whatever I find laying around. Sometimes it’s a thick piece of cardboard. Sometimes it’s an old scrap of leather. Stropping a convex grind usually does the trick, and never requires an angle guide to get the job done. I SELDOM have to sharpen (remove a lot of material) a convex ground edge.

Dan – I did do the Dozier test, where I had samples of nearly identical knife models, one with a standard hollow grind and one with a full convex grind. I’m afraid that the test was rather unsuccessful. The convex ground blade could have been shaped in a million and one ways. As it happened to be, the convex blade was shaped with a very steep curve close to the edge of the blade. This yielded a huge amount of durability, but with vastly reduced cutting performance. The hollow ground blade outperformed the convex blade in every category, except where I was push cutting through very shallow material. I assume that the convex blade performed well here solely because of the high level of polish on the edge. If the convex ground blade had been ground different, I believe the test could have come out lopsided in the other direction, with the convex ground blade winning most of the tests. Convex ground blades are just an entirely different animal, I guess.

I received a Bark River Highland on Friday, and am just starting to learn the knife. First impressions are VERY high. Mike Stewart’s group really knows to put a high performance convex grind on a knife. The fit and finish is pretty good too. If it weren’t for some minor dimpling around the scale rivets, I wouldn’t be able to tell the production knife from an expensive custom job.
 
I'm not even going to try to reply to Cliff. That was the most ludicrous thing I've ever read. He took one miniscule piece of a rather large puzzle, and made it sound like it was the only piece. Shame on you, Cliff. You know better than that.
 
Pendative, you are confusing stropping with sharpening. If you have no appreciable amount of material to remove then chromium oxide works fine and will remove neglegiable material except at the edge (and sometimes at the spine). The green buffing compound will polish the side of the blade and will not impact the finish, providing that you had a mirror polish that was created by buffing across the blade rather than along the blade axis. With time the edge will grow increasingly obtuse since you are only removing material at the edge. When this wear or some heavy blade usage requires serious resharpening you get into the high-overhead situation that Cliff refers to. At this point you will need to use a coarser abrasive on a stiffer pad to restore your acute edge angle. This will impact the blade finish and cause you a lot of work. An easier fix is to sacrifice the purity of your convex edge and use flat hones and a little wrist action to approximate a convex finish for the last 1/8" or so. Then you can try and blend the finish on the edge to the finish on the remainder of the blade.

I have had another problem. I like blades with a lot of edge relief for easy cutting of sticky stretchy material (fine skinning work). For this I like a nearly flat or hollow ground blade (since I am not worried about blade toughness). If you strop blades with these contours you can really see transitions between the strop finish and the factory finish. A nearly flat full convex blade suffers from some of the same effect. As you noted the stropping process primarily removes material near the edge (and also near the spine). This is conspicuous on a nearly flat convex blade. You have shiny strips adjacent to the edge and close to the spine and a distinctly different finish in the low pressure strip along the middle of the blade.

One of the reasons that I largely abandoned using convex sharpening techniques over 30 years ago was cosmetic. One summer while I was in college I couldn't find a job so I started a business going door-to-door sharpening knives, scissors, and garden tools. The customers would not accept scratches and large areas of compromise to the finish of their knives from a strop-based honing technique. What could be provided in a moderate amount of time was a nice and precise combination straight bevel. I would reduce the edge bevel down under 15 degrees then apply a micro bevel around 20 degrees. This looked professional, did not impact the blade finish (mirror finish and chrome finish were not uncommon), and gave a sharp edge with reasonable durability. I even backed off from stropping the blades a little at the final stage since it often impacted the finish.
 
Make no mistake, Jeff. I have never said that the convex edge is the "only edge you'll ever need".

Stropping is part of sharpening (final step). No confusion here. :D

I agree with the majority of your first paragraph. However, abandoning sharpening the conex edge because of scratches on the finish is more about technique, than principle.
 
Thanks for the lively discussion fellas! :)

Sounds like a convex edge likely does not have any clear cutting advantage over a traditional v-grind, and so ease of maintenance tips the scales in favor of using convex edges.

Sound about right?

Matthew
 
I essentially put a convex edge on my AFCK. It cuts fine, but I did it with slack sandpaper and scratched up the coating. I cant get as fine an edge in terms of low angles, because the sides of the blade will be scratched up. I suppose this is where the full convex has an advantage. I'll probably go back to flat soon. Or I could test the edge to see which one cuts better. I even have this weekend off.
 
pendentive said:
You still only sharpen the last bit of the bevel.
Click to expand...

In that case the nature of the primary grind has absolutely no effect on the efficiency of sharpening anyway (unless it is hollow) as you are honing an obviously a distinct micro-bevel. That of course is in fact the exact profile I noted had maximum sharpening efficiency, and why you use micro-bevels in general (they also have durability advantages).

As an example :

http://www.physics.mun.ca/~sstamp/images/aj_skinner.jpg

That knife has a primary convex grind (on a 1/16", M2 blade, 65 HRC) with a very tiny secondary edge bevel, you can't see it on the picture, in fact it is hard to see it by eye. But this doesn't change the fact that you are sharpening a micro-bevel not the full primary grind, which was the central issue being contended.

To be clear though, unlike Dan seems to imply, lots of people do sharpen full convex bevels from spine to edge, with NO micro bevel. Often with hand stones, you just work the hone from spine to edge. It isn't as difficult as you might think to keep the right edge angle, but it is time consuming, as I noted, the most inefficient way to sharpen. All my khukuris, parangs, axes etc., are done this way.

[To be really nitpicky, I put a micro-bevel on them, but is is really small, just 1-2 passes to finish, but when I sharpen I hone the entire grind.]

Mel Sorg, made convex blades, small utility ones, out of D2 and honed them in this manner, he (or his father specifically) used sandpaper. To be really clear, he *cheated* just a little by rolling the blade on the last of the stroke to give it a bit of a micro-bevel, but the flat was polished with every pass.

[In that case I would argue that a hollow relief would have been more optimal, something like this :

http://www.physics.mun.ca/~sstamp/images/aj_utility.jpg

here the full strokes of the sandpaper would only hit the edge and spine because everything in the middle has been ground out - it also cuts better as well]

With a lot of traditional blades, parangs, etc., when honed fully with no distinct edge, this has the benefits of polishing the primary (the entire blade in most cases) which removes rust, scratches etc. . As noted previously, the rate of blunting is so low you don't really care about efficiency of sharpening as its only a half hour once a week for a heavily used blade.

You do see it on occasion though, like a few weeks ago I used a khukuri to cut off some used carpet from the sides of a compost heap (used it to frame out some walls I stuffed to keep the temperature up in the winter). After the cutting the edge was seriously dulled and I had no desire to spend a half an hour planing the full primary down to get the sharpeness up. I just applied a secondary edge bevel and kept going.

[never fear khukuri purists, later on I used a belt sander to restore the full bevel]

With other knives though, like that M2 skinner in the above, if used as a utility knife a full convex grind would be a horrible choice because of the low machinability of M2. The small secondary edge bevel eliminates this problem, while raising the durablity several times over at no significant reduction in cutting ability.

-Cliff
 
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