CONVEX EDGE, why is it so popular?

[Lorien]

I have zero problems sharpening my convexed bevel knives whatsoever in the field. I actually find it to be quite easy. They sure don't look beautiful, but they work like a hot damn.

 

[Fred.Rowe]

The pictures showing the different grinds is not very representative of an actual blade. They show the spine as being extremely wide relative to the other dimensions of the blade.
I understand the effectiveness of a convex edge on a chopping blade, but cannot see how it would outperform a true flat ground blade when it comes to splitting hairs, so to speak.
If you produce two blades that are dimensionally the same in all aspects, one a fully flat ground blade and the other a convex ground blade, I would think that the flat ground blade would have the keener edge of the two.
The only hollow ground blades I have around the shop are the ones I use to cut leather. They do this better than either a flat grind or a convex ground blade.

Good thread here, Fred

 

[Broos]

It's just geometry, eh hlee? I'm with ya, though I'm a convex edge guy, too. I think this emphasizes that talk of geometry is less meaningful when thickness is not tied into the discussion. I recall an old argument over what is a convex edge and what isn't (how many degrees of convexity are required for it to be a convex edge?). I think when you are looking at cutting ability, the thickness and sharpness (edge diameter or edge width) of the blade is more important than geometry. And sharpness defined by edge width or edge diameter is in theory independent of edge angle.

I think in some materials a polished bevel can help more than some would think to reduce friction and increase penetration. Certainly will not hurt.

 

[Lorien]

removing material from the transitional edge which separates the hollow grind from the flat edge grind results in a convex bevel(?right word?) and a sharper knife. Losing that material diminishes strength. An appropriately applied grind will dictate the functionality of the knife for its given purpose, use.

I prefer to convex all of my knives, as I find that most are designed strong enough, some much too strong, for their given task. Doing this gives me a finer edge. To me, a machine grind is a blank canvas, awaiting the perfect sharpening so that it will perform at its most efficientest. Unless you either have, or are a machine, your using knives will always end up convexed to some extent.

The most important thing is for the bladesmith to take this into account whilst applying the finishing touches to the bevels which terminate into an edge, given their customer's individualized needs, and/or the purpose of the knife.

 

[Jerry Hossom]

removing material from the transitional edge which separates the hollow grind from the flat edge grind results in a convex bevel(?right word?) and a sharper knife. Losing that material diminishes strength. An appropriately applied grind will dictate the functionality of the knife for its given purpose, use.

I prefer to convex all of my knives, as I find that most are designed strong enough, some much too strong, for their given task. Doing this gives me a finer edge. To me, a machine grind is a blank canvas, awaiting the perfect sharpening so that it will perform at its most efficientest. Unless you either have, or are a machine, your using knives will always end up convexed to some extent.

The most important thing is for the bladesmith to take this into account whilst applying the finishing touches to the bevels which terminate into an edge, given their customer's individualized needs, and/or the purpose of the knife.

This is the best explanation I've read of what most of us who use convex edges know and have failed to communicate. You've also mentioned that most edges end up convex anyway as a result of hand sharpening, simply because it's humanly impossible to precisely hold a fixed angle. If you examine old tools, knives, chisels, whatever, that have been repeatedly hand sharpened on stones you will see they all have convex edges as a result of the angle change that naturally occurs as the edge moves away from the person holding it. The top on the bevel is rounded - a convex edge. That's why many of those old tools worked so well. It's just taken technology a hundred years to reproduce what our grandfathers did everyday. Good thread IMO.

 

[the possum]

Well, I was half hoping this topic would just die off, but now I can't seem to keep away from it.

Quick survey:
What kind of thickness and angles are folks actually thinking about here when discussing this topic? How many of you regularly carry & use blades with an edge formed at 5-7 degrees per side, with a thickness above the edge bevel of .006" or less? (Great if you do!) How many finish up their sharpening with the angle at the very edge more like 10*-15* per side?

I think a good reason for the convex edge's popularity is that factory made knives come with edges that are too thick 99% of the time. This is one way to thin things out where the edge bevel and primary grind meet, without having to regrind the entire blade.

In the South, something called whittling is a favorite pastime in small towns. The best grind for this is either a full hollow, with a very shallow "V" edge, or a true Flat Grind where the flats extend all the way to the edge. Similar to a Japanese Swords edge. This is sometimes called a Zero grind. Many old timers will lay a blade flat on a stone to sharpen it. These knives are like razors. If you know anything about whittling, or carving, you know that you can't run a curl with a convex edge. it will skip out of the cut, unless you use a very steep angle in the cut. Then it will simply dig in. Ruining the cut.

Mike, this contradicts much of my own experience as well as stuff I've seen from others on the subject. First off, within reason none of these edges should have any trouble whittling a fine curl as long as they're sharp. They will all have a little bit of trouble coming back up out of the wood (depending on wood type) on deeper concave cuts. But convex will do a better job. Contrast it with a full flat grind/zero edge. As the blade is scooping a curve into and back out of the wood, the only parts of the blade contacting the wood will be the very edge and spine. As you try to twist/guide the edge back up & out, you are using the spine as a leverage point against the bottom of the cut. Since the spine is so far away from the edge, you have less control. With a full convex grind, you can lever against the sides of the blade, which are much closer to the edge.

I have seen this exact phenomenon spelled out clearly with several illustrations in a big wood working book. I've seen it myself in action in all kinds of applications. Like when I'm carving wood with a draw knife- use the flat side for making straight cuts, and flip it over to the "wrong" side for better control on concave curving cuts. Or clearing brush with a machete- I make sure to sharpen the edge with a flat V, so the blade will continue cutting on course as the sapling bends. Whereas a convex edged machete will tend to jump right back out of the cut, and directly back at my legs eek if the angle of attack is too steep. Or even on the bucket of our loader tractor- Dad welded an old grader blade to the front wrong way up, and now it only wants to dig deeper. I've seen Japanese sword guys discuss this in regards to cutting tatami.

Regarding the swords- Maybe I misunderstood you; were you only referring to the fact katana don't have a secondary edge bevel? Because while there are all kinds of obscure grinds used on Japanese swords, by far the most common is basically a full convex grind, with a relieved spine area. Not a flat V like is more common on modern competition swords.

 

[Jerry Hossom]

Whereas a convex edged machete will tend to jump right back out of the cut, and directly back at my legs eek if the angle of attack is too steep.

That differs from my personal experience, unless the angle of attack is so extreme it would have deflected or tended to slice down with the grain or bark of the wood regardless of how it was sharpened. I spent a couple years hauling a machete around the jungle in the Army and have seen them used and abused, and in at about all levels of edge deterioration imaginable. If one deflects towards your legs, I'd suggest you might change the position of your feet. The normal position for a right-handed person chopping standing wood with a machete is with the left leg forward, protected by the tree, and right leg back, well clear of how the machete will travel if it deflects.

I've convexed a number of machetes for friends and they seem to prefer them to the standard factory edge. (somewhat understated )

 

[jdm61]

That differs from my personal experience, unless the angle of attack is so extreme it would have deflected or tended to slice down with the grain or bark of the wood regardless of how it was sharpened. I spent a couple years hauling a machete around the jungle in the Army and have seen them used and abused, and in at about all levels of edge deterioration imaginable. If one deflects towards your legs, I'd suggest you might change the position of your feet. The normal position for a right-handed person chopping standing wood with a machete is with the left leg forward, protected by the tree, and right leg back, well clear of how the machete will travel if it deflects.

I've convexed a number of machetes for friends and they seem to prefer them to the standard factory edge. (somewhat understated )

Excellent example, Mr. Hossom. As most commercial machetes are made from relatively thin stock and have no bevel to speak of other than the edge, I would think that they would be one of the best blades on which to use a convex edge. How high up do you take the convex edge "bevel"?

 

[mapper66]

Just wanted to say great discussion guys. I have learned quite a bit. Thanks!

 

[Jerry Hossom]

Most of my experience is with the Standard Ontario machete, which begins at ~0.110" at the spine and has a primary bevel that tapers to about 0.090" at the top of their secondary bevel. The secondary bevel is ~0.250-0.300" wide and I use all of that for the convex. These usually come with little or no edge, so I probably remove about 0.050" of what should have been edge in producing the final convex edge, which I polish.

Here's a little tutorial I threw together on turning a $20 machete into a great woods tool. Sorry for the crappy photography. Lots of people have sharpened machetes like this on little $40 Enco 1 x 30" belt sanders, and even at my age I can still pretty easily drop a 2+" sappling with one of these.


http://hossom.com/sharp/

Edited to add: I wrote this several years ago and having just reread it, I no longer use a buffer to polish sharpened edges. For one thing it's dangerous, but as importantly, a leather belt is much more efficient as well as a LOT safer. I really need to update this.

 

[the possum]

That differs from my personal experience, unless the angle of attack is so extreme it would have deflected or tended to slice down with the grain or bark of the wood regardless of how it was sharpened.

Yeah, that's more what I was getting at- there's a point there with high angles when it can make a difference.

And further to clarify, I used convex edges on machetes too; it just depends on what I'm doing. Actually, I probably should have not even brought 'em up since I use other tools more now... My main point though was just that a convex edge can be turned in the cut more easily; intentional or not.

If one deflects towards your legs, I'd suggest you might change the position of your feet. The normal position for a right-handed person chopping standing wood with a machete is with the left leg forward, protected by the tree, and right leg back, well clear of how the machete will travel if it deflects.

Interesting. I chop from both the right and left. Brush and smaller saplings is most of what I'd use a machete on, and they don't really do any "protecting" since the blade just snicks right through 'em. So on a cut from the right, I'd have my right leg forward.

 

[Jerry Hossom]

Interesting...

Actually in thinking about this further, and this probably should be in a thread about using a machete, I do chop with my right foot forward, but only on materials I am pretty sure I will cut with little difficulty, and then it will be with my right hip in line with what I am cutting. The principle danger in cutting with a machete is when you cut the material too easily and the blade swings through, pivoting back towards you. You need to quickly drop your shoulder to control that. Earlier, I was speaking of cuts where you were unlikely to snick right through, but might even require a couple cuts to complete the felling. IMO, machetes are inherently safer than hatchets or axes and I've never noticed a convex edge causing an increase in the tendancy of the blade to deflect. In fact, they tend to penetrate more easily, actually reducing that eventuality.