Convex Edges?

M

MJR

Joined
Oct 29, 2004
Messages
174
I read somewhere how to convex an edge and decided to try it out, and it was fairly easy. I used my 556 mini grip with 440C. Turned out fairly good I must say for the first time. My question is, What are the advantages of this type of edge and will it work good for most of my EDC'S ? :confused: My main rotation is Benchmade 705, 940TI and small regular Sebenza.
 
MJR said:
What are the advantages of this type of edge ...
Click to expand...
Most of the advantages argued such as cutting ability are not due to the fact the edge is convex but simply that the edge was thinned. You will often see someone take a thick edge blade, put a high convex bevel on it and then proclaim the huge improvement in cutting ability - yes, but this isn't due to the convex nature of the bevel just that a huge amount of metal was removed.

There are some wedging issues with full convex primary grinds such as on splitting axes, but for simple bevels on knives it mainly comes down to ease of resharpening. If it is easier for you to apply a convex edge than a flat one it usually a better choice. A dual v-grind can offer near identical performance and versatility.

-Cliff
 
Cliff Stamp said:
A dual v-grind can offer near identical performance and versatility.

-Cliff
Click to expand...

Thank you. Thats what I needed to find out. :cool:
Now to try it on a blade with better steel. 154CM.
 
IMO, convex edges last longer, and are more robust. My Swamp Rats and CRKs all have convex edges, and I’m constantly impressed with how well they cut and chop.
 
edge of choice for cutting competition champions. ;)
 
Are their any other tips for convexing an edge using a mousepad? Started out with 400 grit sandpaper then 800 grit then 2000 grit. I think it turned out fairly well for my first time. Is their a proper angle to try and achieve? On my mini grip I lifted the edge just enough to ceep the thumb stud from touching the sandpaper, or should I remove them and try a shallower angle? This could be fun learning a new way to sharpen my toys!
 
You want the angle as shallow as possible, optimally it is just thick enough to prevent damage and no more. How far down to go depends on you skill, physical ability and what you are cutting. Generally most edges come vastly thicker than necessary to safeguard against accidental damage and for the majority of people would benefit from being slimmed out. The only way to know is to try it.

-Cliff
 
MJR said:
Are their any other tips for convexing an edge using a mousepad? Started out with 400 grit sandpaper then 800 grit then 2000 grit. I think it turned out fairly well for my first time. Is their a proper angle to try and achieve? On my mini grip I lifted the edge just enough to ceep the thumb stud from touching the sandpaper, or should I remove them and try a shallower angle? This could be fun learning a new way to sharpen my toys!
Click to expand...
The big problem most people have is that they press too hard into the mousepad, thus rounding off the edge.

I just did a friends SAK main blade with a diamond stone, then 1000 grit waterstone, then green compound on leather.

It is the sharpest it has ever been, with the exception of the time I used my EdgePro, but the convex edge cuts thick(thicker than the edge thickness) materials MUCH better...

Thicker materials drag on the transitions between bevels on standard grinds.
Until you are cutting faster than the speed of sound in the material you are cutting, rounded edges will pass through with less resistance...
Look at planes... Only when you get to supersonic planes, do you start to see sharp angles.

The actual edge of a convex does not need to be any more acute than that of a normal grind, and in a lot of cases can be more obtuse, because you do not have to have the longer bevel to get it to push through materials easier...

Something that a lot of people do not understand(understandably) is that sharpness and cutting ability are almost compleatly different things...
Have you ever got a paper cut??? was the paper SHARP??? No, but it had a lot of cutting ability!
Sharpness has to do with how perfect the edge is...Cutting ability is all concerned with edge geometry.
You can get a one inche thick piece of steel and put a shaving edge on it... Now, try to push cut through a four in thick piece of balsawood(you know, the stuff you can cut with your finger-nail). It is going to take a LOT of pressure, because of the wedge effect that is going on...The bevel is forcing the wood apart so fast that it cannot compress far enough to let the blade in...So when you press hard enough the bottom of the wood will break before the edge even touches it. Now try the same thing with a dull be thin knife...
You can try this yourself with something like a carrot... Get a dull THIN kitchen knife and cut it. then try a sharp thick knife...The sharp thick knife will usually cut part way through, and then the carrot will snap apart.

A convex edge slowes the thickening of the blade for the same angle at the contact area...
 
jamesraykenney said:
The big problem most people have is that they press too hard into the mousepad, thus rounding off the edge.

I just did a friends SAK main blade with a diamond stone, then 1000 grit waterstone, then green compound on leather.

It is the sharpest it has ever been, with the exception of the time I used my EdgePro, but the convex edge cuts thick(thicker than the edge thickness) materials MUCH better...

Thicker materials drag on the transitions between bevels on standard grinds.
Until you are cutting faster than the speed of sound in the material you are cutting, rounded edges will pass through with less resistance...
Look at planes... Only when you get to supersonic planes, do you start to see sharp angles.

The actual edge of a convex does not need to be any more acute than that of a normal grind, and in a lot of cases can be more obtuse, because you do not have to have the longer bevel to get it to push through materials easier...

Something that a lot of people do not understand(understandably) is that sharpness and cutting ability are almost compleatly different things...
Have you ever got a paper cut??? was the paper SHARP??? No, but it had a lot of cutting ability!
Sharpness has to do with how perfect the edge is...Cutting ability is all concerned with edge geometry.
You can get a one inche thick piece of steel and put a shaving edge on it... Now, try to push cut through a four in thick piece of balsawood(you know, the stuff you can cut with your finger-nail). It is going to take a LOT of pressure, because of the wedge effect that is going on...The bevel is forcing the wood apart so fast that it cannot compress far enough to let the blade in...So when you press hard enough the bottom of the wood will break before the edge even touches it. Now try the same thing with a dull be thin knife...
You can try this yourself with something like a carrot... Get a dull THIN kitchen knife and cut it. then try a sharp thick knife...The sharp thick knife will usually cut part way through, and then the carrot will snap apart.

A convex edge slowes the thickening of the blade for the same angle at the contact area...
Click to expand...


So, if I'm understanding you correctly, i'm basicly rounding off the corners of the bevel to improve cutting ability and performance?
 
jamesraykenney said:
Thicker materials drag on the transitions between bevels on standard grinds.
Click to expand...
A convex bevel doesn't address this, and drag isn't really significant for most materials as they simply are not sticky enough. The force that you apply when cutting comes from three aspects; the pressure needed at the edge to cut the media, the force needed at the sides to drive it apart, and the frictional/drag forces along the flats. There are some types of cutting which are high drag influenced, cheeze for one, but generally the first two are more important.

You can check if drag/friction will be important by pressing the flat of a knife into the media and then hauling down and seeing how much the knife will stick to the material. The more it sticks, the more friction you will get, generally proper hollow grinds like granton edges are designed to deal with this, convex edges tend to enhance a splitting effect which is why they are used in wood splitters.


The actual edge of a convex does not need to be any more acute than that of a normal grind, and in a lot of cases can be more obtuse, because you do not have to have the longer bevel to get it to push through materials easier...
Click to expand...
I don't quite follow that. A convex profile still needs an acute final edge to have a high cutting ability on most materials because the very edge bevel is important, on some materials it can be all that is seen, ropes and hardwood whittling is very critical on the very edge for example.

I just recently re-edged a Fulcrum IID, the primary grind is still sabre-flat, I didn't make it any higher, I just worked on the same angle reducing the edge thickness and the edge angle. It now has a primary flat grind of ~11-12 degrees which tapers to a ~0.005" edge ground at 20 degrees.

It can cut 3/8" hemp and whittle hardwoods just slightly ahead of a Spyderco Manix which has a high convex edge, simply because the edge is a *lot* thinner on the Fulcrum. The Manix pulls ahead quickly on thicker materials like cardboard and stiff vegetables.

A convex edge slowes the thickening of the blade for the same angle at the contact area...
Click to expand...
I am not sure what this means, convex bevels actually thicken faster than flat which thicken faster than hollow simply by the nature of their curvature, meaning if you connect two points by those curvatures, convex is thicker than flat which is thicker than hollow.

MJR said:
...rounding off the corners of the bevel to improve cutting ability and performance?
Click to expand...
This can make a drastic effect, but it does not have to be a convex bevel, J.J. of Razor's Edge notes the same effect with a flat grind, you can go even more extreme with a hollow grind. It is hard to do hollows by hand though.

-Cliff
 
I am not sure what this means, convex bevels actually thicken faster than flat which thicken faster than hollow simply by the nature of their curvature, meaning if you connect two points by those curvatures, convex is thicker than flat which is thicker than hollow.
Click to expand...

I think what JK is referring to when he said, " A convex edge slowes the thickening of the blade for the same angle at the contact area... " is that the using life is longer on a convex because as you sharpen it the general profile stays the same but the knife gets thinner throughout the life of the tool. In this respect what you said quoted above is actually just the opposite in practical use over time regarding a full convex edge. I think convex edges are preferred because they do in fact last longer due to the very way in which they are sharpened. Once you reach the end of a saber grind it thickens significantly. Same with a hollow grind where it gets too thin the tool pretty much becomes worthless and needs retired.

See here. http://home.nycap.rr.com/sosak/convex.htm and in particular I'm noting the "effects of prolonged sharpening" picture.
 
Thanks for sharing your knowledge Cliff. It is a real treasure. I have learned a lot from your post and articles.
 
STR said:
the using life is longer on a convex because as you sharpen it the general profile stays the same but the knife gets thinner throughout the life of the tool.
Click to expand...
There is nothing stopping you from doing the same with the flat ground profiles. Here is a Fulcrum IID with a low sabre flat grind :

http://i7.photobucket.com/albums/y269/CliffStamp/extrema%20ratio/fulcrum_modified.jpg

The edge on this will thicken rapidly with repeated sharpenings, but this takes a *long* time even with heavy use, as in many years, unless of course you are constantly removing major damage, which was mentioned to me by a local maker who forged spring parangs in the tropics. I asked how he dealt with corrosion, he replied it never builds up because every week the sharpening removes all of it.

Of course if you want to prevent this from happening on flat ground blades you can on occasion just work the primary grind. If you do this the exact same thing happens to the evolution of the knife as you described with the convex grind. It is also much faster to do as well as you can just put it on a really coarse abrasive and lap like a madman. Sal talks about this in the Sharpmaker video in regards to occasional back beveling which addresses a related issue.

Now with a hollow grind it is not as trival to grind them by hand without an actual wheel so you are almost forced to accept the edge thickening. However high cutting hollow grinds have such long lifetimes on quality steels I would really wonder for how many people is this an issue - and this thickening takes place much slower than on flat/convex grinds. My hollow ground blades can have 1/4" of steel wore off them and the edge is still ~0.015-0.020" thick.

Look for example at the Sebenza's in the recent request for user pics thread, how many of them had a significant amount of the edge wore back. How often do people retire knives due to this factor? You could always have it reground of course, after a few years of service, it isn't hard to find a custom maker and presto the knife is back to 100% again. Now look at the total time spend sharpening vs working the full primary profile of the other blades.

For most I would think hollow grinds would be more of an advantage in long term cutting because very few people are willing to work primary grinds, especially considering how many people won't even sharpen their knives. I prefer them for light cutters because I don't have to work the primary because the edge is stable in thickness for a *long* time, and if after 10 years or so I need to get it reground I'll have it done - or probably actually have got a wheel by that time anyway.

-Cliff
 
I probably shouldn't speak for JK actually. My apologies if that is out of line JK. I may be off on what you were referring to also. It was just a guess.

Cliff, I guess I'm thinking of people that work their convex or hollow or saber grinds everyday, sharpen them twice or three times a week or more. A lot of guys do like the ones they can put on a wheel because after a hard day of working the last thing you want to do is convex the edge of your axe back so they take short cuts and file it, or grind it. Nothing looks quite as bad as a convex axe reground with a thin uneven secondary bevel and a scratched up head because someone didn't want to take the time to do it right after dulling it up good. I saw one of those just the other day at a yard sale. My wife asked me if I wanted it, "its only two dollars she tells me". I took one look at it and said, 'Now I know why'.

Once they start grinding it doesn't take long to see the steel diminish though.
 
STR said:
....after a hard day of working the last thing you want to do is convex the edge of your axe back
Click to expand...
About a year or so ago I took out my Wildlife hatchet to do some baseline work against a large fixed blade and the relative performance of the knife was much higher than expected for its size and geometry. I took some measurements of the axe head and sure enough while I had not applied a secondary bevel, I had increased the curvature and the cutting abilitied suffered.

This was probably due to a relaxed reason to what you noted, simply being in a hurry and needing to do a touchup. I take out a file grind the bevel down to full flat and then recreate just a hit of convex thickening to the edge to allow knot work. The cutting performance took a large increase. GB axes don't need to be sharpened every day, I now try to sharpen mine once a week when in season.

If the edge is just a little under I start with a 4000 grit stone and then CrO, so it is just a light polish. On a heavier week, or if I was busy on the previous week I go back to 1000 and then work up. I also pick a time when I have enough time to work on it without being rushed as it does take about 15 minutes to do a full sharpening as the bevel is so large.

Yeah secondary edge bevels look bad, the performance is horrible too, great sod cutters not much else. Among the first thing you need to do to most axes is blast that off.

-Cliff
 
You must log in or register to reply here.
Back
Top