Terminology for convex edges?

[EdgePal]

About convex edges.

Everybody knows that the sharpening angle on a straight edge is important for the knifes performance. We can communicate about the sharpening angle in degrees and that make it easy for us to communicate edges and perfomances.

Not so many people knows (?) that a convex edge are also depending on the sharpening angle for its performance in exactly the same way as straight edges are.

The problem in most discussions about convex edges is how they shall be sharpened. What is the right way…as it was only one way (the right way) to sharpen convex edges. That discussion can we have elsewhere, not here.

I have thought about how to describe the sharpening angle on a convex edge. It is difficult and it must be easy to understand and communicate.
It would be nice if it’s also is easy to measure for all of us…
A convex edge is a curve. Many of us cannot measure the sharpening angle on a straight edge properly and that is much easier to do then a convex edge.

Suggestion:

10/13-5 is a description on a convex edge. It starts from 10 degrees and stops in 13 degrees and the edge is 5 mm long. We get the starting point, the end, and the distance. With those numbers can we, if we like to, draw the curve on a paper.

If I make a steeper convex edge, lets say 10/15-5, the edge can also be described and we can communicate the convex curve, and we also all can understand the difference in the performance of the edge. 10/13-3 says also something about the edge – or?

This means that when some of us shall by a knife with a convex edge, we can ask what sharpening angle the knife has, before we by it. It is good information to have and it tells a lot about the performance of the knife.

If we accept this try to describe a convex edge, or if we can find some other way to do it, is not important, the important is that convex edges needs to get a description of the curve so we can communicate about it -based on facts.

After that, we can talk about how to sharpen convex edges - because it first then we really can talk about it. If we not even can describe the angle of a convex edge – how can we say that this or that way is the right way to do it?

Thomas

 

[Cliff Stamp]

Not so many people knows (?) that a convex edge are also depending on the sharpening angle for its performance in exactly the same way as straight edges are.

Yes, this is the biggest source of hype about the grind which is commonly promoted as being inherently superior due to curvature and not cross section. The reason most people see a difference in performance is simply a relef grind added to the edge, not the actual influence of curvature.

10/13-5 is a description on a convex edge.

I would suggest something like 10/14-0.020" which notes the more obtuse edge sweep starts in under 0.020" thickness of the edge. This is how for example Cook describes the profiles of axes in his book. You want to know how much of the bevel is at the more obtuse angle as this determines the extent of the curvature.

For example 10/14-0.005" is very different than 10/14-0.050" even though they could be the exact same total width. On the wider convex bevels you can use more points of comparison. So 6/10/14-0.050/0.025". I have measured the full profiles on some convex bevels and noted the curvature in detail in the reviews.

-Cliff

 

[EdgePal]

Hi Cliff.
I try to follow you but I am not sure I understand everything you write.

I like simple descriptions. My sugestion to get a description is probably not a right way to do it – but it is a way, I think, people understand.
I think normal human beings have problems to measure decimals with many zero’s, it must be simpler and we must be able to communicate about it.

I think your way is absolutely correct, but I do not think everyone understands it, or, can measure their convex edges and communicate with other people who also understand it. That is the problem I se. Perhaps I am wrong.

We need to find something simple, logic and simple. Perhaps not 100% correct, but understandable.


I use convex edges and have done that most of my life – on some knifes, not all knifes. They are great in some ways and bad in some other ways.
The convex edges are like straight edges, the convex angle is very important for the knifes performance. On knifes and on axes. But we can not (normal people) measure them as we can on a straight edges.

Before we can measure convex edges, we cannot tell how to sharpen the best way. We can describe how to do a “bended edge” with convex form – but that is all. When we can measure the convex edge, and communicate about it, then we also can find the best way to make them correct - in correct angles.

Today we experiment, when the convex edge work for us, then we are happy. But we do not know the convex angle. A person in US cant inform me in Sweden about the angle he use because we have no words for it. The words we have is more or less “scientific” words and I do not understand them…

It must be simple… but how?

Thomas

 

[Cliff Stamp]

I think normal human beings have problems to measure decimals with many zero’s, it must be simpler and we must be able to communicate about it.

I think in terms of fractional inch thickness as that is how I measured them origionally. You can however measure them in terms of lengths as well. So 10/14-1 where the 1 would be the distance back from the edge where the more obtuse bevel starts. Thus 10/14-1 and 10/14-0.1 would be immediately different profiles even though the edge could be 5 mm wide in total. A more exact specification would be something like 10/14:5/1 so the edge has a 10 degree shoulder, 14 degree edge sweep, it 5 mm wide and the edge sweep is 1 mm wide.

-Cliff

 

[EdgePal]

I think that will be fine if I understand it right.

10 degree is the “shoulder” = the edge of the knife where the convex edges starts?
14 degree is the end point of the convex edge.

The convex edge is 5 mm wide and 1 mm high?

Did I understand you correct? = 10/14:5/1.

I by that! It is better then what I suggested. That can communicate convex edges and it is not so hard to understand (hmm, if I understand it right&#8230.

A knife who have a edge in 14 degree:
14/18:5/0,75, I understand that, do you?

Thomas

 

[Cliff Stamp]

I think we are saying the same thing, just to clearify :

The black bevels are the convex profile, it can be approximated by the two orange lines which approximate the shoulder and apex angle as shown on the bottom of the bevel. The blue and red lines show the start of the shoulder and apex angle and you can specificy them by distance from the edge.

-Cliff

 

[EdgePal]

Then I misunderstand you…

I understand it like this:

10 is the existing sharpening angle on the blade, the “base”. (A)
14 is the end of the convex angle = 4 degrees above the base (B)
5 stands for the wideness of the convex edge, the distance between where the convex edge starts and ends
1 stands for the cross section of the convex curve compare to a straight line. (A straight line from A to B up to the convex upper surface)

Isn’t that simpler to understand and measure for everyone? For those who can not measure the cross section, they can still communicate with the first 3 figures…that helps up the method in my mind.

Thomas

 

[Cliff Stamp]

I don't quite follow as there are some problems like :

...
1 stands for the cross section of the convex curve compare to a straight line.

A line has a length it doesn't have a cross section which is an area. If you mean to measure the width and height of the bevel at the shoulder then that is problematic as you need to know the transition point. You can make the edge bevel significantly steeper and not effect significantly the width or the height of the bevel by simply moving the red line to the right. A diagram would help.

-Cliff

 

[EdgePal]

Well Cliff, I am an innovator and I think that nothing is impossible.

A Swedish mile is 10 000 meter long. A Danish mile is 7 776 meter long, but it is little wider so the square is the same.
A mile in US is 1 609 meters long, that means that an US mile is wider then it is long. That is no problem in US because US is rather big, but if you try to use that mile in Belgium, it do not fit at all, That is why our miles in Europe is longer then yours…


There is a distance between a straight line from A to B (the length of the “convex” edge, but straight – and up to the convex edge surface. That was the distance I thought of. It can be drawn easy, it is easy to measure for all of us (I think). And we still is measuring on only one side of the blade.

In your suggestion, you measure the cross section of the complete edge in two certain distances from the cutting edge. It is one way to do it and it is possible to make a “catscan” of the complete edge with 1 mm distance and that is nice, but can we communicate about it? Can all of us do that? Can we measure it without and expensive equipment?


I hope that some other members in the forum tells us their opinion about how to make this work in the most easy way as possible. Something like this, perhaps, solves the problem and that will be a big step forwards for development of convex edges in the future.

Hey guys! Come on, tell us what you are thinking!

We have worked out 2 ways to do it, tell us your opinion and come with more suggestions!

Thomas

 

[Cliff Stamp]

In your suggestion, you measure the cross section of the complete edge in two certain distances from the cutting edge.

These distances are necessary to measure the shoulder and apex angle anyway. It is also a natural extension of the standard way people talk about v-ground bevels. Most people in the US use standard so it is all in inches, the blade is 0.25" thick, 2" wide and the edge 0.035" thick and ground at 15 degrees per side. Other places use metric, Fallkniven cites everything in mm/cm for example. Use whichever units you prefer.

Since it is a basic extension of how people talk about v-ground bevels it makes it very straightforward to understand because you just think of it as two flat bevels. It might be more readable as 10/0.055:15/0.025 which says a 10 degree bevel which starts at 0.055" and a 15 degree one which starts at 0.025". So you can think of this as a 15 degree secondary edge on a 10 degree primary profile.

It is also straightfoward to understand the performance if you understand v-ground bevels. So for example if you had 10/0.055:15/0.045 you would know that bevel is far stronger but cuts at a much reduced level because the 15 degree part is much thicker, essentially the 10 degree relief was just barely applied.

Can we measure it without and expensive equipment?

You only need a pair of calipers which you can buy on Ebay. You can get nice digital ones with zeroing and other features for the price of shipping basically.

-Cliff

 

[EdgePal]

If we use mm or inches its do not matter, it is always possible to translate to the other scale, I se no problem with that. The main thing is that we use the same method.

I am not sure about the measuring you prefer, not because it is wrong, is not, it is very god – but I doubt that all people understands it and can use it. It is perhaps the same thing with the way I prefer.

I hope other people will say something about what we have so far. That can guide us in the right direction. I can accept both ways. But, is a third and better way possible?

What we have done so far is two ways who works nice to describe convex edges. That is a nice thing. But is it nice for others then us? Can it be accepted and used of all of us?

I like simple things. The simple things can be very advanced, but they shall be simple to use. I have found out that I am not alone about that. Most people like simple things. That’s why I started this thread, to try to find a simple way as people accept and use to measure convex edges so that we all can communicate about them.

But, it seams that we are just us two here…

Thomas

 

[Cliff Stamp]

I am not completely sure about what you are measuring, and especially how you are measuring it. How are you determining the angles specifically? Can you just sketch a diagram in Paint or similar?

-Cliff

 

[EdgePal]

I hate to say this, but you are absolutely right. My thinking was elsewhere.
Of cause, it must be measured your way.

I am a little worried that it is only you and me who are interested of this topic? In my mind, this is one of the most important things to discuss about convex edges. How to measure them, how to communicate them and how to compare them. When I can do that, I can also change the convex edge and se if the edge performs better – and then I also understand why it performs better. Without that – I have just a convex edge - without a clue of the sharpening angle.

I suggest that you define the numbers and how they shall be used - in a simple way as possible. Your English is better then my English.

Thomas

 

[HoB]

My thoughts to this are the following: A) without a picture (a labled one) this is going nowhere , B) sorry for the crappy picture, I made it in Word and then took a picture of the screen with my camera......

Ok. The way I would usually apply a convex edge were I control the geometry is by applying a relief grind, then an edge grind and then by blending the two bevels together. When I do a flat grind, the relief grind would actually go almost all the way to the edge and the edge grind is just a microbevel. In that case I would call D1 the thickness behind the edge and D2 is not really sensibly measureable. But I use relief grinds of at least 10 degrees per side. I know that Cliff uses much more shallow relief grinds, in which case D1 is clearly not the thickness behind the edge any more, but rather D2 and I would assume that Cliff still puts on a third, microbevel (is that correct, Cliff?). If we are talking about flat grinds I would call the angle at which the blade is flat ground the blade angle. For very shallow relief grinds the relief angle blends into the blade angle.

With this established, it seems to me that the convex edge should be characterized by 3 quantities: The thickness behind the edge (the way I drew it, that would be D1), the relief angle and the edge angle. The thickness is easily measured with a caliper....far more easily that L. In general the thickness is often much more easily measured than the angles (except of course if you set them yourself, but even then you need a good protractor). So, anyone having trouble measuring D1 will have trouble measuring the angles as well, so ease of measurement should really not be an argument here. Besides, it seems to me that someone not knowing how to use a caliper or not owning one, is not likely going to be interested in these details anyways.

Measurement and characterization of an exsisting convex edge would, as far as I can tell, necessitate a point by point mapping of the blade profile anyways. At least I can not see a way, how you would measure the relief angle otherwise. You can measure the edge angle by seeing when it catches the side of a piece of paper.

Cliff, in your picture, shouldn't the straight lines approximating the convex edge be tangents and therefore on the outside of the curve? (Like you see in my picture?)

Well those are just my thoughts.

 

[Cliff Stamp]

... still puts on a third, microbevel ...

Almost always, but it is usually very narrow, 0.1 mm wide or less.

So, anyone having trouble measuring D1 will have trouble measuring the angles as well ...

Yes, that's how you would calculate the angle anyway.

...how you would measure the relief angle otherwise.

You just estimate it from D1, D2 and L1, L2. Specifically atan((D1-D2)/(L1-L2)*0.5) and than atan (D2/L2*0.5) for the apex. These are the angles of the two lines I drew. For very wide convex bevels or primary grinds you may want more than two points but the extension of the nomenclature is straightfoward. I have illustrated this in various reviews of convex blades including the numbers as well as graphs showing the profile in detail. However in most convex blades, aside from splitting axes, the curvature of the primary grind is very close to straight, so much so it usually isn't even visible by eye. The exception to this are dual convex ground blades such as some traditional parangs which have oblate geometries :

That is a real profile, the +'s are the measured points. Note the way it tapers again towards the top. Now characterizing this in a couple of points would be too simplistic but you can just refer to the top and bottom curvatures distinctly. In general though the most common application of convex grinds are on bevels and primary convex grinds are generally very flat so this level of detail isn't necessary. It never is of course as a place to start discussion and you can always say something like "A five degree primary with a two degree sweep" to note a convex primary grind which is well approximated by a flat grind of five degrees. You can estimate this easily by placing a ruler along the apex of the primary and seeing how much rock it takes to cover the grind.

...shouldn't the straight lines approximating the convex edge be tangents and therefore on the outside of the curve?

They are tangents, just translate along a perpendicular to the curve.

In my mind, this is one of the most important things to discuss about convex edges. How to measure them, how to communicate them and how to compare them. When I can do that, I can also change the convex edge and se if the edge performs better – and then I also understand why it performs better. Without that – I have just a convex edge - without a clue of the sharpening angle.

Yes and if you read about convex edges you will note that it is extremely common for it to be implied or even stated that convex by its inherent nature of curvature is superior in both durability and cutting ability and edge retention as in "I convex all my knives they are much more durable/sharper that way.". Well this of course depends on how you convex the bevels obviously. You could take any convex profile and flat grind it and make it cut better or be more durable by grinding at the shoulder and apex angles respectively.

In reality the dominate effects are just related to cross section and not the curvature. The only main reason for the curvature is just to compensate for the nonlinear dependence of torsional strain on distance from the edge. As with all manner of hype, details tend to eliminate the ability for this type of promotion. I congradulate you on your desire for such information but I have been requesting it for about a dozen years and few makers still do it. Just do a random check on a dozen knives and see if the promotional blurb mentions edge thickness/angle, even on v-ground bevels let alone convex.

It never hurts to ask though and the more people that are specific about edge angles the more likely it is that it will become common place information.

-Cliff

 

[HoB]

They are tangents, just translate along a perpendicular to the curve.
-Cliff

Yes, now I see. You just translated them inward.

The profile you showed I would call a convexed median ridge geometry. I assume that (convex or not remains to be seen) Ed Schempps camp knife will have a geometry somewhat similar to that.

 

[Cliff Stamp]

Yes, generally median ridged means a distinct line like Brend uses but a few minutes of blending over the ridge line would produce the above tapers so your terminology is a natural and logical extension. Schempp notes a japanese influence for his knives and I have also seen the same thing on many traditional parangs. It is also exactly what you tend to get if you just press a knife into a belt sander above the platen. My Battle Mistress for example has a grind along the spine because I ground it flat to the belt to maximize the extent of the relief grind. This was before I figured out you could (duh) use the platen to do that much more effectively.

-Cliff

 

[EdgePal]

Well Guys… My English is not good an off to follow your discussions. I think I understand some parts of it, but I am not sure. You are using words I do not understand. The drawings I understand, and I think they are ok. I think I understand most of the words, but not all of them I am sorry to say.

I like convex edges, they fits in my way of living. I have convex edges on some of my knifes, but not on all of my knifes. Convex edges are superior in some ways, and really bad in other ways. I think you have the same experience.
If we look back in history, all edges was more or less convex when all sharpening was made by hand. A convex edge is the oldest type of edge we have. Straight edges is the newest type of edge, and hollow edges the second newest – because of the grinding wheel.

The straight edge was popular for many years, and now the convex edge is back again. It is a normal thing, a circle in time. Things get back again after some years, sometimes decades or centuries.

As I think you know, I construct, marketing and sale sharpening tools. I have solved the problem technical how to sharpen convex edges with the same convex curve all times you sharpen your knife. It was hard to find the solution, but the solution was very easy when I find it. In the same time it was very functional. Two persons who have my solution can communicate about convex edges and convex degrees – but they do it only in technical comparing terms. They can make exactly the same convex curve on their knifes and communicate about it.
They cannot describe the convex edge in numbers or in words. That is the reason I start this thread.
All people need to be able to communicate about the convex curve they have, or like to have, on their knifes.

I think we together have started to found a way to describe convex edges in a way all people can understand.
Perhaps we need to make it more simple so that everybody can understand it. If people do not understands it, they never use it. And our work is soon forgotten. That is the problem. Simple way as possible – but also a functional way.
We can surely produce a perfect way – but I think – it will soon be forgotten.

When people find an edge they like, and who fits their need of performance of the knife, they do not care about what sharpening angle it have. They just use it. The problem they get after a while is how to maintain exactly the same convex curve. They ask other people who use convex edges and, hopefully, they understand that they cannot communicate. We are just in the beginning of the convex edge time. People can still maintain the convex edge – but they cannot grind the convex edge. Most people have not come to this grinding point jet.

Most people do not feel the small difference they get when they maintain their convex edges. The problem will be acute when the edge do not work at all.

What I can se here in Sweden and Norway is that people who have bought knifes with convex edges have start to get big problems to sharpen them. They send their knifes away for sharpening – or, just leave them in a closet at home and go back to straight edges again, the knife with convex edge they really like become a “shelf heater”. (It lay on a shelf and is not in use any more). I think that is sad.

That is why I try to find a way to communicate convex edges sharpening angles – in a simple and functional way. I have made a technical solution but that is just a small part of the complete problem.

Welcome RoB! Now we are three people who understand that this is a very big problem and its needs to be solved so people who use convex edges can start to really communicate about them, in specific terms.

Today they say: I use convex edges – me too… I drive a car – me to… I eat food – me too…

Thomas

 

[Cliff Stamp]

If we look back in history, all edges was more or less convex when all sharpening was made by hand.

Yes, essentially unless you use a jig and nonwearing stones your edges will be convex, though usually what people mean by convex is that the curvature is large enough to see like on an axe bit.

I have solved the problem technical how to sharpen convex edges with the same convex curve all times you sharpen your knife.

Ref :

Ok now I understand how you were measuing the curvature as noted in the above because you were measuring the arm characteristics and you can do this easily with a protractor and ruler. That is a very elegant solution by the way. I would suggest for a field version that instead of a magnet or clamp have it end in a spike as you could then just tap this into a stump to serve as a base. You could even make this a screw in attachment so you could have a magnet, spike or clamp available to be attached to the basic stand.

When people find an edge they like, and who fits their need of performance of the knife, they do not care about what sharpening angle it have. They just use it. The problem they get after a while is how to maintain exactly the same convex curve.

Yes, if you do this freehand there is a tendancy to thicken the last part of the edge and vastly reduce the cutting ability. My convex edges are really shallow in curvature usually and thus I actually flatten them back on a regular basis and then just add in a bit of a sweep, usually 1-3 degrees with sharpening.

What I can se here in Sweden and Norway is that people who have bought knifes with convex edges have start to get big problems to sharpen them.

A lot of this is due to misconceptions about convex edges, that they require special equipment and that you can "ruin" them otherwise.

-Cliff

 

[omniphile]

HoB

Hope you don't mind if I mention a quick tip that might make things easier for you next time.

sorry for the crappy picture, I made it in Word and then took a picture of the screen with my camera......

If you press the "ALT + PRINTSCREEN" key combination, it will place a copy of the window on the clipboard, then in your favourite picture software, you can paste it as an image (using Edit, Paste or SHIFT+INSERT) and save it as any image format you choose.

Cheers
omniphile