V - Sharpening Convex Edges

Nah dude, nah.
Convex edges, in a word, suuuuuuuuuuuuck.
Regrind that soft bellied ring tosser into something useful rather than just something to scrape hair off with.

 
you're not a wood worker are you. Once you start to cut solid stuff with a bunch of force behind it . . . all becomes clear.
Case in point . . . ever see metal cutting tool bits with convex edges or drill bits with roundy edges ?

Nope . . . they are ground with flat facets or a slight concavitude . . . dude.

A book worth perusing
 
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Nope, convex is my favorite.
Glad you can share your opinions.

For me it works the best.

No interest in that book.
Boring.
Take care.
 
Its not that convex edges are better or that flat edges are better, its that the majority of people can produce convex edges. Its not the case when trying to produce a "flat" edge. Few if anyone can produce a "flat" edge without a control system, no one I know. If its free hand its convex.

Regards, Fred
 
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Done properly, both edge mechanics will work fine. Blanket statements to the contrary are formed by a limited understanding and poor sample groups.

The major advantages are that when compared to a V bevel with same cutting angle, there is less meat behind the edge when all the other dimensions are the same (spine thickness, width). The creation of a convex from a V bevel is a reductive action, you remove steel from behind the edge, not round the edge over and leave the sides the same.

They don't penetrate with more wedging but less, hence back to the felling axe vs splitting wedge analogy. Also backed up by CATRA testing to some extent in terms of longevity, a test that tends to favor thinner geometry over other factors such as edge finish.

On a sample of tools ground from thin stock one would likely not notice any real difference between the two for regular cutting tasks, only on chopping jobs involving ballistic immersion cuts with a lot of force are you going to readily feel a difference.

If you're noticing a real failure of the convex edge, the final edge approach is too broad. Arguments that industrial tooling is all flat ground are also somewhat baseless as most of these have multiple bevel and relief facets that mechanically are somewhere between a convex and a two facet V bevel anyway. From an industrial standpoint, there are specialty shops that will repeatably make a cutting tool with smooth convex approach to the cutting edge though this is not as common.

Either one work.
 
Fred.Rowe said:
Its not that convex edges are better, its that the majority of people can produce them. Its not the case when trying to produce a "flat" edge. Few if anyone can produce a "flat" edge without a control system, no one I know. If its free hand its convex. Convex done on a belt shortens the life of a blade because it removes excessive steel when its not needed to make the edge sharp.

Regards, Fred
Click to expand...


I get close to flat edges free hand, but only under a strong light to check for the shadow, and by scraping parallel to the edge, starting with very rough extra-coarse Dia-Sharp hones... I go slightly diagonal a bit before I get down to Coarse, to cancel out the deeper parallel striations. I cannot get anything close to a near-flat freehand below Coarse, which when worn is fine enough to be shaving sharp. I do sometimes break off wire edges at 60-90 degrees with a "Medium" stone, sometimes at the cost of a slight micro-bevel when finishing the break.

It would be near impossible to get a highly polished edge free-hand that would also be near- "flat"... Scraping parallel to the edge was a big breakthrough in free-hand sharpness for me, because my flats became "truer" flats from lessened "rocking". A motion perpendicular to the edge induces more "rocking" in my experience...

One weird thing I noticed happened consistently was that thick edges that required very tall V bevels were never as strong under chopping into wood as thinner edges that required only small V bevels (like Randalls for instance): I think the reason for this "fragility" of "big bevels/thick edges" is that big bevels are less "stable" under the impact on wood: The thick edge has the same angle sharpness, but, being broader, it is more susceptible to "rolling" or "leaning" under impact, which lateral movement causes damage to the edge's apex. A thinner edge, even of the same bevel angle, stays straighter under impact, and so appears indestructible under hundreds of chops, in the same wood were the thicker edge will micro-chip in dozens of chops... And then I think I have found some super-steel that is tougher thinner, but it really is the bevel base thinness that helps, not harms... This is why I think the idea that thicker edge bevel bases help edge strength is only true for outright prying: Thicker edges weaken an apex of the same angle under chopping, at least in most woods... I would say the ideal bevel base thickness for chopping wood with a knife is around 0.020" to 0.030" at most.

If you look at many convex ground blades, they are often ground thinner near the edge, and the appearance of edge durability (while chopping wood) derived from that thinness/stability -while decelerating in wood- may be larger than any inherent strength gained by the convexing...

Gaston
 
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I've had my share of convex and V bevels. Both of them cut and i'm able to place a great edge on both. I believe it's just a matter of being able to keep either sharp to stay useful to the user.

As such, there is no need for ignorant claims on the absolute superiority of either geometry. A lot of our preferences come from our individual experiences, or believing what other people tell us, so play nice.

I finally had the courage to take my convex Bark River PSK to my Spyderco Ultrafine ceramic stone. I wasn't trying to reprofile it with a 'V', not this time. I tried hitting the shoulders a little and made sure to apex the edge to take some tiny chips off the Elmax edge. The ultrafine ceramic takes steel off so slowly so there was some room for error. This was just a minor touch-up run before stropping with green compound. It cuts well now and has a smooth transition into what looks like a mirror-polished secondary bevel.
 
HeavyHanded said:
Done properly, both edge mechanics will work fine. Blanket statements to the contrary are formed by a limited understanding and poor sample groups.

The major advantages are that when compared to a V bevel with same cutting angle, there is less meat behind the edge when all the other dimensions are the same (spine thickness, width). The creation of a convex from a V bevel is a reductive action, you remove steel from behind the edge, not round the edge over and leave the sides the same.

They don't penetrate with more wedging but less, hence back to the felling axe vs splitting wedge analogy. Also backed up by CATRA testing to some extent in terms of longevity, a test that tends to favor thinner geometry over other factors such as edge finish.

On a sample of tools ground from thin stock one would likely not notice any real difference between the two for regular cutting tasks, only on chopping jobs involving ballistic immersion cuts with a lot of force are you going to readily feel a difference.

If you're noticing a real failure of the convex edge, the final edge approach is too broad. Arguments that industrial tooling is all flat ground are also somewhat baseless as most of these have multiple bevel and relief facets that mechanically are somewhere between a convex and a two facet V bevel anyway. From an industrial standpoint, there are specialty shops that will repeatably make a cutting tool with smooth convex approach to the cutting edge though this is not as common.

Either one work.
Click to expand...

Martin,

Reading your post is easy and they are always clear and concise, but for the life of me I cannot wrap my head around this segment of your sharpening philosophy. I think it may be the land surveyor in me that causes this confusion. Its when it states "all dimensions stay the same including the edge angle at the apex; same everything, and stopping at the same point on tangent. When seen through the eyes of a surveyor there is no way to place a curved line inside a straight tangent and end up at the same point on tangent. Some thing has to change in the equation, either the apex angle, the height of the shoulder or the width across the shoulders. Concave, thats easy, convex seems mathematically untenable.
A well done edge no matter the shape if the geometry of the total package is complimentary then its a job well done.

PM me if you prefer. Regards, Fred
 
Small secondary edge on full convex ground knives are easier to maintain and perform the same as Zero convex.My secondary edge on Full convex blades are almost invisible.It is not hard to sharpen these knives on stones,unless you want to zero convex it every time and care about perfect finish on blade:).I sharpen and touch up convex blades on sharpmaker too with great results.When secondary edge gets larger ,than I reprofile whole blade ......Belt sander is great for this.
 
Fred.Rowe said:
Martin,

Reading your post is easy and they are always clear and concise, but for the life of me I cannot wrap my head around this segment of your sharpening philosophy. I think it may be the land surveyor in me that causes this confusion. Its when it states "all dimensions stay the same including the edge angle at the apex; same everything, and stopping at the same point on tangent. When seen through the eyes of a surveyor there is no way to place a curved line inside a straight tangent and end up at the same point on tangent. Some thing has to change in the equation, either the apex angle, the height of the shoulder or the width across the shoulders. Concave, thats easy, convex seems mathematically untenable.
A well done edge no matter the shape if the geometry of the total package is complimentary then its a job well done.

PM me if you prefer. Regards, Fred
Click to expand...

Fred, I'll give it one more try. The diagram shows 1/2 of a blade cross-section. We hold the spine width to a constant, the distance from spine to edge as a constant, and the angle of the edge as best it can be measured to a constant. The original blue line represents the boundaries of our V bevel. The ochre line is the region I ground down to make my convex, and the red line is now the new "gothic arch" of my profile.

You could grind another flat from the top point of the arch, but where it meets the edge it will have to be more acute, only the last tens of microns are available to meet the new facet. Even if you could squeak it in, a few passes on a stone and the upper and lower facet intersections will be gone yet the final angle value will remain the same (and less mass yet again as a result).

Only if you convert the flat primary/cutting facets to a FFG originating at the spine (morphing it from a quadrilateral to a triangle), will I be unable to ground the intersection into a curve. This will also greatly shrink the cutting edge angle.

Anywhere there is an intersection of two straight lines on a polygon (no matter the degree until it becomes a straight line), the two lines can be joined by an arch that will reduce the overall surface area, yet retain the orientation of the lines to either side = less mass on a 3D item while the degree value between the two stays the same. It is always a reduction in area. If you don't care about the final cutting angle, then yes, the FFG will have less mass, but this also violates the three constants we started out with.

Convex_zpshrtridxi.jpg


Gastons latest also reminded me of a recent experience. I had to do a few sets of chisels and a handful of plane irons. Many were no longer at right angles, and the backs weren't (and maybe never had been) flattened.

My thought - grind them back to a 90 on a wheel, then hit em on a Norton Blaze 60 grit, a few quick passes on a 180 mesh diamond plate and on to the waterstones, no more than 10 minutes ea.

What happened instead, was 8 minutes each just to flatten the facets out from the curvature the belt induced, even over a steel platen. On a regular cutting tool you'd likely never see it, but in this case my freehand was much more precise than the tolerances inherent in the belt surface. I realize large wheel or disk systems are preferred for this sort of work, but still surprised at the outcome off the belts - I wouldn't have thought with light pressure it would be so noticeable.

After doing a few this way I switched to the 60 grit loose SiC on a fine stone and re-established the bevels like an improvised Kanaban. Cleaned them up on the 180 mesh in a minute or two per and on to the waterstones.
 
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Ok, I'm no sharpening guru or anything, so here is me nervously trying to make a point:

The blanket groupings of either convex grinds or V grinds suggests, to me, that all convex grinds have the same "angle", and all V grinds have the same angle but, as HeavyHanded's picture points out, a convex edge could easily have a more acute angle than a V grind.

The way it seems to me is that a V grind is just a type of grind, and as such it could have any number of actual angles. The same applies to convex grinds. You can give a knife a V grind with an incredibly acute or incredibly obtuse angle which, of course, is going to effect its cutting performance. As such, you can strop an edge into one that is convex with either a very steep or very shallow angle, which will also change it's ability in use.

I mean...right?

Personally, I've always felt that (for me, this is just for me!) a convex edge is going to be more durable. Plus...look at samurai swords? Who wouldn't want a knife ground like a samurai sword? Also, since I don't have a guided sharpening system, I find it much easier to strop my knives than having to hit the exact same angle over and over again. In my experience it's been a great way to maintain my knives but, as always, YMMV. :thumbup:
 
Personally as long as the stock is reasonably thin I don't see a big difference in cutting for most tools. The only tool type where I am 100% in favor of convex is on my hatchets, machetes and axes. This also makes it much easier to sharpen them in the field with a puck or other small stone using a circular motion. I can often go out for a few nights and not even need to sharpen my smaller camp knife, the hatchet or machete gets touched up at least every other morning, more often if I've been chopping some serious hardwood.

As a topic goes, this one is seemingly inexhaustible. For me the revelation came about after regrinding some overbuilt sabre grinds down to a gothic arch by hand. It isn't that a straight line is shorter than a curved one between two points (it certainly is!), but that an arc can travel a shorter distance between two points than two straight lines meeting in the middle at an angle.

If you push a gothic arch to the limit, it becomes a FFG.
 
HeavyHanded said:
...The diagram shows 1/2 of a blade cross-section. We hold the spine width to a constant, the distance from spine to edge as a constant, and the angle of the edge as best it can be measured to a constant

... if you convert the flat primary/cutting facets to a FFG originating at the spine (morphing it from a quadrilateral to a triangle)...
Click to expand...

This is where HH has so much trouble, recognizing that the "spine to edge" distance is irrelevant to the geometry of the bevel. The bevel geometry is from the edge to the base of the bevel, not the spine of the knife which could as easily be larger than 12" or smaller than 12 microns. By basing his argument on that distance, he is equating a pentagon with a triangle, not recommended. He knows his argument is wrong because he is forced to admit that an FFG blade proves it so - one cannot "convex" such a blade to be thinner than it already is without changing the bevel height and/or base thickness, although one CAN "concave" it thinner via a hollow grind.

What HH is saying is NOT that convex is thinner than flat, but that if you have a blade with a "shoulder" between two bevels, he can cut another bevel or set of them (a bevel being a physical property with a precise width that starts at one point and ends at another) in place of that shoulder. The bevel he adds would of course be thinner if he cut it flat between those two points. But the point he is trying to make is NOT that convex edges are thinner, they are demonstrably NOT, but rather that a thick blade can be back-beveled with a convex profile that is thinner than its previous profile. This is so.
 
HeavyHanded said:
Fred, I'll give it one more try. The diagram shows 1/2 of a blade cross-section. We hold the spine width to a constant, the distance from spine to edge as a constant, and the angle of the edge as best it can be measured to a constant. The original blue line represents the boundaries of our V bevel. The ochre line is the region I ground down to make my convex, and the red line is now the new "gothic arch" of my profile.

You could grind another flat from the top point of the arch, but where it meets the edge it will have to be more acute, only the last tens of microns are available to meet the new facet. Even if you could squeak it in, a few passes on a stone and the upper and lower facet intersections will be gone yet the final angle value will remain the same (and less mass yet again as a result).

Only if you convert the flat primary/cutting facets to a FFG originating at the spine (morphing it from a quadrilateral to a triangle), will I be unable to ground the intersection into a curve. This will also greatly shrink the cutting edge angle.

Anywhere there is an intersection of two straight lines on a polygon (no matter the degree until it becomes a straight line), the two lines can be joined by an arch that will reduce the overall surface area, yet retain the orientation of the lines to either side = less mass on a 3D item while the degree value between the two stays the same. It is always a reduction in area. If you don't care about the final cutting angle, then yes, the FFG will have less mass, but this also violates the three constants we started out with.

Convex_zpshrtridxi.jpg


Gastons latest also reminded me of a recent experience. I had to do a few sets of chisels and a handful of plane irons. Many were no longer at right angles, and the backs weren't (and maybe never had been) flattened.

My thought - grind them back to a 90 on a wheel, then hit em on a Norton Blaze 60 grit, a few quick passes on a 180 mesh diamond plate and on to the waterstones, no more than 10 minutes ea.

What happened instead, was 8 minutes each just to flatten the facets out from the curvature the belt induced, even over a steel platen. On a regular cutting tool you'd likely never see it, but in this case my freehand was much more precise than the tolerances inherent in the belt surface. I realize large wheel or disk systems are preferred for this sort of work, but still surprised at the outcome off the belts - I wouldn't have thought with light pressure it would be so noticeable.

After doing a few this way I switched to the 60 grit loose SiC on a fine stone and re-established the bevels like an improvised Kanaban. Cleaned them up on the 180 mesh in a minute or two per and on to the waterstones.
Click to expand...

I was thinking you were maintaining the same geometry that is used with the "V" and placing the convex edge inside that geometry. I see by the drawing that is not what your saying. The convex edge being used for demonstration purposes intersects the primary bevel above where the "V" edge terminates. This of course would make the cross section where the convex intersects the primary, [wider] than the cross section where the "V" edge terminates. My engineering mind was overlaying the "V" edge with a convex edge using the same actual distances and thicknesses which means you have to use two different set of criteria when assessing these two edges. Now if we were to do as stated above and place both edges inside the same size triangle; the "V" edge would be a more efficient cutting shape. The same is true in reverse, overlay a "V" edge using the triangle that makes up the convex edge shape and the "V" edge will perform better due to its more acute edge angle.

We were just speaking in apples and oranges.

Regards, Fred
 
chiral.grolim said:
This is where HH has so much trouble, recognizing that the "spine to edge" distance is irrelevant to the geometry of the bevel. The bevel geometry is from the edge to the base of the bevel, not the spine of the knife which could as easily be larger than 12" or smaller than 12 microns. By basing his argument on that distance, he is equating a pentagon with a triangle, not recommended. He knows his argument is wrong because he is forced to admit that an FFG blade proves it so - one cannot "convex" such a blade to be thinner than it already is without changing the bevel height and/or base thickness, although one CAN "concave" it thinner via a hollow grind.

What HH is saying is NOT that convex is thinner than flat, but that if you have a blade with a "shoulder" between two bevels, he can cut another bevel or set of them (a bevel being a physical property with a precise width that starts at one point and ends at another) in place of that shoulder. The bevel he adds would of course be thinner if he cut it flat between those two points. But the point he is trying to make is NOT that convex edges are thinner, they are demonstrably NOT, but rather that a thick blade can be back-beveled with a convex profile that is thinner than its previous profile. This is so.
Click to expand...


The prime reason to define a spine height is that it then defines the width of the spine relative to overall shape. Yes, not needed to determine edge angle, but needed to determine what the options are. As long as both approaches use the same value, we can calculate comparative surface area and mass.


With multiple facets, the intersection of every bevel could be converted into an arc, further reducing the enclosed surface area. Take a grinder and convert a stop sign to a circle.

The point I've been making is that if we define at the very least a spine thickness and final edge approach as our criteria, an arc will join those two and encompass less surface area than two lines joined by an angle (it will be thinner), no matter the value. Only if one resorts to a straight line can we undercut the arc, but then we're changing our final approach angle.

In a purely mathematical sense this could dwindle down to unrealistic scales with neither emerging as a definitive end and I think everyone should realize that by now. In reality, once the angle between the primary and cutting bevel shrinks down to a few degrees of 180, the tolerances of the abrasive process will turn it into an arc anyway - instead of a hard transition you will have a soft shoulder - an arc, joining the two facets.

Convex%20_2_zpsgrh5myir.jpg



Amazing, I'm not even the biggest fan of them!
 
HeavyHanded said:
The prime reason to define a spine height is that it then defines the width of the spine relative to overall shape.
Click to expand...

Spine "height" isn't defined in your argument at all, nor does it define spine "width" or thickness in any way. A blade that is 12" "tall" and 1/8" thick can have the same bevel geometry as a blade that is only 1" tall.

Regarding overall shape, we are talking about a cutting blade = a wedge, "surface area" isn't the prime concern, the prime concern is mechanical advantage which is achieved via the slope of the bevel and calculated as the ratio between bevel width and bevel thickness. The other primary concern is edge strength which is determined primarily by bevel thickness.
 
HeavyHanded said:
Convex_zpshrtridxi.jpg


.
Click to expand...

As has been pointed out many times before, this is not an accurate comparison: It is apples to oranges.

An accurate comparison would have the V bevel half join the same point as the curve, which would inevitably make the V bevel thinner...

Now your argument will then be: "But that is not an equal comparison initial apex angle to initial apex angle!"

Why would you want to deliberately choose the more open -duller- apex angle, when you could have the less open -sharper- angle? Let's put aside the "splitting" aspect, which is not versatile in soft or ductible materials, and probably doesn't help knives anywhere near as much as it does much burlier axes...

You want a duller initial angle because the duller apex is stronger? The idea that the duller apex is stronger is highly debatable, especially when chopping into wood: As I pointed out, in wood, and maybe in many other materials, the more brutal deceleration of the duller apex means your apex is more fragile, being subjected to higher lateral loads from deceleration when not perfectly straight and centered, which is all the time...

The sharper V edge will be stronger in wood just by virtue that a sharper apex means a softer deceleration into the wood... That being said, a knot could reverse that situation, but that is the basic situation.

As far as slicing soft materials being better with convex edges, that is really just so nonsensical, being a low energy situation, that it hardly deserves mention...: The drawing provided by Wowbagger should provide ample illustration...

The stropping argument is even more nonsensical: All that stropping can do is open up even more the initial apex angle... Yes it can be made to shave hair easier, but shaving hair is just about the only thing stropping will improve...

The reason the recent fallacy of convex edges has so proliferated is that so few large chopping knives have truly thin V edges... In general, what held true when I was a child still holds true today...: The bigger the knife, the duller the edge... There are exceedingly few exceptions to this even today: Truly thin-edged (0.020"-0.030") big fixed blades with V edges are almost non-existent in common production knives, about the only ones in my experience being big Randalls, which are rarely used, a few isolated Al Mars and just one of my two Liles, which are even less used...

One must remember that convex edges, defined as something desirable, not a consequence of sloppiness, are a very new idea: They were formally defined by Bill Moran in the 1970s, to the point they used to be known as "Moran edges"... His explanation of how they worked was all wrong of course, always placing the curved line inside the V edge, which is clearly an unscientific non-geometric comparison... Ancient convex edges are just sloppy sharpening, or mainly V edges with washed-out shoulders... It is important to remember that a free hand V edge will nearly always produce an "unintentional" convex edge...

Today factory knives with convex edges will usually be ground much thinner near the edge than all V-edges (Bark River Knives, and Blackjack, being good examples), and the same apples-to-oranges fallacy is perpetuated in the steel as it was in the minds of its proponents...

Gaston
 
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Gaston444 said:
I get close to flat edges free hand, but only under a strong light to check for the shadow, and by scraping parallel to the edge, starting with very rough extra-coarse Dia-Sharp hones... I go slightly diagonal a bit before I get down to Coarse, to cancel out the deeper parallel striations. I cannot get anything close to a near-flat freehand below Coarse, which when worn is fine enough to be shaving sharp. I do sometimes break off wire edges at 60-90 degrees with a "Medium" stone, sometimes at the cost of a slight micro-bevel when finishing the break.

It would be near impossible to get a highly polished edge free-hand that would also be near- "flat"... Scraping parallel to the edge was a big breakthrough in free-hand sharpness for me, because my flats became "truer" flats from lessened "rocking". A motion perpendicular to the edge induces more "rocking" in my experience...

One weird thing I noticed happened consistently was that thick edges that required very tall V bevels were never as strong under chopping into wood as thinner edges that required only small V bevels (like Randalls for instance): I think the reason for this "fragility" of "big bevels/thick edges" is that big bevels are less "stable" under the impact on wood: The thick edge has the same angle sharpness, but, being broader, it is more susceptible to "rolling" or "leaning" under impact, which lateral movement causes damage to the edge's apex. A thinner edge, even of the same bevel angle, stays straighter under impact, and so appears indestructible under hundreds of chops, in the same wood were the thicker edge will micro-chip in dozens of chops... And then I think I have found some super-steel that is tougher thinner, but it really is the bevel base thinness that helps, not harms... This is why I think the idea that thicker edge bevel bases help edge strength is only true for outright prying: Thicker edges weaken an apex of the same angle under chopping, at least in most woods... I would say the ideal bevel base thickness for chopping wood with a knife is around 0.020" to 0.030" at most.

If you look at many convex ground blades, they are often ground thinner near the edge, and the appearance of edge durability (while chopping wood) derived from that thinness/stability -while decelerating in wood- may be larger than any inherent strength gained by the convexing...

Gaston
Click to expand...

Gaston,

I find that interesting about durability along the edge when a blade is thinner above the apex, than one with a taller cutting bevel. There might be something to that. It interest me. I'll make up a test blade or two with geometries that will offer some insight and see how they function side by side. I wonder if it has something to do with the taller edge being pinched or wedged, the taller edge allowing this to happen.

When I regrind an edge on my wet belt machine, using a Bubble Jig to maintain the chosen angle, the end result is a [close to flat] edge, taking in mind that a belt machine does not, produce a truly flat [controlled angle] edge. Close but not truly flat. I switch to a VS disc grinder also using the BJ and the edge will be flat. I finish with an ERU set at a matching angle as the one used on the belt machine as well as the disc machine. The ERU's carbide inserts are perfectly aligned along all dimensions. This does what you were stating above, "sharpening parallel with the edge" This step removes any misalignment along the edge and removes the wire edge in the same process. For a refined slicing blade I'll continue by changing the angle in the ERU by a degree and just touching the apex. This is the strop function.

I'll post on this forum when I get a couple of blades forged for testing.

Regards, Fred
 
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