Thoughts on axe grinding jigs

Au_Craftsman said:
Yeah that's a fair point but I am a perfectionist and when restoring the more high quality vintage axes it is preferred to do the best grind possible so it does not diminish the value.

I have been given advice from a very knowledgeable bloke who grinds the professional race axes. Although a convex grind on a knife may be preferred a flat grind is much superior for chopping on an axe and because of this it is used in racing and timber sports. Flat grinds are much better for wood which is why chisels and planes are flat ground and why a flat grind on an a axe is called a chisel.

Not saying you are wrong a convex grind can be good on some axes since it Is more durable so fine for most, but if efficiency is the way to go then usually a flat grind with small micro bevel is what you want.
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Again, convex isn't more durable. Thicker is more durable. And for a given edge angle, flat is thicker at the edge shoulder than convex is. Racing axes are for racing and just like a stock car would be horrible for daily commuting, so too would a racing axe be poorly optimized for normal axe work. They're heavy and so thin in the edge that anything short of a perfect cut causes the edge to blow out. The flat grind is used in that context chiefly because having slightly more supporting material at the shoulder enables a slightly lower edge angle without pushing it past the point where it no longer has the requisite strength to withstand even those perfect cuts.

The reason why flat (or slightly hollow) bevels are used on chisels and planes is because of how they function. Those are paring tools, and when making straight cuts the flats assist in riding the wood in a straight line, despite their single-sided bevel that offsets the deflection forces they experience. When used bevel-side down, a chisel will want to lift in the cut or turn towards the flat face because of the offset of the deflection forces and the relief (empty space) behind the bevel face. In the case of a plane it's locked securely in position by the plane bed, and the body of the plane further prevents that tendency to turn. The flat bevels serve multiple roles, but they're specific to the context in which the tool is used, and their single-beveled nature makes them different from an axe. The flat-ground edge of a racing axe is called a chisel grind because it is flat like a chisel, not because flat grinds are inherently better for use on wood nor because the flat bevel is performing the same role it would in a chisel. :)

Edit: Fixed minor typo.
 
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FortyTwoBlades said:
Racing axes are for racing and just like a stock car would be horrible for daily commuting, so too would a racing axe be poorly optimized for normal axe work. They're heavy and so thin in the edge that anything short of a perfect cut causes the edge to blow out. T
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Yes!

The other thing to remember is that competitions use clear knot-free wood so that each competitor has equal material to cut.
 
Square_peg said:
Yes!

The other thing to remember is that competitions use clear knot-free wood so that each competitor has equal material to cut.
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Yeah mate it's also dead wood so its much harder to cut than green. It just depends what you want it for. Taking a thin ground axe (under 17*) and using it on a dead hardwood will ruin it, just as using a steeper grind will take so much longer on green wood. Different horses for different courses
 
Square_peg said:
You can use a chisel grind when you have the opportunity to resharpen your axe after every 60 seconds of use as in competition. During a day in the woods it would be counter-productive to have to sharpen your axe so often.
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Yeah I don't disagree but once again it comes down to the intended use. I'm not going to be carelessly throwing around $600 aud racing axe in the woods like I would a cheap beater.
 
I understand it's called a chisel because of the shape not as it's named after the tool haha. I think it depends which way you are convexing to if it's going to be more durable/thicker. Convexing the back is often done to increase penetration and rolling or convexing the front makes it stronger.

I understand I'm probably wrong but I always thought convex was thicker behind the edge? Everywhere I have looked says full flats are thinner than a convex https://agrussell.com/knife-articles/blade-grinds . If thicker is more durable and flat is thicker then isn't flat more durable? If so I personally dont understand why a convex be used if flats chop better and are also stronger but im sure there must be something I am missing.

FortyTwoBlades said:
Again, convex isn't more durable. Thicker is more durable. And for a given edge angle, flat is thicker at the edge shoulder than convex is. Racing axes are for racing and just like a stock car would be horrible for daily commuting, so too would a racing axe be poorly optimized for normal axe work. They're heavy and so thin in the edge that anything short of a perfect cut causes the edge to blow out. The flat grind is used in that context chiefly because having slightly more supporting material at the shoulder enables a slightly lower edge angle without pushing it past the point where it no longer has the requisite strength to withstand even those perfect cuts.

The reason why flat (or slightly hollow) bevels are used on chisels and planes is because of how they function. Those are paring tools, and when making straight cuts the flats assist in riding the wood in a straight line, despite their single-sided bevel that offsets the deflection forces they experience. When used bevel-side down, a chisel will want to lift in the cut or turn towards the flat face because of the offset of the deflection forces and the relief (empty space) behind the bevel face. In the case of a plane it's locked securely in position by the plane bed, and the body of the plane further prevents that tendency to turn. The flat bevels serve multiple roles, but they're specific to the context in which the tool is used, and their single-beveled nature makes them different from an axe. The flat-ground edge of a racing axe is called a chisel grind because it is flat like a chisel, not because flat grinds are inherently better for use on wood nor because the flat bevel is performing the same role it would in a chisel. :)

Edit: Fixed minor typo.
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Call it a chisel grind or not, the term being misapplied anyway when speaking axe talk (I know it, I know it, all the knife guys do it so why not?) for the most part since typically the geometry is symmetrical. But who's checking, we all know what it's supposed to mean, right;);)? "Like a chisel but then with an imaginary line down the middle bisecting the the bevel angle at the apex..." Personally, for what it's worth, I stick to the actual description, flat, hollow, convex... just to avoid confusion, you know.

I happen to use an axe beveled asymmetrically, yes, the old custom grind,flat one side, convex on its other and experience no notable diminution in its durability, a sharpening cycle comparable to other axes, relatively speaking because this sees more frequent use than others similar . I'm convinced that it works better for me that way than otherwise.
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Au_Craftsman said:
I understand it's called a chisel because of the shape not as it's named after the tool haha. I think it depends which way you are convexing to if it's going to be more durable/thicker. Convexing the back is often done to increase penetration and rolling or convexing the front makes it stronger.

I understand I'm probably wrong but I always thought convex was thicker behind the edge? Everywhere I have looked says full flats are thinner than a convex https://agrussell.com/knife-articles/blade-grinds . If thicker is more durable and flat is thicker then isn't flat more durable? If so I personally dont understand why a convex be used if flats chop better and are also stronger but im sure there must be something I am missing.
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Again, remember that if a convex is thicker than a flat edge, it's only because the edge angle is thicker. If you hold edge angle consistent and permit visual bevel width to fluctuate, a convex is like a flat edge that has had the shoulder knocked off it, and it will have a wider visual bevel width as a result. If you hold visual bevel width as fixed (which is silly, because that's not how it works in reality) then a convex will be thicker, but the edge angle will also be thicker, and that's the chief reason why such an edge would be stronger than a flat one. A flat edge of equal angle to that convex would be stronger still, since it would have more material supporting the edge shoulder, but it would also have a narrower visual bevel width. When talking about full height primary grinds instead of edges, that's where you run into fixed width. :)

TL;DR Version:
•Fixed edge angle = convex is thinner total geometry, but wider bevel.
•Fixed bevel width = convex is thicker, but also edge angle is thicker.
 
Yeah I need to stop calling it a chisel haha, I was so confused when I first heard that as on knives a chisel grind normally means one bevel. That's actually really interesting I have never seen someone do that, I mean I guess if it works for you then it a good option. Might have to give that a try one time.

Ernest DuBois said:
Call it a chisel grind or not, the term being misapplied anyway when speaking axe talk (I know it, I know it, all the knife guys do it so why not?) for the most part since typically the geometry is symmetrical. But who's checking, we all know what it's supposed to mean, right;);)? "Like a chisel but then with an imaginary line down the middle bisecting the the bevel angle at the apex..." Personally, for what it's worth, I stick to the actual description, flat, hollow, convex... just to avoid confusion, you know.

I happen to use an axe beveled asymmetrically, yes, the old custom grind,flat one side, convex on its other and experience no notable diminution in its durability, a sharpening cycle comparable to other axes, relatively speaking because this sees more frequent use than others similar . I'm convinced that it works better for me that way than otherwise.
p8091862.jpg
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Now I am so much more confused, I understand when you convex a bevel at a lower angle than the current grind it removes the shoulders and visualy increases the length of the bevel which is what i mentioned before saying it is sometimes used to increase penetration.

What do you mean by if you "If you hold visual bevel width as fixed (which is silly, because that's not how it works in reality)"
If you mean that how far back the bevel goes im not sure why that dosent work in reality. Grinding at a steeper angle wont effect bevel length?

"A flat edge of equal angle to that convex would be stronger still" I'm sorry but how can a convex and flat grind be the same angle when a convex is multiple angles?There is also more than 1 type of convex and it depends heavily how many degrees it is convexed. Say the flat grind angle is 17 degrees and ignore micro bevels for now. Are you talking about a convex grind first done at 19, then 18, then 17 to knock off the shoulder or 17, then 18, then 19 to put a more durable edge on?

FortyTwoBlades said:
Again, remember that if a convex is thicker than a flat edge, it's only because the edge angle is thicker. If you hold edge angle consistent and permit visual bevel width to fluctuate, a convex is like a flat edge that has had the shoulder knocked off it, and it will have a wider visual bevel width as a result. If you hold visual bevel width as fixed (which is silly, because that's not how it works in reality) then a convex will be thicker, but the edge angle will also be thicker, and that's the chief reason why such an edge would be stronger than a flat one. A flat edge of equal angle to that convex would be stronger still, since it would have more material supporting the edge shoulder, but it would also have a narrower visual bevel width. When talking about full height primary grinds instead of edges, that's where you run into fixed width. :)

TL;DR Version:
•Fixed edge angle = convex is thinner total geometry, but wider bevel.
•Fixed bevel width = convex is thicker, but also edge angle is thicker.
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Au_Craftsman said:
Now I am so much more confused, I understand when you convex a bevel at a lower angle than the current grind it removes the shoulders and visualy increases the length of the bevel which is what i mentioned before saying it is sometimes used to increase penetration.

What do you mean by if you "If you hold visual bevel width as fixed (which is silly, because that's not how it works in reality)"
If you mean that how far back the bevel goes im not sure why that dosent work in reality. Grinding at a steeper angle wont effect bevel length?

"A flat edge of equal angle to that convex would be stronger still" I'm sorry but how can a convex and flat grind be the same angle when a convex is multiple angles?There is also more than 1 type of convex and it depends heavily how many degrees it is convexed. Say the flat grind angle is 17 degrees and ignore micro bevels for now. Are you talking about a convex grind first done at 19, then 18, then 17 to knock off the shoulder or 17, then 18, then 19 to put a more durable edge on?
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Not bevel angle. Edge angle--right at the apex. The angle of an intersection of two curves is a real thing. The angle along a convex bevel will change, constantly reducing the further back from the edge it goes, but the edge itself still has a specific angle. An easy way to visually approximate it is to lay the blade on a flat surface and gradually tilt it upward until you see the edge "touch down" on the surface. Think of it like sliding a shovel along the ground. At too low of an angle of approach, just the shoulder of the shovel's blade is riding the ground and the edge is sitting up in the air above the ground. If you rest the edge parallel with the plane of the ground that's sort of like using a chisel with the beveled face up, in that you'll only scoop up dirt that sits above the elevation of the blade. At any angle higher than that, though, and that's when you're actually biting into the dirt. Does that make sense? Imagine a 90° angle, like a floor and wall. If you have two convex arcs with an angle of intersection of 90° or less, the edge formed by that intersection will be able to contact the interior edge of that 90° wall/floor, but if the angle of intersection is any greater than 90° then the cheeks will contact the wall/floor before the edge can reach it.

Visual bevel width is a symptom or artifact of imposing a certain geometry on the base steel stock. If you impose a certain flat angle on a bar of steel, the bevel will be of a certain visual width, but if you put that same angle on a thicker bar the visual width increases, and on a thinner bar it would decrease, despite all three having the same angle. So you shouldn't pursue a certain bevel width other than using it as a rough visual indicator of how steep or shallow of an angle is being imposed on the metal for its stock thickness.

Au_Craftsman said:
Do you mean only when the angle is increased? I do this on a few of my hatches so not sure why it's not how it works In practice
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Increasing the angle won't change your visual bevel width because you already have material removed behind the region being worked. When you set out to put an optimized geometry on a cutting tool you don't say "I'm going to make the bevel have a visual width of 1/4 inch." You might shoot for, for instance, 15° per side, and the bevel that results will be the width that it is purely as a result of geometry that you imposed on preexisting form. Fortunately edges are typically only in need of approximations of angle rather than very specific ones, so we have literal wiggle room for human error, but target angles give you a way of quantifying your target.

I'll need to see if I can put together some clear diagrams. It's all pretty simple geometry, really, but if you're not used to thinking about spatial relationships it can be a little confusing at first.
 
Fair enough if you are talking about edge angle but wouldnt that just be your micro bevel? If both axes have the same micro bevel then it's the angle of the grind behind that which determines how much steel is there not whether it is flat or convex or atleast that's what I thought. You said a flat grind with the same edge angle as a convex grind will be stronger, if they both have a 30* edge and the convex is ground to 16-17* and the flat is 16* how would the flat grind be stronger?

I get that the visual bevel width is how big it looks but I'm still not really sure what you mean. In your previous post you said its silly and doesnt work in reality to hold the bevel width but now your saying that it works? "Increasing the angle won't change your visual bevel width because you already have material removed behind the region being worked." That's literaly what I just said in my last post "If you mean that how far back the bevel goes im not sure why that dosent work in reality. Grinding at a steeper angle wont effect bevel length?" Obviously the thickness effects how wide it looks but I'm not sure how that comes into this besides maybe when grinding different patterns since bit sizes vary.

Yeah the angles don't HAVE to be exact but I'm still trying to get them as close as possible and have picked up a digital angle gauge for it. Even the most skilled grinders in the world would have trouble putting a 100% flat grind and even with a jig it is likely there will be some convex unless you do it very well.

Thanks very much diagrams are always good especially since things can be harder to explain online.

FortyTwoBlades said:
Not bevel angle. Edge angle--right at the apex. The angle of an intersection of two curves is a real thing. The angle along a convex bevel will change, constantly reducing the further back from the edge it goes, but the edge itself still has a specific angle. An easy way to visually approximate it is to lay the blade on a flat surface and gradually tilt it upward until you see the edge "touch down" on the surface. Think of it like sliding a shovel along the ground. At too low of an angle of approach, just the shoulder of the shovel's blade is riding the ground and the edge is sitting up in the air above the ground. If you rest the edge parallel with the plane of the ground that's sort of like using a chisel with the beveled face up, in that you'll only scoop up dirt that sits above the elevation of the blade. At any angle higher than that, though, and that's when you're actually biting into the dirt. Does that make sense? Imagine a 90° angle, like a floor and wall. If you have two convex arcs with an angle of intersection of 90° or less, the edge formed by that intersection will be able to contact the interior edge of that 90° wall/floor, but if the angle of intersection is any greater than 90° then the cheeks will contact the wall/floor before the edge can reach it.

Visual bevel width is a symptom or artifact of imposing a certain geometry on the base steel stock. If you impose a certain flat angle on a bar of steel, the bevel will be of a certain visual width, but if you put that same angle on a thicker bar the visual width increases, and on a thinner bar it would decrease, despite all three having the same angle. So you shouldn't pursue a certain bevel width other than using it as a rough visual indicator of how steep or shallow of an angle is being imposed on the metal for its stock thickness.



Increasing the angle won't change your visual bevel width because you already have material removed behind the region being worked. When you set out to put an optimized geometry on a cutting tool you don't say "I'm going to make the bevel have a visual width of 1/4 inch." You might shoot for, for instance, 15° per side, and the bevel that results will be the width that it is purely as a result of geometry that you imposed on preexisting form. Fortunately edges are typically only in need of approximations of angle rather than very specific ones, so we have literal wiggle room for human error, but target angles give you a way of quantifying your target.

I'll need to see if I can put together some clear diagrams. It's all pretty simple geometry, really, but if you're not used to thinking about spatial relationships it can be a little confusing at first.
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Au_Craftsman said:
Fair enough if you are talking about edge angle but wouldnt that just be your micro bevel? If both axes have the same micro bevel then it's the angle of the grind behind that which determines how much steel is there not whether it is flat or convex or atleast that's what I thought. You said a flat grind with the same edge angle as a convex grind will be stronger, if they both have a 30* edge and the convex is ground to 16-17* and the flat is 16* how would the flat grind be stronger?

I get that the visual bevel width is how big it looks but I'm still not really sure what you mean. In your previous post you said its silly and doesnt work in reality to hold the bevel width but now your saying that it works? "Increasing the angle won't change your visual bevel width because you already have material removed behind the region being worked." That's literaly what I just said in my last post "If you mean that how far back the bevel goes im not sure why that dosent work in reality. Grinding at a steeper angle wont effect bevel length?" Obviously the thickness effects how wide it looks but I'm not sure how that comes into this besides maybe when grinding different patterns since bit sizes vary.

Yeah the angles don't HAVE to be exact but I'm still trying to get them as close as possible and have picked up a digital angle gauge for it. Even the most skilled grinders in the world would have trouble putting a 100% flat grind and even with a jig it is likely there will be some convex unless you do it very well.

Thanks very much diagrams are always good especially since things can be harder to explain online.
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I said that if you held visual bevel width as fixed, which it shouldn't be, because that prevents you convexing an edge while still preserving the same edge angle. It has to be allowed to adjust according to the geometry being imposed on the steel. And convex geometries are only thicker than flat ones when you hold the start and end points of the line as fixed, which only happens with full-height grinds like primary grinds or full zero grinds (which are uncommon.) As far as a steepened angle not altering the already-present visual bevel width...that's because there's already one there. Technically you're making a new one, but it's visually indistinct because of the bevel that's already present. This is actually what commonly happens when an edge thickens over time from variations in the stroke used by the sharpener.

The principle I'm discussing applies in both cases where you're applying a new edge to an old tool or beveling a tool from scratch, in which case there wouldn't be any starting visual bevel width, eh? ;) I'm saying that in reality you don't go "oh I have to keep the bevel exactly this wide" -- instead you go "it needs to be made this thin through this region, with this kind of transition into the rest of the tool", and that's a matter of angles and arcs (which are effectively/conceptually like an infinite series of connected straight lines) being imposed onto...whatever you happen to be starting with that isn't already in the shape you want it to be. So visual bevel width is not being held fixed for the basis of comparing flat to convex--the edge angle is. And in that circumstance, convex geometries are thinner than flat ones because of having nearly equal material nearest the apex, but less material at the shoulder.

I'll try and see what I can whip up for diagrams but I do have my hands pretty full right now with things I have to do, and it may be tricky for me to illustrate everything I'd like to without going through many iterations of drawings. It's all stuff I could sketch out on a napkin while explaining in person in about 15 minutes, but getting it across through typed words and digital diagrams is a bit more arduous (even if the diagrams are more accurate than rough hand sketches.)
 
FortyTwoBlades said:
I said that if you held visual bevel width as fixed, which it shouldn't be, because that prevents you convexing an edge while still preserving the same edge angle. It has to be allowed to adjust according to the geometry being imposed on the steel. And convex geometries are only thicker than flat ones when you hold the start and end points of the line as fixed, which only happens with full-height grinds like primary grinds or full zero grinds (which are uncommon.) As far as a steepened angle not altering the already-present visual bevel width...that's because there's already one there. Technically you're making a new one, but it's visually indistinct because of the bevel that's already present. This is actually what commonly happens when an edge thickens over time from variations in the stroke used by the sharpener.

The principle I'm discussing applies in both cases where you're applying a new edge to an old tool or beveling a tool from scratch, in which case there wouldn't be any starting visual bevel width, eh? ;) I'm saying that in reality you don't go "oh I have to keep the bevel exactly this wide" -- instead you go "it needs to be made this thin through this region, with this kind of transition into the rest of the tool", and that's a matter of angles and arcs (which are effectively/conceptually like an infinite series of connected straight lines) being imposed onto...whatever you happen to be starting with that isn't already in the shape you want it to be. So visual bevel width is not being held fixed for the basis of comparing flat to convex--the edge angle is. And in that circumstance, convex geometries are thinner than flat ones because of having nearly equal material nearest the apex, but less material at the shoulder.

I'll try and see what I can whip up for diagrams but I do have my hands pretty full right now with things I have to do, and it may be tricky for me to illustrate everything I'd like to without going through many iterations of drawings. It's all stuff I could sketch out on a napkin while explaining in person in about 15 minutes, but getting it across through typed words and digital diagrams is a bit more arduous (even if the diagrams are more accurate than rough hand sketches.)
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"So visual bevel width is not being held fixed for the basis of comparing flat to convex--the edge angle is. And in that circumstance, convex geometries are thinner than flat ones because of having nearly equal material nearest the apex, but less material at the shoulder." Yeah I definitely agree that if you are convexing down in degrees then it will be thinner behind which is why these grinds are often done on axes for green wood so it can penetrate more.

That being said it depends on the type of convex as I mentioned before, if your adding steel behind the edge (Grinding at a steeper angle) then it's increase strength and decreasing penetration but as you said if remove steel behind the edge (grind a shallower angle) then it will be thinner thus making a edge which penetrates better but is more prone to damage.
 
Au_Craftsman said:
"So visual bevel width is not being held fixed for the basis of comparing flat to convex--the edge angle is. And in that circumstance, convex geometries are thinner than flat ones because of having nearly equal material nearest the apex, but less material at the shoulder." Yeah I definitely agree that if you are convexing down in degrees then it will be thinner behind which is why these grinds are often done on axes for green wood so it can penetrate more.

That being said it depends on the type of convex as I mentioned before, if your adding steel behind the edge (Grinding at a steeper angle) then it's increase strength and decreasing penetration but as you said if remove steel behind the edge (grind a shallower angle) then it will be thinner thus making a edge which penetrates better but is more prone to damage.
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Exactly, and because you can't really "add steel" unless you're welding it on, you're constrained to either establishing a convex with an edge equal to or lower than the angle that's already on it, or grind back further into the bit, shortening it slightly. And in those cases where you've done the latter, a flat geometry of equal edge angle would be taking off less metal because it would be leaving more at the shoulder. So what's really making it stronger is the fact that it's thicker, not the fact that it's convex. Rather, it's thicker in spite of the convex on account of having increased the edge angle (and also having worked back into thicker material, in most cases.) Technically the bevel width would become narrower in this case because you're taking material off the apex, but it'll usually be by a small enough degree that you won't be able to notice it by eye. :)
 
DB_Cruiser said:
edgeangles.jpg

This is how my mind is seeing what is being discussed. The first represents a convex (green) vs flat (black) grind over a particular bevel width. The second is convex vs flat at a particular edge angle. I hope I am interpreting this correctly.
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Yup! Looks like you've got it! And this is why a lot of these discussions can go back and forth forever, because for every flat geometry, there's an infinite number of equal-angle convex variations that can fit inside it, and then for every convex there's a lower-angle flat geometry that can fit between the start and endpoints of the convex and an equal-angle flat that fits outside of it. So it all depends on which tradeoff makes the most sense for your particular steel/heat treatment for edge stability and your context of use: more metal at the shoulder to support a lower angle, or an equal angle with material shaved off the shoulder? In general a convex will be the stronger of those two options on account of being thicker at the apex itself than a lower-angle flat with more shoulder support, but thanks to the thinned shoulder it isn't sacrificing much cutting performance in doing so.

The closer you get to the edge itself, the greater the magnitude of influence the geometry has on the cutting performance, so material at the edge shoulder still has a lot of influence on how the blade cuts, and reducing drag/wedging there helps squeeze some extra performance out of the blade. This is similar to how you can increase cutting performance by re-grinding the primary grind of a knife or tool to reduce the bevel width and the thickness behind the edge, but working from the edge side instead of the spine side. :)
 
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