- Joined
- Sep 19, 2001
- Messages
- 8,968
before I put forth my lunacy, here's a headache inducer from a few months ago 
http://www.bladeforums.com/forums/showthread.php?t=407022
I just read through it again while contemplating the subject of this new thread
So anyway, I was thinking,what's the big deal with edge angles. I got on this train of thought (which quickly derailed) from reading the zero edge thread. If anyone wants to doodle it out (easy with a chisel grind), you can see that if you start with a zero edge, to add an edge bevel you need to 1) shorten the height of the blade, which happens if you simply grind a new edge bevel onto a zero edge at the cutting edge 2) reduce the stock thickness, leaving a portion of the zero edge angle beginning from the cutting edge, and removing metal from the spine downward, creating a primary grind.
Now, if you do 1), then you make your edge more obtuse, and still present the same shape wedge immediately behind the edge. In 2), your edge angle is the same, but you present less of a cross section behind this edge as you press the blade further into the media being cut. What follows doesn't really jibe with this completely, but that's why people say I'm weird.
This was discussed a bit in the older thread, mentioning cutting with wires and line, presenting no material to bind behind the actual edge. Jeff Clark also mentioned other forces at work while cutting. Seems that if you could take your knife and grind it down to the thickness of the atoms in their metallic bonds, you'd have a heckuva cutter with an actual zero degree angle-but it wouldn't be a durable chopper...
But, what if it were thicker? Cabinet scrapers are zero or 90 (180 included?) I guess it's how you want to look at them. I'd say 90, since that means that you can rotate 90 degrees from the centerline at the base before you quit seeing steel away from the single dimension centerline. If it were really an atom thick, you couldn't rotate an atom wide line (which is as reasonably thin as you can make it) at all while staying within the steel-so it's a zero angle. You can't push cut with the scraper, (well, maybe if you took it to that atom thickness level) but even without burnishing, that 90 degree 'edge' will remove wood. It isn't the angle, but the fact that it has been jointed, and the vertex isn't rounded.
But what is too round/wide for more than superficial scraping? I don't know, but here's a guess. Some steels have relatively large carbides, or big lumps of carbides. Carbides can be very hard, which could be good for lots of cutting. But they may be larger than a 'fine' edge-10 microns or so. To hold them in place, you need surrounding metal. Depending on how big the carbides are, I guess that could determine the edge angle needed to leave enough metal around the carbides to keep them from pulling out too easily while cutting. This leaves you with an edge that is sorta round (the sides can't intersect at a single point, it's as wide as the carbides wherever they are on the edge) and an angle great enough to hold the carbides in place. I visualize it as jello with fruit cocktail mixed in. How much gelatin do you need around the grapes to keep them from falling out? If the grapes fall out, the jello is ragged, meaning it remains suitable for slicing. Wait, that analogy fell apart.
So, we need maybe some smaller carbides, some 'fine-grained' steel. Well, we could grind these edges more acute, which means we move more toward that zero angle of the spectrum. So, we get this stuff and get our edge angle single digits or something. But, that's not a lot of steel in the cross section, so it can fracture or warp depending on how hard it is. Instead of an edge too obtuse to cut really, really, really well from the get-go, we have one that's extremely fine to start, but falls apart if you look at it funny.
So, the long winding road I just took, probably full of bad directions and wrong turns, comes down to me wondering this-Cutting edge cross sections get as close to a single point in space as we can manage, and the amount of metal behind that is determined by the edge angle. Where's the chart for strength, toughness, etc. at these dimensions? I'm nowhere near a metallurgist (as if you couldn't tell
) but it seems even stuff like charpy numbers and such are still a bit abstract for knife edges.
And wherever I was horribly wrong, please correct me.
http://www.bladeforums.com/forums/showthread.php?t=407022
I just read through it again while contemplating the subject of this new thread
So anyway, I was thinking,what's the big deal with edge angles. I got on this train of thought (which quickly derailed) from reading the zero edge thread. If anyone wants to doodle it out (easy with a chisel grind), you can see that if you start with a zero edge, to add an edge bevel you need to 1) shorten the height of the blade, which happens if you simply grind a new edge bevel onto a zero edge at the cutting edge 2) reduce the stock thickness, leaving a portion of the zero edge angle beginning from the cutting edge, and removing metal from the spine downward, creating a primary grind.
Now, if you do 1), then you make your edge more obtuse, and still present the same shape wedge immediately behind the edge. In 2), your edge angle is the same, but you present less of a cross section behind this edge as you press the blade further into the media being cut. What follows doesn't really jibe with this completely, but that's why people say I'm weird.
This was discussed a bit in the older thread, mentioning cutting with wires and line, presenting no material to bind behind the actual edge. Jeff Clark also mentioned other forces at work while cutting. Seems that if you could take your knife and grind it down to the thickness of the atoms in their metallic bonds, you'd have a heckuva cutter with an actual zero degree angle-but it wouldn't be a durable chopper...
But, what if it were thicker? Cabinet scrapers are zero or 90 (180 included?) I guess it's how you want to look at them. I'd say 90, since that means that you can rotate 90 degrees from the centerline at the base before you quit seeing steel away from the single dimension centerline. If it were really an atom thick, you couldn't rotate an atom wide line (which is as reasonably thin as you can make it) at all while staying within the steel-so it's a zero angle. You can't push cut with the scraper, (well, maybe if you took it to that atom thickness level) but even without burnishing, that 90 degree 'edge' will remove wood. It isn't the angle, but the fact that it has been jointed, and the vertex isn't rounded.
But what is too round/wide for more than superficial scraping? I don't know, but here's a guess. Some steels have relatively large carbides, or big lumps of carbides. Carbides can be very hard, which could be good for lots of cutting. But they may be larger than a 'fine' edge-10 microns or so. To hold them in place, you need surrounding metal. Depending on how big the carbides are, I guess that could determine the edge angle needed to leave enough metal around the carbides to keep them from pulling out too easily while cutting. This leaves you with an edge that is sorta round (the sides can't intersect at a single point, it's as wide as the carbides wherever they are on the edge) and an angle great enough to hold the carbides in place. I visualize it as jello with fruit cocktail mixed in. How much gelatin do you need around the grapes to keep them from falling out? If the grapes fall out, the jello is ragged, meaning it remains suitable for slicing. Wait, that analogy fell apart.
So, we need maybe some smaller carbides, some 'fine-grained' steel. Well, we could grind these edges more acute, which means we move more toward that zero angle of the spectrum. So, we get this stuff and get our edge angle single digits or something. But, that's not a lot of steel in the cross section, so it can fracture or warp depending on how hard it is. Instead of an edge too obtuse to cut really, really, really well from the get-go, we have one that's extremely fine to start, but falls apart if you look at it funny.
So, the long winding road I just took, probably full of bad directions and wrong turns, comes down to me wondering this-Cutting edge cross sections get as close to a single point in space as we can manage, and the amount of metal behind that is determined by the edge angle. Where's the chart for strength, toughness, etc. at these dimensions? I'm nowhere near a metallurgist (as if you couldn't tell
) but it seems even stuff like charpy numbers and such are still a bit abstract for knife edges.And wherever I was horribly wrong, please correct me.