Please explain "behind the edge" geometry to me

[Tucson Tom]

I've done some searches and by and large have gotten more confused than educated.

Here is what I think I understand. You can tell me where I have gone off the rails and please do so and/or add to this.
I think that it would benefit me as well as others.

Let's take a full flat grind blade, like most that come from Spyderco (take the Manix to be overly specific). The knife blank is ground at a certain angle. It doesn't matter much for the current discussion, but let's say 10 degrees inclusive for a nice round number to talk about.

Then the factory sharpens (grinds) a steeper taper to yield the edge, let's say 30 degrees inclusive. At the time that taper is ground, a decision can be made about how far back on the knife that taper is taken. That decision will determine the "behind the edge" (aka BTE) dimension. If I take a micrometer or a set of calipers I can measure the width of the blade at the place where the original blade grind meets the taper that produces the edge and that is BTE. That measurement will be challenging to make, but it is what people talk about.

What I think I hear people saying is that knives cut better when this dimension is smaller. And I think these people are correct and know what they are talking about.

Now, here is where I get confused. As an end user, there is little if anything I can do to reduce BTE. Any sharpening I do at the original (let's say 30 degree) angle will just move the "junction" of the two tapers back further on the knife and increase BTE. If for some reason I decide to reprofile the knife to say 25 degrees inclusive, that will move the junction even further back on the blade where the original grind is even thicker.

I particular, I don't see how I can do anything in my sharpening to reduce BTE without having the entire blade reground -- unless I sharpen at a steeper angle and start talking about BTE as the junction between my new steeper bevel and the original bevel the knife was sharpened with. So I sit here scratching my head.

What am I missing or misunderstanding? I have skipped talking about what some people call a "microbevel" because I don't think it is part of the BTE business at all, but maybe I am wrong about that.

[JSumm]

It sounds like you have a pretty good grasp on it to me. Better than I could explain.

You are correct, the more you sharpen a Manix let's say, the larger the BTE number will get as you get closer to the spine. I think this number is only important with a model from the factory to judge the original grind. Without a full regrind like you mentioned your only other option to increase performance is reducing the edge bevel at which you are sharpening.

All this is fun to discuss, but I think in many things it doesn't make too much of a real world difference. I think there are very specific cases where it will stand out. In the kitchen for example, but most kitchen cutlery is so thin to begin with. Spyderco's geometry probably makes more of a real world noticeable difference. I have cut boxes with a Chaparral and a Shaman now. The Chaparral definitely goes through quicker, but the Shaman is no slouch with its overly thick blade. I have not felt let down by either.

[gunmike1]

Having a knifemaker regrind the the blade thinner is the most convenient, and expensive way to thin out the blade. You can also use an XX Coarse stone hammer at the blade by hand and thin the whole blade out, then run through a couple different grits to pretty up the blade if you care about that. It takes a lot of time and can get ugly, so most won’t do that. The most convenient thing I do to thin out my knives for a little better cutting is change the edge bevel to about 10 degrees per side, then I sharpen the very edge at around 15 degrees per side microbevel. Some blades I go even more acute depending on the steel and how I want to use the knife, but I’ve yet to find durability problems with 10/15 degree edges. I prefer to have my whole blades reground far thinner than from the factory, in the .005”-.010” range, and have many knives that thin. They cut great, but you do have to keep in mind that you need to cut straight, avoid hitting packing staples or heavy wire, etc or you might get a chip, maybe a big one. Same with avoiding big impacts. If you have heavier uses then factory thick edges will be what you want, but again running 10/15 degree edges on factory think blades like a Manix 2 has never given me a hint of durability problems for my EDC uses.

[gunmike1]

Double tap

[vandelay]

All this is fun to discuss, but I think in many things it doesn't make too much of a real world difference.

It depends how thick behind the edge it is. I had an old knife where I'd ground off the partial serrations years ago and it cut pretty badly. I went back and thinned the blade out to fix it and the difference was night and day.

[JSumm]

All this is fun to discuss, but I think in many things it doesn't make too much of a real world difference. I think there are very specific cases where it will stand out. In the kitchen for example, but most kitchen cutlery is so thin to begin with. Spyderco's geometry probably makes more of a real world noticeable difference. I have cut boxes with a Chaparral and a Shaman now. The Chaparral definitely goes through quicker, but the Shaman is no slouch with its overly thick blade. I have not felt let down by either.

It depends how thick behind the edge it is. I had an old knife where I'd ground off the partial serrations years ago and it cut pretty badly. I went back and thinned the blade out to fix it and the difference was night and day.

I included the full quote there. I think it is important in this case. I did not intend to say it doesn't make a difference at all or not noticeable at all. Most every day task I don't think BTE will prevent you from doing real work. Does a Chaparral go through a box easier than a Shaman yes. So does a box cutter. Spyderco really does a great job with the geometry of their knives. I think that is why the Shaman as thick as it is still cuts great. No, it will not cut as good as a Chaparral or a box cutter. In the kitchen, you start to see a real difference, but Kitchen Cutlery starts with very thin stock. It is not to say that none of this matters. Of course it does. And it is fun to discuss. I just think the average knife user out there will not be able to tell a huge difference, and I think that is always good to keep in mind when the topic comes up.

Does that make since? I am probably not describing my thoughts well. I just think it is important to reiterate for the average user that I don't think it is something for most to worry about. Well, as long as they are buying a Spyderco knife to test out of course.

[Evil D]

Just keep in mind it's only one slice of a bigger pie chart that makes up cutting performance. It's a fun detail to be aware of but it only tells that part of the story.

You could have a blade grind that is super thin and your behind the edge (behind the bevel) thickness could be extremely thin, like half as thick as any production Spyderco but then if the edge apex angle is 90 degrees inclusive it probably won't slice as well as the 0.020 knife with a 15 degrees inclusive edge apex. The best performance is at the optimal combination of blade grind and apex thickness.

[Josh Crutchley]

Having a knifemaker regrind the the blade thinner is the most convenient, and expensive way to thin out the blade. You can also use an XX Coarse stone hammer at the blade by hand and thin the whole blade out, then run through a couple different grits to pretty up the blade if you care about that. It takes a lot of time and can get ugly, so most won’t do that. The most convenient thing I do to thin out my knives for a little better cutting is change the edge bevel to about 10 degrees per side, then I sharpen the very edge at around 15 degrees per side microbevel. Some blades I go even more acute depending on the steel and how I want to use the knife, but I’ve yet to find durability problems with 10/15 degree edges. I prefer to have my whole blades reground far thinner than from the factory, in the .005”-.010” range, and have many knives that thin. They cut great, but you do have to keep in mind that you need to cut straight, avoid hitting packing staples or heavy wire, etc or you might get a chip, maybe a big one. Same with avoiding big impacts. If you have heavier uses then factory thick edges will be what you want, but again running 10/15 degree edges on factory think blades like a Manix 2 has never given me a hint of durability problems for my EDC uses.

I'm working on thinning out my Chaparral this way. It probably won't look good but I don't care about that. I have it down to .014" bte so far and plan on going to ~.010". When I started it was ~.025"bte.

[vandelay]

All this is fun to discuss, but I think in many things it doesn't make too much of a real world difference. I think there are very specific cases where it will stand out. In the kitchen for example, but most kitchen cutlery is so thin to begin with. Spyderco's geometry probably makes more of a real world noticeable difference. I have cut boxes with a Chaparral and a Shaman now. The Chaparral definitely goes through quicker, but the Shaman is no slouch with its overly thick blade. I have not felt let down by either.

It depends how thick behind the edge it is. I had an old knife where I'd ground off the partial serrations years ago and it cut pretty badly. I went back and thinned the blade out to fix it and the difference was night and day.

I included the full quote there. I think it is important in this case. I did not intend to say it doesn't make a difference at all or not noticeable at all. Most every day task I don't think BTE will prevent you from doing real work. Does a Chaparral go through a box easier than a Shaman yes. So does a box cutter. Spyderco really does a great job with the geometry of their knives. I think that is why the Shaman as thick as it is still cuts great. No, it will not cut as good as a Chaparral or a box cutter. In the kitchen, you start to see a real difference, but Kitchen Cutlery starts with very thin stock. It is not to say that none of this matters. Of course it does. And it is fun to discuss. I just think the average knife user out there will not be able to tell a huge difference, and I think that is always good to keep in mind when the topic comes up.

Does that make since? I am probably not describing my thoughts well. I just think it is important to reiterate for the average user that I don't think it is something for most to worry about. Well, as long as they are buying a Spyderco knife to test out of course.

I don't think you're going to notice that much of a difference from anything in spyderco's lineup unless you're doing fine work, but if you're dealing with something particularly unusual (knife with major repairs, bizarre design, etc.) that is really thick behind the edge, you can end up with cardboard deforming rather than cutting.

[Tucson Tom]

Thank you!

Somehow I had thought that I had this all wrong, but I really didn't. So I'll restate what I think I just learned from you guys so far, especially the explanation by gunmike1.

If I have a knife from the factory with a BTE that I don't like and want to reduce, I have two options.

1) I can have the whole blade reground. I can have that done either at the original angle or a thinner angle. If I had it done at the original angle, then returned to me with no edge grind on it, then I could do that myself with all due care not to carry it back too far on the edge, because the farther back I go, the bigger the BTE value.

2) I can play the game gunmike1 describes of grinding a fairly broad bevel at 10 per side, then my tiny bevel for the edge at 15 per side. Then I just talk about the BTE value of that tiny 15 degree final bevel and ignore the much bigger BTE for where that broad 10 degree bevel meets the original grind of the kinfe.

I am not so much interested (at this point anyway) of doing any of this, but in understanding all of this. As some of you have already said this is just one of many factors in sharpening, and maybe not the most important for me at this point.

[JSumm]

I think Mike was just saying the performance is better at 10 DPS with a 15 DPS microbevel. Without the regrind 10 DPS with the microbevel still would not change the BTE thickness. I think you would still measure behind the main edge bevel (10DPS) to keep things standardized if you will. Only the regrind would change the BTE thickness.

But his edge bevel change improves the cutting performance from the factory edge bevel even without the regrind. Clarifying cutting performance here because some may want a thicker edge for durability in what they are using the knife for.

[Bolster]

So I'll restate what I think I just learned from you guys so far...

That's how I understand it too. If you're wrong then I'm wrong also.

[gunmike1]

To clarify the BTE measurement can be very deceiving if you don’t specify what angle your edge bevel is. A 20 degree bevel will be much thinner BTE than a 10 degree bevel, but at any point along the bevel the 10 degree edge is much thinner than the 20 degree bevel. An extreme example of this are my old Jess Horn ZDP models that I sharpened to a zero grind. So at its thickest point my edge bevel is .097”, which seems thick. But the bevels are 7 degrees per side, so that knife actually is a great slicer, especially out at the tip where it is thinner and more acute. I generally run a 10-15 degree microbevel on that knife with very good results.

[gunmike1]

Having a knifemaker regrind the the blade thinner is the most convenient, and expensive way to thin out the blade. You can also use an XX Coarse stone hammer at the blade by hand and thin the whole blade out, then run through a couple different grits to pretty up the blade if you care about that. It takes a lot of time and can get ugly, so most won’t do that. The most convenient thing I do to thin out my knives for a little better cutting is change the edge bevel to about 10 degrees per side, then I sharpen the very edge at around 15 degrees per side microbevel. Some blades I go even more acute depending on the steel and how I want to use the knife, but I’ve yet to find durability problems with 10/15 degree edges. I prefer to have my whole blades reground far thinner than from the factory, in the .005”-.010” range, and have many knives that thin. They cut great, but you do have to keep in mind that you need to cut straight, avoid hitting packing staples or heavy wire, etc or you might get a chip, maybe a big one. Same with avoiding big impacts. If you have heavier uses then factory thick edges will be what you want, but again running 10/15 degree edges on factory think blades like a Manix 2 has never given me a hint of durability problems for my EDC uses.

I'm working on thinning out my Chaparral this way. It probably won't look good but I don't care about that. I have it down to .014" bte so far and plan on going to ~.010". When I started it was ~.025"bte.

You will love it when you get it all the way thinned out. If you spend a lot of time on finer grits you can usually make the blade look pretty decent. I generally just do enough to make it not look too terrible and use them since you are just going to scratch them up anyway if you use them. The Chaparral coming with a .025” thick edge is really disappointing. A thin slicer like that begs for a far thinner edge, but I understand Spyderco not wanting a bunch of warranty returns from people that don’t know the limitations on strength for very thin edges. To me the my thin knives cut so much better that it is worth the hassle of not twisting out of heavy cuts, avoiding staples, etc. it’s just second nature now. I also usually carry a very thin blade along with a little bigger folder for harder cutting tasks so I’m covered.

[Tucson Tom]

I sit here wondering just what the DPS is for the original grind on a typical knife like the Manix 2 I have sitting right here. I was telling myself I needed some fancy gear to measure it, but I can get pretty close with just my dial calipers and some trigonometry.

I take a ruler and find where the blade is 1 inch broad (spine to edge). I make a mark there on the spine and measure the spine with my calipers. I get 0.10 inch (what a coincidence?!). So divide that in half (0.05 inch) to get the half angle and use your handy calculator to do arctan(0.05). I get 2.86 degrees per side.

So unless I confused myself along the way somewhere, the grind on my Manix is about 3 dps. That is a number I had never thought about much before. So, what do you guys ask for when you get a regrind done?

[gunmike1]

I generally use trig to guess at the factory angles. I believe the sharpening robot on the Golden Co models are standardized at 17 degrees per side if my memory serves right. Pre sharpening robots most Spydercos I got were between 12-15 degrees per side, generally with close to 15 degrees being the most common angle I’d see.

I think starting with regrinds that are about .010” at the top of a 15 degree per side bevel is good to get a feel for the increased cutting ability. The edge will still be plenty durable but it would definitely take damage if you twist out of a deep cut in wood if something like that. For me this is a good general purpose EDC geometry, but I don’t baton or have to do any sort of truly hard use, so your mileage may vary.

Once I got used to .010” I went to trying high hollow grinds with .004”-.005” edges on more like 8-10 degree angles, so extremely thin. Those cut amazingly but you quickly learn you need to cut straight and be cognizant of any possible staples or wires that could be hiding in the boxes you cut.

I generally carry a very thin knife with an 5-8 degree bevel and 10-15 degree microbevel and something like a .008”-.010” regrind, or a factory ground knife with an 8-10 degree bevel with a 15 degree microbevel. Those cover my needs well, and the factory thick blades are very durable and cut better than when they had 17-20 degree factory bevels.

[Bemo]

Thanks for the discussion Tom. My belief is that anytime people think the the answer is "obvious" it's time to go back to basics and make sure that everyone reaches a common understanding. If you've ever seen an old knife sharpened to near death, you would understand the necessity of thinning a blade as you go. Remember at the turn of the 20th century, folks didn't buy very many pocket knives. Thinning before re-sharpening was very common.

[Sharp24/7]

Thanks for the discussion Tom. My belief is that anytime people think the the answer is "obvious" it's time to go back to basics and make sure that everyone reaches a common understanding. If you've ever seen an old knife sharpened to near death, you would understand the necessity of thinning a blade as you go. Remember at the turn of the 20th century, folks didn't buy very many pocket knives. Thinning before re-sharpening was very common.

I’d never heard of thinning until I got an EP Apex as a Christmas gift—it’s mentioned in the instructions. This is definitely something I’ll have to look into.

[Bemo]

Murray Carter briefly addressed this in one of his sharpening videos way back when.

[Bolster]

All other variables being equal, a 20 dps edge would have a smaller BTB (behind the bevel) than a 10 dps edge. Because the 10 dps bevel 'terminates' further up the blade where the blade is thicker. Yet it's logical that the 10 dps edge would be slicier, despite being "thicker behind the bevel." As Gunmike said, BTE/BTB doesn't tell the whole story. The more I think about it, BTB by itself isn't a terribly informative variable. It's somewhat informative but needs to be given in conjunction with dps stats.

To clarify the BTE measurement can be very deceiving if you don’t specify what angle your edge bevel is. A 20 degree bevel will be much thinner BTE than a 10 degree bevel, but at any point along the bevel the 10 degree edge is much thinner than the 20 degree bevel....