"Thin-Behind-the-Edge" Geometry

[AJF]

I know that when I was even newer than I am now, Paul (Deacon) patiently tried to explain to me the concept of "thin behind the edge", as in "the Caly Jr. is a great slicer because it's so thin behind the edge".

Darned if I can find that old thread, so I am hoping some kind soul will please provide a basic explanation of the geometry of this term, and how it differs from the bevel angle, the grind, the blade width, etc. And if anyone happens to have a drawing or diagram of some sort, that would probably make the whole thing easier to explain, and (for the geometry challenged like me) to understand.

Thanks,

Andrew

[jackknifeh]

I know that when I was even newer than I am now, Paul (Deacon) patiently tried to explain to me the concept of "thin behind the edge", as in "the Caly Jr. is a great slicer because it's so thin behind the edge".

Darned if I can find that old thread, so I am hoping some kind soul will please provide a basic explanation of the geometry of this term, and how it differs from the bevel angle, the grind, the blade width, etc. And if anyone happens to have a drawing or diagram of some sort, that would probably make the whole thing easier to explain, and (for the geometry challenged like me) to understand.

Thanks,

Andrew

It involves the grind and blade width. Look in "edge-u-cation" for diagrams of grinds. When the blade grind is Full Flat Grind (FFG) the shape is straight from the edge bevel to the spine. Thin behind the edge means the blade is thin so it won't spread the item being sliced any more than necessary making it easier for the blade to travel through the material. A hollow grind blade increases resistance as soon as the thicker part of the blade tries to go through the material because it tries to spread it farther apart. The difficulty would be very apparent between slicing a tomato or a piece of soft wood which is much harder of course. Did that make any sense? That is what I think "thin behind the edge" means. If I'm wrong, someone please correct me so I'll know.

Jack

[Ankerson]

It helps with slicing a lot as in the blade with slip through the material being cut easier.

However once you are cutting harder materials it's good to have it thicker to support the edge.

Just depends on the use for the knife.

[gunmike1]

Krein grinds are very popular (especially with me) due to being thin behind the edge. I measure the thickness at the top of the bevel where it meets the primary blade grind. Most Spydercos are nice and thin at .020" or less (very good for a factory grind as it slices good but is also durable) behind the edge, however my Krein grinds are mostly .010" or less for great slicing. On knives likes my Millies and Manix and Manix 2 .020" is quite appropriate for some hard use, but for most of my cutting a .010" edge is plenty strong and an excellent slicer. BTW, my Caly 3 ZDP was right around .010"-.011" and 9-10 degrees per side from the factory, which is just astoundingly good for a factory knife. That thin edge is why the Calys are such famous slicers.

Mike

[ChapmanPreferred]

Nicely explained folks!

[npueppke]

"Geometry behind the edge" is a function of grind, blade thickness, and blade width. It basically comes down to how shallow of an angle you have right where the grind starts. This dictates how easily the knife will cut through something as soon as the material comes up past the grind. Very important for shallow cuts, but for deeper cuts, you also have to take into consideration the geometry of the entire blade.

If you ignore the actual bevel on the knife (a 'scandi grind' knife), and you consider a blade that is 3mm thick, 20mm wide, and flat ground, you end up with a certain angle right at the edge of the blade. Now, you you keep the blade stock at 3mm with a flat grind, but you have a 10mm wide blade, you end up with an angle that is more obtuse because you have to transition from the 3mm down to the edge in a much shorter time.

Now if you keep the width and thickness of the blade constant and change from a flat to a hollow grind, you are changing the angle instead of having a constant angle, so you end up with a much shallower angle right behind the edge of the blade but a steeper angle at the spine. This means that you can have a much thinner "behind the edge" angle which would cut more easily if you are making a shallow cut, but if you cut all of the way through a material you end up with a 3mm "kerf", just like you would if your blade were flat ground (and actually, this changing angle tends to get stuck in deeper cuts).

Hope this helps!

[Bluefish]

Mike is right, the caly and a couple sg2's by kershaw are thie thinnest behind edge's I have ever seen. I have four krein grinds and one of his customs and the cally, jyd2 and blur sg2 are right there. Russ

[dj moonbat]

It is frankly amazing to me that the steel makers have managed to come up with steels that will a) survive the kind of thin grinds that some people are discussing above; and b) not rust easily; and c) not degrade by chipping or rolling more-or-less instantly.

[AJF]

Thanks, gents. That helps quite a bit.

So, if 2 knives have the same flat grind and the same blade thickness, the one with the greater blade width will win the "thinner-behind-the-edge" contest?

Or, if both are flat ground, and have the same blade width, the one with the lesser blade thickness wins?

Andrew

[ChapmanPreferred]

AJF,

It really depends on what the thickness of the blade steel is prior to sharpening. You can see the difference in the Edge-u-cationsection of Spyderco's website (non-forum side). Scroll down to the "Grinds" section. Compare the "Full Flat Grind" image and the "Zero Grind" image. Give particular emphasis to your viewing right where the edge begins (the left side of each image). The edge on the "Zero Grind" is much thinner behind the edge than the "Full Flat Grind".

[gunmike1]

Correct, Andrew, but of course with Chapman Preffered's post clarifying things. But, all else being equal ( for instance, flat grind with a 10 degree per side angle), a wider blade with the same blade spine thickness, or a thinner spined blade of the same width as a thicker spined blade, will be thinner behind the edge.

By the way, my Caly Jr. ZDP has a Krein grind to .005" behind the bevel with a very high hollow grind. It is my best slicer by far, and it is always riding deep in my front left pocket. It does most of my cutting, as there really isn't much I run into at the office that requires more strength. That knife just glides through cardboard amazingly well (and most anything else for that matter). That knife has really proven to me that as long as you don't side load the edge and avoid things like packing staples that a really thin blade can make a really good EDC blade for urban chores. I always have at least 2 other knives on me in case a stronger edge is needed, and sometimes I enjoy testing out my S90V Manix 2 or the great M4 in my Bradley (or whatever other knife i may be carrying at the time) just for fun, but ZDP 189 in an ultra thin blade really can cut great and it stays sharp a really long time. Even when the Caly Jr has lost a lot of it's sharpness it still takes less force to cut many things compared to a sharper HD knife with a .020" edge.

Mike

[jackknifeh]

Thanks, gents. That helps quite a bit.

So, if 2 knives have the same flat grind and the same blade thickness, the one with the greater blade width will win the "thinner-behind-the-edge" contest?

Or, if both are flat ground, and have the same blade width, the one with the lesser blade thickness wins?

Andrew

AJF,

It really depends on what the thickness of the blade steel is prior to sharpening. You can see the difference in the Edge-u-cationsection of Spyderco's website (non-forum side). Scroll down to the "Grinds" section. Compare the "Full Flat Grind" image and the "Zero Grind" image. Give particular emphasis to your viewing right where the edge begins (the left side of each image). The edge on the "Zero Grind" is much thinner behind the edge than the "Full Flat Grind".

To keep a blade "zero grind" when sharpening it the stone would need to touch the entire blade, edge to spine. If you increased the angle of the bevel on a zero grind blade it would become a "full flat grind" blade. That's correct isn't it?

Jack

[chuck_roxas45]

What about a chisel grind? Does that make for a thinner behind the edge geometry, assuming that the inclusive bevel angle is the same as a conventional V grind?

[ChapmanPreferred]

Yes Jack, that is on the money!

[dj moonbat]

What about a chisel grind? Does that make for a thinner behind the edge geometry, assuming that the inclusive bevel angle is the same as a conventional V grind?

Well, if you're using a 40 degree chisel grind blade, that just doesn't make any sense.

[AJF]

Thank you, guys. This forum is the best school around.

CP, thanks for the link---I need to spend more time in the Edge-u-cation section---that is good stuff.

And Mike, quit making me want a Caly Jr. any more than I already do! :D

Andrew

[jackknifeh]

What about a chisel grind? Does that make for a thinner behind the edge geometry, assuming that the inclusive bevel angle is the same as a conventional V grind?

**** Chuck, you really want to complicate things don't you? :confused: :) I think you are correct that the inclusive angle would be the same. But you wouldn't want that kind of edge on a slicing knife I don't think. Every time I've used a chisel sharpened knife (once or maybe twice) the knive wants to turn based on the side the bevel was on. Just like a rudder on a boat. The bevel on both sides keeps the blade going straight without "forcing" it to go straight. In my experience the only place for a chisel grind or edge is in a chisel. For that tool (which my experience is not limited) the chisel grind is essential and appropriate. I would be interested in different opinions of the chisel grind. I've always wondered but this forum is the only place the subject has come up (blade edges I mean) for me.

Jack

[ChapmanPreferred]

You got it Andrew.

[chuck_roxas45]

**** Chuck, you really want to complicate things don't you? :confused: :) I think you are correct that the inclusive angle would be the same. But you wouldn't want that kind of edge on a slicing knife I don't think. Every time I've used a chisel sharpened knife (once or maybe twice) the knive wants to turn based on the side the bevel was on. Just like a rudder on a boat. The bevel on both sides keeps the blade going straight without "forcing" it to go straight. In my experience the only place for a chisel grind or edge is in a chisel. For that tool (which my experience is not limited) the chisel grind is essential and appropriate. I would be interested in different opinions of the chisel grind. I've always wondered but this forum is the only place the subject has come up (blade edges I mean) for me.

Jack

I asked about chisel grinds here Jack, because I know that I'll get an unbiased answer here. I always wanted to know about those dang grinds. This thread gave me the chance to ask.

[Evil D]

Think of an ax and maul when thinking about this topic. The ax may be thick by "blade" terms but compared to a maul it's pretty thin. The purpose of the maul is to be a thick wedge that's used to split wood that an ax would get stuck in. The maul is created purposely extra thick behind the edge because you want it to split the wood as opposed to trying to slide through it. Thin behind the edge is the opposite of this concept. Think of trying to slice an apple in half...a maul would cut a little bit and then split the apple apart in a ripping/breaking fashion, whereas a Caly 3 would glide through it, cutting the apple as opposed to splitting it because it's so thin. The thickness of the maul and the splitting action of what you're cutting would make a knife almost useless in a slicing task, as mauls are usually hammered through a log with a sledge or the back side of an ax to force them through and split the wood. Basically, if you want the best slicing ability (as in, easiest/least amount of effort to get through the cut..or in other words "not hammered through") then the thinnest you can get the blade, the better. This is how i picture it in my head anyway.