Edge geometry

thats right any of the 3 types of grinds are able to be pushed to 0 edge be it hollow flat or slackbelted in

a chisel grinds is all things keep the same jsut a offset flat grind and a sabor grind is jsut a flat grind that stops half way up the blade a 20degree chisel grind is the same as a full flat grind at 10per side (i had to show this to a few kitchen guys once as i tricked them with pictures of a few edges and they were aall the same angle in total )
 
tryppyr said:
Since nobody chimed in with an mathematical algorithm for edge geometry, I have to assume that for most folks it has become simply a matter of trial and error over the years. Please correct me if I'm mistaken about that. If so, that's fine, and I can certainly understand why that would be the case.

I've done a little digging to try and understand more on the subject. What I've come up with is more questions than answers. I'll start with the most basic question first. There seem to be about six "main" types of grinds used in creating knife blades:

1) Hollow (concave)
2) Flat
3) Sabre (Scandi?)
4) Chisel
5) Compound (double bevel)
6) Convex

The question, then, is what are the strengths and weaknesses of each type?

My assumption is that hollow grind is much sharper (strength) but more prone to edge damage (weakness), and harder to maintain (weakness).

I suppose the follow on question is what sort of applications each grind type is most suited for. "Chisel" grind seems to speak for itself, but perhaps that is only one of the main applications for that grind. The others are less obvious.

Also, I acknowledge the above is an oversimplification, and that there are other cross sections and grinds (especially for swords), but I think this gives us a basis for discussing knife edge geometry in more detail.

One last thought... presumably, these grind types were developed, at least in part, because the steel of the day was limited in strength and flexibility. Do the new modern steels make any of these grind types less relevant to modern knife design?

- Greg
Click to expand...

The machining industry has just the calculations, or very close to it, that you are talking about. The problem is they are very specific w/r to material being cut, and they assume the blade is held much more rigidly than a human hand can hold it. Also, they have the advantage of practically unlimited force that can be applied to the cutting tool, while knives are limited to just what a human can do. In the end, the huge variety of uses for knives, along with the "sloppyness" of the human hand, combine to make trial and error just as good a way of determining edge angle and geometry as a precise formula.

Certain industries have guidlines and geometry ranges for knives, such as carving, meat cutting/butchering, etc., but as Ed said, one knife will work beautifully for one user, only to be next to useless for another. These industries probably came up with these ranges by trial and error and experience. The only thing I can say for edge angle is keep reducing it until it starts taking some light damage during use, then thicken it back up just a little. Edge geometry would follow this guidline too, but it's much easier for the end user/customer to adjust edge angle than thickness behind the edge, grind type, etc.
 
I love these threads and this forum. So much information and opinion to consider.

Let me start by saying I am not a knife maker, I am an engineer among other things. I think or process most things on the left side of my brain so I like black and white answers. So an algorithm for the dimensions of a knife would be nice, however I know the beautiful custom knives I own were not made this way.

Here are some observations. If I take 2 pieces of high quality steel and use industry standard HT I will get edges that fit the thinness check at 0.050”, but may not really be good knives. The pieces are 1 1/2”x1/4” bar and 1 ½”x1/2” bar stock. Then grind them to a “full flat” (isosceles triangle). At 0.050” I get 0.01667” thick for the ½” spine and 0.00833” thick for the ¼” spine. Based on the preferences discussed here they should fall into a thin to very thin range.

My interest is in the cross section of edged tools/weapons made for cutting. I use my kitchen knives for pure slicing. Beyond that I want blades that cut efficiently through media. I don’t think my life will ever be saved by slicing thin pieces of tomato. A blade that can handle a little abuse and be quickly fixed up on a strop or ceramic rod is more useful to me. By slicing I mean finer work and by cutting I mean going through media where the entire cross section even the shoulder matters.

Those are some of my thoughts. I hope this thread keeps up because I would like to learn more.
Greg
 
gga357 said:
Here are some observations. If I take 2 pieces of high quality steel and use industry standard HT I will get edges that fit the thinness check at 0.050”, but may not really be good knives. The pieces are 1 1/2”x1/4” bar and 1 ½”x1/2” bar stock. Then grind them to a “full flat” (isosceles triangle). At 0.050” I get 0.01667” thick for the ½” spine and 0.00833” thick for the ¼” spine. Based on the preferences discussed here they should fall into a thin to very thin range.

My interest is in the cross section of edged tools/weapons made for cutting. I use my kitchen knives for pure slicing. Beyond that I want blades that cut efficiently through media. I don’t think my life will ever be saved by slicing thin pieces of tomato. A blade that can handle a little abuse and be quickly fixed up on a strop or ceramic rod is more useful to me. By slicing I mean finer work and by cutting I mean going through media where the entire cross section even the shoulder matters.
Click to expand...

Great observation! Dan Farr and I were discussing this thread, and he suggested I posit a 'down and dirty' method of measuring the blade's overall profile:

First, magic marker or Dykem your blade:

5612492211_1c17fc3aa4_b.jpg



Next, set your calipers for the first thickness scribing (in this case, .020"):

5612492449_5084dc7c28_b.jpg



Straddle the blade with your calipers, and gently slide it the length of the blade. Repeat with greater thicknesses (in this case, .050" and .100"):

5613072018_cd862d95d7_b.jpg


This will tell you a couple things... how much meat you have behind that edge along the entire length, and whether or not you've got high spots or dips (intentionally or not!). Thanks to Dan for presenting this to me at his shop this weekend... I'd been doing it on a point-by-point method, making a smear of measurements along the length of the blade instead of all at once!
 
Thanks for sharing that, Matthew! I was curious(and nervous) to try this on the 4" woods blade I had posted earlier. There has been some work done on it since and it is freshly etched. The final sharpening still has to be done but this is where it sits at .020",.050" and .100". It is right where I wanted it, with a gradual thickening toward the tip. The spine is 1/8" thick.

IMG_0025-4.jpg
 
Very cool Matthew, like making a topo map. That's a great, understandable way to illustrate the overall geometry of the blade. :thumbup: Please pass my thanks on to Dan.
 
You guys are going to way too much trouble doing this!

I just leave mine out on a tarp and check 'em out on google maps for the topography... way easier!


All kidding aside, great information here; I'll have to put it to use.
 
test to see if the razor pic showes up and ues i added the .010 this blade is 3/4 tall from edge to spine and ground witha 2 inch wheel
217234_1643989390239_1551997027_31281501_3822152_n.jpg
 
awesome thread matt.

i wish i had time to get to respond to this before just about everything i had to say was already said.

geometry is everything. roman put it perfectly. It's not just the edge thickness prior to sharpening though. like stated earlier, the whole geometry of the blade is what counts. i bet no-one here is grinding blades as thin as Butch's straight razors, but try to slice through an apple with one and you'll be SOL. it's all based on the intended use, but the thinner you can go is better

i don't make very large knives, that permit a ton of abuse. i generally finish them at about .010" to .015" depending on the grind. kiridashis that are hollow chisel ground 1" tall with an 8" wheel on 5/32" thick stock go about .005" but if i have a big bolo that's full flat double ground on 3" wide 1/4" thick stock i'll leave it a little heavier at about .020"

basically push the edge as thin as the steel will handle, based on the given job. that same bolo might cut better just that much thinner at .015" or so until you get to the point where the edge can't hold up

matt i think you need to make this a regular thing. it's awesome to see input on why we do things not just how we do them. it makes you re-think "why the hell do i do it like that? is there a better way?"

has anyone read this thread that is going to take a look at changing their geometry of their edges or overall blades? or any users collectors that didn't pay much attention to this before that are going to now? it would be cool to see what type of positive impact this thread has. seems like it has some good feedback with a lot great questions.

great thread everyone
 
Michael J. Spangler said:
has anyone read this thread that is going to take a look at changing their geometry of their edges or overall blades?
Click to expand...

I'll certainly be measuring mine from now on and make at least a couple of my "standard" blades and edges progressively thinner to see how they hold up with the same steel and HT. If nothing else it will help me check the straightness/evenness of my grinds better than holding it at arm's length and going "yeah I guess that looks pretty good" :o
 
butcher_block said:
test to see if the razor pic showes up and ues i added the .010 this blade is 3/4 tall from edge to spine and ground witha 2 inch wheel
217234_1643989390239_1551997027_31281501_3822152_n.jpg
Click to expand...

Butch, I swear, I'm not reading anything you post until you promise to turn on your frigging spell check and form a complete sentence. JEEEEEZZZZZUUZZZZZZ!!!! How about just some punctuation, for crying out loud!!!:D
 
So true I would never want to use that razor for anything other then shaving. I just wanted to show just how extreme a grind could be made and still have a use (you should see how this shaves garlic down tho ).
BTW the razor has a spine thickness of .207

The other side is this kitchen knife made for veg. cutting.
Its fully flat ground with distal taper (you can see in the lines) and is a bit over 2 inch tall at the heel slightly tapering as it gets to the tip
at the before sharpened edge it is under .007 .
This blade has a spine thickness of .093
imag0038sm.jpg


edit to make it better for matt :)
 
Wow, a lot of thin geometry. Are you using thin steel to start or do you like something with meat at the spine? My go to thickness for an outdoor knife is 5/32. but I use 9/64 to7/32. D
 
What a great thread, Matt. I'm not going to add anything at the moment. Instead I decided to just read and learn on this one. Thanks everybody for all the great contributions. This gives me a lot to think about in my own shop. I've been very happy with my edges so far in my knife making, but taking some of this info to heart might help me improve them even more. Good stuff:thumbup:
 
I'll mic a folder or 2 i have and my wavy edge chopper to show that i cam make a thick knife (is that really the goal of this thread :) )
The one folder needed to be thicker and more obtuse edge angle cause it was cpms125v @ rc63-64 but the cpmM4 is a bit thinner. Im also harder on folders then any other knife
 
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