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Edge geometry optomization : 12C27

Posted by CliffStamp 
Edge geometry optomization : 12C27
January 03, 2015 06:48PM
This is an example of how I typically determine the edge configuration for a knife :





In this case I want the hardest task to be cutting hard woods with high force and moderate twisting.

The first configuration is :

-knife full convex ground, edge is ~7 dps, 15 dps micro-bevel

This however leaves the edge too weak and it twists/deforms to a depth of about 0.005" at maximum. The second configuration is :

-apply a 10 dps secondary edge bevel, ~0.003" thick
-then a 15 dps micro-bevel

This is much better and after a couple of hundred passes there is only one small point of visible deformation. The third configuration is :

-apply a 12 dps secondary edge bevel, ~0.003" thick
-then a 15 dps micro-bevel

Now it does 1000 slices into the hardwoods with no visible damage. The edge and apex are both strong enough to resist deformation and thus the blunting is by slow wear. After 1000 slices the knife can no longer do a true push cut but does 45 push cuts and easily makes looping slices.
me2
Re: Edge geometry optomization : 12C27
January 03, 2015 07:27PM
So here is a question for another steel. I have a nice Leatherman multitool from my wife. It has only one blade on it, which is about 2" long. I'd really like to optimize it for the type of cutting I see for a 2" blade. Right now it has about a 20 dps apex bevel about 0.02-0.015 inches at the top. Is 7 dps too low for light wood carving? Not the hogging you're doing here, but trimming and carving? I have a sharpening block set to 7 dps and want to work that blade, the RADA, and the Cold Steel Scalper down to 7 dps with a 12-15 dps microbevel, though the Scalper should probably stay where it is for the yard work it sees (12/15 vs. modifying to 7/15).
Re: Edge geometry optomization : 12C27
January 03, 2015 09:25PM
For softer woods, or for less force in the cutting, the original geometry is fine. It is what I use on clear pine, cardboard, ropes, etc. even on very basic steels.
me2
Re: Edge geometry optomization : 12C27
January 04, 2015 10:15AM
The original 20 dps/0.015" bevel?
Re: Edge geometry optomization : 12C27
January 04, 2015 10:27AM
For softer woods, cardboard, ropes, 6-8 dps, micro-bevel. I have dipped under 4-6 dps, but at that point you need to use either a transition bevel or not a micro-bevel. It is also a more complicated geometry to maintain.
Re: Edge geometry optomization : 12C27
January 04, 2015 12:08PM
So did you just prove that a full convex ground knife can benefit by secondary bevel and a micro-bevel?
Re: Edge geometry optomization : 12C27
January 04, 2015 12:11PM
Great work, by the way. You make a good example of how to methodically test and get the best perfomance out of the knife. It is a simple concept, I wish more people would adopt a simple method that works, instead of a lot of complicated mess, then agruing and defendng why their way is right, and the only way to do it.
Re: Edge geometry optomization : 12C27
January 04, 2015 03:21PM
Quote
jasonstone20
So did you just prove that a full convex ground knife can benefit by secondary bevel and a micro-bevel?

The problem with convex grinds is that saying a knife has a convex grind really doesn't tell you that much. For example I have a splitting wedge which has a convex grind but does that in general cut well?

With any geometry you have to look at the cross sectional thickness. Ideally you want the minimal cross section which gives you the necessary durability. This hold for most light cutting.

For heavier work you often need cross section to prevent binding, hence while a 1/16" blade may not work well for some woods as though it will have exceptional penetration it won't be very fluid.
Re: Edge geometry optomization : 12C27
January 04, 2015 06:12PM
Quote
CliffStamp
Quote
jasonstone20
So did you just prove that a full convex ground knife can benefit by secondary bevel and a micro-bevel?

The problem with convex grinds is that saying a knife has a convex grind really doesn't tell you that much. For example I have a splitting wedge which has a convex grind but does that in general cut well?

With any geometry you have to look at the cross sectional thickness. Ideally you want the minimal cross section which gives you the necessary durability. This hold for most light cutting.

For heavier work you often need cross section to prevent binding, hence while a 1/16" blade may not work well for some woods as though it will have exceptional penetration it won't be very fluid.

You are right, I was just pointing it out to be funny. I have heard many times that just because something is convex, it is better, which is silly, because as you pointed out, convex covers a lot of range, and the acutal blade geometry matters more that the grind geometry.
Re: Edge geometry optomization : 12C27
January 06, 2015 03:24PM
Similar process, different goal on a Spyderco Lum in VG-10 :

I flattened it with a waterstones (Suehiro 'Chemical' 320) which due to wear / slurry will add a curvature to the blade approaching the edge. This will bring the edge up to 5-6 dps vs ~3 dps if it was flattened with a true-flat plate like the DMT/Atoma's. I wanted the extra few degrees as a ~3 dps edge is in general fairly fragile and I have to be too careful with it for it to be generally effective/efficient. Here is the edge after a polish with a Naniwa Aotoshi 2k and a x-coarse DMT 15 dps micro-bevel :



That picture is 1 mm wide, so that micro-bevel is ~30 microns or ~ 1 thousands of an inch wide, barely visible. Here is the edge after 500' of cardboard (a mix of 1/8 to 1/2" cardboard cut through the corrugations at a speed of ~1'/s) :



The micro-bevel has been polished away and the apex has not significantly deformed or fractured, it is just blunting by slow wear. This is the ideal result so this is the geometry that I would keep. Note that if the x-coarse DMT was not used, and a finer abrasive was used to set the micro-bevel then the micro-bevel width would decrease and the apex could then blunt by deformation or fracture on the same work. Thus you have to keep the apex finish in mind.

To be curious I wanted to know how much damage would happen if this was used past the original goal so I cut up some of the cardboard which had those big globs of hot glue that is commonly used to make packing boxes. This is the edge after about 50 cuts, note again this is high magnification, the image is 1 mm wide :



+



The apex, and even edge, clearly fractures when over loaded, not unsurprising considering it is a fairly hard (~60 HRC) high carbide steel. In order to do that type of cutting without damage I would have to add an actual secondary edge bevel, ~10 dps, < 0.005" thick.
Re: Edge geometry optomization : 12C27
January 06, 2015 06:14PM
Hey Cliff....

"Note that if the x-coarse DMT was not used, and a finer abrasive was used to set the micro-bevel then the micro-bevel width would decrease"

Is this because using the corse stone removes more material making a larger micro?
Re: Edge geometry optomization : 12C27
January 06, 2015 06:56PM
Quote
Fbhandler


Is this because using the corse stone removes more material making a larger micro?

Yes, it takes me a set number of passes to ensure there is minimal burr and with the x-coarse DMT even a few passes makes this micro-bevel much larger. With a very fine stone the micro-bevel is actually on the scale of ~micron wide.
Re: Edge geometry optomization : 12C27
January 07, 2015 06:44AM
Is this to say that you prefer micros set with a course stone in general I r is it situational ?
Re: Edge geometry optomization : 12C27
January 07, 2015 08:57AM
The finish of the micro-bevel depends on the goal of the knife. If it is to have long term edge retention in slicing soft materials then the finish would be very coarse. I don't see any reason not to micro-bevel from a practical viewpoint.

I believe only Bark River is staunch against them and like most arguments made by Bark River they are crocoducks : [www.knifeforums.com] .

A micro-bevel is on the order of a micron wide, anyone who claims they increase "resistance" in cutting has never actually measured anything or doesn't know what micro means.
Re: Edge geometry optomization : 12C27
January 27, 2015 07:07PM
I was wondering what would happen if you used a coarser finish for the microbevel that the base finish, and if coarse microbevels could be of benefit for slicing, but I don't have a microscope, just a 40x loupe, so thank you for showing what would happen.
I am thinking of trying a DMT Blue (coarse) or fine Norton India microbevel for EDC.
Re: Edge geometry optomization : 12C27
January 27, 2015 08:41PM
The grit of the apex has a strong influence : [www.cliffstamp.com]

I can't see how a coarse grit on the edge would help unless you were looking to enhance the self-sharpening/fracturing. It would make for an interesting experiment.
Re: Edge geometry optomization : 12C27
January 27, 2015 09:09PM
It was your work you did on the topic of that write-up that gave me the idea, because I am looking now for EDC edges and a balance between slicing and push cutting, which I was able to get with the Norton Economy/Norton IC6 Sic/Fine India/#325 grit diamond. Then I got to thinking, what if you apply a mirror finish to the bevel, and add a coarse microbevel, what would that combination yield? But I was worried about ruining the apex, like what happens when trying to finish on an ultra-coarse stone like the TASK UC.
The self-sharpening aspect also is insteresting, I am just not sure how to procede without being able to closely examine the edge, and I, not being big on edge retention, am not sure how to keep track of it. I have just been going on the loss of push cutting w/x-grain, slicing w/x-grain of phonebook paper and beard hair whittling as signs of the edge dulling.
Re: Edge geometry optomization : 12C27
January 27, 2015 09:29PM
As a note, at times I may give off the wrong impression that comparisons/experiments need to be some kind of perfect to be useful, that isn't the case at all. At times, often on Spyderco's forum, I note the problems with lack of controls but it isn't because the experiment method it inherently problem but that conclusions are drawn from it which are not supported by the data.

To start off you always have to ask yourself what question exactly are you trying to answer. One I would ask if if the difference in performance is so small that you can't detect it by slicing paper then how practical of a difference are we talking about? At times I do really precise measurements but it is because I am trying to calculate something like edge retention dependence on grit or angle, if you just want to see if something makes a change or not you don't need this kind of precision.

Plus, and this is like the zero-law of experimentation, everything is always a work in progress. The experiment you do today just allows you to do a better one tomorrow.
Re: Edge geometry optomization : 12C27
January 27, 2015 10:25PM
Right, it is just useful to have a metric.
I use the sharpening 'tricks' to gauge the dulling of the blades edge, and usually use time as a measure as I am currently working on EDC edges, and between food prep, household utility, I am cutting the same type of material in at steady rate, and try to take note if I cut something out of the ordinary.
Part of my frustration is that in some things I like to be exact, ie spelling and measurements, and not having a metric other than sharpening tricks is difficult. I can take a slice through printer paper, and get a pretty good idea of how sharp the edge is, but communicating that is a different matter. I am a much better talker than writer.
Thank you again for your input.
me2
Re: Edge geometry optomization : 12C27
January 28, 2015 08:10PM
What would the fractured edge look like if the steel were something like Alvin style O1, 1095 or maybe 13C26?
Re: Edge geometry optomization : 12C27
January 28, 2015 08:31PM
That is an interesting question. With the knives I have from Alvin considering their nature they are ground very light. It would be a curious point to compare AEB-L at 62 HRC vs VG-10 at 59/60 HRC when both are over loaded.
me2
Re: Edge geometry optomization : 12C27
January 29, 2015 04:34AM
Therein lies my question. Would a steel like that have failed under those conditions or would you have to push them harder to get failure in order to compare the 2.
Re: Edge geometry optomization : 12C27
January 29, 2015 08:59AM
Quote
CliffStamp

The first configuration is :

-knife full convex ground, edge is ~7 dps, 15 dps micro-bevel

This however leaves the edge too weak and it twists/deforms to a depth of about 0.005" at maximum. The second configuration is :

-apply a 10 dps secondary edge bevel, ~0.003" thick
-then a 15 dps micro-bevel

This is much better and after a couple of hundred passes there is only one small point of visible deformation. The third configuration is :

-apply a 12 dps secondary edge bevel, ~0.003" thick
-then a 15 dps micro-bevel

Now it does 1000 slices into the hardwoods with no visible damage. The edge and apex are both strong enough to resist deformation and thus the blunting is by slow wear. After 1000 slices the knife can no longer do a true push cut but does 45 push cuts and easily makes looping slices.

This is inspiring thanks Cliff!
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