Edge retention as a function of edge angle / steel
- This topic has 46 replies, 10 voices, and was last updated 02/07/2015 at 8:40 pm by
.
-
I am not where where this should go, this seemed like the best fit.
This is a piece of work which intends to look at the edge retention as a function of micro-bevel angle, steel type and possibly grit finish in an exhaustive manner using the Wicked Edge to set the micro-bevels.
It will be part of the Mule-Library program I have started a test run which I will do on two Spyderco knives. The procedure is basically this :
-sharpen a knife on a Wicked Edge/1000 grit diamond with a specific angle micro-bevel
-cut 1/2″ hemp and periodically measure the sharpness
-determine the edge retention statistics
-repeat a few runs until stable
-then repeat this whole thing across a range of micro-bevel angles
-then repeat this whole thing again across a range of steels
-possibly repeat this whole thing across multiple grit finishesNote the work is blinded to prevent bias.
The results of this comparison when finished allow you to answer some very interesting questions such as :
-At what angle/grit do I need to sharpen 420J2 to have the same edge retention as as 21 dps – 1000 grit WE edge on S110V cutting half inch manilla hemp?
This question in fact many people don’t even realize has an answer at all. In fact the more dramatic question :
-At what angle/grit do I need to sharpen 420J2 to have twice the edge retention as as 21 dps – 1000 grit WE edge on S110V cutting half inch manilla hemp?
also likely has an answer as well because the influence of edge angle / grit on edge retention is very large.
Here is one run only on a VG-10 blade from Spyderco (Lum Chinese) :
-primary grind full zero
-edge bevel is approx. 6.5 dps in the final sweep (slop from stone wear, not intentional)
I would like to get 11 dps, but it is not possible easily with the WE, I am going to play with this a little. In fact very low angles would be interesting because at some point the strength would be compromised and the apex would fail by lack of strength. However hemp is so soft that this takes very low angles, less than 5 dps.
Now before anyone reaches any dramatic conclusions from the particular shapes or relationship between one angle and another, again this is one run only I can not even bound the performance with error bars at this point. However there are still some obvious observations :
-the edge retention appears to depend critically on the edge angle
-further it decreases significantly as the micro-bevel angle is increased
-this change is so large that angle could be as critical (or more) than the steelsA lot of this I have discussed before, however the point to this is to move from these general statements to doing the work in enough volume to allow answering some very specific questions about steels and angles and at what point one can compensate for another.
It will take 3-5 runs before the measurements will stabilize and I think it should be a smooth trend. That large difference between 13 and 15 dps is likely just amplified by random scatter. This is a lot of work, just to compare two steels will take about 100 runs of cutting hemp with just one grit and multiply that by any grit changes (so almost 500 runs if you wanted five grit finishes) so do not expect it completed any time soon.
As a final caveat, you can not generalize from this to edge retention in general. Hemp is so weak (compressible) that it can easily be cut with ultra-low angles in contrast hard wood cutting (plywood etc.) can easily turn edges at 10 dps on weaker steels. Thus if you ask the same question :
-At what angle do I need to sharpen 420J2 to have twice the edge retention as as 21 dps – 1000 grit WE edge on S110V cutting half inch plywood?
The answer might be that it is not possible as you can not reduce the angle on the 420J2 enough to compensate as it would be so low it would not have the strength to cut the plywood and would just deform.
Here is an update with two runs done on multiple angles to show the expected scatter :
I think I will stop at three runs for each and use another steel.
… are you using the sharpness testing jig that you posted plans for?
I think you are referring to Dan’s jig which is a push cutting device.
The simplest way to do it is hang a weight from piece of cord, have a clamp which can catch the other end and then you can put the knife under it, lift the knife and then draw it.
It can be all machine recorded as well. At one point I actually did it with a stand/clamp to hold the knife at a specific height, 1mm above the cord attachment, and had it move on a fixed track and measured the tension in the cord electronically. The problem was that the extra precision wasn’t of benefit as I found there is too much scatter in the results due to random and systematic error.
Consider for example when you are cutting the material you are using to dull the knife, even if you do it very controlled, then all the things that can change :
-the material itself isn’t going to be overly consistent (unless you get ISO rated material)
-speed, force, angle of the cut
-temperature, humidity of the environment
-dirt, dust, debris
-initial sharpness, angle, gritThe you have things like the actual steel itself which is a big mish mash of micro-structures :
-tempered and untempered martensite
-pearlite
-cementite
-alloy carbides
-retained austeniteand the edge is just a random selection of all of that, hence it blunts in a very random manner. Parts of it will wear smooth (well tempered martensite + cementite), parts of it are very weak and roll/deform (austenite, pearlite) and parts of it just chip/fracture (around or through the alloy carbides).
In general, as controlled as I can be, from one run to the next, the most confident I am is about 50% in any run hence I always do at least 3-5 to make sure I have reasonable scatter estimates. Now once you stop doing the controlled comparisons and just do real work, the variability goes through the roof and the performance can change by a factor of ten easily. Hence outside of the “lab” you really want to average over a long time before judging a knife as good, bad or ugly as a knife can easily “fail” simply because it hit a rough patch of material or you were not the best technique wise so you need to find a consistent patter.
Here is a full three runs, the scatter is starting to calm down now and the trend is fairly clear :
This is all the jazz behind it which generates that little graph :
- You must be logged in to reply to this topic.