Cliff Stamp
BANNED
- Joined
- Oct 5, 1998
- Messages
- 17,562
Awhile ago I started cutting rope as one of the methods to look at edge retention. I choose it for a couple of reasons, first many people did it so there was a base of information for comparison, and second it was readily available cheaply in consistent stock. The main goal was of course to allow some measure of comparison against other steels.
As side issues, the rope cutting allowed exploration of various other aspects such as the effect of changing the profile on cutting ability and edge retention. Through looking at the performance of dozens of knives with various profiles, and altering many of the knives I came to have a decent understanding of the influence of many of the geometrical aspects.
However the main goal was still not reached in a concrete fashion. Way back a few years ago on the forum I remember arguing that you could test for edge retention on different blade profiles, assuming you were careful enough about what you were doing. I still hold that this is true in some cases, however now realize that rope cutting is problematic in this area.
I recently finished up four rounds of hemp cutting with the Pronghorn, complete full sharpenings each time. The results were consistent, showing a smooth blunting pattern. However when compared to other knives the results don't make sense. It doesn't show the level of edge retention you would expect based on the quality of the knife.
Now before assumptions are made about the Pronghorn, the same can be said of other knives. I spent some time the last few days trying to sort out all the hemp trials I have ran, and there have been many. What I found was that the largest factor was the geometry of the knife. The more efficient the knife was cutting the rope, in general, the longer the edge lasted.
If the geometry difference was large enough, this effect would swamp out everything else and allow knives with softer steels which were of lower abrasion resistance to out cut knives which they should have no ability to do at all. Thus the obvious question was where is the performance coming from. Or was there a bias in the results.
I looked at the bias issue early on through use of friends who would do random cutting and not tell me how many cuts they had made, until I had finished the run. This had no influece on the trials and they were still consistent. I have also run blind trials in the past where I didn't even known the steels or heat treatment - again no problem with any skew.
During the Fowler hemp cutting I constantly watched the edge under magnifiation as well as testing for sharpness and noted the progress looking at the edge both to the side and straight down. It became quickly obvious that the reason for the skew in the performance was head smacking simple.
Back in the beginning I reasoned that the amount of force used to make a cut could be separated into two parts. A component necessary to cut into the hemp, and another which pushed the hemp apart. Doing just a few cuts you immediately realize that with a sharp knife, almost 100% of the force is the latter.
Based on this I reasoned that if you looked at the *change* in performance, the increase in load would allow an examination of blunting independent of the geometrical influence. I knew there was a small effect due to bunching based on geometry but figured this could be ignored. I also tested this with a few blades and found it was true.
Now it is very critical that the sharpening angle be the same. The microteeth formed by an abrasive are dependant on the angle of the edge. This is why you can see such massive changes in slicing performance when you decrease edge angles. For the hemp trials I always ran the same micro-bevel angle using a v-rod setup.
Here is the kicker, though I did test the influence of geometry, the blades used to test this aspect all cut the hemp very well. I mean *really* well. They could slice the hemp in under 10 lbs on a two inch draw. Try to do that and see just how impressive a performance that illustartes. For reference I have seen this take well over 100 lbs on some knives.
However, on knives that don't cut as well, you don't have as much control on the end of the draw, when you approach the very bottom of the cut. Take a knife which you bare need to press on, ~8 lbs on a draw, and you can finish the cut with minimal contact on the resting surface. Take a knife which needs 40 lbs and at the end of the cut you are mashing the knife into the cutting board.
Thus there is a significant influence of the cutting board (I have always used the same type of board). Unfortunately, the influence of this factor is *strongly* dependent on geometry. So you can increase the edge retention simply by altering the primary grind.
In short, only if the knives cut the rope with a similar amount of force, then you can make pretty much direct comparisons.
What is needed is a way to cut the rope without having it on a backing material, and for this to be easy enough to set up that you can run hundreds of cuts in a short period of time and which doesn't waste too much rope.
Other materials can be cut without such problems as they can be cut freestanding. Cutting cardboard is fairly easy in the manner, as it carpet and such. Thus edge retention comparisons on those materials would be more directly comparable.
As long as that described factor is eliminated, it would seem logical that the change in cutting ability would indicate the blunting and this this could be looked at independent of geometry. This of course isn't an arguement that you should not use identical geometries if at all possible.
-Cliff
As side issues, the rope cutting allowed exploration of various other aspects such as the effect of changing the profile on cutting ability and edge retention. Through looking at the performance of dozens of knives with various profiles, and altering many of the knives I came to have a decent understanding of the influence of many of the geometrical aspects.
However the main goal was still not reached in a concrete fashion. Way back a few years ago on the forum I remember arguing that you could test for edge retention on different blade profiles, assuming you were careful enough about what you were doing. I still hold that this is true in some cases, however now realize that rope cutting is problematic in this area.
I recently finished up four rounds of hemp cutting with the Pronghorn, complete full sharpenings each time. The results were consistent, showing a smooth blunting pattern. However when compared to other knives the results don't make sense. It doesn't show the level of edge retention you would expect based on the quality of the knife.
Now before assumptions are made about the Pronghorn, the same can be said of other knives. I spent some time the last few days trying to sort out all the hemp trials I have ran, and there have been many. What I found was that the largest factor was the geometry of the knife. The more efficient the knife was cutting the rope, in general, the longer the edge lasted.
If the geometry difference was large enough, this effect would swamp out everything else and allow knives with softer steels which were of lower abrasion resistance to out cut knives which they should have no ability to do at all. Thus the obvious question was where is the performance coming from. Or was there a bias in the results.
I looked at the bias issue early on through use of friends who would do random cutting and not tell me how many cuts they had made, until I had finished the run. This had no influece on the trials and they were still consistent. I have also run blind trials in the past where I didn't even known the steels or heat treatment - again no problem with any skew.
During the Fowler hemp cutting I constantly watched the edge under magnifiation as well as testing for sharpness and noted the progress looking at the edge both to the side and straight down. It became quickly obvious that the reason for the skew in the performance was head smacking simple.
Back in the beginning I reasoned that the amount of force used to make a cut could be separated into two parts. A component necessary to cut into the hemp, and another which pushed the hemp apart. Doing just a few cuts you immediately realize that with a sharp knife, almost 100% of the force is the latter.
Based on this I reasoned that if you looked at the *change* in performance, the increase in load would allow an examination of blunting independent of the geometrical influence. I knew there was a small effect due to bunching based on geometry but figured this could be ignored. I also tested this with a few blades and found it was true.
Now it is very critical that the sharpening angle be the same. The microteeth formed by an abrasive are dependant on the angle of the edge. This is why you can see such massive changes in slicing performance when you decrease edge angles. For the hemp trials I always ran the same micro-bevel angle using a v-rod setup.
Here is the kicker, though I did test the influence of geometry, the blades used to test this aspect all cut the hemp very well. I mean *really* well. They could slice the hemp in under 10 lbs on a two inch draw. Try to do that and see just how impressive a performance that illustartes. For reference I have seen this take well over 100 lbs on some knives.
However, on knives that don't cut as well, you don't have as much control on the end of the draw, when you approach the very bottom of the cut. Take a knife which you bare need to press on, ~8 lbs on a draw, and you can finish the cut with minimal contact on the resting surface. Take a knife which needs 40 lbs and at the end of the cut you are mashing the knife into the cutting board.
Thus there is a significant influence of the cutting board (I have always used the same type of board). Unfortunately, the influence of this factor is *strongly* dependent on geometry. So you can increase the edge retention simply by altering the primary grind.
In short, only if the knives cut the rope with a similar amount of force, then you can make pretty much direct comparisons.
What is needed is a way to cut the rope without having it on a backing material, and for this to be easy enough to set up that you can run hundreds of cuts in a short period of time and which doesn't waste too much rope.
Other materials can be cut without such problems as they can be cut freestanding. Cutting cardboard is fairly easy in the manner, as it carpet and such. Thus edge retention comparisons on those materials would be more directly comparable.
As long as that described factor is eliminated, it would seem logical that the change in cutting ability would indicate the blunting and this this could be looked at independent of geometry. This of course isn't an arguement that you should not use identical geometries if at all possible.
-Cliff
