Cliff Stamp
BANNED
- Joined
- Oct 5, 1998
- Messages
- 17,562
Awhile ago I made a post about "self-sharpening" knives :
http://www.bladeforums.com/forums/showthread.php?s=&threadid=214104
Recently I got a large number of heavy cardboard boxes so I had the chance to look at this again in some detail, with a focus on *long* term effects. The boxes were made from 1/4" ridged cardboard, and new so they were very stiff and decently clean and free of debris. The stiffness turned out to be very crucial in the long run.
I cut the cardboard with the blade eyeballed at a 45 degree angle, using smooth strokes to cut off a section about two to three mm wide, cutting as fast as possible to maximize wear and give the fastest rate of blunting, or the worse case senario. The edge was tested for sharpness periodically by shaving, push and slicing photocopy paper as well as push cutting light thread and noting the tension required.
Forgetting about the statistics for a moment, I was mainly interested in a very basic question - how much cardboard would the knife cut before it would need to be sharpened. The knife in question was an Olfa snap-off heavy duty cutter and I was using two types of blades, normal and Ultra-Max, and the whole process was repeated using multiple blades of each type to check for consistency. The finishing point was judged simply by the point at which the blades would rip rather than slice the cardboard.
The blades would lose their shaving ability after just a couple of meters of cardboard, and the ability to slice paper would hang on for awhile longer but the real story is that the blades would never lose the ability to slice the cardboard. Well I assume they would eventually, but I cut over 200 m with the blades and they showed no significant change at all in the last 100m. The blunting slowed to a crawl with a micro-serrated effect being induced, many small chips 0.5 to 1 tenth of a mm, and multiple ones several times larger.
The edge was now acting as a saw. This is where the stiffness of the cardboard comes into play. By this point the blades were by any standard quite dull, they could not for example slice a piece of paper at all, they would just rip it up. Thus this entended "life" of the edge, would only be present on cardboard (or whatever) that was stiff enough to resist the force that the blade needed to induce the sawing motion. Measuring the force required would be interesting, but a side issue. Interestingly, because of the requirement of a high stiffness, you get a longer edge lifetime on 1/4 ridged than on 1/8" ridged, opposite to what you might think on first glance.
The real question that came to my mind was, how would a blade made out of much higher quality steel fare? Take a HSS blade at say 66 RC. The metal would ignore the frictional heating and thus suffer no loss of hardness, and the strength and wear resistance would be so much greater than the edge would behave very deifferently. I would assume that there would be far less micro-chipping. This could mean that the edge lifetime could be less as the smoothly worn edge might stop cutting before the cheaper steel which chipped out creating the micro-saw.
To be clear, this "self-sharpening" is very restricted in focus. It only works for slicing and only on very stiff materials and only with very acute edges (15-20 degrees included) on very thin blades (see previous post for more detail). But in summary, it seems that on some materials it could be the case that using a higher quality steel would lead to a lower edge lifetime. I have a few blades to test this out, but it will take awhile before I get enough cardboard again.
-Cliff
http://www.bladeforums.com/forums/showthread.php?s=&threadid=214104
Recently I got a large number of heavy cardboard boxes so I had the chance to look at this again in some detail, with a focus on *long* term effects. The boxes were made from 1/4" ridged cardboard, and new so they were very stiff and decently clean and free of debris. The stiffness turned out to be very crucial in the long run.
I cut the cardboard with the blade eyeballed at a 45 degree angle, using smooth strokes to cut off a section about two to three mm wide, cutting as fast as possible to maximize wear and give the fastest rate of blunting, or the worse case senario. The edge was tested for sharpness periodically by shaving, push and slicing photocopy paper as well as push cutting light thread and noting the tension required.
Forgetting about the statistics for a moment, I was mainly interested in a very basic question - how much cardboard would the knife cut before it would need to be sharpened. The knife in question was an Olfa snap-off heavy duty cutter and I was using two types of blades, normal and Ultra-Max, and the whole process was repeated using multiple blades of each type to check for consistency. The finishing point was judged simply by the point at which the blades would rip rather than slice the cardboard.
The blades would lose their shaving ability after just a couple of meters of cardboard, and the ability to slice paper would hang on for awhile longer but the real story is that the blades would never lose the ability to slice the cardboard. Well I assume they would eventually, but I cut over 200 m with the blades and they showed no significant change at all in the last 100m. The blunting slowed to a crawl with a micro-serrated effect being induced, many small chips 0.5 to 1 tenth of a mm, and multiple ones several times larger.
The edge was now acting as a saw. This is where the stiffness of the cardboard comes into play. By this point the blades were by any standard quite dull, they could not for example slice a piece of paper at all, they would just rip it up. Thus this entended "life" of the edge, would only be present on cardboard (or whatever) that was stiff enough to resist the force that the blade needed to induce the sawing motion. Measuring the force required would be interesting, but a side issue. Interestingly, because of the requirement of a high stiffness, you get a longer edge lifetime on 1/4 ridged than on 1/8" ridged, opposite to what you might think on first glance.
The real question that came to my mind was, how would a blade made out of much higher quality steel fare? Take a HSS blade at say 66 RC. The metal would ignore the frictional heating and thus suffer no loss of hardness, and the strength and wear resistance would be so much greater than the edge would behave very deifferently. I would assume that there would be far less micro-chipping. This could mean that the edge lifetime could be less as the smoothly worn edge might stop cutting before the cheaper steel which chipped out creating the micro-saw.
To be clear, this "self-sharpening" is very restricted in focus. It only works for slicing and only on very stiff materials and only with very acute edges (15-20 degrees included) on very thin blades (see previous post for more detail). But in summary, it seems that on some materials it could be the case that using a higher quality steel would lead to a lower edge lifetime. I have a few blades to test this out, but it will take awhile before I get enough cardboard again.
-Cliff
