July 4th, 2005 07:45 PM
CPM-10V As Blade Material
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A comparison of the cutting ability and edge holding performance of Talonite vs CPM-10V at a high polished finish
What was done
Using cardboard as blunting stock, the sharpness and edge retention of Talonite and CPM-10V (Phil Wilson custom at 62/63 RC), were compared. In order to examine the onset of blunting in detail, periodic checks on sharpness were made, rather than simply stopping after a set point. Both blades were first completely reset with an x-coarse DMT bench stone and then sharpened using an Edge Pro Apex, finishing with a 2000 grit polishing tape. During the sharpening, the Talonite knife burred rather badly on the medium hone. The first stroke with the hone after flipping the blade over had a very gritty feel as the significant burr was ground off. The burr was much less noticeable on the fine hone and not significant on the polishing tape during which was ground a couple of degrees higher to micro-bevel the edge to ensure complete burr removal.
The sharpness was also quantified in two different ways, looking at both edge aggression or "bite" as well as just alignment. The "bite" is a large factor in all slicing movements, and edge alignment is most critical to push cuts. Therefore a string was push cut and the force required (measured in grams) taken as a measure of the alignment, and to determine the bite a roll of denim was sliced through, recording the amount of pull strokes required to make a cut. Very little force was used on the denim cutting, just enough to steady the blade. To ensure that the force used was consistent, a very sharp knife was periodically used as a check to ensure that as as the blades blunted more force would not be used on the denim slicing to compensate for the loss in cutting ability.
UPDATE : as noted, the denim cutting is problematic because the amount of slices required is very sensitive to how much force is used. When both blades are used at the same time and another sharp blade is used as a reference, user feedback is enough to get decently consistent results. However it is near impossible for example to try and duplicate the testing a week later, how to tell how much force was used. As the testing methods were refined edge aggression was tested by slicing through cord under a specific amount of tension.
There were four rounds of cutting performed, 1000 centimeters of 1/8" ridged cardboard were cut in each round. To minimize the extent of random variations in the sharpness measurements and to get well behaved values, averages were taken over eight to twelve cuts. The following table shows the extent of blunting during the cutting as determined by the string cutting :
Edge deformation shown by force needed to cut string Blade Initial Round 1 Round 2 Round 3 Round 4
10V 163 +/- 7 294 +/- 22 315 +/- 22 331 +/- 16 367 +/- 21
MEUK 168 +/- 8 448 +/- 22 533 +/- 22 500 +/- 21 570 +/- 20
and the loss of bite as shown by the denim cutting :
Loss of edge bite shown by number of strokes needed to cut a roll of denim Blade Initial Round 4
10V 1.4 +/- 0.1 1.8 +/- 0.1
MEUK 4.4 +/- 0.2 11.4 +/- 0.3
The slicing checks for sharpness were not recorded throughout as not enough material was on hand.
What can be learned
First off, from looking at the initial push cutting sharpness, both blades can be formed to a fine edge. This was far past shaving sharp. However the initial aggression is quite different. This difference may be due to the fact that since the Talonite blade is quite softer there is a smearing out effect of the Cobalt matrix over the alloy carbides. It may also be due to the inherently more aggressive structure of the alloy carbides in the CPM-10V blade because it has been cryogenically heat treated which will act to change the fundamental nature of the carbides formed. Deep cryogenics (liquid nitrogen) acts to induce the formation of highly wear resistance eta carbides.
As for the results of the string cutting, it is readily obvious that the CPM-10V blade keeps a much crisper edge for much longer. Specifically after cutting 4000 centimeters of cardboard, the CPM-10V blade has not even blunted to the extent that the Talonite one had just after cutting through 1000 cm of cardboard. The basic reason that the CPM-10V blade stays much sharper in this respect is that the hardness is much greater, and therefore the blade is much stronger and the edge can resist rolling much better. Specifically the tensile strength of CPM-10V at 63 RC is 375 000 psi, Talonite's is only 190 000 psi and thus it is very weak, the steel is almost 100% stronger.
Looking at the denim cutting, the Talonite knife is even further behind the CPM-10V knife. After the full 4000 cm of cardboard were cut the CPM-10V knife has lost very little aggression, however the Talonite knife has little bite left at all. The much larger difference here may be linked to the same possible reasons why the CPM-10V blade was initially more aggressive.
In short, the Talonite blade was found to be significantly inferior in regards to holding a crisp push cutting edge, and vastly outperformed in regards to holding an aggressive slicing edge when compared to CPM-10V with both sharpened to a high polish.
UPDATE : when this cutting was originally performed, back in 2000, I was under the mistaken impression that blunting happened in two stages, first as the edge bent and then as it wore. This seemed logical and appeared to explain the results. However when checked more carefully, the rate of blunting was found not to be simply two stages but a smooth nonlinear decrease [ref].
Side issue : aggression or "bite"
It was interesting to note how the slicing and push cutting ability were different in both the initial performance as well as the rate of blunting.
UPDATE : in the years since this work was done, this has been noted many times. High alloy and heavily carbided steel can retain more of a slicing ability due to an inherent aggression in the edge. However this "natural" aggression is only noted at a high finish. If steels are compared at rougher finishes, the grit chosen to finish the sharpening easily is the most dominating factor in determining the slicing aggression.
The main problem with the above cutting, is that there is a fairly different grind on the blades. Because of thicker geometry of the Talonite blade it requires more force to push it through the cardboard. Now while this force simply goes to wedging the cardboard apart, and doesn't directly mean the edge is under more force, the edge could taken more stress because of the cardboard being under more strain due to the greater wedging. Thus it would have been thus more informative to regrind the Talonite blade to a similar profile. This wasn't done mainly as it would require too much time on hand hones. Power equipment can't readily be used as Talonite is very hazardous, and a quality filtering system needs to be used.
UPDATE : as noted, this was one of the earlier tests done on edge retention and the extent of the influence of edge angle was still being worked out. Current comparisons performed will usually have the edges of the blades reprofiled to similar angles. The CPM-10V knife had an edge of around 15 degrees per side during the above cutting, and the edge on Talonite blade was around 22 degrees per side. In addition, comparisons of this sort should be repeated with full sharpenings to ensure the minimization of random factors.
The first polish of the Talonite blade to the 2000 grit tape resulted in a complete loss of cutting ability. The blade could not even slice into paper. The reason - the blade had been used for awhile and the edge heavily stressed. This was the reason that both edges were first recut with the x-coarse DMT hone - this reformed the edge with fresh metal.
To check the slicing aggression, a custom D2 blade was also polished. It went through the fabric just as easily as the CPM-10V knife. D2 has a fairly coarse grain size (30 or so microns) which enables it to retain some bit at a high polish. CPM-10V has a much finer grain size (2-4 microns), its slicing ability at a high polish may be due to the carbide contrast (Cr / V).
Comments and references
You can comment on this write-up by dropping me an email : email@example.com . Feedback can also be seen in the following ARCHIVED thread on Bladeforums :
Talonite vs CPM-10V at a high polish
Followup work can be found here :
Cutting ability and edge holding performance of Talonite vs VG-10 at a medium finish
Cutting ability and edge holding performance of Talonite vs D2 at a coarse finish
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