Influence of edge grit finish on edge retention slicing cardboard: 10V and CTS-XHP


Introduction

In a comparison of multiple steels slicing abrasive/dirty cardboard, it was noticed that the high carbide steels had the edge form differently T0.1μ . As seen in the images on the right with both steels having the edge set at approximately 8 dps with the Suehiro 'Chemical' 320 grit stone there was a marked difference in the observed finish. The CTS-HXP edge had the common parallel scratch pattern and a slight irregular nature to the apex due to the low grit. However in contrast the 10V blade

The bevel showed signs of both fracture and burnishing. These effects are likely caused by the very high volume of vanadium carbide in 10V which is harder than the alumina abrasive in the Suehiro stone. As the carbide is harder than the abrasive it cause rapid wear on the abrasive and since the bond of that stone is strong enough to prevent fracture under the pressure used (10 to 15 psi) the surface of the stone goes slick as the abrasive wear and stops cutting and starts rubbing and thus burnishing.

The question then raised was is this obvious strain and fracture damage to the edge having a significant effect in lowering the strength and/or toughness of the apex once the micro-bevel was set?

In order to examine if the edge was having an effect on the edge retention by compromising the properties of the apex a comparison was ran by looking at the edge retention with two different edge finishes :

The Naniwa has a very similar grit but it has a weaker bond and so it readily produces a slurry. The slurry also tends to contain some of the bonding agent and the abrasive itself possible also readily breaks down. The net effect is that the finish produced is very different. It is like neither of the above edge finishes but instead has a much more random pattern due to the random grinding action of the slurry. Note there is very little signs of burnishing and no significant fracture in the apex line.

Method

The following knives were used to slice 1/8" ridged cardboard on a 2" draw with an approximate speed of 1 fps :

All knives had the edge bevels at ~8 dps with a 15 dps micro-bevel. The edge bevels were set with the following two stones :

Nine trials were completed with each blade at both edge finishes. The edge retention was calculated as the TCE or total cutting efficiency and the stopping point was set to 1.5% of optimal. The weighted average was used to look at the combined results as well as a direct summation.

Results

The results show that there is no significant difference in the edge retention with either edge finish. More specifically, there was no observable change produced if either the Suehiro or Naniwa stone were used to set the edge before the 15 dps DMT/fine apex bevel was set. In this regard the original idea or concern that the Suehiro may have been compromising the edge retention does not appear to have been a practical problem.

There was however a consistent, but small, difference in edge retention between the two steels. Due to the scatter from random variation it was not statistically significant in either set however by combining the runs of both stones into one larger data set it was possible to calculate the advantage in favor of 10V was 26 (11) percent. Given the nature of the random spread typically produced in such work that kind of small difference is very hard to see unless a lot of data is collected even when the cutting is semi-controlled.

The amount of passes on the stones were also recorded and this produced a much larger difference. The CTS-XHP grindability was much higher. The pass count was about 3:1 vs the 10V (~100 vs 300 on the Suehiro). This large change would be mainly due to the fact that the carbides in CTS-XHP are mainly chromium based and they are softer than the alumina abrasive and thus the steel is much more readily cut. In fact the observable effect of the vanadium carbides wearing the stones were so obvious that on the Suehiro if the surface was not reconditioned after each session then it would essentially stop cutting the 10V blade at all and even ~1000 pps would not reset the edge.

Discussion

In regards to the edge retention difference, this mainly happened due to the way the steels blunted. The 10V blade tended to blunt by fracture and these small chips would enhance the slicing edge retention by increasing sharpness on a draw. In comparison the CTS-XHP blade tended to blunt more by slow wear. The difference in behavior is readily obvious even under 50X magnification as seen to the right.

This kind of "self sharpening" was likely one of the factors that enabled David Boye to become infamous for extremely large volumes of rope that he could cut with his blades. Boye used steel in the as-cast form without rolling and normalization to break up the dendretic networks formed in the cooling from the melt. It is likely that these very large carbides produced that kind of "self sharpening" effect.

Of course Boye also :

all of which would increase edge retention on a slice.

Recap

More extended commentary can be seen in the forum thread :

References


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Written: 30/07/2015 Updated: Copyright (c) 2015 : Cliff Stamp