Numerous comparisons were made in the past to examine the edge retention as influenced by the angle of the apex. These comparisons were usually restricted to a few angle and often the edge retention statistics were minimal and other aspects of the knife were changed so that while the influence of angle was demonstrated, the magnitude and nature of the influence was not so well quantified. As an example, in the review of a D2 hunter from Mel Sorg, the edge retention on 3/8" hemp was compared with

- 22 dps apex, fine ceramic finish
- 5 dps, 100 grit finish

The result shows a very large increase in edge retention, almost 20:1, but only one run was done with the 5 dps finish and two aspects were changed at the same time and both will act to increase edge retention so it isn't clear how much of the increase is due to angle and how much is due to the grit finish. For clarification and quantification, what follows is a large scale comparison using two well known stainless steels, slicing hemp across a range of micro-bevel angles.

The following knives were used, both made by Spyderco Knives :

- Lum Chinese : VG-10
- Paramilitary : S30V

The knives had the edge set at 6-8 degrees per side using the following stone progression :

- Suehiro 'Chemical' 320 grit
- Naniwa Superstone 400 grit -or- King 1000 grit
- Naniwa Aotoshi 2000 grit

and then a micro-bevel was applied with the Wicked Edge sharpening system using the 1000 grit diamond stone. The micro-bevel angles were examined from 13 to 25 dps in 2 dps increments. The image on the right at 50X magnification shows the edge and micro-bevel of the Paramilitary during one run.

The sharpness was measured by a draw cut on bergia spinning thread under a 35 gram load, the length of cut is the measure of sharpness. The edge retention statistic used in the comparison was the TCE or total cutting efficiency which is calculated very simply as the sum of :

- material cut (meters) * sharpness (percentage)

The more intervals used, the more precise the TCE. The stopping point used was 1.5% of optimal. Multiple runs were made with each knife, median based statistics and linear regression were used to analyze the results.

The results for the S30V and VG-10 blades are shown in the image on the right. There are a few immediate observations :

- even with multiple runs the scatter is fairly high
- the pattern of edge retention is the same in S30V and VG-10
- the edge retention increases as the micro-bevel angle decreases
- the scatter is so high it takes a large range to see it practically

The last point raises an interesting question about the practical answer to the question of edge retention as a function of micro-bevel angle vs the statistical or raw answer.

The data can be further analyzed by using a slightly different plot which makes it obvious that the relationship is linear and this can be quantified through linear regression. Again there are a few immediate observations :

- the relationship is strongly linear
- the fit of the data is quite high
- the nature of the line appears to be the same for both steels
- the relationship appears to extend to extreme high and low angles

The result that the edge retention increases as the apex angle decreases is due to two factors :

- the coarseness of the finish increases as the micro-bevel decreases
- a lower micro-bevel angle requires more material to be removed to reach a given thickness

The second point is shown in the image on the right which shows two apex angles at two very different angles. The red line on both apex's is the same width but note the much larger amount of material which has to be removed in the lower apex angle.

Now to be clear, this behavior can not extend forever to extremely low apex angles. At some point the apex will become so weak that it will just fold / collapse due to lack of strength. In order to find the point of optimal edge retention then the apex angle has to be set at the minimum angle which makes it just strong enough to resist deformation so it blunts by slow wear.

In regards to the data there are two ways to look at the performance of S30V vs VG-10 :

- how they compare at the same angle
- how they compare at the same edge retention

At the same edge angle, S30V has 31 (12) % increased edge retention over
VG-10. This is in agreement with the CATRA data from Crucible.
^{1}. Another way to look at the data
is that if the VG-10 blade was reduced in angle to 2.4 (2) degrees
then it has the same edge retention as the S30V blade.

However, and this is a fairly important and interesting point. If the S30V blade was 2.4 (2) degrees higher than the VG-10 blade, while both would have the same edge retention, the S30V blade will be significantly stronger and tougher. With that perspective, the increased abrasion resistance of S30V can be sacrificed as it were to increase strength and toughness.

The Lum Chinese in VG-10 and Paramilitary in S30V were used to slice 1/2" hemp with micro-bevel angles from 13 to 25 dps. The following results were determined :

- the edge retention increased in a linear manner towards lower edge angles
- the relationship was not influenced by the steel type

Additionally an angle change of 2.4 (2) degrees was enough for the VG-10 blade to have the same edge retention as the S30V blade.

In a similar type of comparison, two very dissimilar knives were compared
in edge retention slicing cardboard
^{2} :

- 10V
- basic stainless no-name steel (likely 3Cr13 class)

and the same result was seen. In this case the stainless Chef's knife was able to match the edge retention of the 10V knife (K2/Farid) simply by lowering the edge angle. In this case the angle of 5-6 dps was enough to allow the stainless steel no-name to equal the edge retention of the K2/Farid at 9-10 dps.

For more details/discussion, see the following forum threads :

- Wicked Edge
- Spyderco threads : Summary, Toughness discussion
- T0.1 micron

and the YT video.

1 : Crucible data sheet on S30V

2 : T0.1μ

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Written: 17/01/2015 | Updated: | Copyright (c) 2015 : Cliff Stamp |