Influence of apex grit (finish) on edge retention slicing half inch hemp


Introduction

Numerous comparisons were made in the past to examine the edge retention as influenced by the finishing grit of the apex. These comparisons were usually restricted to a few grits and often the edge retention statistics were minimal and other aspects of the knife were changed so that while the influence of grit 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

The result shows a very large increase in edge retention, almost 20:1, but again only one run was done with the very coarse finish and two aspects where 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.

More recently a Delica in ZDP-189 looked at the edge retention as a function of apex angle alone, holding the angles constant and the media cut was used 1/2 inch polypropleyne rope. It was found, as is shown in the image on the right, that while the very coarse CBN finish does have a lower initial sharpness, it has a significantly higher edge retention.

However while this kind of comparison is useful, it is only two grits and doesn't well answer the question of in general how the edge retention on a slice is influenced by the grit finish. Is what is seen when moving from the very high grit ceramic to the very low grit CBN rod a general trend, or it is just some particular nature of the actual grit itself being shown, i.e., does CBN make a better edge than the alumina based ceramic?

For clarification on these questions and quantification, what follows is a large scale comparison using a simple stainless steel (3Cr13 / 420) in a very basic knife slicing half inch manilla hemp on a two inch draw.

Method

The knife had the edge set at 6-8 degrees per side and then a 15 dps micro-bevel was applied with the finishing grit (all sharpening was freehand, angles were determined/measured by basic trigonometry). The edge angle was brought to a finish which ensured that the micro-bevel when set would be representative of its own finish. In general this meant that that the edge bevel finish had to be at least half the grit finish of the micro-bevel. The apex was also cut off before each sharpening to ensure all strained material was removed from the edge before sharpening.

An example grinding of the edge for a very high grit is as follows :

This would allow a micro-bevel with a Henckels 8000 grit waterstone or as shown in the image at the right, a Sharpmaker medium rod.

If the micro-bevel was much more coarse as noted in the bevel on the right then the process could be as simplified as :

and then micro-bevel formed with a 36 grit aluminum oxide sanding belt. The only complication with using ultra coarse abrasives is preventing the apex from catching and snagging on the grits. The techniques used to prevent this are :

The sharpness was measured by two methods :

The edge retention statistic used in the comparison was the TCE or total cutting efficiency which is calculated very simply as the sum of :

The more intervals used, the more precise the TCE. The stopping point used was 1.5% of optimal.

Results

The results of the edge retention as as shown in the image on the right (click on the image to show it on a much greater scale). There are a few immediate observations, taking into account that the abrasives are graded (based on the manufacturers micron ratings, from left to right, in increasing grit or finer abrasives ) :

However there are a number of deviations where the general pattern doesn't hold and the edge retention is lower than the grit would predict :

and further note that diamond Sharpmaker rods score much higher than the CBN Sharpmaker rods even though they are the same grit

There are also some obvious odd points in regards to initial sharpness where again the pattern seems to break and the sharpness is lower than would be expected :

Discussion

Why do the coarse grit edges have increased edge retention on a slice? It is mainly because they are acting like a serrated edge or a saw on the scale of a micron. This means they follow a sort of staged blunting pattern :

This pattern continues as the knife wears and thus the sharpness can actually increase periodically as new fractures make those new tips/recesses. After a long time it has all worn away and the apex will look similar to the image at the right which is a very worn coarse edge. It now has multiple flat regions but still show some evidence of the intial very coarse finish in the irregular chips along the apex.

The behavior of the low grit stones which have poor edge retention can be partially explained by the low sharpness the exhibit. This indicates the apex is not properly formed. This can be directly seen with some magnified shots of the edge as illustrated on the right which is from the 36 grit dressing stone. Note that the edge shows large scale damage where the apex appears to be pushed in and deformed versus being cut which leaves it in a weakened state and simply not very sharp.

Note that this stone isn't actually intended to be a sharpening stone, it is made to recut actual grinding stones and thus it has to have an extremely strong bond and a very tough abrasive to allow it to cut the grinding stone and not wear or fracture itself in the process. Unfortunately the same properties mean that the grits tend to round easily and then they don't have sharp edges and they tend to plough vs cut steel in contact. This stone has been used for some dressing and flattening and thus the grits are no longer sharp enough to cut steel well, though it can still cut other stones and work as a dressing stone.

In contrast, note the edge on the right which is formed from the 120 grit Sigma Power stone which is much sharper as it is formed by very sharp cutting grains. This stone however does have the same problem which is that the grits can wear and the bond of the stone can be so strong as to not allow fresh grit to be exposed. In order to keep it cutting at its optimal level then either enough force has to be used to fracture the grit and/or the surface has to be periodically recut with an even more coarse silicon carbide abrasive to keep the stone cutting and stop it from ploughing.

Now in regards to the initial sharpness observations which do not fit the general pattern, there are two distinct influences :

In order to quantify the polish of the apex, the initial sharpness on a push cut was also measured and it explained why some of the stones, such as the Sharpmaker medium rods have much lower edge retention than what would be expected from the grit rating. To be specific here are the recorded force measurements push cutting Espirit baisting thread (lower number is higher sharpness) :

Even though the MXF DMT is rated much finer than the Sharpmaker medium, the MXF DMT produces a much more coarse edge. There are two reasons for this :

In some detail, if you look at the image on the right which is the edge formed from the MXF stone you can see that the apex has a decently fine scratch pattern however there are occasion very large and deep scratches (the black gouges). This is a known problem with the DMT stones as the grit place is irregular.

In regards to the Medium rods being worn. These rods are solid sintered ceramic and thus no new abrasive is released, no slurry is formed. The surface of the stone gradually gets finer in use until at some point all of the abrasive is well rounded and it starts ploughing vs cutting. At this point while it produces a very high polish, the edge retention will suffer. Note however that the stone can be periodically recut with a diamond (or CBN) abrasive which is slightly finer. The stone will then initially cut to the coarseness of the scratch pattern which will again wear in and smoothen out.

In regards to the second factor, waterstones tend to produce a slurry which is a suspension of the metal ground off the edge and the abrasive broken away from the surface of the stone. This slurry does two main things :

John Juranitch was one of the first people to note the damaging effects of the second factor and argued it is why all abrasives should be used dry 2. A very thick slurry, which is common with waterstones will tend to round out the apex and makes producing a refined apex non-trivial to the extent it isn't uncommon that they can be a source of frustation or at least challenge vs non-slurry abrasives 3 .

Recap

In short, the edge retention in slicing hemp tends to increase as the grit is decreased and the sharpness hits a peak at a medium grit level. There are exceptions to this however which are due to various factors such as :

As a point of clarification, the above work concerns edge retention on a slice through a fairly soft and lightly abrasive material. It is critical to realize that if any of this changes then so will the general conclusions, or more specifically, the patterns observed. For example, the image on the right shows a very simple comparison between the edge retention with a Stanley folding knife and utility blade with the as-boxed finish which has a very high grit and the CBN rods which have a very low grit (400) finish.

In contrast to slicing cardboard as shown in the above, here the much lower grit gives a reduced edge retention. This is because of two main factors :

In general, push cuts tend to favor higher grits, and as the material gets harder and stronger then higher grits are also favored. This can even be seen cutting hemp as if the cut is made on a push rather than a slice, some of the very coarse finishes in the above actually can fail to make a cut and dramatically collapse immediately.

For more details and discussion, note the thread on the T0.1 micron forum.

References

1 : discussion of the MXF stone on T0.1 micron

2 : Sharpening secrets of a pro : John Juranitch

3 : Sharpmaker vs Waterstones, a discussion on T0.1 micron


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Written: 16/01/2015 Updated: 22/01/2015 Copyright (c) 2015 : Cliff Stamp