Precision Sharpness Testing Device

[Willy]

Hi all,

I read a lot here in the forum and noticed that there can be some problems with the measurements if the user makes mistakes unconsciously. That is why I built a device that should deliver absolutely reproducible results (sorry, german language only, but I'm sure you will understand the function):





I used the BESS line and various fishing lines with the device. The lines are:
CE = Carbon Carp Expert 0.25 mm, measured 0.29 mm
TC = Tectan FCC (Fluorocarbon coated) 0.25 mm
TF = Tectan Fluorocarbon 0.25 mm
PF = Perca Forellen-Schnur Spezial Coating 0.25 mm
CF = Cormoran Profiline Forelle 0.25 mm
Unless otherwise stated, the thread tension is 100 cN for BESS and 300 cN for the other lines (1 cN ~ 1 g).

The first series of measurements was disillusioning. I expected better reproducibility:

   

Now other fishing lines were compared with the BESS thread:

   

The BESS values show the greatest spread. Scattering? No, always an increase. I noticed this during the measurements (i.e. without a diagram) and before the last 3 measured values, the blade was wiped with dry kitchen paper. The following value was significantly lower. The last two values show the typical increase again. Hypothesis: the BESS thread leaves tiny deposits on the cutting edge that cannot be seen with a magnifying glass. Another thesis: these deposits were removed by the CE thread (decrease in the required cutting force).

Now the cutting edge was cleaned with dry kitchen paper after each cut:

   

Up to the tenth cut, the cutting force decreased and the values are below those in the last figure. After that there is no further improvement, but the reproducibility is finally as good as I expected.

The knife position remains unchanged. In order to see whether all threads require a higher cutting force with increasing number of cuts, 10 cuts were carried out without intermediate cleaning. This was followed by thorough cleaning of the cutting edge with dry kitchen paper. The findings:

   

The Tectan Fluorocarbon line works best, which according to the packaging consists of 100% PVDF (polyvinylidene fluoride). The BESS thread shows the greatest increase and the greatest spread. Originally I had the secret assumption that the BESS thread was just a fishing line. Now a special coating seems likely to me. In addition, the fishing lines only require about 30 cN cutting force with a razor blade, while the BESS line requires the expected 50 cN.

Now the cutting edge has been cleaned after each cut. The following diagram shows that a defined thread pre-tension must be applied for reproducible values:

   

[grepper]

Impressive machine you made there! Great job with that.

What exactly is kitchen paper? What is the "cleaning" procedure?

Could the cleaning procedure effect burr or straighten a rolled edge?

[Willy]

> Impressive machine you made there! Great job with that.

Thanks 

> What exactly is kitchen paper? 

Paper for cleaning in the kitchen:

   

> What is the "cleaning" procedure?

When cleaning, the kitchen paper is moved from the back of the knife to the cutting edge and slightly beyond. This is done at both sides. The thread holder is at the rear end and not near the knife as in the picture. But I just didn't have a better picture.

   

> Could the cleaning procedure effect burr or straighten a rolled edge?

Yes I think that can happen and this is why I am not happy with the cleaning required. In my experiments, however, there shouldn't have been much burr. Otherwise the cutting force would have had to increase with all threads as the number of cuts increased. But in the second diagram the cutting force decreases after the BESS thread. So my thesis was that the deposits were removed by the thread that followed after BESS.

[Mike Brubacher]

A very impressive effort there Willy! Thank you for bringing it and your findings to the BESS Exchange.

[grepper]

Ah... Paper towel! 

I would like to see the same tests performed using a double edge (DE) razor blade.  A DE razor is often used as a control standard for BESS testing as they are very consistently around 50 cN for most makes. The Feather brand is a bit sharper.

The condition of the edge of that knife is an obvious and significant variable.  Testing with a DE blade would help to ameliorate and mitigate those concerns.

[Willy]

Paper towel ... not so easy with the translation 

So far, I've made relatively few cuts with a DE razor blade (Wilkinson Sword, around 30 years old). The results show the same tendency in a somewhat weaker form. I'll try to do more tests with significantly more values over the weekend.

   

[Willy]

OK, here are more results obtained with a DE razor blade (~30 year old Wilkinson sword):

   

In contrast to the knife, cleaning obviously doesn't bring any improvement here, but the cutting edge seems to be damaged after the 66 cuts (not visible with a magnifying glasss). With 100 cN thread tension, the required cutting force is slightly above 50 cN. At 150 cN and a new razor blade, it should be slightly below. So the results are as expected. What does this mean for the results with the knife?

[grepper]

"What does this mean for the results with the knife? "

My first thought when I saw your knife results was burr.  Tiny burr (can't see it with a hand magnifying glass) gets smashed against the edge during edge testing resulting in higher readings.  Your "cleaning" procedure probably unbent the burr or wire edge which further supports the idea of burr on the edge.  Just a guess of course.  But at first blush, that's what comes to mind.

BESS media is certified to produce accurate results using BESS test instruments and can indicate burr in test data.  The results of BESS media used in other test equipment is undefined.

Additionally, there are so many variables with edges.  DE razor blades are probably your best bet if you want to establish a calibration standard for your (very nice) machine due to the consistency of their edges and edge finishes.

[KnifeGrinders]

This thorough test tells us that apex of the edge of the knife used in the test is not a clean steel, but the root of the burr - a weak apex that has been shaped from micro-fractured metal deformed in sharpening.
We cannot see the root of the burr under microscope, we need Scanning Electron Microscope for this.

The knife position remains unchanged through all the testing.
With repeated pressure of the test line on the same point, a micro-dent forms


The bottom of this microdent is "mushroomed", widened - on SEM it looks like this:



The wider becomes the bottom of the dent, the worse becomes the sharpness score.

BESS test line diameter is about 0.22mm, and under the same load it exerts more pressure per point, than the fishing line of 0.25mm diameter.
Because of that, the BESS line goes deeper into the root of the burr than the fishing line, and causes more widening at the bottom of the micro-dent.
This is the reason why the test shows faster worsening of the sharpness score of the BESS test line than the fishing line.

If the knife edge were of a clean steel, you would see the same little scatter in the sharpness score that you see in the Wilkinson razor.

Willy, Dr. Werner is finishing translation of my Knife Deburring book to German, email me at KG@knifeGrinders.com.au and I will send you the Deutsch book in PDF.
It explains the root of the burr and wire edge, and what they show on the BESS tester.

To me, your device is an advanced detector of the wire edge and root of the burr, and can be used to evaluate quality of the sharpening process.
Talking of the knife in the test, the initial score of 150 is an indicator of a residual burr on the edge. When with the repeated testing it worsens to 300, you should be able to see the micro-dent with a loupe - something similar to this:

[KnifeGrinders]

The burr root is shown on the following SEM images of a hardened metal.
Top view /  Cross-section

Die Gratwurzel ist auf den folgenden REM-Aufnahmen eines gehärteten Metalls. 
Ansicht von oben / Querschnitt



The arrow shows the direction of grinding at the original surface.
Above the arrow is the burr. Below the arrow is the root of the burr composed of the metal deformed by grinding.
The blue line shows limits of the subsurface damage caused by grinding.

Der Pfeil zeigt die Schleifrichtung an der ursprünglichen Oberfläche an. 
Über dem Pfeil befindet sich der Grat. Unter dem Pfeil befindet sich die Wurzel des Grats, die aus dem durch das Schleifen verformten Metall besteht. 
Die blaue Linie zeigt die Grenzen der durch das Schleifen verursachten Untergrundschäden.