Measures of cutlery Sharpness (Review) : Cliff Stamp


Introduction

The sharpness of cutlery is a critical element of performance. It is determined as-sharpened to rate the skill of the sharpener and quality of equipment and method. It is also measured after use to determine the ability of the blade to retain the initial sharpness, i.e., the edge retention. Sharpness is commonly measured by subjective methods such as shaving arm hair as illustrated on the right by Henning Groenzin1. However there are many problems with such a method. First it is defining sharpness as a measure of cutting ability but the manner of the cutting is not well defined. Is the hair cut on a slice or on a straight push? Is the blade held close to the skin or above the skin? Is the cutting against or with the grain of the hair? All of these variations in the method of cutting will influence the result. However even if the method is well constrained, shaving arm hair is still subject to further complications as illustrated by an experiment carried out by Mike Spinak :

I went from table to table, asking almost all of the knife makers whether their blades were shaving sharp. All assured me that they were. I asked them to show me, and most every one did use their knives to successfully shave off a small patch of their own hair. Then I took the same knife (with the maker's permission) and tried to shave the hair off my own arm. In almost every case, I was unable to. At that point, I asked them if they would mind trying to shave the hair off of my own arm. They, too, were unable to shave the hair off my arm in most cases. -- Mike Spinak2

It could be contended that a specific individual could produce a consistent measure of sharpness on their own arm hair, however it is still at best a fairly coarse ranking. Variations on hair cutting as a test of sharpness have been proposed by others such as Tai Goo who has demonstrated the ability of knives to whittle hairs. This has been adopted by many to demonstrate sharpness such as illustrated on the right by maximums otter3. In order to remove the variation of hairs from one individual to the next, a less variable test medium was proposed by Alvin Johnston :

The paper test is where you get a sales receipt ;) or the edge of a phonebook page and turn about a quarter of an inch of it up 90 degrees and press the blade straight down on the paper. -- Alvin Johnston4

Similar paper tests have been used individuals such as Sal Glesser of Spyderco who has demonstrated sharpness by cutting a circle in light paper. Jimmy Fikes uses paper cutting to illustrate sharpness in his blade testing 5. Paper cutting is a significant improvement over arm hair shaving as both the material and the method are usually better defined, though they still lack a numerical represenation and thus the precision is still low. As most paper in general has significant composition variance, users wishing to compare results should ideally use the same paper to eliminate a possible source of error.

These arm hair and paper tests of sharpness, obtained by pushing the blade into a material, are also only one aspect of sharpness. There is also the ability of the blade to slice through material. Some comments on this issue by Dave Sutton in reference to time spent working as a diver in 1977 :

In any case, the BEST rope cutting edge, one that would last all day, was one gotten by sharpening on a coarse stone (the coarse side of your basic Craftsman/Sears sharpening stone) and then 'maybe' once a week hitting the edge 'very' lightly with a fine mill bastard file. This gave a 'very' light serration to the blade edge, that was very aggressive to manila. -- Dave Sutton6

Mike Swaim also contended strongly on rec.knives that tests of sharpness should not simply be constrained to push cutting as for many applications of knives it was not the critical property :

As I've mentioned here many times before, the "shaving sharp" edge that so many folks rave about simply won't really cut things like foam rubber, meat, and rope very well. It's one of the modern myths that persistantly bug the knife world that an edge capable of shaving a small section of soft arm hair, is really capable of doing significantly better on items cut more frequently than will an edge that won't feel as polished. The type of edge that I'm trying to talk about will be a lousy shaver. It'll probably cut you if you try to make it shave, but for exactly that reason it will be significantly better for cutting meat, rubber and twisted fibers. -- Mike Swaim7

Thus measurements of sharpness have to be specific to both the media and the method of cutting. With arm hair shaving and paper cutting commonly being used to measure push cutting sharpness, rope cutting was adopted by many as a method to measure slicing sharpness :

I simply cut through pieces of 3/8" hard-lay black polypropylene rope sucessively until the blades were so dull that they couldn't reliably make the cut in one pass over a cutting board without leaving ragged edges. A good sharp thin knife will leave a glassy looking smooth cut that almost looks fused. The duller the knife the more ragged the cut. -- Mike Swaim 8

Similar methods were used by Wayne Goddard to measure the sharpness to evaluate the edge retention of various blade steels 9 . Ed Fowler also used similar methods to measure sharpness in edge retention comparisons of 52100 against other blade steels10. Phil Wilson initially measured sharpness in a similar manner :

Here is a series I recently ran on 3/4 inch rope. The cuts were all on the same 7 ft long piece of rope from the same roll. All cutting was done against an Alder wood block clamped to the bench. I judge the blade dull when it starts to slide at the bottom of the cut. At this point it will still cut paper but somewhat erratic. The blade will also skiv leather but with some force needed. -- Phil Wilson11.

These rope cutting measures of sharpness, while informative, still lack a numerical represention of sharpness and are still fairly subjective and thus it would be difficult for users to compare work and the precison would be fairly low.

Quantification

Early experiments by the author on measuring sharpness also ranked edges by feel and nature of the cut12 similar to the previously described rope cutting. The first quantitative method personally developed used the edge length required to slice a roll of fabric under a set force13,14. However the force was still set subjectively and thus this method did not have a high precison or the ability to be repeated well by others.

Cutting several types of ropes and recording the force was used but found to be too dependent on the geometry of the knife and was at best a coarse, though quantitative, measure of sharpness15. To reduce the effect of the cross section of the knife and focus sharpness to a measurement which reflected the condition of the very edge itself (meaning the lack of a perfect bevel intersection), materials were chosen that produced minimal wedging and frictional forces. Working from the viewpoint illustrated by Swaim and Sutton, tests were developed to measure both the sharpness for a push into a material as well as a slice across a material.

The push cutting test used a knife to cut directly through a loop of Strikmaster fishing line suspended from a scale with the force required taken as a measure of sharpness16. The fishing line was later switched to Esprit Baisting thread as it was thinner and thus measurements were more precise 17. The sharpness of the knife during slicing was measured by having the knife drawn under quarter inch polyurethane, with the rope under a given tension supplied by a calibrated weight and the length of edge was taken as the measure of sharpness17. The polyurethane was later changed to thinner cord such as 48 lbs hemp to increase the precision of the measurements18.

A variation on the thread cutting method was simultaneously described by Mike Spinak 19 with variations proposed in regards to the manner of measurement20. This measure of sharpness has been used by several individuals to study sharpness in a systematic manner, 21. On the right is a picture from Henning Groenzin of his setup used for such measurements22. A large body of work on various issues of sharpness has been produced using this method by Steve Elliott who has performed extensive testing on planer blades of various steels23. CATRA uses a similar method to measure sharpness by recording the force used to press a blade through a piece of rubber24.

The use of newsprint as a medium to test sharpness was also refined with a suggestion by the author to quantify the measurement by using the distance from the cut to the point of restraint of the paper. This was adopted by various individuals which also included significant refinements and standardization to the method 25,26.

Physical characteristics

While some have been focused on a direct measure of sharpness by cutting various materials, others have evaluated sharpness from a direct inspection of the edge. John Juranitch of Razor's Edge Systems had high magnification shots of sharpened edge taken so to quantify sharpness as did Lee and John Henry of FuriTechnics 27,28, 29. With the increase in availability and low cost of digital photography equipment more work of this type has been contributed to on-line discussion of sharpness as a significant number of knife users are investigating the condition of edges directly under magnification and sharing their results with the internet cutlery community 30,31. While very informative in several regards, such investigations lack a clear numerical representation of of sharpness, a problem solved by Dr. Verhoeven and Dr. Landes who performed similar inspection of sharpened edges under high magnification and used physical measurement of the imperfections in edges to quantify the sharpness32, 33. An example of Landes work is illustrated on the right. A similar approach (under much lower magnification) was applied by Brent Beach and Steve Elliott in studies of planer blades 23, 34. This viewpoint of sharpness is also given by Steve Bottorff in his sharpening text :

Any edge thickness under a few thousands of an inch could be considered sharp. ... Ideally the surfaces would come together to form an edge with zero thickness. --Steve Bottorff35

Summary

Taking sharpness as a measure of how the condition of the very edge influences cutting ability, it is thus dependent on how the cutting is performed, specifically in regards to a blade pushing through a material as opposed to cuts made with a slicing motion. To maximize precision, the material which is cut should cause very low binding and frictional forces. The maximum force or edge length required under a set force to make a cut is taken as the measure of sharpness. Alternatively, quantification of sharpness can be achieved with the edge observed and the irregularities measured under high magnification.

References

1 : Henning Groenzin, Outrageously expensive sharpening setup, Bladeforums.com, 2006

2 : Mike Spinak, Re: "shaving sharp" and "hair-popping sharp", Bladeforums.com, 2003

3 : WadeF, Bladeforums.com, 2006

4 : Alvin Johnston, rec.knives, 1998

5 : Jimmy Fikes, DVD

6 : Dave Sutton, Purpose of a knife, rec.knives, 1997

7 : Mike Swaim, Sharpness checking methods, rec.knives, 1997

8 : Mike Swaim, Review: Custom Cutlery Fillet Knife, rec.knives, 1998

9 : Phil Wilson, private communication (email), 1999

10 : Wayne Goddard, users.ameritech.net

11 : Phil Wilson, private communication (email), 1999

12 : Cliff Stamp, Cutting performance and edge retention of a Buck Zipper, mini-AFCK, Vaquero Grande, Operator and custom from Mel Sorg and Ang Khola khukuri,1999

13 : Cliff Stamp, Cutting ability and edge holding of INFI and M-INFI , CPM-10V and D2, 1999

14 : Cliff Stamp, rec.knives, 1999

15 : Cliff Stamp, Ray Kirk's ABS Journeyman Smith Bowie, 2001

16 : Cliff Stamp, Wildlife Hatchet from Gransfors Bruks, 2001

17 : Cliff Stamp, Ray Kirk test blades in 1084, L6, 52100 and D2, 2001

18 : Cliff Stamp, K2 from Bob Dozier, 2005

19 : Mike Spinak, BladeForums.com, 2002

20 : Nozh2002, How to test sharpness in digits,BladeForums.com, 2005

21 : Ghost Squire,BladeForums.com, 2005

22 : HoB , Joining the thread cutting club...with surprising results,BladeForums.com, 2006

23 : Steve Elliott, Summary of Results, bladetest.infilplane.com.

24 : CATRA,SHARPNESS TESTER, Catra.org.

25 : Dog of War,Using newsprint as a repeatable test for comparing sharpness, BladeForums.com, 2006

26 : gunmike1,Native flat to the stone & pushcuting newsprint vs. yellow pages observations, BladeForums.com, 2006

26 : John Juranitch, Sharpening Secrets of a Pro, Popular Science, Feb, 1977

27 : Leonard Lee, The Complete Book of Sharpening, The Taunton Press, 1995

28 : John Henry, The No Bull Zone

29 : kel_aa, Some homemade micrographs, Bladeforums.com, 2006

30 : yuzuha, Some homemade micrographs, Bladeforums.com, 2006

31 : Dr. J. Verhoeven, Experiments on Knife Sharpening, 2004

32 : Dr. R. Landes, Messerklingen und Stahl, 2. Auflage, Wieland Verlag, Bruckmühl, Germany. Copyright 2006

33 : Brent Beach, Sharpening and Testing Plane Irons, 2003

34 : Steve Bottorff, Sharpening made Easy, Knife World Publications, 2002

Comments

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Written: November, 2006 Copyright (c) 2006 : Cliff Stamp
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