How sharp is sharp?

[Ken S]

For many years I have had a hobby interest in the history of machine tool technology and measurement. We owe much of our present standard of living and computer age to the nineteenth century pioneers who gave us accurate and repeatable measurement standards.

While mathematician Pierre Vernier lived during the time of the Mayflower pilgrims, the Vernier caliper was not invented until the mid nineteenth century. This, and the micrometer, which was invented around the same time, gave machinists the ability to measure within .001". By the 1890s, with the invention of Johannson measuring blocks in Sweden, we could measure in millionths of an inch.

Today, an engineer can email drawings of parts to machinists throughout the world and expect the parts to fit. The drawings include the acceptable tolerances of size and degree of finish. We have standards for different fits: sliding fits, friction fits; fits for threads, including different fits for the precision or lack of precision of the thread application.

These tolerances can be measured with very sophisticated lab equipment or with tools generally available on the shop floor. Naturally, the shop floor equipment will not have the precision of the lab, but it is usually sufficiently accurate.

Surface roughness can be gaged with sophisticated digital readout equipment with probes. Or, it can be measured on the shop floor by using samples of accurately finished small pieces of metal of known finish roughness.

Do we as knife sharpeners have something which will be more accurate than our fingernails or slicing pieces of paper? How would we communicate to other members of the forum or other sharpeners around the globe how sharp an edge actually is? We do not need several thousand dollars of digital lab equipment. Is there a simple piece of equipment which can produce accurate and repeatable sharpness measurement?

Yes, fortunately, we now have the Brubacher Edge Sharpness Scale (abbreviated BESS). This is an accurate simple standard which measures how much weight is necessary to cut through a polymer thread of known strength with a knife or straight razor edge. This feels very natural to me.

As a telephone installer and repair technician, I routinely measured circuit noise. Listening for noise with only an ear is very primitive. By using test lines and measuring in decibels, I was able to precisely measure and verify if the circuit was within guidelines or needed to be repaired. I was able to do this with a small meter I carried on my shoulder. The BESS system allows knife sharpeners the same precision and repeatability.

I have recently begun working with a BESS machine. I will post my results when I have more experience with it. I believe this advance has much to offer knife sharpeners who work in a home shop instead of a company lab.  Stay tuned.

Ken

[stevebot]

This spring I tool part in a knife sharpening competition. We had use of two commercial testers plus the subjective opinion of a local chef. The results - 3 tests times 3 knives per sharpener - were overwhelming. I quite agree a simple test method is desirable.
I was the contact person for the CATRA tester from the UK and had two weeks use of a CATRA  tester. The CATRA uses a slow motor drive to press the knife into an 8mm rubber rod, which is curved so it parts after cutting to prevent side friction.  WAY too complicated.
The KN-100 sharpness tester is a model of simplicity, using gravity to power the cut, simple test media, and a single number (weight) as the test result.
I saved 10 test samples from the CATRA and I was impressed that they showed excellent correlation between the $160 KN-100 and the $12,000 CATRA.

[Ken S]

The knife edge sharpness tester I have been using is the model KN100 by Edge on Up. This cleverly designed machine tests the sharpness of an edge by determining how much weight is required for the edge to cut through a polymer thread of known strength.

In many ways, this edge tester reminds me of the Tormek. The Tormek can be used to quickly replicate an existing edge to full sharpness.  With a skilled operator, the Tormek can also be utilized to modify edges for individual applications. The versatility of the gouge and drill bit sharpening jigs are examples of this flexibility.

With the KN100 edge tester, it can be set up to quickly decide "pass-fail" for an edge or to acurately determine just how keen or dull the edge is. The testing can easily be repeated and noted using different sharpening methods; during different stages in sharpening; or measuring edge degradation after varying periods of use. As with the Tormek, the limitations are mainly determined by the limitations of the user's skill and imagination.

The unit is very well conceived, designed and machined.The minor amount of assembly is easily finished in a minute or two. Included with "set up" is gathering some components. A digital postal scale is most useful. There are a variety of these available in the $15 to $40 price range. I purchased an American Weigh Scales model PS-25. It was just under $20 on Amazon. It has four modes, Kg and grams; grams; pounds and ounces; and ounces. It works with either two AA batteries or the included AC adaptor.  An important feature to be aware of is the size of the weighing table. The PS-25 has a weighing table of 5 ½" x 7" (140mm x 180mm). The smaller dimension is critical as it allows the KN100 to set on the scale.

Another consideration in choosing a scale in the maximum weight. The PS-25 can weigh up to 25 Kilograms or 55 pounds. This is certainly usable, but it far exceeds the amount needed. Ten pounds is more than enough. The constraint with larger weights is the weight of the calibration weight. The weight used to recalibrate the PS-25 scale is 10 Kilos, or 22 pounds. I made up a satisfactory test weight by combining the milling table from my drill press with a peanut jar partially filled with misc. hardware. It is accurate, but clumsy.

In my opinion, this is the easiest, most efficient way to use the KN100. Just set the KN100 with the knife in place on the weighing table. Push the "tare" button on the scale. The scale subtracts the weight of the KN100 and knife from the measurement. Therefore, the readout is just the measure of the added weight used to cut the polymer thread. No math is needed. The readout is the final measurement.

Small lead shot is the manufacturer's preferred weighing media. BBs, sand or water can also be used. Lead shot or BBs must be used carefully to avoid scattering them. One useful formula is that one liter of water equals one kilogram of weight. I have thought of using water, as I happen to have a collection of graduates left over from my photographic darkroom.

I followed the manual's recommendation of purchasing a pack of 2000 BBs from Walmart. The cost was around ten dollars US. I have numerous empty peanut butter type jars and 35mm film cans.  The smallest plastic peanut butter jar will easily hold all the measuring weight one could use with the KN100 (2000 grams).  I made up and labelled a jar with 375 grams. 375 to 475 grams is the listed weight in the manual for "High End Cutlery (factory edge)". I made up a film can with 100 grams.

I tested my two paring knives. The second knife tested, the knife I rarely use, curt through the polymer test thread with the 375 gram jar. (I placed the jar upside down on the flat lid to mate well with the KN100 surface.) With this test, I knew that the edge, at least the area of the edge I tested, was at least at the top sharpness of factory edges for high end cutlery. Using my "pass-fail" mode, this knife was definitely a pass.

If the 375 gram jar would not cut the polymer, I would have added the 100 gram film can. If these together cut the polymer, I would be within within the range of "factory edge".

My other paring knife, the one I use a lot, required approximately 625 grams to cut the polymer.  According to the chart, 650 – 700 grams is "Typical Knife Edge in Need of Sharpening". I agree. My other paring knife stuck to my thumb nail; this one did not.

1200 – 1400 grams is listed as "This Knife Edge Found in Many Kitchens". 2000 grams is the inner flat edge of a double edge razor or a butter knife.

When I attended elementary school in the 1950s, the grading system was Excellent, Good, Satisfactory, and Unsatisfactory.  I would grade edges which can cut the polymer with less than 375 grams as Excellent. An edge needing 375 to 475 grams woul be Good or Satisfactory. Any edge needing 650 grams or more would be unsatisfactory. For edges duller than 650 grams, we might switch to the ABCDF grading system. The range of dullness from 650 to 2000 grams would give us a whole new letter system of dullness.

In practical terms, anyone sharpening knives need only be concerned with the range from 0 to 475 grams.  Edges duller than that require work.

The KN100 is designed to be accurate within +/- 10 grams of pressure. This standard is more than accurate enough for knife work.

I believe the KN100 is a very useful tool for a knife sharpener. It certinly gives the beginning sharpener a measurable, repeatable standard of what sharp is. It is flexible enough for expert sharpeners. For a sharpening business wishing to provide premium service at premium prices, the test results before and after could be posted on the customer's invoice.

The KN100 sells for $159 US. In Tormek terms, that is a  little more than the three knife jigs and less than a replacement SG-250 grinding wheel. In my opinion, that is a lot of useful firepower for a bargain price.

Ken

[Herman Trivilino]

I saved 10 test samples from the CATRA and I was impressed that they showed excellent correlation between the $160 KN-100 and the $12,000 CATRA.

How did they correlate with the opinion of the local chef?

For me, I see complications. Any meaningful judgement of sharpness depends on how well the knife cuts. And that in turn depends on what you're cutting. And then there's the issue of how well it cuts after you've been using it for a while. A knife that cuts very well for five minutes before its performance declines to mediocre may not be as desirable as a knife that cuts well enough and continues to do so for several hours of heavy use.

The factors that effect these performance issues are the edge angle and the tooth. Sometimes you want a larger edge angle, sometimes a smaller edge angle. Sometimes you want lots of tooth, sometimes you want very little. It all depends on what you're cutting and how much cutting you need to do.

A knife tester that allows for a slicing motion won't give the same results as one that doesn't. When we use knives sometimes we slice and sometimes we chop.  I think it may be too subjective a thing to be measured.

Not everything that counts can be counted!

[Ken S]

This spring I took part in a knife sharpening competition. We had use of two commercial testers plus the subjective opinion of a local chef. The results - 3 tests times 3 knives per sharpener - were overwhelming. I quite agree a simple test method is desirable.

Herman,

I agree that an initial sharpness measurement is not the full story of an edge's usefulness. This would apply whether the sharpness was tested with  KN100, a thumbnail, a piece of paper, or a tomato.However, as stated in the quote, the competition also included being judged by the subjective opinion of a local chef.

I believe the KN100 allows the user the opportunity to measure the sharpness of a given edge at a given time with sufficient accuracy and repeatability. I suggest you download the instruction manual from the edgeonup.com website. If you have trouble downloading it with your browser, email them. They are working on the problem and will email you a pdf. I think you would find the more in depth details fascinating.

The KN100 can test much more than initial sharpness. The manual mentions the measurable difference after five 1/4" (6mm) cuts are made in a standard business card. It discusses the measurable difference after the knife sits overnight (due to minor oxidation). It discusses the measurable differences after using a strop and when using different grits.

I have noticed that some paper works better than other varieties for edge testing. Using one's thumbnail with repeatable accuracy seems scientifically iffy at best. I use these tests all the time as rough indicators. I also use the palm of my hand as a first test for fever. This indicator test is no substitute for a good thermometer.

I would expect an experienced sharpener to have a good knowledge of the effects of varying bevel angles and things like "tooth". I would also expect that sharpener to have a rudimentary knowledge of metalurgy and its effect of the keenness and durability of edges.

I believe the KN100 gives us a much more accurate starting point in being able to produce consistently sharp knife edges. It can also be used to track that sharpness throughout both the sharpening and use processes. It allows us to be able to repeat the process and to share the results meaningfully with others.

I also believe the KN100 is capable of much more in the hands of an experienced sharpener with a solid physics background.

Ken

[stevebot]

Two years ago I prototyped a slicing tester that graphed sharpness along the blade length. It was 100% mechanical, no computer or load sensor, and was made from about $50 of common parts. It worked, but resolution was poor and it required a high resistence test sample to overcome the friction of the cheap slides I used. I intended to refine it, then along came the KN-100 BESS tester.

This year I was doing some difficult slicing and stumbled upon a quick and easy edge holding test method. Now that I am retired I plan to work on that idea - right after I finish the honey-do list and get my sharpening video into production.

[Ken S]

I suggest taking early retirement to allow adequate time for the "honey do" list......

Ken

[Rob]

around 75 years should do it :-)

[SharpOp]

Hi, everyone.  I've been lurking for a while.  I first used a Tormek perhaps 7 years ago and I recently bought a second.  I may need a third soon, as I develop my little part-time retirement business.  It's a truly remarkable machine.

For the past few days, as I redevelop my sharpening chops, I've been playing with my newly-acquired KN-100 (thanks, Steve!).  I have to say that it is proving extremely valuable in comparing knives in the "incoming" queue with the same implements after sharpening, and in comparing the results of various sharpening techniques and procedures.

I understand the reservations Herman has expressed (and the rather harsher criticism I read on Blade Forums), but I think that a quantifiable, repeatable way to measure edge sharpness (even if it is "only" a single-point push test) is a very useful tool for serious sharpeners. 

  ~Doug

P.S.  Like Ken, I spent a long time in the telecommunications world and I grew rather attached to accepted, communicable, measurement standards.  Complaints about noisy circuits, for instance, were much more meaningful when accompanied by signal-to-noise measurements and references to agreed standards for the same.

[Elden]

   Welcome to the forum, Doug. Thank for your comments in regard to the sharpness tester. I am sure it would be advantageous for comparison purposes.
   I imagine some of us hate the thought of laying down that much money when we already know the knives we sharpen are sharp. I am not knocking the tester or those that are willing to lay down the money for one. As I mentioned previously, a person could fabricate a working unit.
   Could it possibly be that I don't want to face a real test of sharpness? Surely not!?
   

[Ken S]

Welcome to the active forum, Doug. It's nice to have another phoney aboard.

We live in a culture of measurements. One of the unfortunate words is "POTS" (plain old telephone service). The somewhat derogatory term does not do justice to the rather sophisticated set of service electric measurements to comply with the standards required by the regulating agencies. We need these standards in many areas because expertise is not universal. Elden, you have many years of sharpening experience and some high level training (You mention your Belsaw-Foley training.). What a beginner might consider "sharp" might not seem that way to someone of your experience.

What the BESS scale does is help level the playing field. It will not in itself make a beginner into an old hand. However, it will provide an objective, repeatable measurement of sharpness. I think you could pass a test of real sharpness, however, if it was me, I would do the initial tests where no one else would see them!

The KN100 costs $159 US. For those of us old enough to remember when a very usable second hand car could be had for a hundred dollars, that seems like a lot of money. In more recent times, my present car and former truck, both economy models purchased second hand, each cost more than my first house. Today one could purchase the KN100 with a Tormek Small Knife Holder for about the same cost as a replacement SG-250. Unfortunately, wage increases have been flat for many years.

I do not believe everyone needs the equipment to test edge sharpness. The tried and true methods of cutting paper, thumbnail testing, and arm hair shaving work as well as they ever did. What has changed is the level of possible sharpness testing. The KN100 makes this not only possible, but practical for many sharpeners.

Ken

[SharpOp]

Thanks, both of you, for the friendly welcome.

Elden, I certainly understand that not everyone who sharpens edges needs or wants an edge tester.  Of course you know what is and isn't sharp, and you know a lot more than that about edges.  On the other hand, I expect you know a lot of folks who have no bloody idea ;^) what "sharp" means -- and they want "sharp" knives even though they haven't a clue.

I think it's possible that the BESS scale may help in educating those people and in comparing knife edges, at least to some extent.

[Elden]

   This morning I read through the EST-KN100 Edge Sharpness Tester operating instruction manual. It is available to be read online at:

http://www.edgeonup.com/eou_web_pagelarge_008.htm

Suffice it to say, I am very impressed. The product sounds great, but the manual is an excellent work. There is a little bit of redundancy in cautionary statements, but they are validly pertinent.

   The section "A little more on wire edge removal" was extremely interesting. It starts on page 41 of the manual. Yes you read that correctly, PAGE 41! There are 46 pages in total that are easily understood.

   No, I have not bought one. However, I am very impressed. Thanks for bringing the EST-KN100 to our attention, Steve B.

[Jan]

   This morning I read through the EST-KN100 Edge Sharpness Tester operating instruction manual. It is available to be read online at:

http://www.edgeonup.com/eou_web_pagelarge_008.htm

Suffice it to say, I am very impressed. The product sounds great, but the manual is an excellent work. There is a little bit of redundancy in cautionary statements, but they are validly pertinent.

   The section "A little more on wire edge removal" was extremely interesting. It starts on page 41 of the manual. Yes you read that correctly, PAGE 41! There are 46 pages in total that are easily understood.

   No, I have not bought one. However, I am very impressed. Thanks for bringing it to our attention, Steve B.

I have found the page 41 also very interesting  , especially the term metal memory.

I know the term memory metal, which is an alloy with a shape memory, which returns an object to its original shape, when heated (an example from the nanoworld  is Nitinol = nickel-titanium).

But Steve B. coined the term metal memory to describe the situation when the stropped edge returns to its original (pre-stropped) geometry after several hours.

Really interesting mention and for me worth of further investigations.

Jan

[Ken S]

I agree, Jan. This sounds like an interesting topic.

Ken