You are here: Edge Retention, Blunting and Wear Tests: http://HowSharpIsIt.com/Edge_Retention_Tests/

Go back to: MAIN PAGE: http://www.HowSharpIsIt.com

 

For an Overview of this test see the Video:

 

(insert video here..)

In our previous tests we were concerned with measuring Apex Edge Sharpness of a blade just after best honing and sharpening has been applied to it. This told us several things and answered certain questions for us: How successful was our sharpening technique:?  To what degree of sharpness is this particular steel and blade geometry capable?  Do I have the right initial degree of sharpness on this blade for my intended use?

The initial sharpness of a knife falls off very rapidly as soon as it contacts real world work. No matter what the advertising for your favorite brand of "never get dull knives" is, the fact of the matter is that no knife material is infinitely hard, and no work piece is infinity soft. It is only under those unobtainable circumstances that your knife will live up to the claims of the huckster on one of those all night shopping network TV infomercials.

Here is a typical chart showing fall off  "slopes" of blades that are tested against abrasive material:

It is all very well and good having a sharp tool with which to start a cut, but not much use if the sharp edge totally degrades and looses sharpness soon into the cut and becomes dull to where it is no longer useful for the work intended.  The question is: Will This Knife Hold an Acceptable Edge Long Enough For Me To Complete My Work?.  We will see if the Jig can help answer that question. . Some operations require higher initial sharpness than others, and some require less initial sharpness but need to hold a lesser edge for a longer period of time. A general purpose Kitchen Chefs knife might not need to be razor sharp for weeks at a time, but during the time period when it is in use (several weeks)  it needs to degrade slowly and maintain a good long term amount of sharpness. On the other hand, a Sushi or Fish Filleting knife might need a very high degree of initial sharpness, but the operator can only tolerate a wear rate that gives him at least several nights or days of cuts (but not weeks as in the case of the Chefs Knife) as it remains very sharp. This is opposed to the same operators requirements for his general purpose Chefs knife. Eventually all tools will become dull or blunt and not be useful for the tasks intended. A kitchen knife might last three to six months before it needs to be re-sharpened, during that time it is being use by the operator "within" the starting and ending points in a area that represents "acceptable sharpness".  It is important to know the 'ending-point' as well as the 'starting-point'  At some time we must decide for each type of knife in use, "What is the metric of dullness for this knife at which time we will call the tool at the end of its life and in need of re-sharpening?"

The rate of tools wear depends on the cutting conditions, the material being cut, and the material of the tools. Knife blades wear for a variety of reasons, such as abrasive particles in the abrasive structure of the work piece, frictional coefficients on the edge face, attrition, diffusion and even chemical reactions all play a role.

It is important to understand, that particularly with the type of small knives that we are likely to be testing with the jig that : Loss of sharpness during cutting is a nonlinear response. (see the above chart)  This is a very important concept to fully understand. As soon as your super keen razor edge contact anything, and we do mean "anything" in the real world then dulling of that blade begins to take place immediately.  The critical question, and the question that the Jig is going  to help us answer is: "how fast and to what degree is that degradation occurring"  If you plot the numbers from testing with the Jig that "fall-off" from initial sharpness all the way to "dullness" the plot will show up as a sloped shaped line that may appear rather flat inside the areas bounded on one side by the very fast starting slope from the point just after its edge was 'very keen'. all the way to the end of the slope where the bade must be considered 'too dull to be a useful tool'    As a user of this knife you might well as the question: -"Can I find a broad enough area within that "slope of degradation" where the knife blade will be able to produce useful work for me for a reasonable amount of time?"  That is an important thing we want to test for.

A general method used to compare two knives for edge retention is to cut a specific material until a given amount of blunting is reached with both knives with the ratio of the material cut determining the performance advantage. For example: "At a certain point in the test of Knife 1 had cut 25 (5) % more material than Knife 2." An alternate method is to cut a given amount of material and make similar statements about the sharpness of both knives at that point, "Knife 1 was 15 (5)% sharper than Knife 2 at a certain point during the the test." The sharpness ratio in general is not as meaningful as the material ratio because usually cutting tools are sharpened after they reach a given state of blunting not after they cut a specific amount of material. However custom knife maker Phil Wilson noted early in his edge retention tests that the rate of blunting slowed significantly as knives continued to be used, he attributed this to the blunting taking place by different means during early and late use.  Due to this nonlinear response it is necessary to specify any point of comparison as : "Cutting until the sharpness was reduced to 10% of optimal, Knife 1 cut 25 (5) % more material than Knife 2."

So how do we set up the Jig to run some blade edge degradation tests?   Here is one possible way to do it. You will, no doubt, think of others. Keep in mind that you will be able to use the Spreadsheet Template  that was specifically created for this type of testing even as you change your abrasive materials to help you draw your charts.

Note: This is described from the standpoint of using the Scotch Tape Media, but other Media can certainly be used.

When to Use This Test:
Use this test when you want to answer the question: "How Fast and at What Rate (Slope) Does my Blade Degrade in Response to Use"

Use after you have freshly sharpened a blade, and the blade is uniform from heel to tip Select out, in advance the type of abrasion material that you will be using. First, We suggest that you start by running a "mild impact resistance test".    Second, Then move on to "cutting mild abrasives, i.e. 20lb copy paper". Other more abrasive tests may not be suitable for and may be outside the limits of the Jig.  For sharp utility type blades, EDC, Kitchen knives, and so forth these two mentioned tests should give you excellent results and indicate representative and typical slopes for your blades.  For heavy chopping blades, machetes, adzes, hatchets, large axes and similar these tests are probably outside of the range of use of the Jig.

How To Perform the Test:
Begin by downloading and printing out a copy of the Field_Data_Collection_Template.pdf

The next step is to mark the blade, using a Sharpie, or other suitable marker, at TWO marks near a  point in the middle  portion of the blade. These two marks should be at least 5mm from each other.  The test occurs in two parts. We will begin with the mild repetitive impact test. This test results from the fact that the contact of the tools head, even at low gram weights on a sharp knife is not entirely a 'non-destructive event"  Each time the tool head contacts the Apex Edge it transfers enough energy to make the steel of the blade react from that energy. How well a steel , absorbs, transfers, deforms or otherwise deals with that inputted energy can tell you a great deal about the nature of the steel and how it was hardened, tempered, or its microstructure.  During this mild impact test we are looking at a plotted slope showing the blade reaction to repeated  multiple hits at the same Apex location.

 

PART 1:
1.) Place the blade on the force scale, clamp securely and using all good practices of data collection (see http://www.howsharpisit.com/Data_Collection/) run 5ea Apex Push Cut tests at the FIRST  position marker. Do not move or readjust the blade position away from the first marker point. On each successive test at that FIRST markers position move the test tool head 1mm to the right and proceed with another data collection point until you have a total of 5 datum for that marker point.

2.) Reload a fresh piece of Tape Media.  Do not move or readjust the blade position away from the first marker point but re-adjust the tool head to begin another series of 5 impacts exactly at that FIRST marker point, again moving the tool head  1mm away from the left hand extreme edge of the test collars tool head with each of the 5 impacts.  

3.) Repeat #1 and #2 at each marker point until you have recorded a total of 75 test results from marker point ONE.

PART 2:
4.) Remove the blade from the Jig and, by hand,  direct push cut ( if necessary, begin the cut using a slicing motion),  as far as possible through the 8 1/2" dimension of a standard piece of 20lb copy paper. Repeat for 5ea push cut slices through the copy paper. Return the Knife to the jig and run 5 test impacts at Marker Position TWO as per instructions in Step#2 above. Record all data.

5.) Repeat, in turn, Steps #4 and then Step#2 above until you have collected 75 data points, You will have used about 15 sheets of paper to perform the tests.  If the blade becomes so dull during the test that it will no longer slice cut or push cut paper then discontinue the test at that point.
 

Data Spread Sheet Template:
Download a copy of:http://www.howsharpisit.com/Edge_Retention_Tests/Template_Degradation_Testing.xls and load it into Microsoft Excel or OpenOffice Spreadsheet. You may obtain a free copy of OpenOffice from: http://www.openoffice.org/download/

To prevent confusion the template has Example Data pre-loaded in it. Type over the Example Data. Turn any unused data points in the template to NULL  value. That is done by pressing your "DELETE KEY" in that spreadsheet cell. Do Not turn the value to 0 (Zero) or that will interfere with the computations!

Do not disturb other columns or data cells on the spreadsheet. Enter your data only in Column B (Bold High Lighted Numbers).  As you complete entry of all data the spread sheet will automatically compute its output and show charts that map your blade. Keep a record of this spreadsheet on your computer for later reference, save it under a unique file name.

 

-end