Introduction

Hi There

Over the years I have written a web site on sharpening at Brent's Sharpening pages. I started that site in 2002 when I started to investigate various sharpening systems. I added to the site as I made discoveries. The result - a large tangle of pages with no overall plan. There is duplication and contradiction in those pages.

I was lucky back then to have started out with a copy of Leonard Lee's The Complete Guide to Sharpening. Published in 1996, this is still one of the few very good books on sharpening hand tools (and just about every other edge tool).

In 1996 Leonard Lee recommended 3M microabrasives and microbevels. Both were little known within hand tool groups at a time. After all my testing I still use those abrasives and have come to understand why microbevels are an essential part of sharpening.

Just Do It?

Over the years I have taken a lot of abuse in various woodworking fora. One of the main complaints is that I make sharpening hard to understand. That in fact sharpening is easy. That people should just use some quick and dirty sharpening method and get on with the woodworking (or other edge tool using craft).

The web site has 87 pages with a total of 1.56MB of html. It has 691 images. Yes, it is a complicated web site.

The sharpening system it describes has one serious flaw - you cannot just buy it. You have to make it. You have to buy 3M abrasives, sheets of glass, backing boards, some nuts and bolts. Worse, you have to make your own sharpening jig. 

Several people have gone to the trouble over the years and had great results. Many more people have not gone to the trouble and have only bad things to say about my results. One person took the time to duplicate my experiments - Steve Elliott. I recommend his site.

You can collect the abrasives, glass, etc, and build the jig and then sharpening is actually very easy. If you watch the Youtube video of a typical sharpening session, you will see that it takes me less than 3 minutes to sharpen a plane iron. 

I will explain in this page why the resulting edge is excellent. It has exactly the desired geometry and why the steel at the edge is minimally affected by the sharpening process.

The Explanation

I know that most of you are not interested in why this particular process, with these abrasives, produces a superior edge. You just want a sharpening system that works and is affordable.

However, I hope that some of you are interested in understanding how abrasives affect steel and how, given that knowledge, you can sharpen edge tools quickly and effectively. I am hoping that a few intrepid souls will read the following material, will do some experimentation on their own, and will report their results here.

What Leonard Lee Did Not Know

After several years of sharpening and testing edge tools a discovered a book called 
Metallographic Polishing by Mechanical Methods, by Leonard Samuels. This book was in the fourth edition by the time I ran across the second edition. Eventually I found and read the third edition and then the fourth edition. 

Metallography is the engineering discipline that examines metals (and later other materials) to determine their underlying characteristics. The motivation was usually to determine why a widget failed by looking for flaws in the metal. It turns out that this study required that the engineers first completely understood how abrasives interact with metals.

Abrasion using coarse abrasives leaves scratches on the surface of the metal that prevent inspection of the underlying metal for flaws. All you can see is the surface scratches. Back in the 1860s, the first metallographer was able to polish metals then etch the metals with various acids, to discover the hidden structures of steel. His methods were time consuming - he hired a person to polish for many hours with very fine abrasives.

Over 100 years, metallographers understand the polishing process but that knowledge is not common outside the profession. Had Leonard Lee had access to this book, I am certain he would have understood how it applied to sharpening edge tools and how important the results from Metallography are.

The connection between metallography and hand tool sharpening is not obvious. The machines used by metallographers to polish metal samples cannot be used to sharpen hand tools.  Metallographers are usually given a broken casting and asked to determine why the part failed. They are never given a hand tool and asked by the edge dulls so quickly.

The connection is that metallographers spent 140 years figuring out how to remove all the effects of abrasion from the metals they wanted to study. If specimen preparation added scratches to the surface, then those abrasion artifacts obscured the flaw in the specimen. It is essential that the surface of the specimen be free of artifacts added during specimen preparation.

They learned how, by using a series of finer and finer abrasives, they could polish out the artifacts, leaving only the underlying crystal structure.

Sharpening is actually similar to metallographic sample preparation. You begin with a hand tool that has the wrong profile - it has a dull profile rather than a sharp profile. In going from the dull profile to the sharp profile, metal must be removed. Most sharpening systems leave the resulting edge damaged by abrasion artifact. That is, changes to the metal that occur during the profiling process are still there in the sharpened tool. These web pages will show you how to use the knowledge gained by metallographers to produce a tool with the correct profile that has no abrasion artifacts.

Having mastered specimen preparation, metallographers went on to study the internal structure of the metals. They looked not just at broken parts. They also looked at steel in the various stages of manufacture.  The steel in the original ingot. The steel after the ingot had been rolled to sheets having as little as 5% of the original ingot thickness (compression artifacts). The steel after it had been drawn into wire, 20 times its original length (tension artifacts). The steel after various heat treatments.

Finally, they looked at steel that had been abraded by various types of abrasives.

During this latter study they learned that abrasion creates artifacts within the metal - not just on the surface. Abrasion changes the fundamental crystal structure - disrupting the atomic bonds that make the metal hard and durable. The changes that happen during abrasion are exactly the same as the changes that happen during rolling and drawing.

I am going to take interested readers through 12 points from this book. I expect that having read these 12 points, those interested readers will understand how this information is essential knowledge for the design of sharpening systems. I hope a few of them will build a jig and undertake some testing and report their results.

Of course, understanding those 12 points is not essential for those who simply want to use a sharpening system. Even I do not review those points each time I sharpen a plane blade, or a knife, or a straight razor.

1 The Vickers Indenter

Vickers Indenter

There are several commonly used methods for determining the hardness of a metal. Each method uses a very hard indenter under a specified load to dent the metal. The dent is measured and the hardness evaluated using a formula based on the applied load and the dent measurements.

It happens that the indenter used in the Vickers system looks most like a flattish abrasive grit. The indenter is a diamond. The angle at the tip is 136 degrees. 

It is a fundamental property of metals that if you force this indenter into the surface, then the ratio of the applied force divided by the "projected area" of the indent is a constant - The Vickers hardness. The harder you press the deeper the indent but the ratio of the force divided by the projected area remains the same.

Indenter Profiles

The depth of the indent depends on the shape of the diamond and the force. In this diagram from Samuels, Figure 3.12, he shows 4 different shaped indenters. 

The first indenter (abrasive grit) has a dull point and makes only a very shallow dent before it reaches the dent area for the applied force. The second indenter has a sharper point resulting in a deeper dent, but with the same dent area. The third has the same angle as the second, but the tip is gone (grit particle has shattered) so the dent is shallower, but the dent area is again the same. The fourth has a flat equal or greater than the appropriate dent area, so no dent occurs.

Because each grit particle is an indenter, the depth and shape of the indent it makes depends on the shape of the grit particle. It will penetrate until the area of the dent divided into the applied force equals the hardness.

2 One Grit vs Two Grits vs 10,000 Grits

The implications are surprising.  This drawing from Samuels makes the first point: it is the shape of the contacting point that determines the depth of the indent, not the size of the grit.

Why then do coarse abrasive leave deeper scratches than fine abrasives?

The answer is that typically finer abrasives have more grit particles per unit area than coarse abrasives. A 1 micron grit particle has average diameter 1 micron. A 10-micron grit particle has average diameter 10-microns. There are therefore 10 * 10 = 100 1-micron grit particles in the area occupied by a single 10-micron grit particle.

Consider a 100 micron by 100 micron area of two abrasive sheets - one 1 micron, one 10 micron. Assume the 1-micron abrasive has 100 * 100 = 10,000 1-micron grits, the 10 micron abrasive has 10 * 10 = 100 10-micron grits.

Because the dent area is a function of the force per grit particle, and since the 1-micron abrasive has 100 times as many particles in contact, each 1-micron dent will be 1-one-hundredth of the area of the 10-micron dents.

Not all of the grit particles will have exactly the same height, so not all will make contact. This is true for both the 1-micron and the 10-micron abrasive. As larger particles wear down quickly (tips broken off, particles dislodged) on both sheets during the break-in period the number of contact points will rise. The number of grits making contact with the 1-micron abrasive will remain 100 times the number making contact with the 10-micron abrasive.

3 A few large Grits in a Fine Abrasive

What happens if we have 1% 10-micron grits in a 1-micron abrasive. A little tricky here, but consider again a 100 micron by 100 micron area. Each 10-micron grit occupies 100 square microns. Each 1-micron grit occupies 1 square micron. Denote by n the number of 10-micron grits. There are 99*n 1-micron grits. The n 10-micron grits occupy 10*10*n square microns. The 99*n 1-micron grits occupy 1 * 1 * 99*n square microns. The total is 199*n square microns in the 100*100 square micron space. So there are n = 10,000 / 199 = 50.25 10-micron grits in the area, 4974 1-micron grits.

At the very least, the number of grit particles per 100 square microns is just over half what it would be in a pure 1-micron abrasive. Thus, at the very least, the applied force per grit particle is twice what it would be for a pure 1-micron abrasive. The area of each indent is at least twice what it would be in the pure 1-micron abrasive.

In practice, the 10-micron grits would dominate - would stand above the 1-micron grits. Rather than 10,000 abrasive grits in contact, there would be 50 abrasive grits in contact. Each indent would then be 200 times as large, with a corresponding increase in indent depth.

15 micron abrasive, 150X

Interesting. The mix of 10-micron and 1-micron grit particles produces an abrasive that is 200 times  more aggressive than a pure 1-micron abrasive.

It is also interesting that the mix of 10-micron and 1-micron grit particles produces an abrasive that is 2 times more aggressive than a pure 10-micron abrasive - there are only 50 grit particles making contact per 100 by 100 micron area, rather than the 100 we would have in a pure 10-micron abrasive. 

Adding 99% 1 micron particles to a 10 micron abrasive changes the effective grit size to 14-micron (10 * square root of 2). It does not reduce the effective grit size, it increases the effective grit size.

The photo on the right is of 3M 15-micron abrasive. Notice that the particle size is very uniform. By digitally magnifying the this image and counting grit particles in a small area in the middle, I estimate that there are over 20,000 grit particles in this image.

4 Sparse Grits mean Deep Scratches

There are other sparse abrasives.

Norton 3X, 220 grit, 150X

Open coat abrasives - used in drywall sanding - typically have sparse grit particles to allow more room for the abraded material to collect on the surface of the abrasive. 

Norton 3X abrasives - used for paint stripping, sanding and finishing on composites and fiberglass - are very sparse.

This picture of 220 Norton 3X open coat shows how sparse the grit particles are. 220 grit corresponds to 60 microns.  Filling the open space with 1-micron grits would not change the fact that this is sparse 220 grit.

Compare the 220 grit Norton 3X open coat abrasive on the right here (fewer than 40 grit particles) to the 15 micron 3M abrasive above (20,000 grit particles). The lower grit particle density and smaller grit particle diameter means that the 3m 15-micron has 500 times as many grit particles in contact with the tool. That means that the each dent area is 500 times as large with the Norton 3X abrasive.

Diamond abrasives, used in sharpening, also typically have fewer grit particles per unit area than other abrasives.

It is not the nominal grit particle size that determines the dent area. It is the number of grit particles per unit area.