sharp/dull blade drawing The geometry of Honing small map
Finest abrasives.
Microbevels front and back.
Use a jig.
Copyright (c) 2002-12, Brent Beach

Contents

The geometry of Honing

The sharpness of any tool is defined by the geometry of the edge - the included angle at the edge. The smaller the angle the sharper the tool. Using good abrasives you can bring any woodworking tool to the desired geometry - the expected sharpness. This page is a discussion of initial sharpness.

The durability of an edge is determined by the geometry of the tool - the included angle at the edge. The larger the included angle, the more durable the edge.

The included angle we use balances sharpness with durability. Having selected an included angle, your sharpening system should use the best abrasives and hold the tool at exactly the angle you selected during honing. In my view this means you must use a jig.

Why use a jig?

sharp, dull edge To use a jig, or not to use a jig? A no brainer really.

This drawing is a profile view of a dull plane blade in working position. It shows the last 0.01" of the blade. The third microbevel, and all blade wear, are in this very small region of the blade. The drawing is explained fully here.

The outer black line is an ideal cutting edge profile - perfectly flat bevels front and back at the desired included angle. The inner red line is the profile of a worn blade - showing the upper and lower wear bevels. The dimensions are based on micrographs of a plane blade in its just sharpened state before use, and its dull state after testing.

You cannot start with a better profile than the black profile. It represents the ideal cutting edge shape given the included angle you have chosen. As you work, the profile gradually morphs from the ideal sharp profile to the usual dull profile. During this time you are getting good results, but it is requiring a little more effort. However, you can use a blade with almost any intermediate state.

If you use a jig, use microbevels, and hone front and back, then you will always stand a very good chance of turning your dull blade into an ideal sharp blade.

If you use a jig, use microbevels, but do not hone the back, you will get a usable edge, combining a flat front bevel with a rounded back wear bevel. It will work, but will not be nearly as good.

If you do not use a jig, you will get some intermediate result (one hopes it won't be worse than the dull blade profile, but you are depending on luck so worse could happen). This is why using a jig is a no brainer - you cannot get a better result, you will almost certainly get a worse result.

Unfortunately, almost any profile between the ideal and the dull will still cut wood. So, all those woodworkers who don't need jigs, who just touch up the edge now and then, are working all the time with blade profiles closer to the dull end of the range than the sharp end. As a result, they regularly have to take time out from woodworking and touch up their edges. Touch up in reality means go from a noticeably dull blade to a not quite as noticeably dull blade. They spend more time "touching up the edge" than I do sharpening. It is my view that a freehand sharpener decides that freehand sharpening works when they have sufficiently mastered their tools that they are able to produce acceptable work with dull tools. They don't learn to freehand sharpen, they learn to use dull tools.

Freehand - Ian Kirby style

I have a large collection of explanations of the stupidity of using jigs - many quite amusing. None make any sense in light of the physics of the problem. Arguments are made that with practice you can freehand hone - you can turn your body into a jig. One of the most famous proponents of freehand honing is Ian Kirby. His book Sharpening with Water Stones gives precise directions on how to stand, the height of the working surface, hand position on the blade, all in an effort to turn the human body into an inferior jig. He recommends honing the entire primary bevel on a 1200 grit water-stone until you feel the burr (wire edge). This makes no sense, since 99.9% of this bevel will never touch the wood.

Freehand - Hollow grind style

As finer and finer water stones became available the futility of honing the entire primary could no longer be ignored. Having the finest grit is of course important - from a promotional perspective. Knowing how to use it effectively not so important, it would appear. You must use microbevels with these very fine abrasives because they remove metal so slowly. You need a jig to use microbevels. All jigs available then rolled on the abrasive surface. Jigs rolling on these soft water stones ruin the stone surface. Had I invented my jig and stone vise earlier, there would have been no problem. Absent my jigs, promoters of these stones made a virtue out of a necessity. They used the hollow grind as a jig.

Rather than using different grind and hone angles, they grind at the honing angle using a wheel. This produces a hollow grind. When you rest the blade on the stone with both the front and back of the hollow making contact, the hollow becomes the honing jig. Problem solved!

Not solved! Consider what happens when you are using the finest abrasive. You are honing the full bevel (on both sides of the hollow) left by the previous abrasive. Your honing time on this abrasive depends on having both of those surfaces a narrow as possible. Using microbevels, when you start with the finest abrasive you begin with a bevel of zero width and stop when the microbevel is about 0.005" wide. In the hollow ground case, each bevel begins at over 5 times this wide (remember, we are dealing with the last honing step, after the coarser abrasives have widened the bevels off the grinder). To remove as much metal you would have to hone 10 times as long. (In fact, you have to work far longer, since microbevels remove most of the metal right at the edge and honing a hollow grind removes the same depth over the all of both bevels.)

You could partially solve this problem by using a bunch more water stones with intermediate grits. By the time you got to the final very fine grit the bevels would be so wide you would be wasting your time.

People try to make the honed bevels as narrow as possible during the grinding step. This means removing all of the wear bevel at the edge (what I call grinding through the edge). This is not a good idea - grinding through the edge fractures the metal at the edge.

Because grinding is an extra step and takes time, people try to get several honings from a single grind. The honed bevels are too wide on the first honing - on subsequent honings you may as well omit your finest (and most expensive stone) for all the good it does. Or you could hone on that ultra fine water-stone for 5 or 10 minutes.

And then, when you are all finished, you have no back bevel, so your blade is only half sharp.

When is it time to sharpen?

The sharp edge in this standard diagram can actually be visually distinguished from the dull edge. If you hold the blade, even in the plane, in the right position relative to a bright light, light will reflect off a dull edge, but not off a sharp edge. This is very clear when you see it, but very difficult to actually photograph.

What is the best sharpening system?

I don't know what the best sharpening system is, if by best you mean the best of all time. Some sharpening system may come along in the future that is better than anything available today. Rather than talk about best, lets think about how we can compare two sharpening systems to decide which is better. Rather than use two particular systems, I will look at the factors that determine edge quality. If we evaluate those factors for any two systems, we can decide which system is the better of the two.

After 9 years of sharpening and testing plane irons, followed by reading many books on Metallography and Metalurgy, I have a reached a number of conclusions. While these conclusions apply to sharpening of any edge tool, most of my experience is with plane irons.

Abrasive Grit Size Matters

deformation This diagram, from Samuels - Metallographic Polishing by Mechanical Methods, shows how abrasive affects steel. Samuels uses different terminology for the various phases - he separates honing into abrasion and polishing. Abrasion uses larger grits with hard abrasive backing. Polishing uses smaller grits with soft abrasive backing.

For each grit size, the diagram shows the relative depths of the two kinds of fracturing - shear and deformation. The diagram also shows the relative rate of metal removal.

While the depth of teh shear deformation layer is about the same as the scratch depth, the depth of the layer of significant deformation is several times that of the scratch depth. This means that a single large grit that causes a deep scratch produces a layer of significant deformation well down into the tool.

Notice that both the depth of deformation and the metal removal rate (during grinding, abrasion, polishing) depend on grit size.

Deformation is the term Samuels uses for metal in which the crystal structure has been disrupted in some way. Deformation makes metal less wear resistant. In use, the deformed metal would wear away first, followed by slower wear of the underlying metal.

If we use a tool straight from the grinding phase, rapid wear of the deformed metal leaves a dull tool. As we progress through the abrasion phases, the tool has less deformed metal. If we used the tool after any one of these steps, after wear of the deformed metal the tool would be less dull. You can see that the polishing steps leave a shallow shear deformed layer. Wear of this layer during use leaves the tool in very nearly the sharp state.

The First Metallographer

Henry Clifton Sorby Metallography, the scientific study of the structure of metals, dates from around 1860 when Henry Sorby did his pioneering work on the structure of steel and iron. He developed a multi-stage abrasion technique followed by acid etching that revealed the true nature of steel. Sorby actually used abrasives on glass - he originated Scary Sharp for use with metals.

Sorby developed these sample preparation techniques while studying the structure of rocks and minerals. He used the same techniques on metals but no one was interested so he went back to studying minerals, including meteorites. Serious work on metals did not actually really get going until the 1930s.

The depth of the deformation layer was not known by Sorby. It was not completely understood until the 1950s. What Sorby did know was that if he used a series of grits, each finer than the previous, and polished long enough, he was able to get a surface free of scratches. Sorby used only naturally occurring abrasives, learning from the Jewellery trade which abrasives produced the smoothest surface.

Sorby did not understand enough about his technique to describe it well enough that others could duplicate his results. In fact, it was not until the whole process was redeveloped in the 1930s that people were able to reliably remove the deformed layers.

The Metallography Process

Referring back to the chart above, you can see the steps used by metallographers in preparing a sample for inspection.

The grinding step for them involves cutting a bit of the metal object to be examined from the larger piece - perhaps a casting. They use all the usual metal cutting machines - band saws, cut off wheels. The abrasives here are quite coarse - comparable in fact to grinding wheels and coarse grinding belts.

Having cut off a representative piece, they usually embed the sample in a suitable casting - epoxy or other suitable material - before starting the abrasion phase.

For the abrasion phase, they mount the casting in a machine which holds it above a rotating abrasive disk while applying the desired pressure. Because they are starting with a very deep deformation layer they work through three abrasives ending up where I start - at about 15 microns.

The polishing phase uses the same machine but different abrasives. It usually involves two abrasives, but may involve a third extremely fine abrasive for some samples. These two polishing steps correspond to the 5 micron and 0.5 micron steps I use.

In all steps the abrasive is continually lubricated with a wetting agent which keeps the filings from collecting on the abrasive under the sample.

Metallographers finish with an etching step, using a weak acid, which increases the contract between the various surface components when viewed with a light microscope.

Grind only away from the edge

Grinding Tips
Some of the time, in exceptional cases, you will have to grind the edge. If you knock a chip out of the edge for example. Or if you want to change the edge profile. See shaping the edge.

Most of the time, when you are sharpening a dull blade to the same profile, you should grind the primary up to the edge, but leave a very narrow bit of the old honed/wear bevels. These pages contain several examples of this technique - see this discussion of grinding and honing a new plane iron for pictures of the primary bevel as I grind up to the old edge.

No deformed layer

Given this quick overview of metal deformation, what are the consequences for sharpening? How does this knowledge allow us to compare two sharpening systems?

The first consequence relates to very coarse abrasives and the tool edge. A tool which has been ground through the edge will have a deep deformation layer at the edge. Unless this entire deep layer is removed the tool will be less durable.

My testing uncovered another interesting fact - all of the wear on a plane iron during use occurs within 0.01" of the edge. This means that grinding farther than 0.01" from the edge has no effect on edge durability.

This idea of leaving a bit of the worn edge is not new with me. As early as 1870, writers were saying "unless the iron be notched, it is advisable to avoid grinding it to an absolute edge". (Handbook for the Artisans, 1870).

Woodturners
One group of people who sharpen a lot and use their tools directly from the grinder are wood turners. In order to skip the honing step most turners have moved from high carbon steel tools to high speed steel tools.

This practice avoids two of the problems with grinding through the edge. First, high speed steels do not lose their temper during grinding unless they are severely abused. Second, with wood turning the exact shape of the edge is not crucial - the turner can adjust for small shape errors.

This practice does not avoid the third problem with grinding through the edge - the deformed metal at the edge. Turners would greatly increase the time between sharpenings if they left the absolute edge and added a honing process involving a series of abrasives and microbevels.

Although artisans knew that grinding to an absolute edge was not a good idea - they probably observed that the resulting tool was less durable - the explanation outlined above was only uncovered by metallographers in the last 50 years.

If you leave the wear bevel and a bit of the last honed bevel, the metal at the edge will have the wrong shape but will have no deformed layer.

Why the wrong shape? Because the wear bevel has a rounded side profile.

Why no deformed layer? While you use the tool, the wood is slowly abrading the tool producing the rounded edge. As an abrasive, wood acts like a sub-micron abrasive, producing almost no deformed metal.

Retain edge shape

Aside from leaving unfractured metal at the edge, leaving a bit of the old edge has a second advantage. Think about preparing a piece of wood for a project. You draw a line on the wood and cut to the line. You plane to the line. You chisel to the line. Once you go past the line you have no guide.

If you grind through the absolute edge you no longer have any visual guide to a correct edge shape. You will quickly get a convex, concave, or slanted edge (or all three across a wide blade) edge. You are then forced to buy elaborate grinding jigs that try to retain the correct edge shape.

Retain steel temper at the edge

The third advantage of retaining the absolute edge is it reduces the chances of over heating the metal at the edge. Powered grinders (wheel or belt) can overheat the tool. Water cooled powered grinders won't. Hand grinding on a coarse silicon carbide bench stone won't. [Even up to the 1940s in London, England, most edge tools were hand ground. Back then they used mostly natural abrasive oil stones.]

The closer you grind to the edge, the thinner the metal in contact with the wheel or belt. The thinner the metal, the less the ability of the metal to carry away the grinding heat and the higher the chance of drawing the temper.

It seems like a small thing - retaining the old wear bevel and a bit of the old honed bevel - but it makes a big difference in edge durability for these three reasos.

A sharpening system in which the grinding step retains the absolute edge is better than a sharpening system in which the grinding step removes the absolute edge.
  1. No deformed layer at the edge.
  2. No change in edge profile.
  3. No loss of temper from over heating.

Hone using a series of abrasives, ending with a sub-micron abrasive

deformation Looking at the chart by Samuels again, and having learned from the depth of deformed metal resulting from the use of coarse abrasives that we should not grind through the edge, we not turn to the finer abrasives.

Our goal is to produce and edge with as little deformed metal as possible. It is clear from this diagram that:

  • the finest abrasives do the least damage to the metal, and
  • the finest abrasives remove metal very slowly.

We would like to use only the 0.5 micron abrasive, but it would take too long. So, we begin with the 15 micron abrasive knowing that we will be introducing a deformed layer. Then we move to the 5 micron abrasive, removing most of the deformed layer left by the 15 micron, but leaving behind a much thinner deformed layer. Finally, we use the 0.5 micron abrasive.

If you want to achieve these results, you cannot use natural oil or water stones. No natural stones are less than about 9 microns with any uniformity. Yes, they may have some particles less than 9 microns, but they also have many more particles that are more than 9 microns. The deformed layed is produced whenever there are large abrasive particles present.

Further, you cannot use any synthetic oil or water stone, or stropping compound which contains large particles. This means, for exmaple, that stropping using the Lee Valley green abrasive crayon will not produce the desired final result. There are pure Chromium Oxide powders available that have a uniform composition. You could use those. If you do, then you must use also microbevels. A pure Chromium oxide abrasive will cut too slowly to be useful if you use them on anything other than a very small area at the edge (see next part).

If you use only micron-graded abrasives (as opposed to a proprietary grading system which allows a much wider range of grit sizes) you can avoid misunderstandings about what grits are present in your abrasive.

A sharpening system that uses a series of micron-graded abrasives, beginning with 15 micron, then around 5 micron, then around 1 micron, will produce a better edge than a sharpening system which uses a substantially different abrasive sequence.

Hone using microbevels

deformation Ah, the Samuels chart again.

We know to avoid coarse grits at the edge, we know to use a series of successively finer abrasives, what can we still learn from this chart?

This time we concentrate on material removal rate. The removal rates shown in this chart are relative rates, intended to show how much more slowly the finer abrasives remove metal. The chart is based on actual experiments conducted in Samuels' labratory and in other laboratories. They are not made up numbers like those commonly found in vendor ads.

The most important thing you can learn about material removal is that this chart does not lie. If someone sells you an aluminum oxide abrasive that they grade at 1 micron which removes metal at a 20 micron pace, then the abrasive is a 20 micron abrasive. There is no magic. Samuels tested all know abrasives on many different materials. Other researchers have replicated his results. Bottom line - a 30,000 grit stone that cuts like a 1,000 grit stone is a 1,000 grit stone. A 0.5 micron honing compound that cuts like a 5 micron honing compound is a 5 micron honing compound.

Given this reality - finer abrasives remove metal slowly - the importance of microbevels is clear. With microbevels, achieved by slightly increasing the honing angle at each step, we begin the 5 micron and 0.5 micron honing steps with the tool resting on the edge left by the previous abrasive. Even though the 5 micron abrasive removes metal slowly, because the contact area between the tool and the abrasive is so small (initially nearly 0), we quickly produce a narrow microbevel. Further, at the edge where it counts, the depth of metal removal is quite deep while back from the edge where metal deformation does not matter, depth of metal removal is quite shallow. The microbevel achieves quick results because it concentrates metal remove in the area where metal removal is necessary.

Again, when we increase the angle slightly and switch to the 0.5 micron abrasive, we begin with a contact area that is nearly 0. For this reason, we much begin with a pull action - a push action would cut the abrasive. Even with this extremely fine abrasive, the extremely small contact area means that we quickly produce a microbevel that is over 0.01" wide - the full width of the contact area between the tool and the wood. Again, the deepest metal removal is at the edge, the shallowest is away from the edge.

A sharpening system that increases the honing angle slightly for each finer abrasive, will produce a better edge than a sharpening system which use each abrasive at the same angle.

Hone both sides of the tool

Using a tool results in wear on both faces at the edge. The bevels page has many images of worn edges. Sharpening a tool requires removing the wear bevel on both sides, each time you sharpen.

A sharpening system that hones both faces at the edge is better than a sharpening system that hones only one edge.

About the jig.

This jig is so simple that people often underestimate its precision. Here are some questions I have been asked, and my answers. As well, some questions and answers on related topics.

  1. What is the best jig for plane blades?

    There is no question that my simple wooden jig is the best jig for honing plane irons.

    1. It enables precise, repeatable angle setting, to any angle.
    2. It supports back bevels as an integral part of honing. Back bevels produce better edges.
    3. It holds the blade firmly, with no slipping.
    4. It holds the blade flat -- side gripping jigs can bend the blade.
    5. In conjunction with two wooden slips, it permits the quick use of microbevels on the front and back of the blade. Microbevels greatly reduce the effort (the super fine abrasives work right at the edge) and improve the quality of the edge.


  2. Where can I get the hardware?

    I found this company on the net that sells the T-nuts and machine screws at pretty good prices (less than I have paid for them!). If you cannot source them at a local hardware store, or special order from a local store, this might be a good bet. I have not used them - just found them when answering a question. If you use them, let me know how it worked out.

  3. How fast does the jig wear out?
    They wear very slowly. This is a picture of the edge of my jig (about 3 years old) at 10X using the microscope. The jig slides on glass, no lubricant, wood on glass. The worn side is on the right.

    The jig is hard eastern maple - a wood that is very good for making jigs that you use a lot. It is hard enough that it wears slowly, yet not brittle. I made a few jigs out of tropical hardwoods. The wood splits much too easily.

    Having trouble seeing any "wear bevel"? Well its there, just hard to see.

    		jig


  4. Why does the blade thickness affect the angle?

    The table of blade extensions has different extensions depending on the thickness of the iron. Why does iron thickness matter?

    Think of the triangle formed during sharpening. Two of the points are the plane iron edge, where the back face of the plane iron meets the front bevel, and the front edge of the jig. The third point in the triangle is where the back face of the plane iron meets the jig. So, the length of this side of the triangle is the sum of the height of the jig and the thickness of the blade.

  5. Can I use the extension tables with my Eclipse/Veritas jig?

    At your peril. Any jig with a roller has a geometry that changes with the angle you are putting on the plane iron. My jig slides - its roller has diameter near zero. The larger the roller the greater the change in the lengths of the other two sides of the triangle.

  6. Why do the jigs come in different sizes?

    The weakest link in the jig is the short jaw. If you make the jig a lot wider than the iron and tighten the bolts too much, the short jaw bends. This can change the geometry (not that big a deal) or break the jig.

    A second problem is the thickness of some irons. I have irons that vary from 0.07" (blade for Parplus planes) to Steve Knight's 1/4" irons. The t-nuts that capture the bolts cannot handle this variation in thickness.

    A third problem is short irons. I like to be able to use as much of the abrasive as possible. This means gripping the iron well up the blade and using a tall jig. However, many back plane irons are so short that a tall jig is not possible. The Mujingfang (an exceptional plane which uses a short High Speed Steel blade) plane has a very short iron.

    I have also had to build special purpose jigs for Japanese irons - to compensate for shortness and for the tapered iron.

  7. What about slip thicknesses?

    The slips increase the honing angle just enough to ensure that the microbevel removes all the scratches left by the previous abrasive.

    My standard slips are 0.06" and 0.10" thick. I made these slips for use with my first jig. That jig had a 1.5" large jaw. If you make jigs with larger jaws you should resize the slips as well. Experiment using the extension calculator to determine appropriate size.

    For example, if you make a jig with a 2" jaw (rather than 1.5") then you should use slips 0.08" and 0.13" thick.

    I make the slips by ripping a piece about 12" long and 1/8" thick and planing it down to the desired thickness. I plane with the slip resting against a stop that is about half the final slip thickness above the bench top. This stop fits in a slot in the bench that is about 1/4" deep. Start with a stop that is about 3/8" thick, screwed into the slot in countersunk holes, then plane it to the desired height above the bench.

    With a sharp plane you can get down to close to the desired thickness. Small errors, less than 0.01" are not a problem. Given that you should be taking shavings that are 0.002" or less, you should be able to get closer than this.

    People with very accurate table saws might be able to rip the slips to thickness.

  8. The Human Jig

    Throughout these pages I say that you should not, indeed, you cannot, turn yourself into a human jig the equal of the simple jigs I make.

    However, with a lot of effort you actually can come pretty close.

    I met a human jig this spring (2009) at the Lee Valley store in town. Lee Valley was sponsoring a workshop by Konrad Sauer. Konrad makes infill planes in the classic style of the British makers like Norris of London. Konrad hand makes his planes (he personally does almost all of the work using files, sheet abrasives, along with a little rough work done using a band saw or a scroll saw).

    Konrad makes many planes a year - 50 or so. Each plane requires many hours of work - much of it shaping the plane on sheet abrasives by hand. Konrad has trained himself, though many hours of effort, to be able to perform the very accurate sanding operations required to finish these planes.

    He also freehand hones the blades. I have looked at his plane irons and they look good. I could not detect multiple microbevels and did see that he does not use back bevels, so they are not as good as they could be. However, they are better than I thought anyone could do freehand.

    Very few people spend 20 hours a week freehand with abrasives. For Konrad, much of this time is spent finishing the plane, not honing the blade. However, the skill level he has acquired over the years he has built these planes has enabled him to get away without using jigs for honing.

    Should you aspire to this skill level? If your main goal is finishing infill planes by hand, or similar occupations, then go for it. If your main goal is woodworking, then spending this amount of time to acquire - and maintain - the skill makes little sense. It is time you could spend working wood. If you want to try your hand at making infill planes check out Konrad's site. Heck, check out his site anyway - the planes he makes are works of art.

Visual checks during honing

 sharp, dull edge It is important to pause while honing and check that everything is going as you expect. Even the best honing jig possible cannot make up for a badly ground primary bevel.

This is a scan of a scrub plane blade I was sharpening. The blade was last sharpened by the former owner - this was my first sharpening of this blade. I ground the primary at 25 degrees almost to the edge then started to hone the 29 degree bevel on 15 micron abrasive as usual. This is what it looked like on my first visual check.

There are three areas of the blade, from the top, the old honed bevel, the new 29 degree bevel, the new 25 degree primary bevel.

If the old honed bevel were at 29 degrees, the new 29 degree bevel would be at the edge, not well back of the edge. This means the old final bevel was well over 29 degrees, perhaps even over 35 degrees. I suspect the previous owner sharpened this blade by hand - it is not easy to sharpen such a blade with commercial jigs. The result was a blade with too high a final bevel. A blade with this final bevel will not cut because the clearance angle is too small. That might be why I got this Stanley scrub plane for a very good price.

It is good honing practice to stop early and look at what is happening. In this case I could never hone on 15 micron abrasive enough to reach the edge. I have to go back to the bench stone and bring the 25 degree bevel almost up to the edge.

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