sharp/dull blade drawing The geometry of Honing small map
Finest abrasives.
Microbevels front and back.
Use a jig.
Copyright (c) 2002-09, Brent Beach

Contents

The geometry of Honing

The quality of the edge you get from honing depends on the quality of the abrasive and your ability to hold the blade at a precise angle during the honing process. 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 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 it dull state after testing.

You cannot start with a better profile than the black profile. It represents the ideal cutting edge. 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, 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). 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 dull blades. In the end, they probably spend more time "touching up the edge", but they can use the resulting not dull but not sharp tools, so continue in their deluded ways.

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 practise you can freehand hone - you can turn your body into a jig. The most plausible of those arguing for freehand honing start with a hollow ground primary bevel, rest the blade on the front and back of the hollow, then hone. That is, their grinding operation produces their jig. Seems clever, but it cannot produce as good an edge for 3 reasons. First, for most people grinding the hollow bevel on a grinding wheel means grinding through the edge. Look here to see why grinding through an edge wrecks the metal at the edge. Second, because you must hone the full area of the already honed bevels with each abrasive, you are doing far more work than you do when using a jig and slips as outlined below. Third, this method makes no provision for back bevels, ensuring your tool is only half sharp.

Remember - this diagram represents the last 0.01" of the blade - the working part of the blade. The profile can only be realistically obtained using microbevels (see why use microbevels below) and microbevels can only realistically be achieved using a jig.

sharp, dull edge This scan is from a scrub plane blade I was sharpening. 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. It is 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 of this blade sharpened it 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 right up to the edge.

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?

After 6 years of sharpening and testing plane irons, 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.

The best sharpening system will form the tool with the grinding step. This step will use a coarse abrasive and will remove metal from the primary bevel, but will leave a bit of the old honed/wear bevel at the edge. This remnant of the old edge acts as a grinding guide, ensuring both that the resulting edge shape is correct (has not been changed by grinding) and that the coarse grits used in grinding have not damaged (chipped) the steel at the edge. The other way you might damage the steel at the edge is through over heating. 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 best sharpening system will hone using a series of finer abrasives, each at a slightly greater honing angle (microbevels). This series should start below 40 microns and continue to at least 5 microns. Abrasives with larger grits can chip the edge, causing damage that further honing cannot remove. Some tools (knives used with a sawing action) can stop before reaching 5 micron abrasive size. The finer the abrasive, the more slowly it removes metal. How much metal do you need to remove with each abrasive? Enough to remove the scratches and metal damage left by the previous abrasive. If you do not increase the honing angle slightly, the honing time required for each successive abrasive is simply too long.

The precision required here is difficult, if not impossible, for many woodworkers to achieve without the use of a jig. It is however easily achieved with my jigs and slips.

Finally, the best sharpening system will treat the tool as a double bevel tool if at all possible - it will hone both sides of the blade. While not news for people sharpening almost all knives, this is news for people sharpening plane irons and most chisels. I believe that almost all chisels (carving, turning, bench) are best treated as double bevel tools. Failure to sharpen in a way that accounts for wear on both sides of the tool cannot produce the best results. Even in cases where a single bevel seems required - single bevelled axes - a very small bevel on the back side can improve performance.

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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.
    		my jig, plane blade, positioned to hone


  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 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. 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.

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