Sharpening Bevel Up Irons

	Bevel Up
	Finest abrasives.
	Microbevels front and back.
	Use a jig.

	Copyright (c) 2002-09, Brent Beach

Summary

Bevel up irons have been traditionally restricted to blocks planes, shoulder planes, and a very few larger planes. The historical failure of these large planes to become more popular is puzzling in light of their apparently very good performance.

This page is not setting out to prove whether or not bevel up planes work. They do work. This page sets out to discover if there are any sharpening problems particular to bevel up planes.

I think that you will see that the problem is quite subtle. If you sharpen well, you won't have any particular problems. If you don't sharpen well, the planes will fail to perform up to expectations. Sharpening well is more important for bevel up planes, especially low angle bevel up planes, than for bevel down planes. The fact that most people don't sharpen well may have held back the more widespread use of bevel up planes for finishing work.

Contents

The problem
The test
Sharpening
Front honing only

The Problem

Several other pages in this series discuss the shape of the wear bevels. By wear bevel I mean the worn surfaces produced during use. I call them wear bevels because in images taken by the QX3 Microscope they look much like the microbevels produced during sharpening.

In summary, friction with the wood wears the blade. This wear is confined to a very narrow region extending back from the edge. In experiments conducted on over 45 plane irons of many different makes, some irons tested several times, while taking over 1,300 images with the QX3, I have made some observations.

Upper wear bevel

Shavings colliding with, then sliding up, the upward facing surface produce the upper wear bevel.
The upper wear bevel width increases with use, but reaches a maximum value of around 0.005" long before the iron feels dull. The width of this upper wear bevel is about 2 to 3 times the average shaving thickness. (Incidentally, this means that unless you position the cap iron within 1/200th of an inch of the edge, it will perform no "chip breaking" function).
Wear is not uniform across this wear bevel. There is more wear within one shaving thickness of the edge.
There may be a very narrow high wear bevel, as little as 0.0002" wide, right at the edge. It appears that most of the force on the shaving occurs in this very narrow region.

Lower wear bevel

The wood being planed sliding under the blade produces the lower wear bevel.
The lower wear bevel width increases with use, reaching a maximum width at which the plane no longer performs adequately.
The final lower bevel width at which the blade feels dull depends on the blade thickness. Very thin blades may stop working when the lower wear bevel is only 0.0007" wide. Thicker blades may work well until the lower wear bevel is 3 times this width!
The sense that the blade is dull is often not a function of the quality of the edge itself - the edge may still be very straight and smooth. One standard edge test is to drag a thumb nail along the edge, expecting defects in the edge to catch the thumbnail. An edge that does not catch the thumb nail may still seem dull in use.
While sharpness of a razor blade depends largely on included angle, this is not the case for plane irons. A plane iron may perform well with included angle up to 45 degrees, while razor blades usually start out with included angles around 10 degrees.
A plane iron seems dull when the width of the lower wear bevel reaches a point a which the forces required to push the edge into the wood (which increase as the width of the lower wear bevel increases) are large enough that the plane is no longer able to sustain these forces and still hold the blade in the correct position. Making the plane more rigid at this point may not help - the user may not be able to exert the force required while retaining the control needed. In any case, further wear increases with the force applied, so using a blade with largish wear bevel is only possible for a short time.

Why are Bevel-up Planes a Problem

The above observations apply to all planes. It does not matter whether the iron is bevel-up or bevel-down, there will be wear bevels on both sides of the iron. The upper wear bevel will look the same whether it is on the bevel side of the iron or the other side. The lower wear bevel will look the same whether it is on the bevel side or the other side.

And this is the problem!

Conventional sharpening techniques handle the conventional problem: the problem of sharpening a bevel down iron. Conventional sharpening techniques concentrate on the bevel side of the iron. Conventional sharpening techniques do a good job on the bevel side, do little on the back face, but this works out pretty well for bevel down planes. The back face of the iron gets the upper wear bevel, which is slightly rougher than a well honed bevel and is at a slightly greater angle than expected. The net effect of not working the back face of the blade is a slight increase in effort, with little decrease in surface quality except perhaps for soft and stringy woods (where the increased effective planing angle is a negative).

With bevel up planes, the lower wear bevel is on the back of the iron. Standard sharpening techniques do not work the back of the iron enough to remove this wear bevel. Most jigs for sharpening irons do not even allow you to work the back face effectively while the iron is in the jig. The net effect is that the lower wear bevel remains on the iron after sharpening. While the edge seems sharp, the lower wear bevel is still wide enough to prevent effective use.

The Test

Bevel up bench planes are increasingly being used to face planing difficult woods - woods subject to tearout. In the old days they made bevel down planes with beds greater than 45 degrees to increase the planing angle. They used bedding angles of 50 and 60 degrees. Lie-Nielsen offers bench planes with 50 and 60 degree frogs.

With a bevel up plane you can increase the planing angle without changing the bedding angle if you increase the included angle. You can leave the primary the same and add a steeper microbevel. With a 12 degree bed, you get a 60 degree planing angle if the final microbevel is 48 degrees. The first series of test comare the rate of wear bevel formation as a function of the included angle.

Block planes are bevel up planes that are used for planing end grain. Typically they use a small included angle and a small bedding angle which combine for a small planing angle. This small planing angle and the excellent blade support inherent in bevel up plane design are both helpful on end grain. Planes used on end grain get similar wear bevels.

The Test - Included angle and Wear Bevels

To set the problem, here are some results from tests with a Lie-Nielsen #62 which uses a very thick iron made from O-1 high carbon steel. I tested this blade 4 times, varying the conditions slightly.

In the first test, the blade had my standard bevels: a primary bevel at 25 degrees, then three microbevels on the front and back, with the first at 29, the second at 30.2, the third at 30.9 degrees. The back bevels are 2.4, 3.5, and 4.3 degrees.

Images are of worn blades, taken after 150 passes along the 4' Douglas-fir board. While performing well, this blade shows very high wear levels. This may in part be caused by the geometry - the low bedding angle means an initial low clearance angle. To be strictly comparable to results from bevel down planes with 45 degree blade bedding angle and 30 degree front bevel angle, we would need a bedding angle here of closer to 15 degrees.

This is the downward facing, back face.

4. The wear bevel (the darker band along the edge) is 0.0013" wide. The darker line right at the edge (about the same width as the top yellow line) is the high wear bevel that is 0.0003" wide.
3. The 0.5 micron microbevel.
2. The 5 micron microbevel.
1. The 15 micron microbevel.

This is the upward facing, bevel side.

The wear bevel (the darker band along the edge) is 0.0023" wide, with a high wear bevel at the edge (slightly different appearance right at the edge) that is 0.0003" wide.

There is an interesting second dark area just back of the wear bevel that I don't understand.

In the second test I increased the angle of the first front microbevel from 29 degrees to 34 degrees. The second microbevel was 35.3, the third was 36.4. Increasing the front bevel angle on a bevel up plane changes the effective planing angle. This has the same effect as changing the bed angle. Planes with higher planing angles can reduce tear out in highly figured woods.

This is the downward facing, back face. The wear bevel (the darker band along the edge) is 0.0015" wide.
This is the upward facing, bevel side. The wear bevel (the darker band along the edge) is 0.0023" wide.

In the third test I increased the angle of the first front microbevel to 39 degrees. The second microbevel was 40.5, the third was 41.8. Adding the 12 degree bedding angle gives an effective planing angle of 53.8 degrees.

This is the downward facing, back face. The wear bevel (the darker band along the edge) is 0.0018" wide.
This is the upward facing, bevel side. The wear bevel (the darker band along the edge) is 0.0023" wide.

One problem with analysing these images is that the 0.5 micron abrasive leaves a surface that does not look a lot different from a wear bevel - both lack visible long scratches.

As an aid to analysis, I repeated test 3 with a slightly different sharpening sequence. First, 15 micron at 39 degrees as before. Second, 5 micron with the iron angled during sharpening and the jig resting on the 0.1" slider. No third microbevel. The final microbevel angles are the same as in the previous test; the final abrasive has changed.

So, in this test the second and final microbevel is at 41.8 degrees.

These pictures were taken after 150 passes along the 4' board. The last 10 passes were quite difficult - the plane wanted to skip along the board.

This is the downward facing, back face. The wear bevel (the darker band along the edge) is 0.0020" wide. The boundary between the 5 micron bevel and the wear bevel is pretty clear here.
This is the upward facing, bevel side. Again, the boundary is much clearer in this picture. The wear bevel (the darker band along the edge) is 0.0023" wide.

Composite Images

The next image shows the upper wear bevels after each test. The width does not appear to change much; if anything, it might be getting smaller as the angle increases.

upper wear bevels compared

The next image shows the lower wear bevels after each test. Here the wear bevels are getting wider as the last front microbevel angle, and hence the effective planing angle, increases.

lower wear bevels compared

Discussion

Bevel up planes require a sharpening system that removes the wear bevel on the back of the blade.

The width of this wear bevel increases with increasing included angle. The larger the angle the more important it is that the back is restored to the desired bevel angle right up to the edge.

You have two ways you can handle this problem.

Use back bevels, or
Hone more, or grind more often.

The Sharpening Problem

Having seen the wear bevels, we are now ready to think about sharpening the worn blade, bearing in mind that the back face must be repaired as well as the front bevel.

Wear Bevel Shape

This drawing is a profile view of a dull plane blade in working position. It shows the last 0.01" of the blade. If you use my jig and the slips, the last 0.01" is the third microbevel. All blade wear take place in this very small region of the blade.

The outer black line is the profile of the sharp blade before use. In this case, the front and back third microbevels. If you use my jig you get perfectly flat microbevels, 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.

The upper wear bevel is long and flat, the lower wear bevel is short and steep. This picture is true for bevel up and for bevel down planes. The difference is that with bevel up planes, the short steep lower wear bevel is on the back of the blade.

In this discussion, we are considering only bevel up planes. That is, the upward facing bevel here is a front bevel. The downward facing bevel is a back bevel, or the back of the blade.

Honing mainly the Front

To indicate the types of problems that can arise, consider this simple model of a sharpening session. In both cases the sharpner works mainly on the bevel side, honing until the wear bevel is gone. Let's look at the resulting blade.

Bevel Down

First the bevel down case. In this image the bevel faces down and honing until the wear on the bevel side is gone is shown by the pinkish line. In the plane this blade is not really sharp, since there is still wear on the upper surface. However, the bevel side is as good as new. The blade has the original clearance and will plane fairly well.

Bevel Up

Now the bevel up case. In this image the bevel faces up and honing until the wear on the bevel side is gone is show by the blue line. This blade is just as sharp as the bevel down blade - it looks exactly the same. However, in the plane the lower surface still has all the wear of a dull blade. If you press the plane down hard enough, this blade will cut. But it will cut poorly.

The LeeValley planes are very strong. Combined with the thick blade, you will be able to plane with a blade sharpened like this because the plane can transfer your downward pressure to the blade and drive it into the wood, even though it has a full lower wear bevel.

Honing with Back Bevels

If you use back bevels, you will create a new sharp blade that has a profile like the blue line. That is, you will remove some metal on the front and some metal on the back of the blade. You will completely remove both wear bevels.

This is an idealized result - the final microbevels ending at the tip of the worn blade. Honing has not shortened the blade at all. You will probably not get this result. In fact, I don't even try for this result. It is the easiest result to draw however.

When I hone, I aim to achieve this blue line on the first microbevels. I hone the front on 15 micron abrasive until the scratches reach the edge - the old front wear bevel is gone. Then I hone the back on 15 micron abrasive until the scratches reach the edge - the old back wear bevel is gone. This would be approximately the situation in the drawing. The wear bevels are removed completely by the 15 micron abrasive.

The second and third microbevels shorten the blade, while retaining the same basic geometry.

The problem with back microbevels is that these bevel up irons are often short, some very short. Say you have a 4" blade and get get a 3" extension of the edge from the jig. If you use a jig like mine, where the thin jaw is 1/8" wide, the extension calculator produces the following results:

Inputs	tall jaw height	1.5	inches
	short jaw height	0.125	inches
	short slip height	0.06	inches
	tall slip height	0.10	inches
	jaw angle	90	degrees
	blade thickness	0.075	inches
	bevel angle	29	degrees
Outputs	Extension	2 + 27/ 32nds	inches
	Base of jig to blade	3.1	inches
	Actual angles
	... no slip	29	degrees
	... short slip	30	degrees
	... tall slip	30.7	degrees
	Back bevel angles
	... no slip	2.5	degrees
	... short slip	3.7	degrees
	... tall slip	4.5	degrees

Notice the final back bevel angle is 4.5 degrees. This reduces the clearance from 12 degrees down to 7.5 degrees.

A better honing strategy might be to not use the slips when honing the second and third back micro bevels. You sacrifice the quality of the surface, but gain 2 degrees of clearance. I suspect the tradeoff favours omitting the slips for the back bevels. You might spend a little longer on the second and third grits to make up for using the same angle.

Front Grinding/Honing

If you work only from the front, you must grind/hone down to this blue line. That is, you must shorten the blade by the length of the back wear bevel.

When during honing your new front bevel reaches the dull edge, you have visually removed the front wear bevel. You can no longer see a bright line at the edge, when looking at the bevel. People may well stop honing at this point. The entire back wear bevel is still present.

If you continue honing the front bevel until you can feel a wire edge, you may still have not removed the entire back wear bevel. You will start to feel a wire edge as soon as you hone through the dull edge.

Other Grinding/Honing Options

One often proposed alternative to remove the back wear bevel is the David Charlesworth ruler trick. You can find a short discussion of this technique here. I have not investigated this technique. It may work, it may not. Something for a rainy winter day. If you use a jig like mine, you already have a precise back microbevel mechanism, so don't need to use this alternative.

Another alternative is to just buff the back of the iron using a charged leather strop. I discuss stropping here. While stropping may smooth the rounded back wear bevel a little, it will not flatten it. That is, it won't create a flat back bevel at the edge that is roughly (within 3 or 4 degrees) co-planar with the back face.

One reader of this page, Peter Michaux, asked: "Your page on bevel up planes has a worst case scenario with the 39 degree blade having a wear bevel of 0.0020" wide. My question is this. Suppose we take that worn blade and only work on rehoning the front bevel. Through rehoning don't we remove at least 0.002" of metal off the front bevel and thereby remove the bottom wear bevel?"

My answer - The above discussion points out that people using bevel down planes should be a little more conscious of the condition of the back of the blade during sharpening than those using bevel down planes. I am not saying that sharpening won't remove the entire back bevel, just that it might not.

When working the front bevel you can raise a wire edge without removing the entire back bevel. As soon as the abrasive reaches the edge, it starts to create a wire edge. So, the presence of a wire edge is not enough to ensure that you have removed the entire wear bevel on the back face.

If you use my jig and work the first microbevel at 29 degrees until the scratches reach the edge, then at 31 until the second microbevel is 0.01" wide, you only shorten the blade by 0.0006". The third microbevel shortens the blade by about half of that again, for a total of less than 0.001". You have shortened the blade by about half of a wide back wear bevel. So, if you are using bevel up blades and my jig, be sure to raise a good wire edge on the 15 micron abrasive, then continue as usual.

I don't know how much metal the average user removes when sharpening a blade. In fact, I have never actually tried to determine the amount of metal lost when going from a dull blade to a sharpened blade. I have only begun using scribe marks on blades to make sure I get the same part of the blade in each image. I usually mark the blade after the first abrasive - which may have shortened the blade already. These scribe marks allow me to determine how much metal is removed by the second and third microbevel, when done on both sides.

I will get some pictures to show the shortening of a plane blade (both bevel up and bevel down) in upcoming tests and add a section to the FAQ page.

LINKS

Check out my jig page for a simple jig you can make in your shop, along with a sharpening set up using sheet abrasives, that reliably produces excellent edges, for all types of irons.

Navigation

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Questions? Comments?

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