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| Back bevels and microbevels |
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| Finest abrasives. | ||
| Microbevels front and back. | ||
| Use a jig. | ||
| Copyright (c) 2002-15, Brent Beach |
Alternate title: If back bevels are so good, why doesn't anyone else recommend them?
As soon as you plane your first bit of wood your plane iron has a back bevel - the wear bevel. So whether you want a back bevel or not, you have one. Your sharpening system must take this back wear into account. There are two options: rework back wear from the front or the back. This is the usual diagram showing the original sharp blade profile (black line) and the dull profile (red line). The drawing is explained fully here.
Reworking from the front must involve removal of substantial metal since the wear on the back is about 0.003 inches wide. You cannot remove this much metal by honing from the front - it would take hours. If you insist on working from the front you have to grind, and you have to grind through the existing edge.
Most people try to sharpen by honing a couple times between grinding sessions. Working only from the front, these people will not get a good edge by honing along. This produces a half-sharp edge - they remove the bottom wear bevel but leave the top wear bevel. The result is a much larger included angle than people expect.
Grinding the iron back 0.003" is also a problem, especially if the abrasive is narrower than the blade (a grinding wheel, a 1" belt sander). In that case the resulting edge will not be straight. As well, the large grits will also create defects in the edge and may over heat the edge, drawing the temper. People who use grinding wheels daily may minimize these problems. People who only grind edge tools once a month will certainly damage the edge. Drawn temper, uneven and fractured edge ... why take a chance?
By using back bevels you can restore the cutting edge by honing alone, most of the time. No drawn temper, no uneven edge, no fractured edge.
When you do have to regrind, hand grinding with a jig allows you to easily stop grinding short of the edge. At no time can you harm the metal at the edge.
Hand grinding and honing do require preparation: time to collect the right materials (glass, sheet abrasives, bench stone) and build the jigs, then time to learn the grinding and honing skills. I believe it is more than worth the effort.
The problem of back wear bevels and honing is covered again in Why flat backs suck.
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There is a mathematical logic to microbevels that I find compelling. In this picture, the wear bevel is at the edge (red), the extended 0.5 micron bevel is next (yellow), the mixed 5 and 15 micron next (green), the primary next (blue). The total width of the wear bevel and the three microbevels is just under 0.02", or about one-tenth of the width of the primary bevel. The wear bevel is about 0.0006" wide.
As an example, imagine you are sharpening a 2" wide plane blade that has a 25 degree primary. On a standard thickness plane blade, the primary bevel is .21" wide. When you hone a microbevel at 29 degrees until it is 0.025" wide, you remove metal at the edge to a depth of 0.017". This is deep enough to ensure that all scratches left during primary bevel formation have been removed. If you simply honed the entire primary bevel to the same depth, you would remove 16 times as much metal, take 16 times as long, and use up abrasives 16 times as fast. That is 16 minutes instead of 1 minute.
Honing the second microbevel at 31 degrees produces more gains. The average second microbevel is about 0.010" wide. At this width the depth of metal removed at the edge is 0.0035". Again, this is enough metal to be sure that you have removed the scratches left by the 15 micron abrasive. If you were to hone to that depth without using microbevels, you would have to remove 40 times as much metal during this honing step, take 40 times as long, and go through 40 times as much abrasive. That is 20 minutes instead of 30 seconds.
The third microbevel is important. When you hone with abrasives in the 1 micron range and below, abrasion no longer affects the structure of the blade beneath the surface - it no longer cracks the underlying metal. With abrasives like the 0.5 micron 3M abrasive, honing results in a plastic reshaping of the surface metal which actually work hardens the blade. (See scratching.)
The geometry of the third microbevel: width 0.005", depth of metal removed at the edge 0.0008". If you were to hone to that depth without using microbevels you would have to remove 91 times as much metal, take 91 times as long, go through 91 times as much abrasive. That is 45 minutes instead of 30 seconds.
Remember that all wear on the blade during planing takes place in the third microbevel. Honing with the finest abrasive outside that region is wasted effort, because the part of the blade outside the third microbevel will never be part of the cutting operation.
With microbevels you spend 2 minutes honing, without microbevels you spend 16 + 30 + 45 = 1 hour 31 minutes to get the same edge. No one is going to spend 1 hour 31 minutes per blade. Everyone who does not use microbevels is going to have a worse edge.
This diagram is the basis for understanding what you are doing when you sharpen a dull blade. It also helps explain what happens when you touch up a slightly dull blade.
A rather complicated drawing (Sketchup model actually - check out the introduction to Sketchup models that shows honed microbevels and wear bevels), it shows all the bevels shapes from:
The volumes removed are pretty small. Notice that wear removes more than twice as much as the 0.5 micron abrasive. Of course, the 0.5 micron abrasive is used for about 30 seconds while it takes a lot more use than 30 seconds to produce the wear bevel.
Notice as well that the 15 micron abrasive removes more than 100 times as much metal as the 0.5 micron abrasive although it is only used for about twice as long (perhaps 3 times as long). The 15 micron abrasive does most the honing work.
A touch up that produced the same results as a 3 minute sharpening takes almost 14 minutes.
Ah, you say, but I don't wait until the blade is dull to touch it up! Good point! I made a second version of this model which started with a much less dull blade. Metal volumes in this model are:
The volume of "Metal removed during use" in this model is about 17% of the corresponding volume in the previous model. The volume of "Metal removed during Touch Up" is about 23% of the corresponding volume in the previous model.
So if you touch up about one-sixth of way between sharp and dull, you have to remove about one quarter of the metal. This is still about 6.4 times as much metal as would be removed in forming the third microbevel. It would therefore take over 3 minutes to touch up this blade - about the same time as a full resharpen of a dull blade.
On the other side there are people who do touch up their blades and are certain that they benefit from that touch up honing. Often these people are skilled woodworkers with long experience. Often these people do not use jigs when they sharpen or touch up, so I cannot model their technique. Nor can I reproduce their technique to verify or disprove their claims. This is not to say they are not getting some sharpening effect from their touch ups.
I can think of several differences that might explain these reported successes:
So, I still don't have a model that explains why touch up works for some people. What this section does show is that if you sharpen like I do then you definitely should not try to touch up your tools. Use them until they are dull and resharpen them.
This has interesting parallels to discussions of stropping. Many excellent craftsmen do strop and say it improves their results. However, if you sharpen like I do then you should not strop.
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