Sharpness Epiphanies

[Derek Cohen]

It strikes me that for most this whole area is a non-issue.

For most, hollow grinding a blade removes waste from the centre of the hollow. The subsequent honing removes the same amount of steel as honing on a flat bevel. All a hollow does is remove the unimportant steel from the equation when honing. Honing here removes the same amount of steel as the next method, but takes longer.

For those with a Tormek or a CBN wheel, it is possible to grind to the edge of the blade. The aim is not to grind away the edge, but just to the edge. You can determine this by feeling for the wire edge. Stop grinding when you feel the finest of burrs. In my case I hone directly on the hollow, and the microbevel so created is coplanar with the primary bevel. In other words, no secondary bevel is formed.

The third method is to use a honing guide and add a secondary bevel. This could be done on a flat or hollow grind. The secondary bevel will shorten the blade more than the other two methods.

Even in the latter method, the amount of steel removed - by one experienced - in minimal: what is the size of a wire edge? It is only the inexperienced who creates a secondary bevel that is larger than necessary, and that thereby shortens the blade more.

I appreciate that professional workmen of Yesteryear attempted to preserve the expensive steel because it was a significant part of their overhead. Today, very few professional furnituremakers would use their handtools enough to wear them out, and they are cheap enough for this to be less of a concern (a friend if mine is a high end furnituremaker who is known as a specialist in handtools. I've watched him work many times, and only a small percentage of his work is done with handtools. His LN chisels still have the first hollow grind he put on them several years ago). It is more likely to be the jobsite carpenter who grinds a cheap chisel on a beltsander that will reduce blades to a stub more rapidly. The amateur who does use handtools may care more, but still is unlikely to use up the steel in their lifetime. There may be a few years of wastefulness until they dial it in.

Regards from Perth

Derek

[Graham Haydon]

Adam, it's great to hear you are getting a level of sharpness that will move you to another level.

Derek I'm not sure I follow how a chisel or plane iron gets shorter by applying a secondary bevel over a hollow grind. Would not any method respective of type shorten by the same amount. All any method does is bring the meeting surfaces of the edge back to sharp. One method does not need more more length removed to do so?

I've always thought it best to avoid grinding up to the edge, if you can feel a burr when grinding you are removing more length than any other method.

[Derek Cohen]

... Derek I'm not sure I follow how a chisel or plane iron gets shorter by applying a secondary bevel over a hollow grind. Would not any method respective of type shorten by the same amount. All any method does is bring the meeting surfaces of the edge back to sharp. One method does not need more more length removed to do so?

I've always thought it best to avoid grinding up to the edge, if you can feel a burr when grinding you are removing more length than any other method.

Hi Graham

A secondary bevel, by definition, is a bevel that is ground or honed higher than the primary bevel. A micro bevel is simply a small bevel, whether secondary or coplanar.

A coplanar bevel is created when a micro bevel is honed directly on the face of a hollow grind. This means that the front and rear edges of the hone are aligned.



As you will see in the picture, although not to scale or angle, a secondary bevel will always remove more steel than the equivalent angle coplanar bevel (the same applies to honing a flat bevel).

About grinding a wire edge - note that I said that the only applies to a Tormek and a CBN wheel (because they will not overheat the thin steel). The aim of the ideal grind is to get as close to the edge of the blade as possible. I stop when I can feel the faintest wire developing. This is a wire about the same as created on a 1000 grit waterstone at most. This is still less steel removed than a secondary bevel. The advantage of grinding this close is that the amount of honing is reduced, and the microbevel formed is smaller. (Note also that the edge is not weakened by grinding this far - I have never experienced an edge breaking down because of this, and I do work with some really hard woods).

One more thing: as you hone the secondary bevel, it is common practice to raise the angle sightly. This shortens the blade (imperceptibly, but still so). Honing on the coplanar bevel (and flat bevel) removes less steel from the length.

Regards from Perth

Derek

[Lee Schierer]

I discovered the scary sharp method a number of years ago and was amazed at the improvement in cutting ability of my chisels and planes. I recently sharpened a spoke shave iron in about 20 minutes which included removing several minor chips.

[Graham Haydon]

Cheers Derek,

I think I must be going bonkers because I can't make it out. The angle of the micro bevel on both the hollow and the secondary are the same no? You would need to remove the same amount of blade length to remove wear and get a sharp edge? The diagram shows a secondary bevel honed more that it needed to be from a fresh grind and honed steeply. The hollow grind diagrams shows a lower honing angle and less removed. It don't think it's comparing apples with apples. But like I said I most likely am to thick to get it.

[Jim Koepke]

Cheers Derek,

I think I must be going bonkers because I can't make it out. The angle of the micro bevel on both the hollow and the secondary are the same no? You would need to remove the same amount of blade length to remove wear and get a sharp edge? The diagram shows a secondary bevel honed more that it needed to be from a fresh grind and honed steeply. The hollow grind diagrams shows a lower honing angle and less removed. It don't think it's comparing apples with apples. But like I said I most likely am to thick to get it.

If one has a sharp primary bevel and then hones on a secondary bevel, some metal has been removed, thus more shortening of the total length.

We are talking thousandths of an inch and semantics.

Because of the area of a single bevel is larger than the area of a secondary bevel, more metal is removed when honing on a single bevel. The difference of the length between the two honing processes may be zero. It is when the secondary bevel is reground that more metal may have to be removed eating up what wasn't removed in the past.

If you are not totally confused, you just don't understand the situation. -Edward Murrow

jtk

[Derek Cohen]

OK Haydon .. have you had your meds today? A soothing hot bath?

I think I see what may be confusing you. I did not include it as it was too obvious. "Obviously" not!

The primary bevel for the hollow grind is higher than the flat grind-plus-secondary to achieve the same cutting angles. For example, the primary for the hollow may be at 30 degrees, while the flat grind may be 25 degrees and the secondary on it at 30 degrees.

Adding that secondary will shorten the blade.

Regards from Perth

Derek

[Steve Voigt]

OK Haydon .. have you had your meds today? A soothing hot bath?

I think I see what may be confusing you. I did not include it as it was too obvious. "Obviously" not!

The primary bevel for the hollow grind is higher than the flat grind-plus-secondary to achieve the same cutting angles. For example, the primary for the hollow may be at 30 degrees, while the flat grind may be 25 degrees and the secondary on it at 30 degrees.

Adding that secondary will shorten the blade.

Regards from Perth

Derek

I don't think Graham is the one who is confused. Your argument is a little like Xeno's paradoxes, where one concludes that the arrow never reaches its target.

As he said, we need to work back a certain distance from the cutting edge, as measured along the back of the blade, to remove damage (or wear), whether it's damage from the grinding wheel or from work. Let's say that distance is .001". We then need to hone until the blade has been effectively shortened by .001. It doesn't matter whether we balance on the hollow or lift the blade off the hollow.
Since balancing on the hollow more closely approximates the initial grind angle, it will take more strokes to remove the wear than if you lift the blade. Perhaps that is what you really mean, that given an equivalent number of strokes, balancing on the hollow will remove less length? But of course we don't use an equivalent number of strokes. Balancing on the hollow requires significantly more strokes than lifting the blade does (I'm not saying one is better, just that one requires fewer strokes).

One other point. Both of the honed bevels in your diagram are "secondary" bevels. In your first diagram, if you draw a tangent line to the arc, right at the cutting edge, the slope is shallower than the honed bevel. It doesn't matter whether the honed bevel is "co-planar," it's still steeper than the ground bevel, and therefore meets your definition of a secondary bevel. Lifting the blade just produces a slightly steeper secondary method than yours.

[Derek Cohen]

Sorry Steve, but we are not on the same path this time. I think it may be terminology (some refer to a secondary as a micro, and vice versa - I do not).

Firstly, a coplanar grind/hone simply means "in a straight line", or "in the same plane". Therefore, when you hone directly on the surface, a 30 degree primary bevel produces a 30 degree cutting edge. There is no secondary bevel. Get the hollow close to the front of the blade, and you will have a coplanar micro (small) bevel.

No one has mentioned which method requires more or less strokes. You introduced that. In this light, you suggested lifting the wear, which would create a secondary bevel. I did not suggest this. I am referring only to honing on the hollow (which creates a coplanar bevel face). As I noted before, this is essentially the same as honing on a flat bevel, but with a much reduced amount of steel to hone. Only in this regard do I mention speed, which is faster with the hollow vs the flat. I am not including the secondary hone here, which can be as fast or even faster to do, but not as easy to re-do.

Regards from Perth

Derek

[Brian Holcombe]

Assuming we're talking about putting the same angle on the end of the blade I also do not see how one approach winds up effecting the length of the blade more than another. I'm not discounting either approach as I quite like sharpening hollow ground blades freehand on the stones (HNT gordon provides them hollow ground), but I have drawn one approach overlaid by the other approach, to scale, and do not see how one is different from the other.

[Graham Haydon]

Not sure about meds Derek but I did have two glasses of scrumpy last night

As Steve said no matter the method we need to remove the wear/damage to refresh the edge. That wear is a fixed point, a distance that can not be changed and a certain amount of steel must be removed regardless of method we find most effective.

Perhaps the image shows what I mean? No matter the method, in theory we must remove the red zone. I don't see how the hollow grind changes this.

Plane iron, oem, worn.jpg

[Ted Friesen]

Well I'm with you. Being relatively new to woodworking I've often heard or read that I will KNOW when a tool needs to be sharpened--but when you're new that doesn't really work--until recently working on a project I thought "Hey this chisel needs sharpening". That's an epiphany!

I use Shapton stones and just a few strokes on the 8000 stone on the secondary bevel had the chisel cutting very well again. A few more of these aha moments and I may be able to begin calling myself a woodworker.

[Derek Cohen]

Hi Graham (and Steve)

Not sure what scrimpy is, or even if I should volunteer to try it ... but we do have some great beers in Perth. I have a couple on ice waiting for you two.

Anyway, it seems to me that nothing has changed (!). The wear is a common factor, and has to be removed from both chisel sharpening formats.

I am not arguing which method requires more or less steel to be removed - the original question was which shortens the blade most. I need a better explanation. Until then we are left with a secondary bevel which grinds at an angle to the primary bevel, which shortens it.

Frankly, I am now as confused as any, and will go and think some more over a beer.

Regards from Perth

Derek

[Graham Haydon]

Always a good idea! And Scrumpy (scrimpy! that's just offensive ) http://en.wikipedia.org/wiki/Scrumpy

[Stewie Simpson]

Always a good idea! And Scrumpy (scrimpy! that's just offensive ) http://en.wikipedia.org/wiki/Scrumpy

https://www.danmurphys.com.au/produc...umpy-flagon-2l Nice brew.