How to sharpen 3/4" mortise chisel

[Jan]

Mentioning steels and its properties Ken, you remind me, that last week when I was celebrating an anniversary the staff gave me as present very nice Santoku chef's knife made of a 33-layer stainless Damascus-patterned blade with a Japanese steel VG-10 cutting core...

The HRC of this steel is 60-61, probably in harmony with the purpose of this universal kitchen knife. (In Japanese Santoku means "three uses": slicing, dicing, and mincing.)

Jan

[Herman Trivilino]

For the rough use a large mortise chisel must endure, hardness or the steel may not be as important as toughness.

It's definitely not desirable to make the steel too hard. Of course you don't want it too soft, either. Extreme examples of this are brass and tungsten carbide steel. A brass chisel would be so soft that it would constantly in need of sharpening because the edge would flatten or bend over. A carbide steel chisel would be so hard that the edge would break off and again, you'd constantly need to resharpen it.

Of course, you'd have a much harder job sharpening carbide steel than brass!

[Jan]

Something else, I thing, is sharpening an old chisel inherited from your grandfather. Such a tool is usually used only occasionally, but all the more shown as a living history artifact.   

I agree that it's very satisfying to take an old tool that was used and abused and bring it back to its former glory by cleaning it up and putting a razor-sharp edge on it. This is especially true of a tool you remember seeing your father or grandfather use. My father never sharpened his tools or knives properly and I saw him struggle with them. Now I can use those same tools and thanks to my Tormek they work better than they ever have in my lifetime.

For this occasions the bevel squarness matters.

And so does the shine! We want to see a mirror finish on those bevels.

Since I read Herman's sentence "And so does the shine!" my thoughts are revolving around the shine of nicely shaped and honed bevel. Why is the shine of a flat bevel so different from the shine of a cylindrical bevel grind on a vertical grindstone? We know that the hollow depth is very, very small, but the difference in visage is visible at first sight.

My current understanding is following. The Tormek bevel is concave cylindrical mirror, called also converging mirror, because it collects light and focuses the rays toward a focus. The focus is at a distance R/2 from the bevel surface, R is the grindstone radius. The image of a distant light source is strongly reduced, inverted and very bright.

On the contrary, the flat bevel reflects light as a plane mirror, it means an erected image of the same size as the light source is produced. The light  does not spread out after reflection from the flat bevel surface. The image is virtual, because it appears to be behind the mirroring flat bevel. 

So, grinding even a tiny hollow causes that the bevel surface reflects light as a converging mirror and the shine of this bevel is more intense than the shine from a flat bevel.

[Ken S]

Jan,

I think it is important to include how one would sharpen a mortising chisel which was not a family heirloom. By that I mean the mortise chisel one might buy new or used with the intention of using it regularly to chop mortises.

Yes, I think you made the right decision to sharpen your grandfather's chisel with a flat bevel. I would have done the same. In our mind's eye, the ideal chisel should have a flat bevel, be razor sharp and polished, both bevel and back, like mirrors.

However, for actually chopping mortises, I think the very moderate hollow grind of the Tormek would not present a problem. i realize conventional theory would disagree. The real world difference is so small. The actual cutting is only done by the very front of the chisel, the sharp edge. The rest of the bevel is just along for the ride. With water or oil stones, one typically adds a tiny micro bevel to make sharpening easier. With the Tormek this is not necessary. The micro bevel is usually ground with a five degree steeper bevel. We can do the same thing with the Tormek by adding five degrees to the bevel angle.

While your ingenious setup certainly accomplished what you wanted, for the day to day sharpening for a carpenter or furniture builder, it would be slower than just sharpening with the Tormek.

I look forward to reading your future posts.

Ken

[Herman Trivilino]

So, grinding even a tiny hollow causes that the bevel surface reflects light as a converging mirror and the shine of this bevel is more intense than the shine from a flat bevel.

I don't know if this effect is large enough to make a difference. If you use this surface to view the image of something that's very close, you're looking at an image formed by neither a spherical surface nor a plane surface. In one direction it's curved and in the other it's flat. This should produce an image that's distorted in such a way that something round, like the head of a screw, would form an image that's oval. This effect is so small I can't notice it.

By the way, the equivalent thing is done by opticians with lenses to correct for astigmatism.

I'm not saying you're wrong, it's just that I want to give it some more thought. It's a very interesting observation.

[Herman Trivilino]

With water or oil stones, one typically adds a tiny micro bevel to make sharpening easier. With the Tormek this is not necessary.

But, as you know better than me, you can take a chisel that's been sharpened on a Tormek and then polish the bevel on a water or oil stone. This gives you two flat spots, one at the edge and the other at the heel. The one at the edge is sort of a micro bevel as it makes the bevel angle more blunt in the same way a micro bevel does.

Some people claim that the shape of the bevel surface makes a difference in the way wood is removed as you pare.

[Ken S]

I think of the paring chisels as being the prima donnas of the production. The mortise chisels are like the chorus, chugging along doing the hard work. Traditionally chisels were sharpened by hand. I don't think minor variations are critical.

Ken

[Herman Trivilino]

I don't think minor variations are critical.

To me, they definitely aren't. My "collection" of chisels consist of the one I bought about 30 years ago when I was working as a carpenter, plus a half-dozen others that were all finds. None of them are expensive chisels. They work fine for the type of work I do. Especially when they are sharp.

[Jan]

Thank you for your response Herman and Ken. The idea behind my post concerning bevel shine was definitively not to evaluate what kind of bevel shaping is better. My post is an attempt to use the well known law of reflection to explain the different shine of a bevel shaped flat and curved.

[Jan]

So, grinding even a tiny hollow causes that the bevel surface reflects light as a converging mirror and the shine of this bevel is more intense than the shine from a flat bevel.

I don't know if this effect is large enough to make a difference. If you use this surface to view the image of something that's very close, you're looking at an image formed by neither a spherical surface nor a plane surface. In one direction it's curved and in the other it's flat. This should produce an image that's distorted in such a way that something round, like the head of a screw, would form an image that's oval. This effect is so small I can't notice it.

Even a tiny hollow is for light large enough because the wavelength of visible light is so small, that a hollow of a height 0.001" can accommodate  several dozen wavelengths of visible light. 

The bevel shaped a standard way on a vertical grindstone is exactly cylindrical. Neither a plane nor a spherical mirror surface does exist in this case. You probably want to say, that the image of the light source depends on its distance from the reflecting surface. That is correct.  But in my post I have described the behavior for a distant light source, which is far behind centre of mirror curvature. That  is the common situation when the bevel is reflecting the strip light on the ceiling or the Sunlight.

Thank you again for your prompt feedback.

[Herman Trivilino]

The bevel shaped a standard way on a vertical grindstone is exactly cylindrical. Neither a plane nor a spherical mirror surface does exist in this case. You probably want to say, that the image of the light source depends on its distance from the reflecting surface.

My point is that a cylindrical mirror produces two image planes. To locate one plane you can assume the surface is planar along one direction, and to locate the other you can assume the surface is spherical along the other. In other words, there doesn't exist a single plane where you have an image in focus. As you say, the curvature is so small in this case that it makes little difference.

You are correct in that the waves are so small we can ignore the wave nature of the light and use the ray model to locate the images, which is what we are doing.

[Jan]

Yes, now I understand you correctly. For you the cylindrical mirror is de facto an elongated spherical mirror. The focus of the cylindrical mirror is not a point, but a line of points parallel with the bevel edge. In my post I was describing the ray tracing in a cross-section plane perpendicular to the bevel edge. Hopefully I will be able to prepare some pictures during the Christmas holidays.

Deeper going discussion about the visage of the bevel may be considered unimportant by someone, but for me the bevel image is crucial for the whole sharpening process.

[Jan]

Here are the promised images.
The photo below shows the bevel grind with Tormek grindstone R = 125 mm = 5". The light source – the light bulb is shown as a line, because the bevel reflects as a converging cylindrical mirror. The focus is at a distance R/2 from the bevel surface.



The two photos below show flat bevel grind on a belt sander. Flat bevel reflects light as a plane mirror. The reflected light is not focused, because the focus is in infinity.



Bellow we can see the image of table lamp shade with a light bulb.



Regards Jan

[Herman Trivilino]

Nice photography, Jan. I agree that we are looking at an image of a lamp shade in that last photograph. Note that it appears round because the reflecting surface is flat. I believe you'd see a similar image produced by a curved bevel, but the image of the round lamp shade would be oval-shaped.

But I do not agree that the image of the light source in the first photo is a line because the surface is curved. Note that we see in that last photo a smeared image of the light source, and it's produced by a flat bevel! Instead, what I think we're seeing is diffuse reflection created by the scratches, just as the ripples in a pond produce a diffuse image of the sun instead of the sharp image produced by the specular reflection off a pond with a smooth surface. Now, the line is narrower in the first photo, and that is due to the curvature,

To get a clear image of the lamp shade on the curved bevel I believe you would have to spend more time dressing the Tormek grindstone with the fine side of the stone grader. And then follow that with a good polish on the leather wheel.

[grepper]

Obviously a rough surface will diffuse light and blur a reflection, and a flat, convex or concave surface will reflect light differently and will distort or focus a reflection if it is not flat.

Is this discussion simply about how shiny a bevel looks?