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Messages - Jan

#1321
Quote from: Herman Trivilino on December 26, 2014, 03:59:15 AM
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.

Thank you Herman. You are correct. Nevertheless for growing distance of the lamp from the bevel the eccentricity of the oval-shaped image will increase and in a limit case it will change to a line. A good reminder how the rays reflect on a curved mirror can be found at the following address:
http://en.wikipedia.org/wiki/Curved_mirror Relevant for our discussion is the table "Effect on image of object's position relative to mirror focal point (concave)".

I agree that the scratches on the bevel complicate the reflected image of the lamp substantially because the light is interfering on it. However for me even the blurred images support enough the explanation why a flat bevel and cylindrically grind bevel looks differently in reflected light.
#1322
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
#1323
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.
#1324
Quote from: Herman Trivilino on December 19, 2014, 02:04:32 AM
Quote from: Jan on December 18, 2014, 02:00:37 PM
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.


#1325
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.
#1326
Quote from: Herman Trivilino on November 13, 2014, 07:10:36 PM
Quote from: Jan on November 13, 2014, 04:35:34 PM
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.

QuoteFor 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.
#1327
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
#1328
SharpenADullWitt (Randal), thanks for providing link to video showing how a good handle can be done using hand tools only. For my ancient tool this would be more suitable than a turned handle.

I agree with you that the handle of my old chisel is still usable. Ken is of the same opinion. Thus I will postpone the chisel rehandling.
#1329
Thank you Ken for showing appreciation for my effort. The simple Universal support montage shown above works fine, but it is fair to admit, that the work is not so comfortable and precise as with unique Tormek water-cooled sharpening system.

The belt sander is noisy and dusty. When the belt is worn out and hot it is often bulged in the middle of its width. That is why I prefer the grinding direction when the belt is running away from the edge,  the belt is pressed down before it reaches the bevel edge.

Thank you for reminding the marvelous Chris Schwarz educational videos. I have still to find the one showing the technique of draw boring. It may be interesting for you that Chris Schwarz recommends the Narex mortise chisels (the company shown in the production video). It is because they are probably the  best value for money set available. The Narex steel is softer than Lie-Nielsen (A2 steel, hardened to 60-62 Rockwell) or Ray Iles (D2 steel), but for a big mortise chisels the steel hardness is not so important as for narrow bench chisels.

Jan
#1330
Herman and Ken thank you for your response. It is surprising that the chisel twist is not easy visible at the first sight. My understanding is that the chisel is twisted from the very beginning. Below please find an chisel production video showing how a company in this country is still making chisels by traditional techniques.
https://www.youtube.com/watch?feature=player_embedded&v=gittWRq2Sjk
Nevertheless they are  surely many ways how an ancient chisel could acquire some geometrical imperfections.
#1331
I have decided to keep the bevel of my old heavy duty mortise chisel flat. That is why I have mounted the Universal support on my belt sander, as you can see on the attached picture.


The Universal support is positioned vertically in sleeves drilled into beech wood prisms. It is possible to grind in both directions, but I prefer the direction when the belt is running away from the edge. I use 240 grit belt. The old iron is used to cool the belt sander.

Because my chisel is to big for the Square edge jig (SE 76), I have removed the special Tormek lower clamp and replaced it temporarily with two small steel plates bolt together by four screws.


Because the chisel is twisted, mounting its axis parallel with the belt, resulted in a skewed bevel edge. However, it was not difficult to find an angular deflection from parallel orientation which resulted in more or less square bevel.


Regards Jan

#1332
As Herman wrote "it's very satisfying to take an old tool that was used and abused and bring it back to its former glory ..."
I am considering replacing the handle of my old heavy duty mortise chisel. The current one has a conical shape with rusty ferrule and rusty metal ring at the top of the handle. The handle is split, the top is smashed. I am not sure about the type of wood. May be it is ash. I have heard that hornbeam is one of the best woods for heavy chisel handles. Hornbeam handle, even without the hoop at the top, should be able to withstand heavy banging with a mallet. I have a peace of hornbeam wood, so I could make the handle on a lathe. Does anybody have good experience with similar chisel rehandling?
#1333
Dear Herman, dear Ken I have highly appreciated your comments. It is a very pleasant feeling to know that all over the world there are people sharing the same values. Thanks also for sharing your own personal stories. I have a similar one, but as a non native speaker I am hardly able to tell it as aptly as you. I am really enjoying posting here.
Jan
#1334
Ken,
thank you for your wise advise concerning the perfect squarness. For chopping of mortises it is really not very important. 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. For this occasions the bevel squarness matters. 
In the past I was sharpening my tools on a bench sander equipped with my own roller assembly which was capable to maintain the bevel angle quite well. Now when I have acquired T7 with accessories I am step by step re-sharpening all my tools when I am sure to improve the bevel quality. In the case of my heavy duty mortise chisel I am really not sure whether the bevel will be improved or not.
Jan
#1335
Herman, your feedback is highly appreciated! I agree with you that an experienced sharpener may be able to grind (possibly even by hand) a square edge on a chisel which does not meet the above formulated geometrical requirements. However, because the geometrical propositions are undoubtedly valid, I came to the conclusion that in this case the "square edge" would de facto be composed of several aligned cylindrical surfaces forming the bevel.
Jan