Hi Everyone,
I need to sharpen an very old heavy duty mortise chisel with blade width some 18 mm (3/4") and blade height also some 18 mm (3/4"). Please advice how to sharpen this chisel with the Tormek system.
Jan
Welcome to the forum Jan,
This should help:
http://www.youtube.com/watch?v=6lJHxoQWO8U
http://www.youtube.com/watch?v=kMCwG8xehGE
Welcome to the forum, Jan.
We had a similar discussion on this forum a couple years ago. Ionut, a longtime member who sadly has not posted recently, was trying to sharpen a 1/2 inch (12mm) mortise chisel. With the smaller chisel in that discussion, using one of the older square edge jigs provided enough depth to hold the chisel. However, even that jig would not be large enough to handle a 19mm thick mortise chisel.
What would work is one of the Torlock platform jigs. The control is not as precise as the SE-76 jig, however, you should be able to put a very sharp and good looking edge on your mortise chisel.
I would suggest that once you have the platform set for the correct angle that you make a template with either cardboard or plywood. it will fit between the platform and the grinding wheel. Mark it with the bevel angle for your mortise chisel. That way, setting up for future sharpening will go more quickly.
http://tormek.com/international/en/grinding-jigs/svd-110-tool-rest/
Keep us posted.
Ken
Hi Grepper,
Thank you for your suggestion, unfortunatelly those demos do not solve my problem. My mortise chisel blade thickness is ¾" which is too much for the Square edge jig (SE76).
Regards
Jan
The SVS-32 Short Tool Jig should work for you.
http://tormek.com/international/en/grinding-jigs/svs-38-short-tool-jig-svs-32/
Otherwise, free hand.
Hi Ken,
thank you for your response, I already have the Tool rest SVD-110 so I can try your suggestion.
If possible I would like to keep full control over the grinding process. I am wondering if the Multi jig SVS-50 will solve my problem.
I am concerned about the hollow shape of the bevel. If you imagine the bevel length is 1 5/8" than the hollow will be well noticeable.
Best regards
Jan
Jan,
I understand your concerns. In a perfect world, mortise chisels should have a flat or even slightly convex bevel. In this case, I would not worry about it. The 250 mm diameter grinding wheel of the Tormek produced much less hollow grind than a smaller (150 or 200mm diameter) grinding wheel. If you are concerned about this, add three degrees to the bevel angle. A very sharp mortise chisel with a bevel of thirty five to forty degree angle will be very stout. Keep in mind that the extra width will spread the force over a larger area than a narrower chisel.
Regarding not having your mortise chisel fully controlled in a jig, if you are reasonably careful holding your chisel with the SVD-110, your bevel and edge will be more precise and probably sharper than the master cabinetmakers of the past could do.
Good luck and continue to keep us posted. Good mortise joints will still be solid long after we are gone.
Ken
Hi Herman,
thank you for your suggestion. Yes, the Short tool jig SVS-32 provides enough space to hold my heavy duty mortise chisel. As Ken mentioned in reply #2 the control is not as precise as with the Square edge jig. The Short tool jig SVS-32 works fine provided the lateral blade edges are well preserved and the top face of the blade is perfectly flat and intersects with the plane of the back face of the blade in a line which is parallel to the cutting edge.
Regards
Jan
Ken,
thank for your reply #6. I agree with you fully. You have to guess my mind.
I have just calculated the hollow depth for my mortise chisel with a bevel length 1 5/8" (40 mm). The hollow depth from a 10" stone should be 0.03" (0.8 mm), from a 8" stone it should be 0.04" (1 mm)and finaly from a 6" stone it should be 0.05" (1.35 mm).
Regards
Jan
Quote from: Jan on November 11, 2014, 09:18:15 AM
The Short tool jig SVS-32 works fine provided the lateral blade edges are well preserved and the top face of the blade is perfectly flat and intersects with the plane of the back face of the blade in a line which is parallel to the cutting edge.
In my experience the SVS-32 is not precise enough for those issues to be a concern. Regardless of how the chisel is mounted in the jig, the operator must pay attention to how the cutting edge makes contact with the grindstone. This is especially important when trying to get a square edge.
Herman,
thank you for sharing your experience with the Short tool jig SVS-32. This jig offers larger number of degrees of freedom what is however compensated by the fact that some part of the tool control is transferred to the sharpener.
The sentence you have quoted is recollection of my thoughts concerning general requirements on the blade geometry which have to be fulfilled when trying to get a square edge. No jig can help us to get square edge when the geometrical prerequisites are not met. Since I have recognized it, I keep calmer even after several vain attempts to grind a square edge.
Jan
You are correct, Jan, in that to get a square edge you do have to make sure all of those geometrical considerations are met. That is, a true grindstone and the tool properly mounted in the jig. The rub is that although these conditions are necessary for a square edge, they are not sufficient. Operator skill is very much a requirement when getting a square edge on any tool using the Tormek.
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
Jan,
I believe we are worrying too much about squareness. Mortises are made with some clearance between the end of the tenon and the bottom of the mortise. This allows the shoulder of the tenon to fit snuggly and provides a reservoire for any surplus glue. The end of the tenon is end grain, with no gluing strength, so the clearance does not reduce the strength of the joint. Whether the bottom is square or just near square makes no real difference.
I suggest you try using the platform jig. Blacken the bevel with a felt tip marker. This is discussed in the Tormek handbook. Grind a little and observe how the marking is being removed. This simple method will give you feedback about the squareness of your grinding. Don't worry about removing tii much steel. A chisel with this much bevel will require quite a bit of grinding. Start with a light touch until your confidence builds.
The real test of your sharpening is how well the chisel cuts. Sharpen your chisel using the platform and chop a mortise on a piece of scrap wood.
Ken
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
Quote from: Ken S on November 13, 2014, 11:31:15 AM
I believe we are worrying too much about squareness.
Those are good points, Ken. The same is true when mortising for a hinge. As long as you're resonably close to square it makes no difference. It's far more important to have the edge sharp and ground to the correct angle.
In my experience the same is true of a plane iron. If it's out of square a bit that can be corrected by an adjustment on the plane itself.
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.
Jan,
I see your point about the chisel having belonged to your grandfather. I, too, enjoy using my grandfather's tools. I also have grandchildren. I hope that if they become woodworkers they will keep the tools sharp. I mostly hope they will use them. The pristine tools we see are the ones which didn't get used. Honor your grandfather by using his tools to make practical and beautiful wooden objects.
Ken
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
Jan,
I am pleased you enjoy being part of the forum. Our diversity of thoughts and background is one of the things which makes this forum interesting. We share ideas and when we disagree, we disagree civilly. Don't worry because English is not your primary language. We all speak sharpening here.
Do keep posting.
Ken
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?
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.
(http://img13.rajce.idnes.cz/d1302/10/10793/10793784_4f5d73aa79e38dff462204b63aedb3f2/images/IMG_3709_72.jpg?ver=0)
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.(http://img13.rajce.idnes.cz/d1302/10/10793/10793784_4f5d73aa79e38dff462204b63aedb3f2/images/IMG_3711_72.jpg?ver=0)
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.
(http://img13.rajce.idnes.cz/d1302/10/10793/10793784_4f5d73aa79e38dff462204b63aedb3f2/images/IMG_3713_72.jpg?ver=0)
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.
(http://img13.rajce.idnes.cz/d1302/10/10793/10793784_4f5d73aa79e38dff462204b63aedb3f2/images/IMG_3718_72.jpg?ver=0)
Regards Jan
Good job, Jan!
Ken
Ingenuous, Jan. Both on the USB mount and the modification to the jig.
That chisel looks ancient. Do you have any idea how it got so twisted?
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 (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.
Jan,
You have done a fine job of pushing the Tormek beyond its limits. Do keep up the good work.
I am not surprised that your chisel has a twist. A mortising chisel of that size would have probably been made for housebuilding rather than furniture making. It may well have been hand forged by a local blacksmith rather than manufactured in a factory. Today we expect our tools to look perfect. Years ago how well a tool functioned may have been the primary concern. (We could all learn from that.)
A good source of information about traditional mortise chisels is Christopher Schwarz. He writes a blog and has written many magazine articles and several books. He prefers the traditional design of mortise chisels, such as your grandfather's tool, to the more generally available "sash mortise" chisels. The traditional design is more robustly solid. The oval shaped handle provides more strength to lever out waste and helps keep the chisel aligned. If you do a google search with "Iles mortise chisels" you will find the chisels of that design still being manufactured in England.
If you get on the lie-nielsen.com website, under educational videos (you tube) Chris Schwarz does a pair of you tubes on the technique of draw boring. This is a very traditional method of securing mortise joints. It is very strong, long lasting, and requires no glue. I suspect your grandfather may have used these joints frequently in his work.
I wouldn't replace the handle as long as it still functions. Lie-Nielsen makes some of the world's finest chisels, and they use hornbeam.
I am pleased you have found satisfaction in restoring your grandfather's chisel. Your work honors his memory.
Lem
https://www.youtube.com/watch?v=8l8T7YOdWpw
I've seen several video's on rehandling (from making them with hand tools, to a lathe, to even a piece of a broken broom handle, like my grandfather would have done), but several did what the above did and it stuck with me. The copper pipe end piece to keep the handle from splitting.
Compared to some I have seen, your looks useable still. Clean and fill the crack with epoxy and refinish to get some moisture back into the handle. (another option)
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
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.
Jan,
I remember reading the famed knifemaker, Hoyt Buck, made his handmade knives for US soldiers softer than he could have made them. He knew these knives would have to field sharpened with whatever was available.
I think it is interesting that Lie-Nielsen also makes its chisels with O1 carbon steel. O1 won't hold an edge as long, however, it will take a keener edge and a more acute bevel.
For the rough use a large mortise chisel must endure, hardness or the steel may not be as important as toughness.
Much can be learned from the humble chisel.
Ken
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
Quote from: Ken S on December 17, 2014, 11:26:59 AM
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!
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.
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
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.
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.
Quote from: Ken S on December 19, 2014, 12:10:50 AM
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.
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
Quote from: Ken S on December 19, 2014, 02:50:40 AM
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.
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.
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.
Quote from: Jan on December 19, 2014, 10:04:40 AM
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.
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.
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.
(http://img13.rajce.idnes.cz/d1302/10/10793/10793784_4f5d73aa79e38dff462204b63aedb3f2/images/IMG_3736_detail_2.jpg?ver=0)
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.
(http://img13.rajce.idnes.cz/d1302/10/10793/10793784_4f5d73aa79e38dff462204b63aedb3f2/images/IMG_3745_rotace.jpg?ver=0)
Bellow we can see the image of table lamp shade with a light bulb.
(http://img13.rajce.idnes.cz/d1302/10/10793/10793784_4f5d73aa79e38dff462204b63aedb3f2/images/IMG_3731_det.jpg?ver=0)
Regards Jan
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.
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?
Quote from: grepper on December 26, 2014, 07:04:40 AM
Is this discussion simply about how shiny a bevel looks?
How shiny and how it forms an image, and how that might relate to the quality and usefulness of the bevel.
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 (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.