Jura T

Hollowing back of woodworking chisels

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Hi,

How would you hollow a back of a chisel as seen f.e. over here:

  damekiri309530b.jpg

Those ones seem to be ground. Any idea what is used for the narrow grooves in production runs? I'm planning to do a set or two, so speed and consistency would be required...

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i always grind them in with the contact wheel on my belt grinder, after heat treat. I have made some in the field (ie middle of nowhere, in a field with very few tools) and forged a hollow in with a fuller, but that was to reduce the amount of filing i would have to do

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Why do you do it after heat treat? I tried doing it with a fuller, but it is difficult to get good looking results. I guess I finally need to get a proper belt grinder.  

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These are Japanese chisels. You can probably find a video showing how they are made. They usually are soft steel with a hard tip. However, Why do you want the groove? When I use a chisel for more exacting woodworking I am very glad my chisels have a perfectly flat side that allows me to cut with precision where  a plane cannot get.

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Well, as Davis wrote it is a lot easier to lap the bottom when it is hollow. Sometimes when I've restored old chisels, I've ground the center hollow to make the lapping faster. Partially it is just the challenge to make Japanese chisels ;) 

I haven't found any videos, despite quite extensive search.

 

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the hollow back makes it much much easier to get it flat and makes sharpening it in the future faster and easier (some say it reduces friction on the wood too, but that's less obvious to me!). For my own purposes a chisel that is flattish is perfectly fine, but the people I sell them to are often a little more anal about things being as flat as possible; so without spending out on a surface grinder this works best for me ;)

 

As for why I grind after heat treating. I do all of my back and edge grinding after heat treating. I don't have to worry about decarb or warpage that way

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is there a reason to do this other than looks?   Or maybe to reduce the weight a bit?   What is the benefit?

Now hollow grinding the cutting edge is worth the effort.   This makes resharpening much faster and is well worth the time in my opinion.

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A wood chisel is flat on one side because it allows it to make a straight cut. The straight side slides on the surface and the edge pares away any unevenness. The operation is not unlike a plane. The flat side is never ground. The bevel on the front is sharpened and it's true that it should be very slightly convex since this facilitates re-sharpening with a hand tool. This geometry is invaluable when shaping tenons and similar stuff. A wood chisel is used a lot by hand only without a mallet and the flatness is a must. 

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I think those were probably ground with a profiled water stone.  That is the way it's done in Japan. Probably with a slotted table that is adjusted so that the water stone just rises above the surface to the depth of the grooves. Most likely with a fence to guide the groove placement.  NOT the cheap way to go... but in Japan the sharpening guy is a well equipped specialist... also HIGHLY SKILLED!  

The real advantage is in making the chisels easier to sharpen by hand with traditional water stones.  A craftsman with a good belt grinder would find those grooves more of a nuisance than an aid!  I am one of those guys!  

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1 hour ago, bigfootnampa said:

I think those were probably ground with a profiled water stone.  That is the way it's done in Japan. Probably with a slotted table that is adjusted so that the water stone just rises above the surface to the depth of the grooves. Most likely with a fence to guide the groove placement.  NOT the cheap way to go... but in Japan the sharpening guy is a well equipped specialist... also HIGHLY SKILLED!  

The real advantage is in making the chisels easier to sharpen by hand with traditional water stones.  A craftsman with a good belt grinder would find those grooves more of a nuisance than an aid!  I am one of those guys!  

I can see how that would be done but in this case - the ends of the hollows are a different shape which would lead me to believe they were mechanically forged (possibly).

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I do the final sharpening of my chisels with waterstones. When taking the back of the chisel (not the whole back but part nearest to the edge) to mirror finish I find that I can do it faster and easier with Japanese chisel (hollow backs) than with western chisels (flat back). I haven't noticed any problems using Japanese chisels for example for tenons.

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A carbide grit hole saw in a drill press would work.  If you took a large block of wood and sawed it vertical to the drill press you could clamp the blade in a central dado.  Clamp a piece of scrap wood into the block, clamp that to your hole saw, and hog out the hole using a wood cutting hole saw/bit of the same size as the carbide grit.  The wood block becomes the fixture that aligns the hole saw and clamps the chisel stock.  You could even drill some small holes angled down into the main hole so you can dribble oil in to cool things down.

I watched a woodwrights shop episode on japanese planes and they explained that the back of the plane (or chisel) isn't "worn out" when the main bevel descends to the top of the relief cuts because you're supposed to carefully hone the back to taper towards the tip.  The relief cuts are only through a portion of the hardened layer's thickness. 

 

 

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I finally managed to find a Japanese video on the whole process. They use a bench grinder to hollow the backs (www.youtube.com/watch?v=6bI_q1gksII, see 18:50). Embedding the video seems to make the the post dissappear...

  

 

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On 1/27/2016 at 7:36 AM, Jura T said:

I finally managed to find a Japanese video on the whole process. They use a bench grinder to hollow the backs (www.youtube.com/watch?v=6bI_q1gksII, see 18:50). Embedding the video seems to make the the post dissappear...

  

 

I'm not sure if this is the video you meant, Jura T, but I found this video that shows how the ura (the hollow back) is formed both by the modern method of grinding and the traditional method of scraping with a sen. Grinding starts at 17:56, and using the sen starts at 18:13.

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Great video, thanks. Hadn't seen that earlier. I was more or less sure that they would have used sen in the old days, and that confirms it.

  

 

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On 1/27/2016 at 8:11 AM, Forging Carver said:

Why do you want to hollow the back of your chisels?

Read above posts

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Yeah I totally agree about hollow grinding making a sharper tool and easier to sharpen, but the con cavities are nowhere near the edge to make a difference. From what I see, it is practically a groove in the area behind the edge. Now I am sure that these grooves work in some way, but I don't see how. My tools that have a hollow grind have the concavity on the edge making the cross section of the edge a u or c shape. Anyone have a decent explanation to how the chisels in the picture hollow grind works?

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A few years back, I was fortunate enough to have spent six days with the premier saw maker of Japan, Yataiki (R.I.P). He was versatile, could make many other tools as well as 113 saw patterns. I had to leave the workshop early, but one of the participants, John Burt, sent me as a gift, a marking knife that Yataiki made. It had the hollowed flat back, which was done with a sen. Yataiki had many sen of varying cutting blade widths.  

The workshop was 30 days long and was held in Fairfield, Iowa. It was designed primarily for woodworkers who used Japanese saws, and the emphasis was on sharpening and repairing saws. I was allowed to make a small hammer under Yataiki's direction.                                 

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