15th Mar 2007, 01:26 PM #16Senior Member
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- Feb 2007
Sorry it's taking me so long to get back to your email, but like I wrote I wanted to take time replying to that one, so give me a bit more time!
Let me just answer these for now.
You were saying that "I would think that as a curved blade is tapped in to the dai, the blade would tend to dig out the grooves to match the curve of the blade" and that's exactly correct. Hahaha, you have answered instead of me
Then you said "In order to make a dai to fit such a blade it would need to have the grooves cut to match the curve of the blade - and I've never seen anything like that before." but I think you have! Every dai should be like that because the groove is cut to match the curve by the blade itself like YOU have suggested!
It is thus very important not to touch (shave, file, cut) the upper part of the slot yourself, but let the blade do the cutting. You adjust the looseness of the slot by shaving the Omote-najimi(the surface where the front side of the blade touches. Front side is the side without the mei=brand.) I will write an detailed article on how to set the blade into the block for the first time, soon(?)
Q2: So when we refit an old blade into an old dai, how much of the curvature should be removed before refitting the dai? In your drawings it only looks like you tried to remove a portion of the curvature. For some blades the curvature is small enough (only .002inch/.05mm or so) that removing it all is no problem - or it's so small that it should just be left alone. However on other blades the curvature is large enough (0.5 - 1mm) that removing all of the curvature at once would significantly alter the ura-suki, could make the hagane very thin at the edge, and noticeably change the taper of the blade so it may become loose in the dai, perhaps requiring a new dai. What would you suggest?
The intention of uraoshi is not to make the ura flat in order to match the dai, but to gain enough flat surface so that you can maintain the flat contact between the steel and the stone. (Someone help me out with the English here!!) If there is only 5mm of flat area, it is very difficult to keep it apressed onto the stone flat, so you'd need about at least 2cm. The amount depends on how curved the steel is.
If the curve is great, and this was the case with my drawing being exagerated, you can only grind off a little, because as you have explained you'd be grinding so much and loosing the ura-suki=the hollow. And you don't want to bend it any further either, thus you need to keep the ura-dashi minimal. And as you use it you gradually grind off the blade side of the ura and try to increase the flat surface. You start off from 1 in the below drawing, and by the time you've sharpened your blade, uhhh I don't know 100 times? you'd be at the #2 line, and #3 when you have sharpened 300 time! The below drawing is super exagerated to make it easier to see.
You need to press the blade side harder, and try to kind of lift the other side (called kai-saki, the lamination line) a little without actually lifting it up, creating more pressure on the tip of the blade and less on the kai-saki side. By doing so you can avoid looking like B in the above article.
If the curvature is just a little, basically you can flatten the whole steel part. Again you need to press the blade side harder, and kai-saki side lighter.
Thanks for the question. I hope I have answered your question well enough.
Talk to you soon.
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15th Mar 2007, 06:02 PM #17
The smallest I've got is around the block plane size. And .....my planes arn't pretty mate like yours .... not in me to really get into that side of it unfortunetly.
But lovely work in those pictures. Great big thick blades are always appealing to me. And the hollowed backs I always thought was a good concept.
15th Mar 2007, 06:05 PM #18New Member
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- Mar 2007
- Oregon (bloody American!)
Thanks So, like many things in life - one often knows the answer, but doesn't realize it yet. I' have been doing the proper things - but the blade with big curvature was outside my experience and threw me a curve - so to speak. Glad to make it easy by providing my own answers.
Dead on - never mess with the top cut of the groove. Which is why they are the critical saw cuts when making a dai from scratch - they need to be straight and parallel to each other or you might as well just chuck it and start over. For the newcomer, rubbing a pencil or something similar on the "underside" of the blade while fitting will help point out the tight spots that need to be relieved. We'll all look forward to an article (or a 3 part series on DVD!) from So on dai fitting when he can find the time.
I guess I didn't look closely enough at the dai to detect any curvature - but since the residual curve in most blades is so small I would have had a hard time seeing or measuring it. I'll go measure the blades I use regularly for curvature and carefully check to see if the dai matches. Some will be old enough that the curvature will likely be gone but others are newer or not used as much.
Very good drawings and description of the balance between needing a flat part on the back so you can actually hold the plane on a stone for maintaining the back while sharpening , and the need not to remove so much that the hollow is lost unnecessarily. I don't think I've ever seen as good an explanation of this before. Just to be crystal clear for others the 1-2cm flat area So refers to is not measured in the middle of the blade - but measured at the sides. That reminds me of the name I have always heard and used for the flat part of the blade between the cutting edge and the hollow "puddle" in the back - we call it the "land" since it separates the puddle from the edge. I've wondered if that name originated in Japan?
Anyway then, for my rather curved blade I should approach the flattening over a period of time - doing just enough for now that I can sharpen effectively and do a bit of maintenance along the way until regular sharpening and periodic adjustments remove the excess curvature.
It does sound as though you (like me) don't think the hagane curvature is left/put there on purpose to aid in securing the the blade in the dai - otherwise you'd be trying to preserve the curvature as much as possible, instead of making a nice wide >1cm flat to work with. I could be wrong though and would like additional confirmation of this since it affects how the blade is actually made, I will try to confirm with a blade maker and a daimaker if I can.
An aside - some folks ask - if you can tap a blade one direction why can't you just tap it the other way and remove the unnecessary curvature. If you think about what's happening, bending the blade so that the hagane side becomes hollow puts the hagane side in compression and tapping the other way puts the hagane side in tension (stretching). Highly hardened high carbon steels can handle the compression pretty well so long as it's fairly uniform - but it does not handle tension very well and the common result is a crack.
15th Mar 2007, 06:58 PM #19Senior Member
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Ichihiro takes extra care for his chisels not to be curved. Like you've explained he makes it convexed and expect it to be flat once he hardens.
Last edited by Groggy; 26th Dec 2007 at 11:10 PM.
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