Blog - general
Back bevel and ito-ura
15 January 2012
A very good question about back bevels was raised in the comments section of the previous blog entry on the ito-ura. I tried answering it in the comments section, but my reply was probably too long for the commenting system to handle, so I’ll answer it here.
The question was: “I understand why a back bevel isn’t a good thing with a chisel, but why is a very slight back bevel not ok with a plane blade?”
The back bevel is convenient on a normal Western plane blade because without the hollow (urasuki) of the Japanese blade, there's a fair bit of real estate on the back of the blade to keep flat. Back bevelling (David Charlesworth's "ruler trick" is possibly the classic example) reduces this to a fraction of a millimetre at the cutting edge. I use this for my block plane blade, and also on the blades in my Stanley planes, which unfortunately are feeling fairly neglected as I haven't used them for a few years.
A properly formed ura with the ito-ura, doesn't have this large area to keep flat. It's only the thin flat (uraba) at the cutting edge and the two thin flat areas (ashi) to the sides of the uraba that have to be kept flat. Provided the blade has the ito-ura, essentially all the advantages and benefits of the Western blade back bevel for sharpening are already inherent in the structure of the laminated Japanese blade.
So as long as you don't allow the atama to sag when polishing the back to remove the burr, it's really quite simple to keep the very thin ito-ura area flat, and a back bevel would simply be an additional unnecessary step.
The second, and probably less significant, reason is that on the final finishing plane (jō-shikō kanna) the chip-breaker should be set back 0.1–0.2 mm, and about 0.2 mm on the intermediate finishing plane (chū-shikō kanna), which is very close (in Japan, this is referred to as "kami no ke ippon" — the width of a hair). A back bevel has the risk of interfering with the contact between the uraba and the chip-breaker.
Forming a back bevel to increase the cutting angle for planing difficult hardwood is a different matter. I still wouldn't give the Japanese blade a back bevel though. For hardwood with difficult grain I'd either use a kanna dai with a higher cutting angle, give one of my Stanley blades a back bevel, or use a bevel up plane with the blade sharpened to a higher cutting angle (I have the Veritas low-angle jack plane for this).
The question was: “I understand why a back bevel isn’t a good thing with a chisel, but why is a very slight back bevel not ok with a plane blade?”
The back bevel is convenient on a normal Western plane blade because without the hollow (urasuki) of the Japanese blade, there's a fair bit of real estate on the back of the blade to keep flat. Back bevelling (David Charlesworth's "ruler trick" is possibly the classic example) reduces this to a fraction of a millimetre at the cutting edge. I use this for my block plane blade, and also on the blades in my Stanley planes, which unfortunately are feeling fairly neglected as I haven't used them for a few years.
A properly formed ura with the ito-ura, doesn't have this large area to keep flat. It's only the thin flat (uraba) at the cutting edge and the two thin flat areas (ashi) to the sides of the uraba that have to be kept flat. Provided the blade has the ito-ura, essentially all the advantages and benefits of the Western blade back bevel for sharpening are already inherent in the structure of the laminated Japanese blade.
So as long as you don't allow the atama to sag when polishing the back to remove the burr, it's really quite simple to keep the very thin ito-ura area flat, and a back bevel would simply be an additional unnecessary step.
The second, and probably less significant, reason is that on the final finishing plane (jō-shikō kanna) the chip-breaker should be set back 0.1–0.2 mm, and about 0.2 mm on the intermediate finishing plane (chū-shikō kanna), which is very close (in Japan, this is referred to as "kami no ke ippon" — the width of a hair). A back bevel has the risk of interfering with the contact between the uraba and the chip-breaker.
Forming a back bevel to increase the cutting angle for planing difficult hardwood is a different matter. I still wouldn't give the Japanese blade a back bevel though. For hardwood with difficult grain I'd either use a kanna dai with a higher cutting angle, give one of my Stanley blades a back bevel, or use a bevel up plane with the blade sharpened to a higher cutting angle (I have the Veritas low-angle jack plane for this).
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11 June 2011
I do all my own website work (as you can probably tell), and I’m still learning all the various bits and pieces. I’ve just changed the RSS feed link to this blog, so a thousand gomen-nasais if this affects your RSS access to this. The RSS feed should hopefully now take you to the latest part.
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