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My main point was that between what you call the apex and the edge heel, the soft iron layer will be thinner...  that's all...   And if you have experience with Japanese blades (especially high quality ones, not the junk that is generally sold through the western tool dealers...) you would know that the wrought iron layer is integral to the support of the thin blue or white steel layer...  and the risk of tapping out a Japanese kanna blade is high enough without thinning out the layer...  (and if you never tapped out a blade, you aren't using the plane very much, or have one of the junk blades that has a huge flat on the edge of the ura...)  You don't tap out a blade at the edge heel, you tap it in the middle, between the apex and the edge heel...  where it would be thinned out...

I'm not saying that you -can't- grind a Japanese blade on a Tormek...  just that I wouldn't (and neither would any of the guys that I know that use high quality Japanese tools...) ever grind a $600 hand made blade, and wouldn't see any reason to grind a cheap one either...  a hollow grind would structurally change the blade, and not for the better...

JH

You really don't want a hollow grind on a Japanese kanna blade (or Japanese chisel blades either...)  The blades of Japanese tools are made from two different types of steels...  there is a very hard steel that is brittle (the cutting edge...) laminated to a much thicker and softer iron...  the thicker iron keeps the more brittle cutting edge supported, and if you use a Tormek to hollow grind the bevel, you will be significantly reducing the thickness of the supporting iron...  mostly because of the thickness of the overall blade, and the length of the bevel...  (most of my kanna blades are around 25 deg. or so, and almost a half inch of bevel...)  Same goes for Japanese chisels... 

On western blades, the whole blade is the same type of steel, so hollow grinding isn't a problem...  but I would think that if and when you go to tap out the blade (which you will have to do at some point...) it's stressful enough to do it with the full support of the softer iron, but I would think that the possibility of cracking the hard steel would go up many times with a hollow grind...

Also, the main advantages of a hollow grind being to keep a narrower bevel "stable" when honing and not needing to hone as much steel on each sharpening pretty much go away with Japanese blades...  I've found that the wide bevel makes it very easy to keep the blade flat on the stones, and the softer iron hones so easily that they are much easier to sharpen on stones than western blades...

I use my Tormek all the time for western blades and knives...  but wouldn't touch one of my good kanna blades on it...

Also, no micro bevels or ruler trick bevels on the back on Japanese blades... 

Hope this helps...

JH