Double wheel grind ?smile?

I was just looking at the Dovo U-Tube video showing production symmetrical hollow grinding using a double wheel grinder. It was showing someone grinding straight spine razors and by keeping the spine flat on the tool rest, the edges would be straight also. No surprise there.

But if you have a curved spine keeping the spine on the rest would produce a matching curved (smiling) edge. How would you get a curved spine and a straight edge? Would you need 'large' wheels to establish the curve of the underside of the spine and then 'small' wheels and a tool rest long enough for both ends of the spine to rest on it to make a second grind straight across?

Just wondering.

You don't.

Curved spines will automatically cause curved edges.
The geometry does not work for straight edges and curved spines unless either the blade is not ground up to the spine (i.e. it is just a straight spine with decorative 'humps' at the heel and the toe) or unless the spine is thicker at the heel and the toe so that the angle stays the same.
Or you could possibly work with narrow wheels, and give different parts of the blade a different grind.

Either way, I doubt the aesthetics would be pleasing.

Take a look at the "a grind issue" thread of mine in the Forge. Ray Cover made a wooden wedge that illustrates perfectly how a smiling wedge works. It might shed some light on the situation for you.

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Originally Posted by Bruno

You don't.

Curved spines will automatically cause curved edges.
The geometry does not work for straight edges and curved spines unless either the blade is not ground up to the spine (i.e. it is just a straight spine with decorative 'humps' at the heel and the toe) or unless the spine is thicker at the heel and the toe so that the angle stays the same.
Or you could possibly work with narrow wheels, and give different parts of the blade a different grind.

Either way, I doubt the aesthetics would be pleasing.

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I guess with a small curve in the spine the bottom of the smiling edge wouldn't be too out of line with the corners, so maybe a very narrow second 'straight' grind, basically just 'kissing' the corners, could take the smile out without leaving a honking great transition line between the two grinds along the length of the blade.

I just looked at one of my curved spine razors (the first one I thought was a curved spine was really 'just' :) a worked back on a straight spine) with a very gentle/shallow curve (maybe a 1/16" swayback over 2 5/8") probably started out as a very gentle smiling blade, but over the years has lost most of the smile. On careful examination I can see the heel still recedes from the edge a wee bit. It looks like a swayback with a straight edge, but really isn't. To my eye, the aesthetics are fine. Maybe with a pronounced swayback, a straight edge might look a bit odd.

Quote:

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Originally Posted by medicevans

Take a look at the "a grind issue" thread of mine in the Forge. Ray Cover made a wooden wedge that illustrates perfectly how a smiling wedge works. It might shed some light on the situation for you.

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Thanks medicevans. TMI. I'm going to have to read and study that thread before it all comes clear. I understand you have to roll the 'X' to hone a smile, but that thread seemed to be describing some kind of warp in the geometry of a smiling blade. I'm probably looking at the 'x' dimension/view when the post is describing the 'y' or 'z' dimension/view. Or it's describing the resulting geometry when grinding with one wheel. Either way, I'm going to have to reread that thread before it makes sense to me. Thanks for the link.


O.K. just reread this reply and I think Bruno's "or unless the spine is thicker at the heel and the toe so that the angle stays the same" is describing the same thing medicevans' "Ray Cover made a wooden wedge that illustrates perfectly how a smiling wedge works." is. It is slowly coming clearer. Thanks guys.