sanford wrote:

Do I understand? Sounds like your iron has two curves. First you talk about the cutting edge itself being curved. That is called a “crown” and people sometimes introduce a bit of crowning intentionally for various purposes. Paul does not, except on his scrub plane. Crowning will allow the plane to take a bit more from the center of the blade than from the edges. Second, it sounds like the back of the plane has a curve, what I have seen called a “belly.” This makes the back convex. That means the back has not been flattened perfectly. If the back to too far off, it might be rather hard to get rid of that belly so as to have a nice flat back.

I am not an expert, but I would think that if the back is convex, then the front (bevel side) of the iron may be concave. And sharpening an iron with a convex back and a concave bevel side will not give you a nice straight cutting edge. It seems to me that sharpening will take more steal off the corners than from the center yielding a crowned cutting edge. Try a little experiment. I just grabbed a post card which is shaped a bit like a plane iron. I bent it a bit so that its back is convex and its front (representing the bevel side of an iron) is concave. When I put the part of the post card representing the cutting edge on the table at a normal sharpening angle, I see that the edges of the card hit the table before the center hits the table. So if I were sharpening that post card on that table, the corners would wear off before the center touched.

So unless I misunderstood what you described, it looks to me like like a crowned cutting edge is to be expected with an iron warped with a convex back and concave front. Hm . . . the flip side is that if the iron were warped the other way, with a concave top and a convex bevel side, more iron would wear from the center than from the edges. That is exactly a problem that some novice’s (me!) have when sharpening a gouge. We take too much metal off the center of the gouge and end up with funny looking “ears” at the top of our gouge’s cutting edge.

Yes, you understood it correctly, and your explanation makes sense!
It must be what’s happening, indeed, I’ve never thought about it that way!
I’ll flatten the back of the iron and sharpen it, and see it the “crown” disappears from the cutting edge.

Thank you for your explanation.

Robin Clark wrote:

Hi there,

I have maybe picked this up wrong, but the curved edges bellied at the cutting edge (knife edge), or do you mean concaved as in when bladed lying flat the under surface is raised? If it’s the knife edge that the issues it maybe your sharpening platform. Are you using wet stones? If you are, and especially 400-600g need flatting before every use, because they go out of flat very easily.

Robin

Hi Robin, it’s the cutting edge of a plane iron.
I’m using diamond stones, the same as Paul Sellers uses.