How do you make a convex edge?

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wacki

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Sep 12, 2009
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Paper wheels are going to make a concave edge. How do you make a convex edge on a knife?
 
I think you're thinking of it backwards. Paper wheels will convex an edge, as will sand paper on a mouse pad. But in reality, all freehand edges are convex to some degree since we are human and nobody can keep a perfectly consistent angle.
 
Convex edge- isn't that what we call a "hollow grind"? Those are made with a big wheel on a belt sander. (I don't know if the big companies use a belt sander or some form of grinding wheel.)
 
bdmicarta said:
Convex edge- isn't that what we call a "hollow grind"? Those are made with a big wheel on a belt sander. (I don't know if the big companies use a belt sander or some form of grinding wheel.)
Click to expand...

You're thinking of a concave edge. A convex edge looks like this:

Convex_Edge.GIF
 
bdmicarta said:
Convex edge- isn't that what we call a "hollow grind"? Those are made with a big wheel on a belt sander. (I don't know if the big companies use a belt sander or some form of grinding wheel.)
Click to expand...
Nope, hollow grinds are concave. Concave=in, convex=out. A convex edge is essentially an edge which has had the shoulder of its primary and edge bevel rounded to blend the two bevels together. You can have a convex edge on a hollow grind. The only way to have a concave edge would be to have a zero hollow grind, meaning hollow ground all the way to the edge with no secondary edge bevel.
 
Dorito Monk said:
You're thinking of a concave edge. A convex edge looks like this:

Convex_Edge.GIF
Click to expand...

OK I go back and re-read the original post with the question. I did respond incorrectly.

I don't know if there is a problem with terminology but paper wheels will create a CONVEX edge. Freehand sharpening depending on how good you ware will eventually create somewhat of a convex edge. The original grind with a big wheel on a belt sender will create a CONCAVE edge, or hollow grind. I think a person that freehand sharpens will eventually create somewhat of a convex edge unless they are really good at it.
 
Handy tip....caves go "in"....like concave grinds. :) And freehand sharpening creates a convex edge to varying degrees. (I made a pun! Degrees!)
 
wacki said:
Paper wheels are going to make a concave edge. How do you make a convex edge on a knife?
Click to expand...

bdmicarta said:
Convex edge- isn't that what we call a "hollow grind"? Those are made with a big wheel on a belt sander. (I don't know if the big companies use a belt sander or some form of grinding wheel.)
Click to expand...

A hollow grind is concave (curved inward, as if looking at the inside arc of a circle). A convex grind is the opposite (curved outward), as if viewing the arc of a circle from the outside.

Hard/firm grinding wheels will put a concave (hollow) grind on a blade, and a slack belt will convex it, as will sandpaper on soft backing (like a mousepad). A soft buffing wheel will tend to convex as well. This is all assuming the operator doesn't vary the angle of contact, as with freehanding. If the angle is varied, a convex will always be the result, regardless of the tool used.
 
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Obsessed with Edges said:
Hard/firm grinding wheels will put a concave (hollow) grind on a blade
Click to expand...

This can be easily seen when sharpening any Mora. Lay the whole bevel on your stone and start sharpening. You'll soon see that the bevel is being ground away near the edge and near the shoulder but not in the center. As you continue to grind away metal, the bevel will flatten and begin to sharpen evenly.

Fro convex edges I use wet/dry sandpaper on leather. I like thick stuff like 8-9oz or so. I find my sharpening style is too aggressive for a mouse pad, which requires a very light touch.
 
you could always just go over a stone and purposly change your angles slightly every time you run the knife over the stone. atleast thats what i did, came out pretty good imo.
 
I'm sure Richard J will chime in soon, but until then..
The paper wheels do give a slightly CONVEX edge. Yup, it sounds
counterintuitive, but the wheels, especially the polishing wheel
have enough surface give to round off and make the edge convex.

To make a true concave edge, as well as a hollow grind, you need a harder backing
than the paper wheels give. Think hard rubber wheel, 85+ durometer for backing a belt, or a
curved platen. These will give a great concave edge.

Look at a blade sharpened on paper wheels using some magnification and you'll
see that the edge really is convex.

Bill
 
Round wheels that are made of hard paper or any other solid object will NOT convex a edge or any other flat surface. It is completely impossible, the wheels have no give to them.
 
I guess you haven't used the paper wheels yet. The polishing wheel certainly is soft
enough to "give" slightly. They're not stone hard. If you watch the blade edge-wheel
interface the distortion of the wheel is easy to see. If you then look at the edge you've
just sharpened, it's clearly convex. Not a whole lot, but convex nontheless. I've
sharpened several thousand blades, I do it commercially, and trust me, it's convex.
Really, really sharp and convex.

Bill
 
I've seen them, touched them, and watched them spin, layers of paper glued together is very hard.
 
As you use the polishing wheel, the area of the wheel right next to the cooling slots
begins to soften and fray ever so slightly. It's this softer surface that gives the
convex edge. Seriously, look at the edge under a magnifying glass and you'll see
that the edge is convex.

Bill
 
Free handing will always convex an edge. You may convex even more, by keeping the resistance feeling the same and slightly lift the spine between the strokes. First thing to do, though, is to remove shoulders.
 
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