Convex grinding question

[Chiro75]

I do occasional convex grinding on my Coote. My problem is that the center portion of the knife (about 1/4" on 1" wie stock, or 1/2" on 1.25" wide stock) doesn't seem to get ground. I've played with grinding closer to the platen, closer to the idle wheel, etc and that doesn't seem to effect it much. If I really push hard, I get lots of metal being removed from the edges of the belt but the center section still pretty much is untouched. "Rolling" the blade doesn't seem to help much, either. Changing belt speed doesn't matter, either. This is 1/8" thick stock, too, if it matters (which maybe it does). My questions:

1) Is this normal?
2) Is this a belt stiffness problem? (I use the regular 3M stiff belts up to 220 then switch to the really thin, flexible Klingspors after that).
3) Is this a belt tension problem? If so, too much or too little (I am using the same setup I would use for platen grinding, only I'm grinding in the space between the top of the platen and the idle wheel, which is about 1' or a little more of space)

Thanks!

 

[AwP]

I haven't had that specific problem, but I have a guess you can try. Try grinding it edge up, but NOT touching the edge itself to the belt, that way it should start grinding from the centerish to the spine. Don't put it at too much of an angle or you'll just toast the spine and still barely touch the center. Hope this helps.

 

[oldwolf]

It has been a common problem with me also. I do a lot of convex grinding as solved the problem by using a soft face on the platen, consisting of some rubber "mouse" material and thick leather sandwich. I made it with wire hooks on the top so it's easy to remove. You can roll the blade some and hit where you want and it won't gouge the blade if your careful. I think I got that off this or another forum.

 

[Raymond Richard]

What I have been doing for years since all my grinds are convex is I put half inch felt on the platon to give it some cusion. I'm still doing that but now I'm also putting a layer of graphite material on the felt so it will last longer and it won't smell like a wet dog. You can also take a piece of wood and either cut or grind a convex indent. Put a layer of graphite on that also. I use Barge Cement to hold it to the platon but any leather glue will work.

 

[Gossman Knives]

chiro the area you are trying to convex grinding in is to narrow. I did the same thing with my 2x48 grinder. This is my slack belt set up on my KMG

The belt has be tight and you really need to push hard when working the grind up the blade. You need to change belts more often because you're not pressing against a hard surface like a platen or contact wheel.
Scott

 

[Chiro75]

Try grinding it edge up, but NOT touching the edge itself to the belt, that way it should start grinding from the centerish to the spine.

That is what I do and it doesn't tough the center of the blade at all. I've even slowly, super carefully made minute changes in angle from grinding the spine all the way to the edge, a little at a time, and somehow it skips that middle zone. It's like an invisible shield on it!

Oldwolf and Raymond, genius! Duh!I'll give that a shot next time...

 

[Gossman Knives]

Here's an example of a convex grind using the slack belt. This is 5/32" thick by 1 1/4" wide O1, 5" long blade. This is convex ground down to 0.020 at the edge before HT.
Scott