I need help with convex grinds!

Snow

Snow

Joined
Jun 3, 2001
Messages
2,233
Fulloflead, I'm sorry for hijacking your thread. Rather than further pollute it, I've started my own.

I am having trouble with my convex Howling Rat. It came kinda dull from the factory and I have been trying to get it sharp with no success. I've read plenty about sharpening a convex grind, but I'm having lots of trouble.

Right now I'm using 320 grit sandpaper backed on a mousepad with moderate pressure. It has removed some of the edge and some of the finish, as you can see in the pictures below. I am holding it at the shallowest angle I can while still having the edge contact the paper, and drawing it backwards while pivoting the knife to also sharpen the belly and tip.

After a good hour with the 320, it's finally feeling a little "grabby," so maybe I'm making progress? It just seems like it's taking forever and I'm getting extremely frustrated. Here are pictures of the edge. I'm sorry these pics are bad, but I'm too worn out to try again. You can see how much of the finish is coming off. I was hoping with this info someone would be able to tell me if I am holding it at an okay angle.

P1010048.jpg

P1010055.jpg

P1010056.jpg



Any advice is welcome, guys. I feel like I'm in over my head.
 
You are grinding at a much reduced angle than the initial profile and since you are grinding soft it will take a very long time to reduce the edge angle signficantly. If you want to keep the origional profile then elevate the blade until the edge makes contact with the paper. This should have the blade sharp on the coarse paper very quickly, as in 1-2 minutes maximum.

If you want to apply a really acute bevel as the above pictures would lead to if you kept grinding. It is much more efficient to use a coarse solid stone and press really hard to do the initial shaping. Or as Thom suggested put the sandpaper on something rigid and press very hard. It should only take 5-10 minutes to really grind off a lot of steel.

320 paper is still pretty fine for reshaping though, I would drop down to 80/100 if you want to acually change the angle of an edge.

-Cliff
 
Well I don't really want to change the profile, I just want it sharp and I've never done it without a Sharpmaker before. I am holding it at a shallow angle because I read to hold it at 13 degrees, which seems pretty acute to me.

I will give it a shot tonight with the 320 but pushing a bit harder and at a steeper angle.
 
Great looking blade!

I would forget the convex edge and just use a double-bevel edge grind when sharpening. It is so much easier than reprofiling the entire blade and trying to match the convex curves on both sides. The double-bevel is the 1st cousin to the convex, IMHO. A double-bevel with some stropping with some CrO on the joint between the bevels will give you a semi-convex edge I think.

Here's a link with pics and diagrams that may help.


http://forums.egullet.com/index.php?showtopic=26036

If there is a custom knife maker near where you live he probably has a belt grinder ( the best way to sharpen a convex edge) and would be glad to help if you are nice and bring him some donuts or liquid refreshments and show an interest in his knives and work. Just a thought!
 
Snow said:
I am holding it at a shallow angle because I read to hold it at 13 degrees, which seems pretty acute to me.
Click to expand...

That is the angle recommended in the BRK&T document but those knives have a very different profile. Convex edges are no different than v-ground bevels in that you have to adjust the height of the blade above the stone to produce the required edge bevel(s). When using a soft backing media which will give somewhat you don't need to be as precise, but you still have to be with a degree or two of the existing bevel.

-Cliff
 
Okay I just got home from school and did a quick 10 strokes per side at a much more obtuse angle. It now feels a lot sharper, but still not shaving sharp. I'm gonna give it some more on the 320, then switch to the 600. Would doing 400 before 600 have any advantage, or should I skip it?
 
Snow, if you'd like, I'll resharpen it for you. Convex grinds and edges are what I do. Let me know if I can help.
Scott
 
Snow said:
Okay I just got home from school and did a quick 10 strokes per side at a much more obtuse angle. It now feels a lot sharper, but still not shaving sharp. I'm gonna give it some more on the 320, then switch to the 600. Would doing 400 before 600 have any advantage, or should I skip it?
Click to expand...

Don't move to the finer grits until the blade is sharp. Knives can shave readily at finishes far more coarse than 320 sandpaper. Once the edge has been honed at 320 and is cleanly slicing paper you need to remove the burr. The easiest way to do this is a few light passes into the paper on a hard medium at an elevated angle.

-Cliff
 
Hey Snow. I'm just south of you down in Spring Hill (work in Lenexa).

If you are still having issues, PM me. Maybe I can help you out a bit. Don't claim to be a convexing expert, but I have convexed a few blades of my own successfully.

--Chris
 
Cliff Stamp said:
The easiest way to do this is a few light passes into the paper on a hard medium at an elevated angle.

-Cliff
Click to expand...


Can you explain this? I'm sorry but I'm a complete noob with sharpening.


Razorback and Saunterer - Thanks for the offers, but for now at least I'm gonna try to do it myself, otherwise the next time it gets dull I'll be SOL again. I'll let you guys know if I make progress.
 
No problem Snow. I was not planning on doing it for you, that would be too easy. Teach a man to fish.... :)

A way I have removed the burr, is to slice a piece of wood down the blade vertically with light pressure. (Think of trying to slice off a piece of luncheon meat using the blade from bolster to tip....)
 
Don't press too hard on the mousepad.
It curves back up and can dull the very edge.
Light pressure and a slightly higher angle (as previoulsy recommended) should do the trick.
 
Snow said:
Can you explain this? I'm sorry but I'm a complete noob with sharpening.
Click to expand...

Clean the sandpaper or use a fresh piece, put the paper on something hard, raise the blade until it is significantly above the as ground edge angle, make a couple of very light and very short passes as if you were trying to cut the grit off the paper, change the sides of the knife on every pass. It should only take 1-2 passes per side. This should remove the burr from the edge and set it clean. You can now proceed to finer grits if necessary. On some knives you may not need to do a specific burr removal step because the edge will form fairly clean. However if the edges meet but the blade isn't sharp then all that remains is to remove the burr.

-Cliff
 
Saunterer said:
No problem Snow. I was not planning on doing it for you, that would be too easy. Teach a man to fish.... :)

A way I have removed the burr, is to slice a piece of wood down the blade vertically with light pressure. (Think of trying to slice off a piece of luncheon meat using the blade from bolster to tip....)
Click to expand...


Yeah I may still PM you if I can't get it. I have a hard time learning to do things solely by reading them. If I could watch somebody, I'd have an easier time.

I did some more passes last night with it, and at one point, the part of the blade closest to the handle would shave poorly, but the rest wouldn't. I then started sharpening again and lost it. I think I am too inconsistent. Lighter pressure and a steeper angle seemed to work a bit better, as many people have said. Today I looked at it again and it has a "grabby" edge on both sides when felt with my fingertips, but still doesn't feel too sharp or cut very well. I am gonna try some more and try to keep all your guys' advice in mind.

Thanks everybody for the advice. It is very much appreciated. I'll post an update after trying some more.
 
Try this...

If you have a piece of leather or a leather belt, take it and lay it flat on a table. Then take your Rat and lay it flat on the belt. Run it blade first with a little pressure across the belt while raising the spine. Once the blade bites into the belt, that is about the angle you need. Try and keep this angle consistantly.
 
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