Convex Grind

B

Benen

Joined
May 11, 2017
Messages
44
Hi!
Any proficient convex grind sharpeners able to give me any tips?
I'm using a global paring knife to learn and it's quite blunt. It can only just cut paper and I've been trying with a mouse pad and sand paper for around half an hour. Gently stropping I back and forth with a very shallow angle and almost no pressure applied to the weight of the knife.
I began with 360 and worked through to 1200 with no improvement.
I've been working on 600 grit since foe around 100 strokes per side with still no improvement
 
I sharpen convex edges on regular stones. It's easier for me that way to make sure I hit the apex at the angle I want. Everything behind the apex just gets blended in until you reach the spine or however high you want the convex to go. Keep practicing on cheap knives, it's not hard if you can sharpen regular edges.

If you use soft backing and sandpaper, mark the edge with sharpie and watch how it behaves, how much pressure you need to apply and which points the sandpaper is hitting. You want tha sandpaper to mae contact with the apex but not so much that it wraps around it and makes a steeper angle than intended. you said you use a very shallow angle.. it might not be hitting the apex. Don't count the strokes, raise a burr, cut the burr off, look for symmetry.

As I said, I prefer stones for more control... plus you can sharpen your convex edges in field that way.

stones+ strop
6yzext.jpg
 
You are starting at too fine a grit! 360 is a finishing grit for the majority of commercial sharpeners.

Drop the sandpaper on your pad to 120 grit to establish your bevel, then work your way up to your finishing grit. I go to 15 Microns which is around 1200 G then I Buff with Green Chrome.
 
A soft backing behind sandpaper shapes the upper portions above the edge pretty easily, to a convex shape. But it's lousy for enhancing or improving the sharpness of the edge itself, which needs to be maintained closer to a 'V' profile to be sharp. Softer backing will also make the sandpaper work much slower, i.e., much less aggressively, for metal removal.

Try using exactly the same method on a firmer backing, like hard/stiff leather or wood. That'll help the abrasive work more aggressively, while at the same time keeping the edge (apex) crisper than it would be, coming off the soft-backed abrasive. Glass or stone as the backing will do even better, though it takes more getting used to, in terms of using that to shape the convex itself. That's really the only good reason a soft backing is so popular for convexing, as it's pretty easy to learn. But results, in terms of sharpness, will not be as good, because the soft backing is too prone to rounding the apex itself. Even more so, if the edge starting out is very thick or blunt, which only increases the likelihood of rounding on a soft backing, like a mouse pad.

With a hard backing, a thin grind like a paring knife should ordinarily be easy to shape fairly quickly on sandpaper, even at 360 grit. Coarser still works faster, but a paring knife can usually be done reasonably quickly, if the grind isn't unusually thick to begin with. I used to use anything up to 400 grit for tasks like this, but 220-320 is a good place to start.


David
 
Last edited:
Its definitely improving. I sat there for around an hour though! It cuts paper but not amazingly. I'll try on some wood
 
Make sure you clean the sandpaper with a synthetic wine cork, eraser etc often while you work - once it loads up its going to cut slow and plug right up.

Also, make sure to use silicon carbide wet/dry when sharpening.
 
Hmm I I don't really get it. In all of the YouTube videos I've seen, they just use super fine grit dry sand paper like 1500 or 2000 followed by a leather strop and get it shaving sharp.
 
Benen said:
Hmm I I don't really get it. In all of the YouTube videos I've seen, they just use super fine grit dry sand paper like 1500 or 2000 followed by a leather strop and get it shaving sharp.
Click to expand...
Hi,
You can do that too,
simply increase the angle so you're hitting the apex ,
starting with a lightly blunted knife also helps with such high grits.

If you're trying to raise a burr,
and its taking more than one or two minutes (however many to do 300 passes ),
even with P360 or P600 sandpaper,
your angle is too low,
and you're not sharpening,
you're shaping/regrinding/reprofiling,
so increase the angle until you're hitting an apex,
use permanent marker trick,
count the passes or sing a 1 minute song,
increase angle again if required
 
I believe the knife was a v grind previously. I've spent another few minutes this morning on the 360 paper and you can very clearly see the convex shape now. I can cut paper ok'ish with the front half of the blade. Push cuts are a joke though. I'll keep at it!
 
Benen said:
I believe the knife was a v grind previously. I've spent another few minutes this morning on the 360 paper and you can very clearly see the convex shape now. I can cut paper ok'ish with the front half of the blade. Push cuts are a joke though. I'll keep at it!
Click to expand...

Shape it with a hard backing under the wet/dry. You want the edge to be as crisp as possible.
 
Heavy I finally got my gunny right after all the damn questions last week. Thanks for helping me with that! Watch some of his videos they helped me
 
Thanks guys, I'm getting somewhere. I've managed to shave a few hairs with the front half of the blade. Heading out to get coarser paper now.
 
I'm getting extremely disheartened. I have spent quite a few hours trying to sharpen knives with a convex grind. I have paper from 180 to 1200 grit.

I processed two chickens yesterday so used the opportunity to sharpen my F. Dick butchers knife as it could shave hairs before the work and had lost its edge.
I spent about an hour trying to sharpen it yet I think it could even have gotten blunter since trying.
I can get it plenty sharp on a cheap stone but really want to learn ago sharpen convex grind knives as I have one that I want to buy.
Everything I read says how easy they are to get sharp and keep sharp yet I'm just not getting anywhere.
 
You probably have your angle too low. Use a marker and draw some lines from the middle of the knife down to the edge so you know what your hitting. Probably just need to raise your knife up a little
 
Benen said:
I'm getting extremely disheartened. I have spent quite a few hours trying to sharpen knives with a convex grind. I have paper from 180 to 1200 grit.

I processed two chickens yesterday so used the opportunity to sharpen my F. Dick butchers knife as it could shave hairs before the work and had lost its edge.
I spent about an hour trying to sharpen it yet I think it could even have gotten blunter since trying.
I can get it plenty sharp on a cheap stone but really want to learn ago sharpen convex grind knives as I have one that I want to buy.
Everything I read says how easy they are to get sharp and keep sharp yet I'm just not getting anywhere.
Click to expand...

If you're already getting good sharpness from the stone, don't be shy about using that to finish or refine the edge after you've shaped the convex on the sandpaper. As previously mentioned, a stone or any other hard substrate will always do better, for keeping the apex crisp. And at some point, after doing that for a while, you might find you can do both on the stone alone. That's sort of the path I followed, in finally using stones alone to do convexing. A back & forth scrubbing motion on the stone, with some of the inherent freehanded rocking back & forth of the angle, has worked very well for me, in shaping the upper portions of the convex, above the edge itself. Then, once that's taken care of, a more traditional edge-leading stroke, done with a lighter touch, works best to refine the edge itself.

In terms of sharpness degradation at the edge itself, most of the issues I ran into were due to the movement of the paper (lifting, rolling/curling) and compressibility of the substrate under the edge, as I worked. These always contributed to some rounding/blunting of the apex, and more so, if pressure got just a little too heavy, or the angle wasn't controlled tightly, when working the apex. Both of those issues are eliminated automatically, in using a stone instead of the paper over a softish substrate.


David
 
Benen said:
I'm getting extremely disheartened. I have spent quite a few hours trying to sharpen knives with a convex grind. I have paper from 180 to 1200 grit.

I processed two chickens yesterday so used the opportunity to sharpen my F. Dick butchers knife as it could shave hairs before the work and had lost its edge.
I spent about an hour trying to sharpen it yet I think it could even have gotten blunter since trying.
I can get it plenty sharp on a cheap stone but really want to learn ago sharpen convex grind knives as I have one that I want to buy.
Everything I read says how easy they are to get sharp and keep sharp yet I'm just not getting anywhere.
Click to expand...

If you're really spinning wheels, switch to a hard stone and form the convex manually.

Per
Gradyw said:
You probably have your angle too low. Use a marker and draw some lines from the middle of the knife down to the edge so you know what your hitting. Probably just need to raise your knife up a little
Click to expand...

Mark up the entire edge and work it down on a stone. Save the wet/dry over conformable backing for the cosmetic work. Once you shape an edge to convex on hard stones once or twice, you'll have a lot more understanding of where its not working on the wet/dry. Or at least use the wet dry over a brick or something super hard so it doesn't have much give.
 
You must log in or register to reply here.
Back
Top