I need some help with Convex sharpening please.

SC_Dave

Gold Member
Joined
Jul 29, 2010
Messages
187
I am at my wits-end. I have always heard the a convex blade was easy to sharpen; not for me. I'll list some details below but I have tried every method I know to try and it's so frustrating to not get a positive result. So I would appreciate some guidance please.

This particular sheath knife is 01 tool steel, 3/16 thickness at the spine, some what of a drop point.

I have tried water stones, diamond plates and finally sand paper with a mouse pad. I have used the sharpie trick to be sure I'm getting to the apex and not going past the apex. I have tried no pressure, more pressure and heavy pressure. All futile.

I seem to get a tiny burr before I switch to the other side but the end result is an edge that will barely cut printer paper.

I understand that teaching someone to sharpen in words is kinda like teaching someone how to drive a car over the phone but any help/advice is appreciated.

Thanks in advance, David
 

Twindog

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Joined
Apr 6, 2004
Messages
3,352
If you're getting a burr, you're at the apex.

The thing about convex edges is that they can be very acute or very obtuse. You probably have a blade closer to the latter, with a lot of metal immediately behind the apex. That kind of edge makes it difficult to slice paper, even when sharp.

You could switch to a 30-degree inclusive V edge to improve the edge geometry. You could then smooth out the shoulders to improve cutting performance and make it more like a convex edge.
 

SC_Dave

Gold Member
Joined
Jul 29, 2010
Messages
187
If you're getting a burr, you're at the apex.

The thing about convex edges is that they can be very acute or very obtuse. You probably have a blade closer to the latter, with a lot of metal immediately behind the apex. That kind of edge makes it difficult to slice paper, even when sharp.

You could switch to a 30-degree inclusive V edge to improve the edge geometry. You could then smooth out the shoulders to improve cutting performance and make it more like a convex edge.

You are correct it is very obtuse. I'll give your suggestion a try. Thanks a lot.
SCD
 

Jason B.

Knifemaker / Craftsman / Service Provider
Joined
Jun 13, 2007
Messages
11,089
99% of people that I see sharpen a knife never look at the edge. I mean really look at the edge... put the blade within a few inches of your face and really look at the edge. Do this every handful of passes too, easier to make corrections when the mistakes are still small..

It sounds like you brought the edge to an apex then kinda lost track of the burr. It also sounds like you have not removed the burr and it is creating an obstruction in your cutting/sharpness tests. Reducing sharpening pressure near the end can reduce burr size, I would start with that before further geometry changes.
 

tinfoil hat timmy

Gold Member
Joined
Aug 21, 2014
Messages
10,135
If you're getting a burr, you're at the apex.

The thing about convex edges is that they can be very acute or very obtuse. You probably have a blade closer to the latter, with a lot of metal immediately behind the apex. That kind of edge makes it difficult to slice paper, even when sharp.

You could switch to a 30-degree inclusive V edge to improve the edge geometry. You could then smooth out the shoulders to improve cutting performance and make it more like a convex edge.
I like this way.

I sharpen to a nice apex than knock the shoulders back using the sharpie method to not hit the nice edge I've made.
 

SC_Dave

Gold Member
Joined
Jul 29, 2010
Messages
187
99% of people that I see sharpen a knife never look at the edge. I mean really look at the edge... put the blade within a few inches of your face and really look at the edge. Do this every handful of passes too, easier to make corrections when the mistakes are still small..

It sounds like you brought the edge to an apex then kinda lost track of the burr. It also sounds like you have not removed the burr and it is creating an obstruction in your cutting/sharpness tests. Reducing sharpening pressure near the end can reduce burr size, I would start with that before further geometry changes.

My eyes don't work as good as they once did, but I use a loop and that helps. The burr that I have gotten (once) was so small it was nearly undetectable and I think I goofed it up when I went to the other side. I tried unsuccessfully for over an hour last night to get that burr back but did not. I don't think I understand where to stop when raising the spine trying to reach the apex. I used sharpie marks but I swear it seems I have to raise the spine really high to get rid of the marks toward the edge and trouble keeping it there with subsequent passes.

BTW right now I'm using 400g sand paper on a thin piece of 60 durometer rubber mounted to wood. I have also tried the paper against straight wood, and with a piece of foam (mouse pad).
Thanks, SCD
 

Jason B.

Knifemaker / Craftsman / Service Provider
Joined
Jun 13, 2007
Messages
11,089
My eyes don't work as good as they once did, but I use a loop and that helps. The burr that I have gotten (once) was so small it was nearly undetectable and I think I goofed it up when I went to the other side. I tried unsuccessfully for over an hour last night to get that burr back but did not. I don't think I understand where to stop when raising the spine trying to reach the apex. I used sharpie marks but I swear it seems I have to raise the spine really high to get rid of the marks toward the edge and trouble keeping it there with subsequent passes.

BTW right now I'm using 400g sand paper on a thin piece of 60 durometer rubber mounted to wood. I have also tried the paper against straight wood, and with a piece of foam (mouse pad).
Thanks, SCD

Your sharpening angle sounds too low, laying the blade too flat is a common mistake and leads to very thin convex grinds that perform poorly. You will also need to change pressure through the belly and tip area or you will overly thin them, less pressure is needed because the contact area becomes smaller making the pressure in that area spike. The use of a harder rubber backing is wise and a good choice over a mouse pad that often has too much give.

Small burrs are good and will be common with a softer backing on the abrasive because it adds cushion. Pressure is what forms big burrs and with light pressure you form small burrs. Go even lighter and the burrs basically go away, then your knife will get sharp. Always switch from side to side on the blade, helps reduce the burr and will make the sharpening more even.
 
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