Full Convex Grind

Big Smitty

Big Smitty

Joined
Aug 30, 2007
Messages
509
Can anyone provide information on making a full convex grind on a knife blade starting with a profiled blade blank. I am not looking for information on sharpening, but actually grinding the blade for the first time. Thanks in advance for the help.
 
i do a roughly 80-90% flat grind to start with, leaving the edge a bit thicker and taking it almost all the way to the spine. then i start grinding on the slack belt the rest of the way until i have the edge the right thickness for heat treat and the grind reaches all the way to the spine at the base of the blade.

the convexed edge comes after heat-treat, and is flowed into the grind overall.
 
I do pretty much what siguy stated. Most of my knives are either full flat grinds, or mostly flat grinds with a convex finish. It definitely helps to start with a flat grind to get the basic blade geometry down.

--nathan
 
when i first started making knives convex grinds were all i done. i would mark a centerline and freehand grind each side down to almost how i wanted it finished. i would leave a little bit for heat treating and final sharpening.
 
when i look at stomper's knives (JK knives) it appears to me that he slack-belt grinds the entire bevel, which seems to result in a curve that is flatter towards the spine and more sloped towards the edge...so its more like a bullet than two arcs meeting.

i understand scott gossman used to do his grinds totally in a slack belt, but that he started a while ago to do a 50-70% flat grind and then finish in a slack belt.

grinding the whole thing on a slack belt vs starting with a flat grind means that you will tend to have more metal in the blade...which is something i would be after for a chopping blade, for instance.
 
Keep in mind, grinding completely on a slack belt will change your blade geometry quite a bit. You will get a ton of meat directly behind the cutting edge. Great for a heavy chopper, but not great for a multi-purpose knife that may see mixed use, for instance in a camp setting. I do prefer a flat grind start; you can still get a solid area of steel behind the cutting edge, but with a better overall geometry for cutting/slicing.

--nathan
 
Thanks for the input guys. As for basics, when you are flat grinding are you holding the edge up with the belt coming down into it or the edge down with the belt running down with it? I watched some videos on the Koster web site on sharpening and he holds the knife blade down for sharpening, just wondering it this is the way the grind should be developed.

I did a full slack belt grind on a D2 blank last night with the blade up. It is a small skinner with 3.5" blade and it is quite thick behind the blade. I will go back and thin it out with the flat platen and see how it comes out.
 
I always do all my grinding (not sharpening) edge up. That way I can see my centerline and watch the "gap" between the edge and the belt to adjust the height of my grind. Start with a 45* angle at the edge to break it in, then begin your bevel at a lesser angle and slowly work it towards the spine by grinding evenly, but with increasing pressure toward the spine. To walk the grind up, it's not so much changing the way the blade is hitting the platen, it's more thinking about changing the way the blade hits the platen. In other words, it's just changing the application of the pressure while staying on the same flat. More pressure towards the edge to thin it out, and more pressure towards the spine to walk the flat towards the spine. Keep the edge a bit thicker than you would on a full flat grind.

When you have it where you want it, switch to the slack and begin blending it all together with increasing pressure towards the edge to thin it out.

--nathan
 
siguy is right, I did my convex grinds completely on the slack belt. Problem is, it doesn't remove enough steel behind the edge on thicker stock. I bought the rotary platen for my KMG and did the grind completely on that. It worked at removing more steel but was wearing out the rubber belts too fast. I now grind on the flat platen for about 70% of the grind then switch to the rotary to finish. Flat grinding first hogs steel off faster and cuts my grinding time down.
Scott
 
Newbie question about this: on flat ground blades I most often see there is a very distinct plunge line between the bevel and ricasso. On most convex blades I see there is what appears to be an imperceptible transition. How is this accomplished when convex grinding? Does it just happen naturally when grinding to the same point every time, or is it manually 'feathered' in?
 
I do a full convex grind on the rotary platten of my KMG grinder.
 
Hey, thanks for the info everyone. I have 2 blades done with full convex grinds and they are looking sweet! I'll post pics when they are done. It will be a little while as I am sending them out for heat treat. Thanks again for the help!
 
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