Convex and edge geometry.

shane wink

shane wink

Joined
Sep 14, 2010
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I am working on another bushcraft/survival style knife with a 3/16ths x 1 1/2 x 5" long blade and want to convex it but I am not sure how to know whether to go 1/2" 3/4" or full height. Should I flat grind HT then place the convex on the blade?

The intended use for this blade is to help me on my trapline this year. I cut and split a lot of wood for snares during trapping season and the knife I was using needs replacing.Cutting trigger notches and batonning through wood , trimming limbs, cutting rope as well as helping process some of the game will be its main function.

The one I am replacing was a blade I made a couple years ago.Flat ground with a convex edge. Many times I would have issue with the flats getting stuck in some hardwoods and I was worried about breaking the tip off but it never did. Now that my grinding skills are growing and knowledge as well I intend to replace it

Can anyone help me understand how high and how to grind the correct convex cross section for a blade like this with the above dimensions?
 
Andy of Fiddleback Forge has some of the best performing convex grinds I know of.:thumbup:
 
Rick Marchand said:
Hmmmm.... I don't agree with the convex portion of that webpage at all. They are showing a secondary bevel. I feel that convex should be a continuous arch to the cutting edge.
Click to expand...

See thats just the kind of information I am looking for from a rough knife 2%er like you. I am open to hear what you have to say as I want to learn the correct way. There just is not much that I have been able to find about specifics and starting points.
 
I can tell you how I do 80% of my knives. It is not a true convex by definition and is more like what that page you linked to showed..... lol... I told you I disagreed with them on "convex" but happen to agree with the merit of a secondary bevel approach.

My blades are brought to almost full flat then convexed to a very shallow arch(too shallow to be considered a respectable edge. Then I finish with a V-microbevel that leaves the shoulder somewhere around .005-.007".(.010-.012 for heavy choppers).
 
It really depends upon whom you ask. Similar to Rick, I make my own personal knives with a full, gentle convex grind, continuing that curve right down to a "zero" edge. I suspect my edges are thinner than his, but we haven't compared them side-by-side. I take 'em pretty dang thin... why not? A knife is meant to cut.

I'm pretty well convinced that that gives the keenest edge and the strongest blade for most types of knives, large or small. I've found that sort of blade cuts really well even when somewhat dull, and is easy for me to sharpen.

The only blades of my own that are truly flat-ground with a micro-bevel, are kitchen knives, precision leather-cutting tools, and test pieces.

But many - no, most - of my clients prefer a full-flat-grind or mild convex with a micro-bevel because it's easier for them to touch up with the sharpening gear/techniques they already have. Is that "wrong"? *shrug*

You've mentioned a "correct way" and "a knife better than I can buy". You're going to have to define that for yourself.

Rick Marchand said:
Then I finish with a V-microbevel that leaves the shoulder somewhere around .005-.007".(.010-.012 for heavy choppers).
Click to expand...

Perhaps I spoke too soon... That's pretty thin! :thumbup:
 
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That is very close to what I am doing now except my shoulders are too fat. I have been grinding flat to .050 HT temp clean the blade up then set the convex edge with the grinder, then work it on the stone to make certain the edge is flat from tip to tail then finish the convex up to the black 600 belt then polish the edge with a 60 then 30 micron at approx 25*.

I have never worked the convex up 1/2 3/4 or full height and wondered if I was missing out on anything. Honestly I think I need to focus on the secondary geometry and thin the shoulders more. I am guessing if the edge rolls I can set a steeper angle and fix the issue?
 
james terrio said:
You've mentioned a "correct way" and "a knife better than I can buy". You're going to have to define that for yourself.
Click to expand...

Thanks for the info James. That part you quoted is in my signature and not part of the post. Just a testament to how addictive throwing sparks and cutting rope can be :)
 
By all means, if you're pleased with your HT, keep grinding 'em higher and thinner till they crumble. Then back off a bit.

If the damn thing won't cut what you want to cut, it simply needs to be more acute. If the edge chips, consider a softer tempering regimen or less-acute final edge. If the edge rolls, it's too soft.
 
james terrio said:
By all means, if you're pleased with your HT, keep grinding 'em higher and thinner till they crumble. Then back off a bit.

If the damn thing won't cut what you want to cut, it simply needs to be more acute. If the edge chips, consider a softer tempering regimen or less-acute final edge. If the edge rolls, it's too soft.
Click to expand...

That's the way to do it.:thumbup: Grind them thinner and thinner until they fall apart. Know what the limits are for a given geometry and hardness.
 
One aspect of convex bevels that choppers take advantage of, is their propensity to split wood instead of wedging the bevels while chopping. Full flat ground blades tend to lodge or wedge themselves into the wood. Put a convex edge on a full flat grind and theoretically it should wedge less, the secondary edge splitting instead of wedging.
There is an advantage to adding a "flat" secondary angle at the edge on a full convex ground blade; it can be made sharper than a secondary convex edge with the same geometry, therefore better penetration when cutting.
In the case of cutting softer materials, that do not tend to grip or wedge a blade while cutting like vegetables and the like, convex bevels or secondary edge grinds make little seance.
 
IMO, the best convex grinds, which are the ones that you see on knives by guys like Bill Moran and Robbie Hudson on field knives, the japanese sword makers and some of the kitchen knife guys too, are darn near flat and done freehand. For us mortals, one way to get close to that is to do like you said, Flat grind a fair bit of the way then convex up. I got a chance to talk to Bill Moran back in 2005 and he told me that he never used a slack belt because it gets the the blade too convex. I use the rotary platen on my KMG and will go back on the flat platen and during hand sanding and flatten/blend it more if need be. Sometimes, you can grind a couple of flats and then blend them together.
 
All good advice. I will go higher and thinner then grind back if it fails keeping record of the heat treat. I wont be changing steels and will be sticking with 1084 so nailing the ht will be easier and focusing on the geometry.
 
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