Convex Grind -- How do I do it?

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Greebe

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Aug 2, 2009
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So I was getting a belt grinder set up and remembered that Ed Fowler's book mentioned that the Convex grind was his favorite. The knives I have made have been forged to shape and finished by hand.

So once I get the belt grinder going, how do I grind the convex blade geometry?

Thanks
Greebe
 
Well, I'm sure there are a variety of ways to approach a convex grind - the one I know best is using a rotary platen from Beaumontmetalworks (here).

TS
 
I start with a series of flat grinds on the flat platen, sort of like facets. Then blend them all together with the slack portion of the belt. You can do the whole thing on the slack belt but working against a platen is much quicker.
 
You will get many different answers on how to do a convex grind. A true full convex grind is an axe grind which when done completely on a slack belt, has poor geometry for a knife blade. It's just too thick behind the edge. You can flaten out the convex by grinding on the rotary platen. My preference is to flat grind then finish on the rotary platen. My grinds are more flat then convex being 75 to 80 percent flat and 20 to 25 percent convex. I've found this gives better grind geometry along with the strength of a convex contour.
Scott
 
I also perform a flat grind to start and then covex it once the geometry is suitable. It's been a while since I've done a convex grind....

--nathan
 
I have owned a couple of Moran knives and Bill's convex grind looked as close to a flat grind as what you would see on a Japanese sword. Even "flatter" that the ones done by guys he taught. Slack belt give you too much convex and even the rotary platen can leave things a bit thick. I usually get it real close on the flat platen and either convex the edge on a slack belt or the lower 2/3 of the blade on the rotary platen. I smooth and "flatten" out the grind during hand sanding. By the way, Bill did his totally freehand using a huge emery wheel, contact wheels and platens.
 
Yes, the term "convex grind" seems to imply the Axe grind, or "apple seed" grind. While a knife is the extreme end of this grind, the geometry is of course different from an axe.

Start with a flat grind, paying attention to keeping it perfectly flat as you would normally. As you approach the edge thickness that you would normally make the secondary bevel from, just let it curve over to form an edge. While it can be done on the flat platen by merely lifting the spine as you grind, it is usually done on a slack belt. This is one place where the rotary platen really shines.
When finishing and sharpening a convex edge by hand, a trick is to use a medium-hard rubber pad with the wet-or-dry paper over it. Lay the pad down and pretend it is a stone. You will get a curved edge, instead of an angled edge.

Regardless of the edge, the main bevels should be the same, or nearly the same.
The key word is convex EDGE, not convex blade.
 
Thanks guys for the suggestions.

OK, so I was thinking that the convex grind was more pronounced.

I am not going to have a rotary platen so I will have to figure it out on a flat platen. When I get a motor and get it all set up I will start playing around with it and see what happens.

Thanks
Greebe
 
bladsmth said:
Start with a flat grind, paying attention to keeping it perfectly flat as you would normally. As you approach the edge thickness that you would normally make the secondary bevel from, just let it curve over to form an edge. While it can be done on the flat platen by merely lifting the spine as you grind, it is usually done on a slack belt. This is one place where the rotary platen really shines.
When finishing and sharpening a convex edge by hand, a trick is to use a medium-hard rubber pad with the wet-or-dry paper over it. Lay the pad down and pretend it is a stone. You will get a curved edge, instead of an angled edge.

Regardless of the edge, the main bevels should be the same, or nearly the same.
The key word is convex EDGE, not convex blade.
Click to expand...

Hmm. So what you are saying is that a convex blade grind really isn't convex but flat. The only difference is that the secondary bevel for your cutting edge gets rounded instead of ground flat.

If this is the case why don't they just call it a flat grind? I thought the purpose of a convex ground blade according to Ed Fowler was to have a thicker blade than a flat ground blade.

Thanks
Greebe
 
The problem is in the semantics of using the word grind instead of edge.

The bevels are flat, or nearly so on most convex grinds. It is the edge that gets the final rounding to zero. It is actually done as a series of tangent grinds from about 50 degrees at the edge, down to 10 degrees by the time you are a little back from the edge. From there it might as well be flat, since the curvature of less than 10 degrees is not noticeable in such a short distance as a blade side.
Here is a very good drawing of how the convex edge is formed.
http://www.kosterknives.com/convexedge.htm

This site explains the different grinds fairly well:
http://faq.customtacticals.com/geometry/

You are correct in saying that the blade is thicker behind the edge than a flat grind or hollow grind. This extra thickness makes for a more impact resistant edge. The trade off is having somewhat more drag in slicing. If the flat ground blade sides go to a fairly thin edge before convexing to the final edge, this is not a problem. On a much thicker convex grind, like an axe or a big camp chopper, the slicing ability will be greatly reduced.

On a belt grinder, after flat grinding the sides to about .030", the final convex edge can be formed by gently laying the blade on the slack belt just behind the upper wheel. Lift the spine so it is a tad above the belt. The flex of the belt will convex the edge automatically. Obviously ,you do this with a trailing edge. The thickness of the edge before the convex is added will determine how tough the edge is. The final edge formed is given a couple quick passes across a buff with a muslin wheel and whatever compound matches the blade metal. If buffing wheels scare you ( and they should), a charged leather strop can finish off the edge nicely. Polish the edge until the wire is gone, and quit. Over doing a convex edge can make it appear dull.

The rotary platen has a backing that gives just enough resistance to make this process repeatable easily. The distance between the wheels determines the amount of flex the backing has.
 
Last edited:
Gossman Knives said:
You will get many different answers on how to do a convex grind. A true full convex grind is an axe grind which when done completely on a slack belt, has poor geometry for a knife blade. It's just too thick behind the edge. You can flaten out the convex by grinding on the rotary platen. My preference is to flat grind then finish on the rotary platen. My grinds are more flat then convex being 75 to 80 percent flat and 20 to 25 percent convex. I've found this gives better grind geometry along with the strength of a convex contour.
Scott
Click to expand...

.

Yep....
 
Are Bark River's and Fallknivens and Busse's given this flat grind before convex edge? Or is the flat grind more for knives starting out at 5/32 or less where good slicing trumps toughness? I thought most people were doing about 4 slightly different angles and then blending on steel 3/16th+.
David
 
These are the tricks I use for the convex grind.
First grind the blade flat removing all dents or irregularities.
Then make your edge grind, it is easy, the edge leaving the ricasso is your rear sight I do this on the rubber wheel of my Burr King..
The tip of the blade is the front sight.
Once you have established these reference points you simply match the sides of the blade up, grind to about a 16th of an inch on the edge with the convex shape flowing gently to the spine. I always grind edge up and do it all free hand.
Move your entire body right to left and left to right instead of just your hands and the emotion you put into the blade will be more you.
I start with a 36 grit belt to a 220 before hardening.

Do not leave a blade laying around with the 36 grit scratches on it for long or they will grow down into the blade below the bottom surface of the scratch to show up later when you etch.

Most important is have fun with it, the convex grind is the strongest grind you can develop in a blade.
 
Greebe said:
Hmm. So what you are saying is that a convex blade grind really isn't convex but flat. The only difference is that the secondary bevel for your cutting edge gets rounded instead of ground flat.

If this is the case why don't they just call it a flat grind? I thought the purpose of a convex ground blade according to Ed Fowler was to have a thicker blade than a flat ground blade.

Thanks
Greebe
Click to expand...

A properly ground blade in the Style that Ed and many others do, starts at the spine and gently curves down to the edge. The way I do this grind the curve flattens a little as it gets closer to the edge. This is a true convex grind and not just a convex edge. The convex edge that I see on many knives actually get steeper right at the edge resulting in a more obtuse angle than is neccessary. of course this all depends on the thickness of the edge before convexing.
 
I agree with Bill, there is such a thing as a "full convex grind" and it is fairly popular. The cross section consists of an unbroken radius from the spine all the way to the edge. I've only done a few knives that way, on customer request, but worked it about like James Terrio describes above.

I do convex edges more often, as Stacy says that's a different animal.

I found full-convexing while maintaining a clean plunge cut to be difficult.

Here's a blade shot of a full convex knife I did. I think you can see the surface is a non faceted, mild curve.

IMG_3588.jpg
 
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