Convexing a concave grind

T

thombrogan

Joined
Nov 16, 2002
Messages
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Now, even with the extra knowledge from Buzzbait and Cliff, I'm still "in the process of" turning a hollow-ground clip-point hunter into a convex-edged thaing. I'm attacking a K-Mart special and it must have been the one stainless knife in China to contain tungsten and vanadium. After my unskilled labor is factored in, it would still have cost less to have bought a new convex-edged knife from Bark River or Marble's and just maintained or polished its edge.

If you, too, have dreams of the perfect edge and are leaning towards convex for practical or Quixotic reasons, please try with a flat ground or already convex edge.

:grumpy:
 
Hollow grinds should be the easiest to convex because you don't have to abrade the primary grind. The rough shaping should be very quick, there is no way the hardness is high nor the carbide content heavy on a cheap knife. However they can be very difficult to hone to a fine and crisp edge because if the metal is poor it can completely fail to form a clean edge because of forming large burrs (a visible strip of metal more than a mm wide) or just breaking apart.

When I sharpen hollow ground kitchen knives I just lay them flat on the belt sander and hone right over the hollow from spine to edge. This only takes a few seconds as the metal is easily hogged off. Without the motor, it still isn't a long process if you just use the belt hanging free :


http://www.bladeforums.com/forums/showthread.php?s=&threadid=208093

If the shaping is taking any length of time odds are that you are using too fine of an abrasive. The initial honing should be done with a very coarse sandpaper, 100 grit at max. If the shoulder is very heavy you can even hit it with a file for a minute to hogg it off before starting with the sandpaper. If the metal is too hard to cut with a file, make a pseudo-file by pasting some 40 grit sandpaper to a piece of hardwood. Lean into it and strip the shoulder off.

If it is a light cutting knife, then you grind right over the hollow grind which will provide the maximum level of cutting ability. Ref :

http://www.panix.com/~alvinj/HSSknife.jpg


-Cliff
 
I was trying to remove the unsightly hollow grind, which may explain why my task was taking so long. I would have been happy with what I had, but I've been making slow and steady progress at grinding away the metal above and below the hollow grind and thought erasing its existence would make my knife that much better at cutting. Yes, I have an imagination on me.

Off to read the "For Dummies" link now. Thanks.
 
Ok that is going to take quite awhile even with a belt sander as you are having to chew large amounts of bulk material off of the primary grind. You might want to stop and periodically check the cutting performance. The critical area is the edge angle and thickness. Once they get down to a suitable level, going further and fully removing the primary hollow doesn't aid much in cutting ability. But does make the knife look a lot better. It also unfortunately greatly increases sharpening time in the future.

-Cliff
 
Full convex grinds need to be hone over their entire length, this is a time consuming process, sharpening time is roughly proportional to the bevel width. If the primary grind has a hollow like in the above picture, you only hit the spine and the edge when honing and thus sharpening is *very* fast. Many blades have hollows ground into them for precisely this reason, Japanese knives for example are filled with them. It also acts to increase cutting ability, but only slightly except on odd materials which are dense and sticky like cheese.

-Cliff
 
I do have several knives with a full convex grind, even though it is not very efficient sharpening wise, as long as the steel isn't that difficult to machine is really isn't much of a problem. Application is also an issue, if the edge gets really worn or damaged, sharpening would be very time consuming.

Most of my blades that have this grind are brush cutters. The convex profile is very smooth and shock resistant, and wood isn't very abrasive so even simple steels like 5160 stay sharp long and sharpening is only 15 minutes or so as they are easily machined. This is still more than ten times longer than if the edge bevel was secondary, but the need for such a sharpening is so infrequent on such blades that it isn't a real concern.

I have a small D2 blade though with a full convex grind which is a real pain to sharpen as it is so difficult to machine, it really begs for a hollow in the primary which it will get as soon as I pick up a wet wheel grinder.

-Cliff
 
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