Why are convex grinds often very sharp????

A

akula57

Joined
Jul 24, 1999
Messages
198
My Marble's (52100) for example. Hair just flies off my arm! I could probably shave!

Is it that the edge is so thin? Also, do they not cut well through certain
"resistant" mediums such as leather for some reason? Finally, are they difficult to sharpen?

Warm Regards
 
No, yes, yes.

Convex edges are not usually particularly thin. I think that makers who go to the trouble of making convex edges tend to use better alloys and take more care sharpening than the average manufacturer. 52100 is a particularly fine grained alloy that takes a sharp edge. Marble goes to extra effort to make a convex edge and make it sharp.

Since convex edges are not particularly thin and don't have relief behind the edge (like a hollow ground edge) they can offer extra drag on particularly stiff material. You will notice this on stiff thin material like leather cut on a cutting board or when you cut a stiff thin material edge-on like slicing through a plastic garbage can. In the leather-on-the-board case the portion of the blade passing through the leather is thicker than a hollow ground blade and there is more drag. In the garbage-can case you suffer from the blade being wedged into the material for a significant depth before the material can start to fold out of the way. A hollow ground edge would be thinner in this high pressure area behind the cut and wouldn't thicken until the region where the material is freer to fold back.

Compared to fully tapered or hollow ground blades a convex edge is hard to sharpen. You need to remove more material to hone down to the edge plus you must hone in a curved contour. Offsetting this is a tendency of convex edges to wear better. If you learn the tricks you can sharpen a convex edge. The edge will last well and be tougher than other edges. One trick to sharpening a convex edge is to strop the edge frequently on stiff leather with an abrasive compound. If you never let the blade get dull it is easier to maintain the convex contour.

If you are not cutting leather on a table or trash cans you may find the convex edges perform well for you and are worth a little extra maintenance hastle.
 
Akula, as Jeff noted, you are probably seeing a better sharpening. I have seen similar levels of performance from high end makers using flat bevels.

do they not cut well through certain
"resistant" mediums such as leather for some reason?
Click to expand...

For a given level of edge durability, a convex grind will have a higher level of cutting performance than a flat bevel :

http://www.physics.mun.ca/~sstamp/images/convex_vs_flat.jpg

In order to get the same amount of material behind the edge the flat bevel has to be much more obtuse, and thus will cut much worse.

are they difficult to sharpen?
Click to expand...

Using sandpaper on a soft backing, a convex edge bevel is easier to sharpen than a flat one as the backing will adjust to the curvature of the convex bevel whereas you have to judge this for yourself on the flat bevel on a hone.

If you are sharpening a full convex bevel then you have to decide if you want to keep the original full profile and thus gradually make the whole blade thinner every time you sharpen or just add a slightly more deep convex bevel to the edge and just sharpen that.

The latter method has the advantage of speeding up sharpening, but of course lowers the cutting performance as over time the profile thickens - as it does with all blades with secondary edge bevels.

-Cliff
 
Cliff took the words out of my mouth, but that is o.k. because I was going to take them out of Wayne Goddard's mouth myself! I just acquired his new book and he really likes the convex grind. In a few words, he likes 'em because you can go thinner with the profile of the edge and whole blade with the convex grind. Also, he says that this type of edge will slice rope like mad if you have a highly polished blade and edge. A strop, then, would be a good item for you to have. When I get a little more energy, I'll try and scan the graphic from the book that says the other 9 hundred and some words that I have left out.

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"Come What May..."
 
Convex edges.....very good edges in my opinion...I recently tried sharpening a knife that had that grind and was very excited at the out come. It was a Jimmy Fikes CabinBoy, I had a friend send me his that he has used for years, with only conventional sharpening on the edge. This is a forged blade with a chisel grind that is convex on one side.

I used a piece of Deer skin as an underlayment and cut sheets of sand paper into three equal widths, full length of the sheet. I used the following grits to do the sharpening; 80, 220, 320 and 600. Stropping the knife across the sand paper and rocking it slightly to obtain a convex edge.

Then I stropped the knife and out to the buffer for a final edge, came out exceedingly sharp.

So that made me think....scary eh?....I have a Boye Basic #3 it was already sharp, well fairly sharp, but when you press cut into thick leather it doesn't glide through it. So through the steps with that and now...wow!

So if you have a David Boye Basic series knife and want a truely shocking edge, give that method a try.

G2


------------------
"The Road to Hell is Paved with Good Intentions!"
Take the time to read your Bible Now, don't be left behind...

G2 LeatherWorks
 
Well... I dunno. My Marble's Fieldcraft came with a convex edge, I guess. But I've sharpened it a few times and it's no longer convex but still cuts like a razor. In the real world, does it make much difference?
 
Ed...sharp yes, but the ~shoulders~ that the conventional bevel has makes push cutting through stuff just a little bit harder, with the convex edge you ~~Round~~ off the shoulders and the blade slices much easier, your knife would be easy to return that convex edge with the method mentioned above.

G2

------------------
"The Road to Hell is Paved with Good Intentions!"
Take the time to read your Bible Now, don't be left behind...

G2 LeatherWorks
 
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