Kukris/bolos and edge geometry

[Orion]

I have been doing some tests lately with these types of blades. The ones I have used are CS ATC, CS LTC, BK&T Machax, Ontario Kukri, Gurkha House Jungle model and the CS Gurkha Kukri.
As stated in a previous post, the ATC out-performs them all as far as brute strength at chopping. It also will take an edge that will pop hair. The only problem I have had with the ATC at all is binding in some types of wood due to the thickness and edge geometry (full flat grind).
The argument always comes up about GH and HI kukris not binding due to the edge geometry. I know thin blades (LTC) bind a lot due to the grinds but why do the thicker blades bind less often?
I have noticed that my GH kukri edge has sort of a convex grind more like an axe or hatchet, as opposed to a concave edge, or hollow grind. I am assuming that this helps to cut down on binding, but why, exactly?
I know concave edges slice better but I would not want one of these on a chopper as it would roll or chip rather easily. The convex edges will take a decent slicing edge, but will not pop hair like the flat ground ones.
I still like my ATC (it doesn't bind VERY often because it has considerable thickness) but it makes me wonder... The design of the ATC makes it awesome. Would it perform better as a chopper if the edge was slightly convex as opposed to flat ground?
Cliff....? Any comments? Anyone else?
A lot of people swear by the GH and HI models as the best but I can never do any long term chopping because the handles are so damned uncomfortable. I don't really like the CS handles much, either, but they are better than the GH or HI models. The GH model I am using is 15". I am wondering if I had a 18"-20" HI model (comparable in size/weight to the ATC) would it perform better than the ATC. I would still be able to work longer and more comfortably with the ATC because of the handles. The best handles by far are the Becker models. I am actually considering carving some slabs like the BK&T handles to put on my ATC but don't know how I would go about attaching them to the tang... ditto on a larger HI model. Any suggestions?

Thanks-
Orion

 

[Crayola]

Interesting comments! That is great that you are testing the blades too.

I think the convex grind doesn't bind as much because of the curve. It should throw out wood chips better and shouldn't wedge its way into a chop like a traditionally sharpened edge would. The edge on my LTC kukri is a convex one. Convex edges shouldn't roll as easily as a traditional edge. They are very strong and ideally suited for choppers. How did your CS Ghurka kukri do? Is it better than your LTC?

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"Come What May..."

 

[Cliff Stamp]

Orion, glad to see you posting your work with various heavy choppers. The ATC seems to get high regards from most if not all that have used it. Strange then that CS discontinued it.

I have noticed that my GH kukri edge has sort of a convex grind more like an axe or hatchet, as opposed to a concave edge, or hollow grind. I am assuming that this helps to cut down on binding, but why, exactly?

Under the same amount of power the thicker khukuri will penetrate less and thus have less wood to bind in. As well, at the same penetration depth, the thicker khukuri will force the wood to compress more increasing the nonelasticity and preventing it from binding strongly to the blade. Lastly, the convex nature of the profile limits the surface area exposed to the wood during the chop and the draw out from the wood.

So to prevent binding you must have both a thick profile as well as it being convex. A lack of either of these will cause the blade to readily stick. If you make a convex blade really thin, say 1/8" with a full convex grind, it will still readily bind in wood, not as much as a 1/8" flat ground, but much more so than the convex sabre ground 3/8"+ thick khukuris.

The convex edges will take a decent slicing edge, but will not pop hair like the flat ground ones.

Shaving is just a function of edge alignment not geometry. My khukuris will all shave after I have sharpened them. It is rare to get a traditional khukuri from HI or GH that has that level of polish though as it is not something that the kamis seem to place a lot of importance on. It should not be that difficult to do though. There is lots of information on this in the HI forum.

The design of the ATC makes it awesome. Would it perform better as a chopper if the edge was slightly convex as opposed to flat ground?

If it was convex in profile as opposed to flat ground then it would be thicker and heavier. If you used the same amount of effort during chopping that you use on the flat ground version, you would get far lower penetration on the convex one and you would see less binding. However, by increasing your effort to take advantage of the extra mass, you should be able to significantly increase the penetration and reduce the binding at the same time.

A lot of people swear by the GH and HI models as the best but I can never do any long term chopping because the handles are so damned uncomfortable.

There are a few common sources of uncomfort. The first is impacts off of the butt and the second, irritation from scraping the ring in the center of the handle. Both of these are signs that you are losing control of the handle during the swing. The solution to both of these is to use a tighter grip and a locked wrist swing.

You could also be getting discomfort from excessive vibration which is either caused from chopping with the wrong part of the blade or problems with the knife itself.

There are a couple of people in the HI forum that will rehandle a khukuri.

-Cliff

[This message has been edited by Cliff Stamp (edited 10-24-2000).]

 

[not2sharp]

The Cold Steel ATC was suppose to have a problem with the handle. The rear of the kraton handle was insufficiently supported causing it to move around and eventually rip. I do not know whether the tang has been extended to correct this in the CS Gurka version.