Couple edge damage pics

T

the possum

Joined
Jul 31, 2002
Messages
2,954
Yesterday my brother got his first deer with a bow & arrow. The broadhead performed impressively; perhaps that should be another topic.

But I helped him find the deer, and skin it. I let him use my bowie knife to lop off the head, and even got a video clip of it complete with maniacal laughter at the end. The blade snicked right through- he said he was surprised how easy it was. (He had already skinned down the fur from the neck, which doubtless made it easier.) This did not damage or even dull the edge at all, as far as I could tell. I've used this knife on the necks of about a dozen deer now, and it has never been damaged by this.

However, then he brought the deer over to the neighbors who have a walk-in cooler, and there were a couple other guys there sitting around, so he brought my bowie knife along. When he returned, I saw that he had used my knife again, and it had some nicks in the edge. Turns out he used it to chop the legs off his deer too. The leg bones are MUCH harder than the ribs or spine.

I took the liberty of snapping a few photos of it for grins. The edge appears rather ragged in these photos, but they are not chips. The bottoms of these nicks are smeared over. Best I can figure, the leg bones really have a tendency to shatter on impact, so probably each little splinter made its own nick in the edge. (?) And it's really not as bad as it looks; I can probably have this fixed in five or ten minutes on my coarse stone. As best as I can tell, the steel is about 0.1-0.15 millimeters thick (.004"-.006" or so) at the bottoms of the deepest scallops. (about half as thick as the lead for my mechanical .3mm pencil or less) Most of the nicks are only about half that.

damage1wq9.jpg

damage2sh5.jpg

damage3ny8.jpg
 
Yep. Same bowie as last year. It didn't take as much damage from the legs this time though; not sure how my brother actually did the cuts.

And I just took a look at the edge with a 20x dissecting microscope, and maybe I was wrong earlier- some of these do look like freshly broken steel in the bottoms of the scallops, but there is plastic deformation too. Not sure what to make of it. There is no way bainite should be chipping (it should dent/roll) but then again this piece of steel was pretty bad to start with.
 
Rat-
Steel banding is pretty hard, so I wouldn't expect any blade to withstand it without some level of edge damage. But, I have to wonder, were the nicks on your blade this deep?

I have never seen a swamp rat knife in person; could you describe the edge thickness & geometry? I've only seen a couple Busse knives at shows, and their edges were at least two or three times thicker than what's on my bowie.
 
I learned that many years ago. The leg bones of a deer are 7 times denser than a cows !! Use a cleaver or better , a saw .
 
the possum said:
Rat-
Steel banding is pretty hard, so I wouldn't expect any blade to withstand it without some level of edge damage. But, I have to wonder, were the nicks on your blade this deep?

I have never seen a swamp rat knife in person; could you describe the edge thickness & geometry? I've only seen a couple Busse knives at shows, and their edges were at least two or three times thicker than what's on my bowie.
Click to expand...

I don't have the blade with me here at work, nor do I own a micrometer, but the knife has a pretty wide 10.5" Blade Flat ground from 1/4" stock, and has a convex edge. here is some older pics of the knife:
ratdaddy2.jpg

ratdaddy4.jpg



I will try to get some pics of the damage, and a shot showing the edge thickness at the choil. The damage was minimal or non-existant where I chopped the banding cleanly at a perpendicular angle. I should have just pried the banding off, in hindsight, since I did that to the last piece, and it worked great.
The damage was caused when I inserted the blade parallel to the banding and used leverage to cut through it- this caused some sideloading on the edge, and the banding slipped along the edge in the process.
 
Cliff Stamp said:
What is the edge angle?

-Cliff
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Before I answer, I'd like to point out that it doesn't really matter. At least not in this case.

You could make the argument that angle would factor into things if we were seeing straight on impaction, like the edge were mashed flat into itself and expanded out on both sides of the blade. In that case, the volume of steel moved determines the damage, and a triangle with longer/taller sides (and the same sized base) has more area than a shorter triangle.

But in this case, it was clear that the damage was due to lateral forces on the edge. (especially under the microscope, I could see the way the edge bent to one side until it dented/broke in each scallop) The steel will bend or break off until it gets too thick, regardless of the angles used to get there.

The edge is convexed, blending into the main grind, but generally I do the primary sharpening at ten degrees. (I was finally able to get a more accurate measurement of this based on the quick formula you posted in the Tinkering forum. Thanks.) Naturally it will be shallower higher up on the edge, but this is a good approximate. The edge rounds to 14 degrees at about the bottom third of the edge bevel. Sometimes, I may add a further microbevel with a couple passes at up to 15-18 degrees per side (if I anticipate whacking steel next time, or if I just need a quick touch up to get back in action when it actually needs a proper reprofiling).
 
mete said:
I learned that many years ago. The leg bones of a deer are 7 times denser than a cows !! Use a cleaver or better , a saw .
Click to expand...

I think we've touched on this subject before, at swordforums, perhaps? I have used a cleaver for this, and also get edge damage. A saw would have been the best tool, but then again I'm not the one who did this. (This time. :D ) And I would have warned my brother not to do it had I known.

I am curious about your figures on bone though. Is that density figured for the bone as a whole, or just the thick outside part? A bird's bones on the whole are not dense at all since they're hollow, but that may not mean they'd be any softer or easier to cut with a knife. I find myself wondering whether density is the real issue, or hardness. (are they directly correlated?)

And, because of my interest in historical sword use, how human bones compare. (not that I'd ever test it!) A deer's 160 pounds or so is supported by four legs, whereas a human warrior's 220 pounds must be supported by only two. If a sword takes damage from a deer's leg, how would it do against a human enemy?
 
[impaction]

the possum said:
In that case, the volume of steel moved determines the damage, and a triangle with longer/taller sides (and the same sized base) has more area than a shorter triangle.
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That is an interesting perspective and it is basically true if you focus on direct impaction like banging a blade into a rock.

But in this case, it was clear that the damage was due to lateral forces on the edge. (especially under the microscope, I could see the way the edge bent to one side until it dented/broke in each scallop) The steel will bend or break off until it gets too thick, regardless of the angles used to get there.
Click to expand...

This I think is indeed the basic idea, the steel will tend to deform under the lateral loads but I don't see why you would conclude the angle isn't relevant. Consider the two fairly extreme cases :

Bowie; (a) three degree primary bevel on 1/4" stock which tapers to a 0.015" thick edge at five degrees per side, (b) the exact same thing except the edge is 45 degrees per side. Now imagine chopping both into a piece of thick bone.

That five degree bevel will have a significant part of it so thin that even light lateral forces will bend it enough to just break it off. However the 45 degree edge thickens so rapidly that there isn't a significant part of it which can be bent under the lateral loads.

In regards to the angles, I chopped a lot of caribou leg bone last year, frozen and thawed and how you cut it made a huge difference. Just taking the knife and smashing it at a 90 ends to make life unpleasant to the knife. However with controlled angle cuts the edge should be fine at 15 degrees. Shearing through the upsweep of the tip is also a lot easier on the knife.

Ron Hood used to (likely still does) heavy chopping on Elk and similar bones as part of evaluating large blades. Numerous knives have shown no visible damage. The edges are however usually far more obtuse than on your bowie. At about 20 degrees it becomes very difficult to damage the edges on the tougher tool steels.

You should try a piece of S5/S7 for your next large bowie to eliminate fracture as a concern as it is much easier to produce martensite than bainite. You could probably get a few samples from Carpenter either as they have a number of low alloy P/M shock steels.

-Cliff
 
Cliff Stamp said:
[impaction]
Consider the two fairly extreme cases :

Bowie; (a) three degree primary bevel on 1/4" stock which tapers to a 0.015" thick edge at five degrees per side, (b) the exact same thing except the edge is 45 degrees per side. Now imagine chopping both into a piece of thick bone.

That five degree bevel will have a significant part of it so thin that even light lateral forces will bend it enough to just break it off. However the 45 degree edge thickens so rapidly that there isn't a significant part of it which can be bent under the lateral loads.
Click to expand...

.....Well, after further consideration, I suppose you do have a point that a more obtuse edge will allow more leeway in misalignment/torsion and still have support behind it because of the angles involved, rather than simple thickness. With a five degree edge, if your swing is misaligned (or the target moves) by only a few degrees, then there is no steel in line behind the edge to support it. But with an included angle of 90 degrees, you could turn the knife 45 degrees sideways in your grip, and there would still be steel perpendicular to the line of the impact force (i.e., directly behind the edge) to support it. If you went beyond 45 degrees of cant, then the shoulder at the junction of the primary and edge grinds would strike the target, and the actual edge wouldn't hit at all.

Furthermore, this is one instance where a more acute edge would take longer to sharpen/repair. Using your example of the two bowies above, if you have a small nick in both edges to the same thickness of steel, and want to sharpen it out, you will have to remove far more metal from the surrounding area on the thinner edge. The "taller triangle" formed by the narrow angled edge has both more volume, and (exterior) surface to abrade away.

So I guess it depends on whether the failure is occuring by direct impaction or lateral strain. The latter seems by far the more common failure method to me. But at the same time I really like the cutting performance offered by the thin & narrow angled edge.

Cliff Stamp said:
I chopped a lot of caribou leg bone last year, frozen and thawed and how you cut it made a huge difference. Just taking the knife and smashing it at a 90 ends to make life unpleasant to the knife. However with controlled angle cuts the edge should be fine at 15 degrees. Shearing through the upsweep of the tip is also a lot easier on the knife.
Click to expand...

As I've mentioned before, the reason I'd do this is to discover what happens when things aren't precisely controlled, as my more "serious" useage is rather dynamic. A while back in Blade, one of the competition cutter guys said he sharpens his comp knife with an edge much finer and sharper than what he'd ever give to a customer. That alone should tell him the competition tests no longer represent real world use. And it may also be why the majority of competitors got disqualified this year when their edges were damaged upon facing a new test- stabbing through a metal cake pan.

Cliff Stamp said:
You should try a piece of S5/S7 for your next large bowie to eliminate fracture as a concern as it is much easier to produce martensite than bainite. You could probably get a few samples from Carpenter either as they have a number of low alloy P/M shock steels.

-Cliff
Click to expand...

I had considered S5 & S7, but was put off by reports of plastic deformation even at moderately high hardness. I want strength and toughness at the same time. (Don't we all?) My L6 stock just arrived a few days ago, and I have enough for two big bowies after I get done making a Falchion for my Dad. I'll try the first one in a marquenched martensitic structure at 58Rc, and see how it does. If I'm not impressed, I may try a different heat treat on the second one, or explore the shock steels in more depth.
 
However said:
Over here we just use a small knife (mora etc.) to take of the legs and head of a moose or reindeer. Just pry the knife in the joint of the knee and break it off. No need to use an axe, bowie or a saw.

Tor
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Just like there's no need to do it with a mora when you have a saw and bowie at hand. ;)

Besides, the whole point of cutting the legs off is to save room in the cooler & such. They would still be too long if we cut 'em off at the knee. We cut 'em further up where the meat starts turning to nothing but tendons.
 
possum, Cliff makes a good point about the angle you chop the bone. Ron Hood does indeed still use the frozen elk leg test on knives he evaluates. He used the Buffalo knife I sent him on a frozen elk leg and it is documented in the Woodsmasters series vol.17. The knife I made was 3/16" thick D2 steel with an 8" blade. Full convex grind and edge. He shaved hair off his arm afterwards.
Scott
 
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