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Hi And Blade Strength

seattle1
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seattle1 Experienced

seattle1

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Hello:

A dealer has recently listed a long and excellent Shinto katana which has bo-hi, and he writes that they "...lighten the blade without its losing strength." I have long thought that it is an engineering axiom that removing metal does not strengthen, however do hi leave the strength of a blade, somehow measured, the same as before, other things remaining equal? I mean only the usual bo-hi we commonly see on swords and not some exaggerated deviation from that.

Arnold F.

Geraint Grand Master

Geraint

Posted
Posted

Dear Arnold.

 

I think the devil is in the wording here.  As a young cyclist I was told that filling down the lugs on a cycle frame to a taper removed metal but made the frame stronger.  Like you I found this implausible but it turns out that it is correct because the thinning of the lug alleviates a stress concentration and hence potential crack.   In this specific case I think the best analogy is an I beam.  Under load the tendency for the beam to distort is greatest at the horizontal planes of the top and bottom of the beam, so the vertical section of the I serves to keep the two horizontal sections in position but adds little to the overall strength of the beam.  Bo hi in swords create almost the same structure and reduce weight without significantly weakening the sword. 

 

Hope this helps.

 

All the best.

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seattle1 Experienced

seattle1

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Hello:

Thank you Peter and Geraint. The typically wonderful Sesko material shows that adding hi seems not to be a put off to anyone, but from a quick reading I don't think the issue I raised was addressed. To be redundant do hi reduce strength, other things being equal? One can imagine any number of reasons for why hi and horimono in general are added, and if added as a trade off they all can make perfect sense. Strength is sort of an elastic term, but I suppose it would refer to breakage in use. If however hi made a blade easier and quicker to use and swing against an opponent, that might be an entirely rationale trade off. However substitute a larger, stronger, quicker user of the same blade without hi, it would be a different comparison I suspect.

Geraint, I'm not quite sure of your argument as you do introduce weakening. Further imagine a bike frame member connecting A and B, suppose it is triangular then hollow it out to any degree and it has lost strength.

Just curious.

Arnold F.

Ed Harbulak Collaborator

Ed Harbulak

Posted
Posted

Hi Arnold,

I'm not a mechanical engineer, so my "theory" may be wrong. But, if one assumes the blade needs its maximum strength when it is used for cutting, then the Hi has not reduced the width of the blade which is where the strength of the blade is most needed. However, I would expect in side ways bending, the strength of the blade IS reduced slightly because of less metal in the blade. Fortunately, when used properly, all the force on the blade is directed from the ha to the mune and since the Hi hasn't reduced the width of the blade there's no significant loss in its cutting strength. I'll let the mechanical engineers provide a more scientific answer.   

John A Stuart

John A Stuart

Posted
Posted

It is stronger because the critical bending moment is increased by changing the center of mass. As you put more area farther away from the sword's cross-sections centroid, the moment of inertia increases thereby decreasing the stress. John

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BIG Grand Master

BIG

Posted
Posted

Hi Arnold, " lighten the sword, without loosing strengh.." probably shows a special "taste" from schools and smiths like the shinto Hosokawa and the shinshinto Naotane school. Special kantei point and a refreshment of blades may be. Do not loose the normal strength but shows a nice and clear effect, demonstrate the blades beauty and the smith reputation...

 

Best

PS ..is this also sword marketing..??

Peter Bleed Veteran

Peter Bleed

Posted
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What an enjoyable discussion. Thank you all

IMHO most "good" Japanese blades were more than "good enough" for the tasks they were asked to preform. They were used in situations were swords broke and guys died. Expectation were also probably pretty low. In this context so marginal reduction in overall strength may not have mattered very much. Thus, even a slight increase in "stiffness" may well have mattered more than a marginally minor reduction in overall strength.

And beyond that, it is likely that Japanese swordsman had few measures of how and when a sword would fail. Thus obvious measures of quality may have guided sword selection. If you can't be sure which sword will break, you might as well look for evidence that a sword is well crafted. And in that regard, a nicely carved groove could be viewed as a evidence of craftsmanship. A guy who could do that probably made a good blade....

Peter

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Ken-Hawaii Grand Master

Ken-Hawaii

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Arnold, a few years ago, I modeled a katana in AutoCAD with & without bohi, & then ran a series of stress test models.

 

As already stated, the blade with bohi was much more stable (stronger) in the vertical (cutting) direction, & less stable in horizontal (lateral) directions. Interestingly, it took nearly a 30-degree deflection off the cutting line before anything resembling a failure mode took place with either blade, so your hasuji would really have be awful to cause your blade to break!

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seattle1 Experienced

seattle1

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Hello Ken:

It is interesting how many things have been knitted into this thread. Could you elaborate on AutoCAD, which I assume means computer assisted design. Does "...more stable (stronger) in the vertical (cutting) direction..." mean less vibration or what?

Arnold F.

Ken-Hawaii Grand Master

Ken-Hawaii

Posted
Posted

Yes, AutoCAD was one of the first computer-aided drafting & design (CADD) software packages, & I happen to be one of the guys who wrote the program back in the dark ages (1980s), so I know it pretty well.

 

I don't want to get into the nuts & bolts of Finite Element Analysis, but I wasn't looking at vibration modes, just at deflection (bending) & failure modes. In short, it took about 25-30% more energy in the cutting direction to make a blade with a bohi fail than it did for a blade without a hi. It's been awhile, & I don't remember the exact number, but that's in the ballpark.

  • Like 4
Ken-Hawaii Grand Master

Ken-Hawaii

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A rendering won't do it, Peter. You need to create an accurate solid model, & assign steel as an intrinsic attribute. Then you can use Ansys or another FEA modeler to do the stress/strain analyses. There's quite a bit more to it, like determining the best way to tesselate the model, especially with the bohi, without bogging down your CPU & dragging the FEA process to a screeching halt, but those are just details.

 

I found the hardest part was determining the depth of the hi, & then subtracting that on both sides of the blade without having them intersect. Those old-time tosho were really masters of their art! What took me quite a few tries to do on a computer, they were doing with solid steel!

  • Like 1
Ooitame Mentor

Ooitame

Posted
Posted

Good thread, I remember AutoCAD from my HS engineering classes and the CNC we used for our designs. I would imagine the groves if done correctly would reduce torsion stress from an off angled slice and promote flexibility by reducing rigidity. As far as not affecting actual strength, I would doubt it. Rarely does removing metal not effect strength. However, flat pieces of metal where ovals or circles are cut completly through, can increase strength at the cost of flexibility and sometimes durability. Of course those HS classes were a long time ago and somewhat basic, so I could be wrong. I would be curious as to how much affect the shingane to kawagane and tempereing proportion would have on strength, flexibilty, and durability...

This thread is quite old. Please consider starting a new thread rather than reviving this one, unless your post is really relevant and adds to the topic..

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