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Post by JH Lee on Apr 22, 2021 15:38:51 GMT
If a tsuka fails during use, where is the weakest or most vulnerable part? My firsts thought was along the top of the mune side of the tsuka. But if the nakago is pivoting toward the direction of the cut, with the bottom of the tsuka moving forward, then the weaker part would seem to be the bottom of the ha side of the tsuka.
What are you guys' thoughts?
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Post by RufusScorpius on Apr 22, 2021 16:42:52 GMT
Assuming the wood is not rotten, a failure could happen in a number of ways. There are a many things that could go wrong: the wood can split where it has been drilled for the mekugi, the fuchi could have failed or moved it's positon and weakened the tsuba end of the tsuka, or the tsuka ito became loose and failed to provide the necessary reinforcement, or the wood itself may have had an invisible crack due to the nature of it being an organic material.
The mode will indicate the root cause of the failure. I would love to see some photos of failed tsukas and then do a root cause failure analysis on the parts. That would be fun. But I imagine the Japanese did that a very long time ago and that why tsuka was designed the way it was to mitigate the failure modes.
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Post by JH Lee on Apr 22, 2021 17:37:08 GMT
Great points, Rufus.
Let me pose the question another way: If you guys were building a tsuka core, which parts would you want to make sure were reinforced, because they are likely to be subject to the greatest stress?
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Post by RufusScorpius on Apr 22, 2021 18:18:49 GMT
That's easy: the wood (ho) and the mekugi. All other parts serve to support those. Using modern materials, I would make the ho out of carbon fiber or the composite plastic stuff that Morakniv uses (that junk is indestructible) and the mekugi from hardened steel. No f'n way you are going to break that. If you do manage to break it, it's because you did it on purpose using stresses that exceed any normal use by a huge margin (like putting the tsuka core in a hydraulic press or something).
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Post by randomnobody on Apr 22, 2021 18:37:53 GMT
I feel like using steel mekugi would be a bad idea. I know a handful of makers, most notably Cheness (RIP), have used copper for one of their two mekugi, but copper is softer than steel and non-reactive, if memory serves.
Some folks seemed, at least at one point, very excited about using delrin as mekugi.
There's also good old fashioned rivets, or epoxy...
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Post by JH Lee on Apr 23, 2021 14:52:29 GMT
Using metal for mekugi seems to me to be a risky proposition, especially if the material could enlarge or warp the nakago-ana. Also, it could, over time, apply pressure to the tsuka and same and enlarge those holes as well--a problem exacerbated if the assembly experiences changes in temperature and humidity over time. This is also my concern over using delrin as mekugi, though I do use them without any issues (so far).
Rather than simply saying "the entire tsuka wood," which is fair and also accurate, what I am hoping for from you guys is your perception of where (at which precise location) the tsuka is likely to fail under stress during use. That is, which area of the tsuka is potentially the weakest link in the chain of parts? Of course, the answer may be, "It depends," which I can appreciate. But let's say that the tsuka has been optimally constructed with flawless shaping, carving, gluing, with samegawa and ito. Under such conditions, all other things being equal, IF the tsuka fails (e.g. due to striking a particularly dense target, albeit with correct technique), since nothing is indestructible, which location of the tsuka is likely to fail first?
Appreciating the discussion. Thanks.
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Post by RufusScorpius on Apr 23, 2021 15:42:00 GMT
I would suspect, given the parameters you laid out, that a failure would happen at the thinnest point of the structure, namely in the corner where the mune meets the shinogi. That is the point of least material, and there is a 90 degree angle at that point. Without getting into complex calculations, I'll just say that the stress waves don't do well meeting at 90 degree intersections. The failure would take on the form of a crack extending the length of the joint area.
That would be my edjewmacated guess.
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AJGBlack
Member
"This world will stress you like Orson Wells on the radio." -RTJ
Posts: 379
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Post by AJGBlack on Apr 23, 2021 16:53:16 GMT
On a traditionally made katana with proper fit I think the mekugi would be the main weak point. Everything else is bound together and relies on both mechanical and chemical support. The mekugi is the only part really that relies on mechanical support alone.
Why would a metal mekugi, if properly fitted, be bad? Knife and sword makers use metal pins all the time.
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Post by RufusScorpius on Apr 23, 2021 17:10:14 GMT
The mekugi has very little stress on it, assuming a properly fitted ho. It only has a linear stress, and actually not much of that. The traditional hardened bamboo would in fact be very strong and a failure would result in splintering instead of shearing. That means even if it fails, the fibers will still have some retention value. I still think the most likely failure point is in the corner.
I also don't see a problem with a metal mekugi. Or even a mekugi made from high impact resin (like the stuff Morakniv uses on their handles- that stuff is indestructible!). But if it were a problem area, then I'm sure the Japanese would have used a metal pin a long time ago. They were quite capable of making them, so manufacturing wasn't an issue.
Component failure in mechanical systems usually happens in places where the components can move relative to each other. The area where the maximum movement happens is the fit between the ho and the nakago inside that pocket. Stress waves would bounce between the air gaps and the two materials looking for a place to release. The weakest point (I assume) would be where the material is thinnest, in the corner.
Without examining some real world examples it's all speculation and best guess at this point. A bit of predictive analytics without having a materials data sheet to build a simulation model. The only way to be sure is to do it the Soviet style: in other words- take a bunch of swords out to the field and see where they break. Then you'll know for sure.
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Post by jyamada on Apr 23, 2021 18:42:22 GMT
I would suspect, given the parameters you laid out, that a failure would happen at the thinnest point of the structure, namely in the corner where the mune meets the shinogi. That is the point of least material, and there is a 90 degree angle at that point. Without getting into complex calculations, I'll just say that the stress waves don't do well meeting at 90 degree intersections. The failure would take on the form of a crack extending the length of the joint area. That would be my edjewmacated guess. It's common for tsuka cores to crack at the mune corners of nakago, and also separate at the seam for the core halves (or 2/3 - 1/3 slabs). If the fuchi is strong and tightly fit, this usually helps to keep things together. It's not uncommon for production swords to have these cracks and seam separations when they're "new". Many production katana tsuka cores are shimmed and forced-hammered on. Changes in humidity and temperature can also cause cores to cracks and separate without usage.
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Post by jyamada on Apr 23, 2021 18:50:24 GMT
... Component failure in mechanical systems usually happens in places where the components can move relative to each other. The area where the maximum movement happens is the fit between the ho and the nakago inside that pocket. Stress waves would bounce between the air gaps and the two materials looking for a place to release. The weakest point (I assume) would be where the material is thinnest, in the corner. ... Yes. Once there is room for movement in fittings and/or core, then things just get progressively worse with more usage. Wood compresses under force, and it's common for swords (even with good materials and build quality) to loosen up when used extensively and/or used to cut heavy and/or hard targets. Mount-tightening is part of routine maintenance for swords that are heavily used for cutting targets with resistance.
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Post by JH Lee on Apr 23, 2021 22:37:42 GMT
Thanks for your inputs, guys. I appreciate the opportunity to think through this stuff together.
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Post by nebulatech on Apr 24, 2021 3:33:11 GMT
I'll try to take some pictures tomorrow of a gunto tsuka. It's cracked at the top, I don't remember where. For one reason or another, I actually cut about 30cm off the top, so we can see how deep the fracture penetrates.
Adding support to RufusScorpius's theory about the corner, I can share my personal experience from making some tsukas. When chiseling out for the nakago, I have had the wood split at the corner on the rear side (opposite from the cutting edge), forcing me to scrap the wood and start over. I used American cherry and sapele mahogany. White oak would be stronger, but I read it's unsuitable as it causes corrosion. Walnut is very hard, but also quite oily and tannic.
Regarding synthetics, I've done a good bit with fiberglass and resins lately working on my boat. That stuff can be literally bulletproof. It's an intriguing proposition that I might try in the next month or two. I'll share any pictures if I do it.
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Post by randomnobody on Apr 24, 2021 13:45:19 GMT
Why would a metal mekugi, if properly fitted, be bad? Knife and sword makers use metal pins all the time. From what little I recall, having read a discussion about the same, certain metals react poorly with each other, causing corrosion among other things. As far as hardened steel, the nakago is usually not hardened, so I would worry that hard steel would deform the mekugi-ana in the event of such impact as to threaten the mekugi. I can't recall the specifics, unfortunately, I just remember the consensus being "metal mekugi bad". Really, good, treated, conical bamboo is obvious the ideal, as it's been used for hundreds of years.
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Post by nebulatech on Apr 25, 2021 0:40:39 GMT
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Post by RufusScorpius on Apr 25, 2021 1:29:44 GMT
Something I just thought of looking at the pictures is the direction of the wood grain. It needs to be like an ax handle that is straight and linear with little to no runoff. Poor grain will weaken the structure
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Post by JH Lee on Apr 25, 2021 16:10:00 GMT
Great photos and info! Thank you! The most striking thing I notice is that the full wrap of same didn't seem to have prevented the damage!!
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Post by nebulatech on Apr 26, 2021 3:23:43 GMT
Also, no damage around the kozuka groove.
The same didn't split until after I cut the piece off the top of the tsuka. Good argument for a full wrap, but since it can mask problems (the cracks were present), one needs to be vigilant in disassembling and inspecting under the fuchi.
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