I don't know why this is happening again, but I've received a lot of e-mails lately asking about three mekugi. So one last time on the topic of mekugi in general, 3 in particular.
1. The historic record.
Traditionally one was the most common number of mekugi. Overwhelmingly. That does *NOT* mean that there weren't swords with 2 mekugi. There were. Just not the majority.
2. What are mekugi for?
Mekugi are *NOT* things that work to hold the blade in the tsuka, at least not in a literal sense. If the mekugi is actually physically the only reason the blade is staying in the tsuka, you have issues. Mekugi help "settle" the blade deeply into the tsuka. Once the blade is settled tightly into a well carved tsuka channel, the tsuka, nakago and by extension sword become much like a single object. If everything is properly tight, vibration, rattling, movement, etc. is all eliminated or at least minimized. If the assembly is loose with each movement the nakago is causing increased wear and tear. Think about a poorly fitting shoe. If it's moving around where there is contact and movement you're going to get a blister *really* quickly. You don't tie your shoes properly to keep your shoe from flying off your foot as you walk. You tie it to get it properly snug so it moves with your foot as one complete thing. With no extra rubbing or pressure. Those pressure point result in damage later. In a tsuka pressure points in the wood core mean compressed wood which means more looseness which means more pressure points which means... You get the idea.
A properly fit tsuka with a mekugi that is holding the nakago properly in place has very little stress going on inside of it. That includes the mekugi itself. As long as the blade is being "pulled" into the wood just a bit, well, everything stays rock solid and all that energy is distributed and transmitted. And doesn't end up as energy dissipated by damaging the internal core. Once things get loose, well, now the mekugi is taking pressure and stresses. If the blade is free to just fly out easily, the mekugi is most certainly the only thing holding it in. But you're also talking about what most of us would consider an unsafe sword already at this point. One pin, two pins, I don't care -- don't swing around a sword that's moving inside the tsuka. There are other failure modes including catastrophic tsuka failures that can send the whole thing flying, 2 mekugi or 20, it won't matter if the entire assembly blows apart. And you don't want to be holding it when it happens.
3. Why 2 mekugi today? Well, a couple reasons. One is that in the olden days folk realized that their tsuka, saya, etc. on blades intended to see battle were consumables. They wear out. You replace them. You fix them. When your life literally depends on the integrity of your sword, a new tsuka isn't a big deal. And the local shogun had guys who sat around fixing, rewrapping, etc. all day long. It is how it worked. Today, especially with production swords, the end-users aren't always as in-tune with those sorts of things. Lots of people enjoy swords and "casual" practice without the sort of supervision or training folk used to have. So tsuka wear out and gather looseness but often the owners don't bother with getting new tsuka or the existing one repaired.
Obata and his Shinkendo group also have done a lot to popularize the double mekugi. He insists all this swords have a second mekugi, and many I've seen have been iron (a topic for a different day). In this instance it is basically a requirement for a particular style and as such those students need a second mekugi. One the one hand because he feels it's a good idea. But on the other hand, it's because he said so. And really that is good enough.
The reality is that someone with the proper tools, experience and know-how can fairly easily add a second mekugi. And sure, if done correctly, the second mekugi is sort of a "fail-safe". But you need to *really* look at this critically. The second mekugi is added (and I do it on all daito I mount) as a backup of sorts. But really, if you brain fart so bad that you forget to replace the mekugi after taking your sword apart, why does having two mekugi make a difference? Well, if you forget to replace one of them, okay, fine. But if you really did just "forget", well, sheesh, you'd probably forget them both. And what are you doing swinging a sword without first checking to see if the pins are in place? NO ONE SHOULD EVER DO THAT. PERIOD. ALWAYS check the pins. Be it one or two, it doesn't matter, you should look, touch, make sure it is tight, and only then swing the sword. To me it's like dry firing a gun without checking first to see if there is a round in the chamber. Stupid, stupid, stupid. And unforgivable.
There is of course the issue of a pin failing or falling out during use. That's another issue and it is important. But it still goes back to proper inspection and maintenance of your tools. You pop out the mekugi every now and then to look at it. Is it intact? Still in good shape? If not, make a new one. Is the mekugi older than a couple years? If so, make a new one. And I don't mean a chopstick or a piece of wood from your scrap pile. I mean using something like the properly aged and very fibrous bamboo traditionally used. That stuff is *SUPER* tough. Chopsticks, wood dowels, etc. are like glass in comparison.
Personally I use delrin pins tapered on a lathe to a very precise taper. And I use a precise tool to ream the mekugi-ana to match the taper of my pin. The delrin has a burst strength of something like 10,000 psi. The stuff will bend, stretch and deform, but breaking it... Ain't gonna happen. And with a machined, precise taper fit it is very unlikely to fall out.
3. Placing mekugi -- a gotcha. There is one problem with multiple mekugi, however. One no one ever seems to talk about. If you've mounted enough blades you'll eventually run into this problem. You place the first mekugi. Check everything out -- tight, clean, perfect. You place the second mekugi. Pop in the pin. Swing the the blade and there is the slightest of fitting rattle. How is that possible. Adding the second pin makes it looser?
It can. Go back to the original bit up top about what mekugi do. They help settle the black into the tsuka. Sometimes when people drill the second hole, the power of the drill causes shifting of the nakago against the front mekugi. You take the drill out and it settles back down. Now you put pin #2 in and press it tightly in place. But what's really happening is that the second pin is actually pushing the blade forward. So now the pins are fighting each other, one pulling it in, the other pushing it out. Eventually one of the pins is going to deform a bit. Or the tsuka nakago-ana will deform a bit. And a sight bit of looseness will form.
The point is that the second ana has to be very carefully drilled by someone who knows what they're doing. The ana need to be properly deburred, fit and sized for the ana size in the nakago so that the kashira end of the ana is what is being pulled into the tsuka core. If there is any movement between drilling the holes, even the slightest shift, one of the pins may longer be pulling the sword into the tsuka. And depending on how it was all done, one pin may actually be pushing out instead of in. In the first case one pin is close to useless and not in contact with the nakago anyway. In the second case one pin is actually working against the other one making for a sword that is less safe than if you had simply not put in the second one.
Properly done two holes is safer. The degree of additional safety is really a complex issue and (IMHO) the reality is that the second pin doesn't add a huge amount. But today many insist on it. For me, I'd rather see someone use a sword with one pin that they check and maintain religiously rather than a person using a sword with 2 pins that they don't check. Better of course is two pins, properly placed, and check them before use. Not checking pins is simply poor safety and one pin, two pins, it doesn't matter. Just like checking to see if a gun is unloaded you always check to make sure your mekugi are in place and tight before doing anything else. And if everything is proper and correct, one pin will work fine. But not everyone is as focused as that, neh?
4. Mekugi angle. Mekugi-ana are usually place perpendicular to the nakago. The pin goes in perpendicularly. Sometimes there is a very slight angle. The reason for the angle is that the opening on one side (the diamond in the ito) is opposite the overlap on the other. When custom makers do the mekugi, they're drilled, placed, tested etc. *before* the tsuka is wrapped. It is a very careful thing because you have to measure out all the parts, the compressed width of the ito, etc. and figure out exactly where the ana is going to be *after* it gets wrapped so you can get it in and out without destroying the wrap. It takes care and planning. You place the hole slightly forward of the diamond to ensure it comes out on the other side just behind the crossover making it possible to get it in and out. Sometimes they end up with the small end under the crossover. Usually that's okay because you just use the point of the mekugi-nuki (the hammer) to get under there and put it through. Sometimes the craftsman may use a slight angle of drilling to ensure the pin comes out clear of the ito, but we're talking a very slight angle. Very little actually.
What you will see on some mass produced swords (and often on iaito) is mekugi ana drilled at a very sharp angle. Basically what they're doing is drilling the ana after the tsukamaki and angling it to make sure they clear both sides and are centered. That's fine with light, soft alloy blades of iaito. They also don't get "impact" stresses since you don't cut with them. But on a heavier, harder, steel blade, the severe angle changes somewhat how the steel nakago-ana and the pin contact each other. Generally most don't consider it a great idea to create a sharp edged nakago-ana (which is what it has on one side) that looks more like a cleaver looking at it sideways. The angle should be a lot less than that if the blade is going to see stresses of actual use.
5. Okay, 3 mekugi... If you read all of the above, hopefully we're on the same page now. The third mekugi is redundantly redundant.
The first does the work. The second is a back up and if done correctly works in conjunction with the first. The second, if done incorrectly the second pin is irrelevant and if it's really off it may actually make matters worse in terms of mechanical properties. Adding a third pins adds nothing and you're only increasing the chance a slight offset is actually working against you keeping the tsuka tight.
6. The bottom line... One pin if the swordsman is attentive and exacting in his practice is fine. After all, you're playing with a deadly weapon so the primary safety feature *has* to be the person's attitude first and foremost.
Adding a second pin does offer some advantage of a backup, but without a sincere safety attitude it won't matter. Sure, if a pin "falls out" during use you have the second one still there. And many sensei insist on the second one anyway so there you go. But if you don't check them it doesn't matter if you have one or two -- the problem is the user again.
A third pin offers nothing more than an extra hole in your nakago. And if the extra pins (the second or third) aren't placed correctly by the craftsman, well, they kinda work against themselves. So the bottom line is one was workable and fine throughout the history. A second pin can add some extra piece of mind *if installed correctly*. But additional pins not done well may actually degrade the fit oh-so-slightly. One is fine. Two is probably overkill, but done right it is a good overkill. Three is silly.
And by the way, done correctly three pins will work just fine. But really no better than two done well. So why bother?
And it doesn't matter how many pins are in the thing if you don't maintain them, check them, etc. User stupidity is the bigger problem.