Gabe has questions regarding handsaw tension, as do I.
I really wish that I had some good answers here, your questions are all good ones.....questions that I share! Tensioning of handsaws is one of those questions that a few of us are asking, but finding answers is proving difficult.
George Wilson (now retired, tool makers from colonial Williamsburg) indicated in some long ago thread on SMC that they didn't actually tension many of the full size long saws that they made on site, and they worked just fine. What the heck is tensioning anyways, and is it necessary? Here is my thinking, such as it is.....
When you cut wood, force and friction cause the blade of the saw to heat up, not like HOT, but the heat is definitely there. The heat effect is greatest at the tooth line, and causes the saw blade to expand slightly, effectively lengthening the saw a tiny amount. The effect being greatest at the tooth line, if there isn't some way to accommodate the extra length, you get wandering cuts or binding. Striking the body of the saw to "tension" it, is essentially making the body of the blade longer than at the tooth edge, so that as the blade heats in use, all things are evened out. To me this would mean that, rather than the edge being "tensioned" in its normal state, what we are actually doing is putting it in a state "compression". Tensioning would then be a misnomer. I'm not sure if this is the case, but that's how it works in my mind, haha.
I doubt that this saw was tensioned originally as I saw no marks on the plate that indicate that it was run through an English wheel....lines of distortion running longitudinal to the body. I think that's how they do it nowadays. The saw plate on this guy is just relatively thick, and very stiff. When I struck the tooth line at the hash marks, to remove or neutralize the tension, it was mostly for forms sake. The idea is that I would be "lengthening" the tooth line to be equal in length to the body, and I used my faux cross-pein hammer to give more directional force, a stronger lengthening effect.
In re-tensioning the blade, if we look back to Hasluck Handyman's Guide,
he calls for the use of two different hammers to fix bends, the cross-pein hammer for specifics and a larger dogs-head hammer for the general area around the bend. The larger hammer is used for tensioning, I am assuming because we need to input more force. Bob Smalser calls for using a 2# hammer for tensioning, so he has something similar in mind.
It would've been interesting to have tested the saw prior to tensioning, but it didn't occur to me. This was one of those cases where I just got pissed, grabbed the hammer and started whacking things. My direction and intent were there, but the pictures came after the effect. Let's all try to do better in the future, because this is helpful information, haha.
As I spend more and more hours staring at these different saw blades, I guess that I am training myself in various ways to see deviations. When I bend a saw blade and look at the way the light reflects, I now see much more than I did a few months ago. I would have to say that my eye is looking differently now, somewhat akin to the way an artist sees the space around an object as much as the object itself, seeing the "negative space". Mark Grable refers to this as right brain/twilight brain thinking, and that would be correct as well. I don't look at the blade itself, but more how the light gets distorted by the bad spots.....kind of. And to be clear, I claim no facility here either, I'm just starting to see this stuff. When I was tensioning this blade, how did I determine the areas that needed extra tension? You could call it a guess and be just as correct as any excuse that I could devise, haha. It just felt like it needed a few extra whacks to bring things back into line.
When staring at the blade side on, if you just bend the blade into an arc there might not be anything to see, but if you squeeze that blade between your hands as you bend (inducing a compressive force) you might see much more.
Somewhat similar, I have two ryoba, one cheap and the other much higher quality. The cheap one bends smoothly and evenly, also rings loud and bright. The higher quality saw bends smoothly as well but as I bend it, it feels like the saw plate close to the edge is stiffer, while the main body is more flexible. It doesn't bend evenly exactly. Smooth, but not even. When rung, the sound it makes it clean, but also lower in pitch or perhaps richer/more complex.
I was looking at a handful of old, rusty saws yesterday, and spent a minute bending them back and forth, sighting the length, just kinda checking them out. One of the saws was an old 1920's Disston, one was a "guaranteed superior", and a couple were just.....crap. The Disston was trashed, but the GS saw was stiff, bent in a way that felt nice to me, but more than just stiff. I don't know yet how to describe it. Most interestingly I thought, were the crap saws. Though they bent clean, smooth, and easy, they just felt flexible.....like if you were sawing something, they would be flopping all over, wandering from one side of the line to the other. It just feels like something was lacking.
I have been looking at some pictures of a nice Maebiki-oga that was on eBay not too long ago.
Of the great variety of hammer marks that you see on these handmade saws, the maebiki have some of the most distinctive.
I'm looking at the upper corner, and the wealth of impact strikes that are common here. Is this to thin out the metal in that entire corner, reducing drag in use? Is it greatly thinner there, to allow a degree of correct-ability when your cut runs away from the line?
Is it to drive mass into the greater body of the saw, as part of the tensioning process?
When I look at this photo, I see hammer strikes that are pushing (fullering, in blacksmithing terms) metal from the tip of the saw into the main body. Then, that mass gets pushed /spread along the length of the saw, as evidenced by the little line marks from a completely different type of hammer.
The opposite side of the saw shows a similar (though not exact) system in play. Obviously this is done for a specific reason and I would love to know.
Gabe, you've got two maebiki right there....what are your thoughts?