Thursday, March 24, 2016

Where the money is

You know, science is about truth searching, isn't it? Well, think again. Money is given to universities to advance a certain agenda, the agenda of those who have money or power (which are usually the same).

Saw tensioning is worth of study as any other subject, but since you make more money selling disposable circular saws to big companies, that's where the science is. Said in other words, you are not gonna get funded to study the tensioning of a ryoba so we need to divert some of the colloidal suspension money into it.

Usually the first step in a research project is to check out the literature and share it with your friends so you start talking the same language. This is what we are doing here.

Our first paper in this study is "Understaind Saw Tensioning" by G.S. Schajer appeared in 1984 in the journal Holz als Roh- und Werkstoff. Here's the link.

I just copy paste the interesting quotes to give you an idea of what the paper is about.

This paper describes the various techniques of circular and band saw tensioning, and explains how they work. It also discusses many of the significant contributions to the literature in this field.

Circular saw tensioninginduces stresses in the sawblade such that the periphery is pulled into tension. These stresses alter the saw vibration fiequencies, and when favorably dis- tributed, they can significantly improve sawblade stability.

The traditional way of tensioning both circular and band saws is by hammering their surfaces. The hammer blows in- dent the saw steel and squeeze it laterally in the plane of the plate. These highly localized deformations induce the tensioning stresses. Harmmer tensioning is very much an art, and great skill and experience is required to achieve good results. When done well, hammering can be as effective as more modern methods. However, hammer tensioning is usually not recommended for general use because the results can be very variable. Also, the hammer blows make the saw blade surface uneven and can initiate fatigue cracks.
A small induction generator heats the sawblade close to the collar to a temp rature in the range 30-80 C. This modest temperature does not cause any permanent changes, such as occur during heat tensioning. Thermal expansion of the heated central region of the saw induces tensile stresses in the unheated outer region. These tensioning stresses exist only while the central region of the saw is kept warm. When the induction generator is turned off; the saw returns to its original state. In practice, the induction generator is controlled so as to maintain a set temperature difference between the inner and outer regions of the sawblade.

Food for thought.

The pictures is from another paper and presents the analysis I plan to make for a ryoba with a shaped surface. These are the normal modes for a circular saw, the way it displaces out of plane. Any vibration of the blade can be decomposed into a sum of these modes, like a string in 1 dimension if you catch my drift.

 What you want to do in a saw is to dampen the low frequency modes so the vibrations are high frequency and low amplitude, thus your kerf is straight. Do you achieve this by carving the centre of the blade with the sen? by doing a surface hardening with a burnisher? I bet you can, I bet you do.

Saw tensioning is an obscure subject, but not because being intrinsically difficult or magical, but because the economic conditions of the world had made the subject not worth of research effort while at the same time making the people who knew this art redundant and obsolete. To change this means to work in a lateral way to usual university/research work, freed from corporate interest, based on friendship and love for knowledge. As it used to be, as it should be.

Stay tuned.
This paper describes the various techniques of circular and band saw tensioning, and explains how they work. It also discusses many of the significant contributions to the literature in this field. 


  1. Oh yeah baby!! Psychedelic harmonic vibes! Love it!

    Aaron J. shared this link with me, an old article from Scientific American, circa 1877....old-school gold here!

    Ive been too busy digging holes, not doing enough sawing, and my tensioning explorations have gone on hold, but I am very interested in this field of thought. One simple key point in tensioning a handsaw is that the field or zone of tension must be contained within the boundary of the saw plate. You induce a bit of tension by hammering along a line that runs parallel to the teeth, effectively stretching the metal. The hammer tensioning must STOP before reaching the tip of the blade, so you only tension to within maybe 25mm from the end. This "contains" the tension, without causing undue distortion. Taken too far though, and you end up with an "oilcan" dent, no fun!

    Share the knowledge, great stuff!

    1. Sorry, feeling wordy this morning.....

      The parallel here is that with a circular saw, the boundary is the perimeter, so the tension induced needs to be contained within the circle. This is obvious for a circle, but less so with a rectangular blade shape. Also....

      VERY(!!) interesting that the thermal dilation induced by the induction generator has a low end of 30C! That's with the range of heat that you generate during vigorous hand sawing or planing. This sounds like confirmation that proper handsaw tensioning has some value (something that I've been borderline sceptical about, to be honest).

    2. I also thought you were talking crap when you mentioned the temperature difference... but seems to be relevant nevertheless.

      look what I also found: "The case of a 54-year-old male suffering chronic schizophrenia is reported. The deceased was found in the workshop of his house lying in a pool of blood. The blood loss was caused by several stab wounds, in particular to the neck, due to the use of a screwdriver. Additionally, two superficial cuts were detected at the front of the neck, with a wave-like course consisting of several linear scratches apparently caused by the teeth of a handsaw found near the corpse."

  2. found another one: "At the tooth point, it is made wider than the thickness of the saw in order to prevent the blade loosing tension as it heats up during the cutting process. The tooth width was the parameter that the saw-doctor varied in cutting timber of different densities. In general large tooth width is for light hardwoods and small tooth width for heavy hardwoods. The saw was first tipped and ground to a large tooth width and used for cutting light hardwoods. After the tooth width has been reduced though sharpening, it was then used for cutting heavier timber till the width was too small and then re-tipped."

  3. You might be interested by Douglas Brook blog I found recently on Woodspotting where you also appear:
    picture of saw tensioning in Japan
    tensioning a diston in Japan:

    1. Hi Silvain

      thanks for the links, he's the chomasaru we talk about in fact, if you check back under saw sharpening tag you will have more links to him and some of his appearances in japanese, in case you are interested in saw sharpening that is. I'd love to have Douglas book too...

  4. Fantastic! I haven't made enough progress to add to this discussion of tensioning, and I actually feel more confused about it at the moment. But I did get a chance to read Brook's first book on wooden tub boats, which contained the method for braiding the bamboo hoops used on coopered vessels, something I hope to try in the future. I'd love to read his second book as well, had no idea he had posts on metate. I find it really interesting to see the guy working in front of a fluorescent light, Mark only works with the vagaries of natural light because he finds the fluorescents too harsh, but conditions have to be just perfect or you can't see what the hell you're doing.

  5. Oh, I forgot to add that the process of removing oil-canning seems to be a direct analog to tensioning, its probably a good place to start in lieu of possibly deforming a saw that is already working. Have any of you experienced the vibration of a poorly tensioned saw?

    1. check this out

      and yes, low frequency modes all over the place, big deformations, woobling sounds...

  6. I think you went through the same process my Dad says teachers go through- the young teachers are always hopeful, fresh out of college armed with the latest methods; the ones who've lasted a couple years are tired, set in their ways, and lost the idealism of their youth.

    Your getting a lot of work done, thanks for putting it all online freely- if you did get a grant to study handsaw tensioning somehow, odds are you'd have to put it all behind a paywall, no? Those things are annoying when I'm simply trying to get work done for school.

  7. in general I put all my papers in arxiv, which is open access. Founding agencies are moving more and more to open source, thanks god.