Panel Saw Tensioning

[Rhyolith]

Got a reply from Spear and Jackson concerning Saw tensioning.

In the early days when materials were supplied in the normalised state, craftsmen would roll tension and hammer the blade to induce spring back and tension.
Below is an extract from an old Woodsaw journal that mentions tension.


In modern times, the blade material is hard rolled and heat treated at the steel works and so is supplied pre tensioned ready for blade production.

The old craftsmen skills have almost died. The exception is the "little mesters" that still survive and are often linked to museums.

 

[Rhyolith]

And more from Lie-Neilsen following more queations, not sure there is anything new here.

I believe all of their saws were hammer tensioned. It seems most likely though that it is the steel itself that made for a nice stiff saw plate. There is no one today that makes steel the way that Disston did, so even though modern steels may be more consistent, the results are not the same. Since that formula and how the steel was treated was a carefully guarded secret, it may not be possible to recreate what they did today. We can analyze old blades and get steel made to the same specs, but no one knows what they did to it after that. Like many things from the past, those oldtimers may have taken the secrets to their graves.

None of our saws are “Hammer tensioned”, but the backsaws are tensioned after they are fitted to the back. All of our Panel Saws are also taper ground, which only a couple of other makers are doing today.

And a little more on the apparent tensioning of Lie-Neilsen's brass back saws... I have asked for more info on this.

We slit the brass backs to receive the saw plates, then rather than just relying on friction and adhesive to hold them in place, we compress the brass back onto the plate, which also increases the tension in the plate.

 

[D_W]

I'd be curious to know how they've concluded that it's the treatment of the steel (presumably by temperature, and not by rollers) that they think has stiffened the saw. Of course, everyone's allowed a guess or five.

I say that because there's no way they heat treat their own saw steel.

 

[Corneel]

A liitle late reply, but I found an scientific article about saw tensioning in circular saw blades and bandsaw blades. If you have access to Springer you can read it here: http://download.springer.com/static...ad4970bed7f4a74c3178ab1b3ef78fc6fb4f4300c4dfc

A short paragraph about hammer tensioning:

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.

 

[Trafalgar]

The firm Spear and Jackson were doing this at least as late as the 1960s, maybe later. Perhaps they were delusional. I tend to think not.

 

[Corneel]

The article I linked to above has something like 50 references. There is not a shadow of doubt by the authors that saw plate tensioning is a real thing, and can be explained perfectly well in scientific terms.

 

[Rhyolith]

The article I linked to above has something like 50 references. There is not a shadow of doubt by the authors that saw plate tensioning is a real thing, and can be explained perfectly well in scientific terms.

Is it soley circular saws though? There is some debate to whether tension those and panel saws is the same thing. There seems plentiful info on circular saws but not panel saws.

 

[Corneel]

The article is about circulair and bandsaws. There is absolutely nothing about normal handsaws, nobody wants to research those and put a lot of money into it.

But I think bandsaws can be compared one way or another to handsaws. Of course, the speed is about 10 times faster.

The whole idea of and the art of tensions saws comes from the handsaw world. It was done long before mechanised sawing.

 

[woodbrains]

Hello,

I only took an interest in this thread when Corneel added a post yesterday, and just read the whole thing. I'm not an engineer, but have always believed the old texts stating that saw blades were tensioned by hammering. I did not know any better, so assumed (automatically) that the info must have come from some real world processes from saw makers. It seems a bit of an unlikely thing to be a fictitious phenomena, stated in so many published texts. Then I read the engineers here who say it is not possible to make a plate stiffer without increasing thickness! Surely this can only be true if all remains the same. Doesn't tempering, case hardening, annealing etc. vary stiffness without grossly changing thickness? Wouldn't stress hardening from hammering alter stiffness? I understand differential cooling of die cast magnesium alloy (for example) introduces tension in the structure, enabling thinner sections to be manufactured for the same stiffness. I spoke to a chap from Vauxhall motors a few weeks ago who mentioned that some car bodies are being made from thinner plate to reduce weight, but being stiffened to compensate--during the baking process involved when curing the paint finish! (I find that idea wonderful, modern manufacturing is truly sophisticated) So in my mind, it is clearly possible to make metals stiffer without increasing thickness.

Mike.

 

[Cheshirechappie]

Doesn't tempering, case hardening, annealing etc. vary stiffness without grossly changing thickness? Wouldn't stress hardening from hammering alter stiffness?

No.

Doesn't say much for Vauxhall cars if they make the bodywork stiffer by painting it, does it?

 

[D_W]

Painting and metal deformation are the same thing?

 

[woodbrains]

Doesn't tempering, case hardening, annealing etc. vary stiffness without grossly changing thickness? Wouldn't stress hardening from hammering alter stiffness?

No.

Hello,

When I was a schoolboy, our metalwork teacher persuaded the weediest lad in class to bend a bit of thin steel plate, whilst holding it, arms straight above his head. After flattening it out, he got the butchest lad to try the same, knowing he would fail miserably. This was to demonstrate stress hardening. Has stress hardening become a myth?

Mike.

 

[Cheshirechappie]

Doesn't tempering, case hardening, annealing etc. vary stiffness without grossly changing thickness? Wouldn't stress hardening from hammering alter stiffness?

No.

Hello,

When I was a schoolboy, our metalwork teacher persuaded the weediest lad in class to bend a bit of thin steel plate, whilst holding it, arms straight above his head. After flattening it out, he got the butchest lad to try the same, knowing he would fail miserably. This was to demonstrate stress hardening. Has stress hardening become a myth?

Mike.

No. But it does not make metals stiffer, it pushes the yield point up the stress-strain curve, making them (in effect) more elastic and capable of standing higher strains before permanent deformation occurs -as it happens, exactly the qualities you want in a handsaw blade. See previous essay a couple of pages ago (I'm not typing that lot again!) for details.

 

[woodbrains]

No. But it does not make metals stiffer, it pushes the yield point up the stress-strain curve, making them (in effect) more elastic and capable of standing higher strains before permanent deformation occurs -as it happens, exactly the qualities you want in a handsaw blade. See previous essay a couple of pages ago (I'm not typing that lot again!) for details.

Hello,

If that is not a definition of stiffer, I don't know what is!

Mike.

 

[Corneel]

I had some trouble understanding that one too at first! Let me try to explain:

Take two strips of steel, same steel, one hardened, the other not hardened. Clamp them on one side to your bench and hang weights on the other end.

You will see, until one of the blades reaches its yield point, they behave exactly the same. So they are just as stiff until the unhardened one starts to permanently deform. They are equally stiff until that point.

The article I posted above described how they increase the stiffness of a plate by putting tension into it. In circular saws and bandsaws this is important to reduce vibrations so the blade can run at higher speed. They describe several methods.
- Hammering. The hammer compresses the steel in a small area which puts the metal around it in tension.
- Rolling. The same but in a manner that asks for less skill from the operator.
- Heating spots up to +/- 400 degrees. While cooling again this causes deformations which act the same as the hammering process.
- Heating during operation. With induction coils, usually near the hub of a circular saw blade, they heat it up to 80 degrees. This creates enough tension in the blade to change its behaviour under speed. It seems this works with temperatures down to 30 degrees!.

Why doesn't the plate deform into a rollercoaster under all that hammering? That's the skill of the operator to keep any deformation so small and well balanced between the various dents that it doesn't effect the plates straightness as a whole.

 

[Trafalgar]

I posted several links in a post on page 10 of this thread dealing with the dimpling process of steel. It changes it. Apparently, for the better if well done.

 

[woodbrains]

I had some trouble understanding that one too at first! Let me try to explain:

Take two strips of steel, same steel, one hardened, the other not hardened. Clamp them on one side to your bench and hang weights on the other end.

You will see, until one of the blades reaches its yield point, they behave exactly the same. So they are just as stiff until the unhardened one starts to permanently deform. They are equally stiff until that point.

The article I posted above described how they increase the stiffness of a plate by putting tension into it. In circular saws and bandsaws this is important to reduce vibrations so the blade can run at higher speed. They describe several methods.
- Hammering. The hammer compresses the steel in a small area which puts the metal around it in tension.
- Rolling. The same but in a manner that asks for less skill from the operator.
- Heating spots up to +/- 400 degrees. While cooling again this causes deformations which act the same as the hammering process.
- Heating during operation. With induction coils, usually near the hub of a circular saw blade, they heat it up to 80 degrees. This creates enough tension in the blade to change its behaviour under speed. It seems this works with temperatures down to 30 degrees!.

Why doesn't the plate deform into a rollercoaster under all that hammering? That's the skill of the operator to keep any deformation so small and well balanced between the various dents that it doesn't effect the plates straightness as a whole.

Hello,

You are saying, if I interpret it correctly, the hammering or rolling or whatever, causes the plate to be differentially hardened. Now if there is no difference in stiffness until the less hard part reaches its yield point, and we never reach that point as it would ruin the saw plate, then there is no benefit to the process at all, apart from straightening any defects during manufacturing. Clearly there must be something else going on. Do we run saws close to the yield point of the less hard steel and the harder steel prevents the yield occurring? Distortion through heat buildup is prevented by the tensioned portion of the plate, so the tensioning process is beneficial to circular saws. This heat cannot be what we would term HOT though, surely the steel dissipates the heat fairly quickly. I guess a few degrees is enough to cause thermal expansion in the metal and cause vibrations in an untensioned plate. So I would assume that handsaws could get warm enough at the toothlike to cause similar unwanted distortions without getting hot enough to draw temper for instance. So it would be beneficial to tension the handsaw plate with hammering/rolling for the same reasons as the circular saw plate. Whatever words we choose to use to define stiff (incorrectly maybe) there is something happening to the saw plate that is hammered to one that is not, that provides some benefit.

I would add, the grain structure of hardened steel is different to unhardened, so we can never truly compare like for like in the side by side tests such as Corneel suggests. Hardened steel has a smaller grain structure and I don't see why this could not be stiffer to a similarly dimensioned unhardened steel. Stress hardening changes the grain structure. In fact hasn't the stress hardened steel essentially become a different material? This would explain quite a lot.

Mike.

 

[Corneel]

Not really (I think). I'm far from an expert in this field, just trying to tell what that article from 1984 was all about.

In my previous message there are two different things, thirst why hardened and unhardened steel have the same stiffness. Well until the unhardened gets deformed of course which happens at a rather small load allready.

The second part is me trying to explain how you can increase the stiffness of a hardened plate of spring steel (a saw). A local deformation, more precisely a compressed spot of steel, causes tension in the surrounding steel. Chappy tried to convince us with a theory that that is impossible. But there seems to be a group of scientists who have studied this back in the 70's and 80's and came up with a plausible explanation. But you really should read the article. I gave a link on the previous page. There is also a whole list of references in that article, which really is not much more then a literature study.

I you can't access the full text, I'll see if I can copy it next week when I am at work again.

 

[Corneel]

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[woodbrains]

Hello,

Wow, thanks Corneel, I'll give it some time to read tomorrow.

I do think that I have stumbled on a good theory, though. If the grain structure in stress hardened steel is different ( it is) than the unhardened steel, then it is, in fact a different material, albeit slightly, which will have different properties of stiffness, elasticity, brittleness, etc. etc. albeit slightly, it must still be different. So should be assumed that it can no longer be directly comparable, any more than a piece of plastic or brass or whatever, of the same dimensions, can be compared to the steel in its unhardened state. How could it, really?

Mike.