Panel Saw Tensioning

[Cheshirechappie]

Mike, you're right that the grain structure of steels is different depending on their heat-treatment condition (annealed, hardened right out, hardened and tempered back), but that won't affect their stiffness (or more technically, their elasticity below yield point). It will drastically affect their yield point and ultimate tensile strength, though.

Earlier in the thread, AndyT mentioned the book, "The New Science of Strong Materials" by J E Gordon. I'd heartily endorse his recommendation. For anybody wanting a deeper understanding of the metallurgy of steels (and other metals), a copy of "Engineering Metallurgy" volume I by R A Higgins should cure any insomnia, but also explain what's going on with grain structure when metals are worked or heat treated.

However, most saw doctoring is about the manipulation of internal stresses, not about grain size modification. Saw blades start out hardened and tempered, so the grain structure is pretty well set by that condition.

I'm a little bit wary of drawing comparisons between circular saws, band saws and hand saws. The reason is that both circular and band saws see stresses in service that handsaws don't.

A circular saw develops stresses in the plate as it spins, and the heat generated in cutting cause thermal expansion of the rim. It was found during the development of such saws during the early to mid 19th century that the detrimental effects those stresses could be alleviated by doctoring the saw before service - basically making it a flat cone or flat saucer shape instead of dead flat. The larger the saw, the more important the pre-service doctoring. The early texts such as 'Grimshaw on Saws' deal with this in some depth - many pages on circular saw doctoring, against a bare couple of pages on handsaws. It's interesting that vibration becomes a factor as rotational speeds increase, and that it's deleterious effects can be reduced by saw doctoring.

A band saw, when in service, IS under tension - that imparted by the top wheel adjustment mechanism. It will also see the load applied by the material being fed to it, and the balancing loads applied by the guide wheels to the back of the blade.

Handsaws see none of these loads - indeed, the stresses on a decently sharp hand, panel or backsaw are very low (you don't need to apply much force to get it to cut, just move it backwards and forwards). What stresses there are will be at the tooth-work interface, and mostly near the working bit of the tooth, the tip. The main body of the saw blade will see pretty well the same stresses it sees when lying on the bench. Thus, circular saw and bandsaw type 'tensioning' are not needed.

Maybe as the saw makers were wrestling with the problems of making reliable circular saws, they thought that the lessons they were learning could be applied to hand saws. I'm not at all sure that it actually does anything worthwhile to the hand saw blade, but it doesn't half sound good in the marketing blurb!

 

[Corneel]

First you tried to tell us that tensioning of sawblades is not possible. That hammering on sawblades didn't do anything for the tension in the blades. Now I gave you a few references where tensioning of sawplates is described, calculated and the effect meassured. And now this all suddenly doesn't apply to handsaws?

As far as I know, hammer tensioning came first on handsaws, and was later applied to circular and bandsaws to cure problems with these. And bandsaws might be under tension, but they have the problem that they find a block of wood in their path. This block of wood slows down the blade, while the motor tries to pull it through. The result is vibrations, and the cure for these vibrations is hammer or roller tensioning.

The effect of hammer tensioning is real. You could at least admit that. Then we could see if it actually does any good to a handsaw.

 

[Cheshirechappie]

Corneel - I've stated that you can't STIFFEN a hand saw blade by hammering it (or rolling it, or whatever). I've put forward a hypothesis about what 'tensioning' might do to a hand saw blade, by the effects of manipulating internal stresses. I haven't said that 'tensioning' doesn't exist - it's just that nobody has yet explained what it is (I don't know either), though one or two have jumped to unsubstantiated conclusions.

The things done to circular and band saw blades are different because they operate under different conditions, and see radically different stresses to handsaws.

 

[D_W]

Two things are mentioned for tensioning:
* stiffening the saw
* increasing the likelihood that the saw returns to straight.

If the tensioning isn't why two of my saws with similar cross section are different in stiffness, I'll gladly take the benefit of the latter.

I doubt there's going to be much literature on tensioning anything other than bandsaws and circular saws for the simple reason that research and documentation would've been met with economic reward with those two. Not since the early 1900s for hand saws, and in that case, in an ultra competitive business where makers did most of the research, I can't see why we'd expect they'd have offered anything other than ad copy publicly.

 

[Cheshirechappie]

Two things are mentioned for tensioning:
* stiffening the saw
* increasing the likelihood that the saw returns to straight.

Can anybody post a reference that states that 'tensioning' stiffens a hand saw blade? I've read a fair bit about saws, though I very much doubt I've read everything, but I can't recall ever having seen any statement to that effect. Most of the references, when they mention 'tensioning' at all, are pretty vague about what it is or what it does.

 

[D_W]

Two things are mentioned for tensioning:
* stiffening the saw
* increasing the likelihood that the saw returns to straight.

Can anybody post a reference that states that 'tensioning' stiffens a hand saw blade? I've read a fair bit about saws, though I very much doubt I've read everything, but I can't recall ever having seen any statement to that effect. Most of the references, when they mention 'tensioning' at all, are pretty vague about what it is or what it does.

There is an article by Bob Smalser where he states both of the above. If we're looking for something in an engineering journal, I think we might as well just hit repeat on the thread as we've gone through that. Thus my comment from above - I don't think it was something that would've been studied at a university level (especially not in the interest of making information public).

The later study material is for things that are commercially viable.

Sort of like the cap iron study - it was not done for planes. It was done for the marunaka super surfacer (which you can look up, you'll know you've seen it if you can't visualize what it is). There was, however, an ancillary paper written for planes, but i think that was out of good intention by the university, and they weren't very precise about that part itself. People are running around quoting distances for cap irons that are in regard to machine testing, but the paper the same university released on the hand plane was much less committal about specifics and instead said that it's a matter of experimentation to get the cap iron in a plane set properly.

 

[Cheshirechappie]

If the Bob Smalser article you refer to is the one about saw straightening, I know the one you mean. He makes a number of statements in that article that he doesn't provide any references for. Have you any idea where his statements came from?

In the 19th and early 20th century literature, I can't recall seeing any statement to the effect that 'tensioning' makes a hand saw blade stiffer. 'Tensioning' is mentioned a several references, but as I said earlier, always in rather vague terms.

 

[Corneel]

I just gave about 60 references about tensioning or pre-stressing of saw blade material. They compress the blade material in a lot of small spots which puts tension in the plate. There is no reason why that wouldn't work on handsaw blade material too. This has already been worked out, calculated and tested back in the 60's- 80's.

It helps to reduce low frequency vibrations. And that is something very usefull for a handsaw too. When you trust the sawplate down into the wood, the forward trust is resisted by the wood cutting action. This would buckle the plate if it is too thin and thus create low frequency vibration. On the back stroke it is also very easy to excite a low frequency vibration, you probably know the annoying flapping of a floppy blade when you don't pull it back exactly straight through the cut.

Hammer tensioning helps to make a blade resist those vibrations so you can use a thinner blade. There is no market for high quality handsaws anymore, so there is no money for research. But there are strong analogies with the powertool blades. How it is done is explained, calculated and tested in those papers I mentioned above. Before this scientific research it was more like a craft, well known among seasoned sawmakers, but not described in detail anywhere.

 

[Cheshirechappie]

I just gave about 60 references about tensioning or pre-stressing of saw blade material. They compress the blade material in a lot of small spots which puts tension in the plate. There is no reason why that wouldn't work on handsaw blade material too. This has already been worked out, calculated and tested back in the 60's- 80's.

It helps to reduce low frequency vibrations. And that is something very usefull for a handsaw too. When you trust the sawplate down into the wood, the forward trust is resisted by the wood cutting action. This would buckle the plate if it is too thin and thus create low frequency vibration. On the back stroke it is also very easy to excite a low frequency vibration, you probably know the annoying flapping of a floppy blade when you don't pull it back exactly straight through the cut.

Hammer tensioning helps to make a blade resist those vibrations so you can use a thinner blade. There is no market for high quality handsaws anymore, so there is no money for research. But there are strong analogies with the powertool blades. How it is done is explained, calculated and tested in those papers I mentioned above. Before this scientific research it was more like a craft, well known among seasoned sawmakers, but not described in detail anywhere.

OK. So all we need now is a demonstration of that working on a hand saw blade. Somebody taking a cheap, floppy, unhammered saw, giving it the hammer treatment, and demonstrating that it's then stiffer. Or taking two pieces of identical spring steel, hammer treating one, and demonstrating a significant difference in stiffness between the 'tensioned' and the 'untensioned'.

I don't think anybody will be able to demonstrate it, for all the reasons set out in this thread. However, there are those who say that a hand saw blade can be stiffened by hammering (or rolling, or whatever). OK - prove it.

 

[D_W]

I just gave about 60 references about tensioning or pre-stressing of saw blade material. They compress the blade material in a lot of small spots which puts tension in the plate. There is no reason why that wouldn't work on handsaw blade material too. This has already been worked out, calculated and tested back in the 60's- 80's.

It helps to reduce low frequency vibrations. And that is something very usefull for a handsaw too. When you trust the sawplate down into the wood, the forward trust is resisted by the wood cutting action. This would buckle the plate if it is too thin and thus create low frequency vibration. On the back stroke it is also very easy to excite a low frequency vibration, you probably know the annoying flapping of a floppy blade when you don't pull it back exactly straight through the cut.

Hammer tensioning helps to make a blade resist those vibrations so you can use a thinner blade. There is no market for high quality handsaws anymore, so there is no money for research. But there are strong analogies with the powertool blades. How it is done is explained, calculated and tested in those papers I mentioned above. Before this scientific research it was more like a craft, well known among seasoned sawmakers, but not described in detail anywhere.

OK. So all we need now is a demonstration of that working on a hand saw blade. Somebody taking a cheap, floppy, unhammered saw, giving it the hammer treatment, and demonstrating that it's then stiffer. Or taking two pieces of identical spring steel, hammer treating one, and demonstrating a significant difference in stiffness between the 'tensioned' and the 'untensioned'.

I don't think anybody will be able to demonstrate it, for all the reasons set out in this thread. However, there are those who say that a hand saw blade can be stiffened by hammering (or rolling, or whatever). OK - prove it.

Here's the difference between real life and the engineering answer. Engineers assume that the knowledge is difficult but the proving is easy ("the whole world is built by engineers" who couldn't butter a brick if you gave them two hours). What you're not grasping is there's no documentation on the rolling or hammering, so what you'd like to prove and what you asserted at the beginning (that tensioning doesn't do anything), you can't prove. You can't prove anything about it because you have no clue whether or not it's done correctly.

 

[Cheshirechappie]

I just gave about 60 references about tensioning or pre-stressing of saw blade material. They compress the blade material in a lot of small spots which puts tension in the plate. There is no reason why that wouldn't work on handsaw blade material too. This has already been worked out, calculated and tested back in the 60's- 80's.

It helps to reduce low frequency vibrations. And that is something very usefull for a handsaw too. When you trust the sawplate down into the wood, the forward trust is resisted by the wood cutting action. This would buckle the plate if it is too thin and thus create low frequency vibration. On the back stroke it is also very easy to excite a low frequency vibration, you probably know the annoying flapping of a floppy blade when you don't pull it back exactly straight through the cut.

Hammer tensioning helps to make a blade resist those vibrations so you can use a thinner blade. There is no market for high quality handsaws anymore, so there is no money for research. But there are strong analogies with the powertool blades. How it is done is explained, calculated and tested in those papers I mentioned above. Before this scientific research it was more like a craft, well known among seasoned sawmakers, but not described in detail anywhere.

OK. So all we need now is a demonstration of that working on a hand saw blade. Somebody taking a cheap, floppy, unhammered saw, giving it the hammer treatment, and demonstrating that it's then stiffer. Or taking two pieces of identical spring steel, hammer treating one, and demonstrating a significant difference in stiffness between the 'tensioned' and the 'untensioned'.

I don't think anybody will be able to demonstrate it, for all the reasons set out in this thread. However, there are those who say that a hand saw blade can be stiffened by hammering (or rolling, or whatever). OK - prove it.

Here's the difference between real life and the engineering answer. Engineers assume that the knowledge is difficult but the proving is easy ("the whole world is built by engineers" who couldn't butter a brick if you gave them two hours). What you're not grasping is there's no documentation on the rolling or hammering, so what you'd like to prove and what you asserted at the beginning (that tensioning doesn't do anything), you can't prove. You can't prove anything about it because you have no clue whether or not it's done correctly.

Should be easy enough to demonstrate. Take one floppy saw, hammer it, demonstrate that it's now stiffer. You've been telling us all thread that that is what happens - 'tensioning' makes a hand saw blade stiffer.

To be frank, I'm getting bored with this thread, and people making assertions they can't prove - namely, that 'tensioning' a hand saw blade by hammering it makes it stiffer. I say it doesn't.

Well, now's your chance to prove that it does.

 

[D_W]

Should be easy enough to demonstrate. Take one floppy saw, hammer it, demonstrate that it's now stiffer. You've been telling us all thread that that is what happens - 'tensioning' makes a hand saw blade stiffer.

To be frank, I'm getting bored with this thread, and people making assertions they can't prove - namely, that 'tensioning' a hand saw blade by hammering it makes it stiffer. I say it doesn't.

Well, now's your chance to prove that it does.

Ahh, the engineer's take again. The solution is in our heads, but once we know it, then it's very easy and trivial to carry out. Of course, you are still ignoring the fact that you have no clue how you would hammer a saw to tension it, and in tensioned saws (circular), where the properties are not at all in dispute, you wouldn't be able to hammer that or tell someone where or how to hammer, either. It would take a skilled sawsmith to do that. Just as it would take a skilled smith to prove something on a handsaw.

If you think you're getting tired of it, imagine how tired everyone else is getting with attempts at theoretical talk when you don't even know what you're arguing against. How could you know what you're arguing against when you don't even know what a sawsmith would do to tension a saw?

Really, I'm tired of it, too. I have top line saws that are stiffer than third line saws I've had of the same cross section, and no amount of supposition on your part will change that.

(I recognize not every engineer is similarly afflicted with this disease where all things are better solved in discussion rather than carried out in practice).

 

[Corneel]

I was called early this morning for an emergency, but everything is quiet again. I'm sitting behind my desktop at work, so I can access the Springer articles again. I scanned the article I posted a link, and this page explains exactly what we have been talking about all the time. It is about bandsaws. Those blades are under tension when the bandsaw is not running, but as soon as it starts running the tensions around the piece of wood being cut is compromised and waves in the blade are possible, hey, just like a handsaw!

Speeds in a bandsaw are higher, but the exact same thing happens in framed gangcutting blades at a sawmill. Pretensioning the blade with one of the methods like hammer tensioning is being done here too, and speeds in those mills are more alike a handsaw used in anger.

I am not going to run experiments with bits of sawplate. I don't have the time and like David wrote, hammer tensioning is an art. Learning to tension and flatten a sawblade usually took up to 6 months of learning on the job. And I also don't need to do the experiment, it has allready been done. It was also done on millions of handsaws at the Disston, Spear and Jackson, Kenyon, Greeves etc saw factories.

 

[Cheshirechappie]

D-W - On page 1 of this thread, you told us all how to 'tension' a saw.

I friend wants to know how to re-tesion panel saws (if they get bent or other wise damaged).

Does anyone know how this is done?

Hammer lightly and evenly on both sides of the saw. Inevitably, the saw won't be straight when doing this, but additional strikes on one side or the other (depends on what's under the saw) will bring the tensioned saw back to straight.

Presume every saw you find (other than japanese saws) will have been tensioned on rollers (if it was, in fact, tensioned).

The 'Engineer's Approach' you take such delight in looking down on is often quite simple - just do it, and prove it works. If the likes of Spear and Jackson, Kenyon etc etc were doing it a hundred years ago, it can be done again. Take a piece of spring-steel, or a cheap, floppy, untensioned saw, hammer it as you described, and demonstrate that it can make the saw stiffer.

I say it won't work on a hand saw, and have tried to explain why it won't work. (I've tried to explain why circular and band saw blades are different, too, and why manipulating shape and residual stresses can be beneficial to them.) You say it does work on hand saws. OK - prove it.

 

[Corneel]

You haven't quite explained why and how bandsaw blades are different.

 

[Cheshirechappie]

1) They are under a uniform, externally-applied tension (from the adjuster on the bandsaw top wheel) - not applicable to hand saws.

2) They have two guide rollers at the back of the blade opposing the feed load - not applicable to hand saws.

3) The feed load is significant, and causes the band to be effectively a simply-supported beam with a more-or-less uniformly distributed load. That causes the loaded band to be in additional tension at the back, and adds compression at the toothed front. When the band is off the saw, adding some residual tension to the front adds to the band-stretch tension, and offsets some of the feed compression, thus allowing higher loading (faster feed rate). Again, not applicable to hand saws, on which the feed loads are very small.

4) It seems that at increased band speeds and feed loads, vibration can become a problem. Again - not applicable to hand saws - nobody is going to get them up to those speeds or feedrates.

 

[Corneel]

You can also look at it differently. A bandsaw is under tension, the blade is fully supported at the cutting end with the guide rollers. And still it is capable of low frequency vibrations. They solve this problem (up to a point) with hammer tensioning the tooth line.

A handsaw isn't supported in any kind like a bandsaw blade is, so it is in a worse situation. It is also quite possible to get undesirable low frequency vibrations. So the characteristics of the cutting action can be improved with hammer tensioning the blade, especially the tooth line. This allows to use a thinner blade then would otherwise be possible.

Modern manufacturers of disposible handsaws simply increase the plate thickness. Disston et. al. used hammer tensioning and roller tensioning to be able to use thinner blades, even grinded thinner towards the back (taper grinding). Those saws work very nicely despite those thinner blades.

 

[Trafalgar]

CC you said:

The 'Engineer's Approach' you take such delight in looking down on is often quite simple - just do it, and prove it works. If the likes of Spear and Jackson, Kenyon etc etc were doing it a hundred years ago, it can be done again. Take a piece of spring-steel, or a cheap, floppy, untensioned saw, hammer it as you described, and demonstrate that it can make the saw stiffer.

I say it won't work on a hand saw, and have tried to explain why it won't work. (I've tried to explain why circular and band saw blades are different, too, and why manipulating shape and residual stresses can be beneficial to them.) You say it does work on hand saws. OK - prove it.

I suppose a theory about why these firms "were doing it" would be more into the realm of psychology (some sort of mass delusion?) but still I'd be interested in your personal theory about why these firms bothered hammering their top of line saws, and paid what has been described as 'highly skilled' smiths to do it, presumably at a higher wage to account for their special skill. The process has been described in links already provided as something quite distinct from straightening, etc. so that's not it. Was it all just a marketing farce common amongst all firms?

Otherwise, the bulk of your argument seems to be a challenge to reproduce a skill that is widely acknowledged to be one in danger of being lost. In other words, if an 'every man' can't do it this somehow stands as proof.

 

[Cheshirechappie]

You can also look at it differently. A bandsaw is under tension, the blade is fully supported at the cutting end with the guide rollers. And still it is capable of low frequency vibrations. They solve this problem (up to a point) with hammer tensioning the tooth line.

A handsaw isn't supported in any kind like a bandsaw blade is, so it is in a worse situation. It is also quite possible to get undesirable low frequency vibrations. So the characteristics of the cutting action can be improved with hammer tensioning the blade, especially the tooth line. This allows to use a thinner blade then would otherwise be possible.

Modern manufacturers of disposible handsaws simply increase the plate thickness. Disston et. al. used hammer tensioning and roller tensioning to be able to use thinner blades, even grinded thinner towards the back (taper grinding). Those saws work very nicely despite those thinner blades.

OK - if, as you suggest, hammering a hand saw blade does something beneficial to it (makes it stiffer, makes it less liable to vibrate, whatever), then demonstrate the fact. At the moment, it's all just assertion and speculation, no clear, unambiguous demonstration.

Earlier in the thread, I posited a hypothesis about what hammering (rolling, whatever) might do to a hand saw blade that would be beneficial to it, other than straightening it after heat treatment. I've no idea whether the hypothesis is right or wrong, it's just a hypothesis, that's all.

Why did Spear and Jackson, Disston et al hammer their hand saw blades? Mainly to get them flat. Any other effect is unproven and undemonstrated - so far. And Charles - maybe, just maybe, the fact that nobody can manage to stiffen a hand saw blade by hammering it is an indication that it doesn't work. I don't accept the idea that if people did it in the late 19th century, it can't be done today. They hammered hand saws in the late 19th century - to get them flat, not to make them stiffer.

 

[Trafalgar]

CC, they did not hammer the saws exclusively for the purpose of making them straight.

"Tensioning" was reserved for higher end saws, at least in Spear and Jackson's case. Unless they sold all their lower-end saws with a slow bend in them, you are completely wrong or would need to explain why they would make their lower end saws straight by some other method and reserve the expensive, laborious method for their best-quality saws.

I'm reasonably sure there are still people out there tensioning saws, Bob Smalser appears to be one, your insistence on finding one on this particular forum (a not particularly heavily traveled one at that) leaves one baffled by your contorted logic and what this means in the real world.

Perhaps all that is needed is a broadening of your horizons?