Panel Saw Tensioning

[D_W]

electro boracic must've been an alternative to carbo magnetic.

 

[Bedrock]

As a non-engineer, I can at least understand how this tension might be applied to a back saw, because the back gives the blade something to brace against, but am I right in deducing that this would have to be done once the blade is firmly fixed into the back?

I don't see how the tension is introduced into a handsaw where there is nothing to hold the top edge against the tension, unlike a bow or coping saw.
Can someone please enlighten me?
I have 4 handsaws, 2 old Disstons, one a D8, and 2 S&Js, one of each rip and the others cross cut. Of any of them, one of the S&Js cross cut, is noticeably more flexible then the others. It's a nice saw to use, but if my hand is not relaxed, rattles in the cut. Does a more flexible blade necessarily indicate a better quality saw, and thus is it more likely to have been "tensioned", however applicable that might be to a hand saw?
I doubt that any of the throwaway hard points so prevalent today, are tensioned in anyway, but seem to perform reasonably well. That being the case, what benefit might any reasonably competent workman notice from such a saw being tensioned?

 

[D_W]

As a non-engineer, I can at least understand how this tension might be applied to a back saw, because the back gives the blade something to brace against, but am I right in deducing that this would have to be done once the blade is firmly fixed into the back?

I don't see how the tension is introduced into a handsaw where there is nothing to hold the top edge against the tension, unlike a bow or coping saw.
Can someone please enlighten me?
I have 4 handsaws, 2 old Disstons, one a D8, and 2 S&Js, one of each rip and the others cross cut. Of any of them, one of the S&Js cross cut, is noticeably more flexible then the others. It's a nice saw to use, but if my hand is not relaxed, rattles in the cut. Does a more flexible blade necessarily indicate a better quality saw, and thus is it more likely to have been "tensioned", however applicable that might be to a hand saw?
I doubt that any of the throwaway hard points so prevalent today, are tensioned in anyway, but seem to perform reasonably well. That being the case, what benefit might any reasonably competent workman notice from such a saw being tensioned?

Other way around...the stiff saws are tensioned and the stiffness makes them preferable.

Less rattle, flop, bind, etc are the reason that stiffness is preferable.

The heavier you use the saws, the more you notice the difference.

 

[Cheshirechappie]

Stiffness is down to the material's dimensions, not any change in it's mechanical properties. It's proportional to the cube of the length, and it's also proportional to the cube of the thickness. Consequently, quite a small increase in a saw blade's thickness gives a noticeable increase in stiffness.

Just to develop this a bit, consider a handsaw or panel saw, clamped to the bench at the handle end, with the blade poking out flat across the workshop. Now, take a loop of string, and put it round the blade a given distance from the handle. Hang a weight on it, and measure the deflection. Now take another saw, identical except in thickness, and do the same. You'll find that the thicker saw has a significantly reduced deflection.

In engineer speak, we'd analyse this as a cantilever fixed at one end. The beam formula for deflection is (weight applied times length cubed) divided by (3 times modulus of elasticity of the material times moment of inertia). The moment of inertia of a simple rectangular section is (breadth times depth cubed) divided by 3.

Thus, the deflection caused by a given force applied (the weight) is proportional to the cube of the length, and the cube of the thickness. The material property quoted, modulus of elasticity, is pretty much constant for steel under any condition of temper, hardness or whatever (it varies very slightly with temperature, but unless you get up to a couple of hundred centigrade or so, the variation is insignificant).

More practically, if anybody cares to measure the thickness of their blades with a micrometer, they'll find that the stiffer ones are the thicker. It's a tad more complicated with hand and panel saws because of taper grinding, but in general the thickness of metal for the inch or two behind the tooth line will pretty much define the saw's stiffness. An increase in thickness from 0.030" to 0.036" will increase stiffness by a factor of 1.73.

Sorry that's all a bit technical, but it's needed to explain why stiffness is related to thickness, not to the steel's metallurgical properties or internal stresses.

 

[Cheshirechappie]

I think I may have an idea what 'tension' is on saw blades.

Back on page 2 of this thread, Worn Thumbs mentioned shot peening. If you look this up in Wikipedia - https://en.wikipedia.org/wiki/Shot_peening - and scroll down to 'History and further developments', you'll find the interesting fact that shot peening was invented in the late 1920s because somebody noticed that blacksmiths peened the tension side of leaf springs with a ball-pein hammer to increase their life. The blacksmiths didn't know why peening springs increased their life, they just knew it worked.

We do know, now. It's to do with the internal stresses in the metal, sometimes called residual stresses. Residual stresses have been known about for a long time (or perhaps more accurately, their effects have), but even now they're not particularly well understood. Wikipedia again - https://en.wikipedia.org/wiki/Residual_stress .

As far as the springs were concerned, the peening worked because it caused a compressive residual stress in the surface of the metal. That was beneficial because when the spring was loaded, and tensile stress built up in it, the stress had to cancel out the residual compression, and only then start building tension. Thus, the spring could bear a bit more load before failing. Another advantage is that cracks, which start easily in components under tension do not in components in compression (which way do you nick and bend a stick to break it?).

It's not hard to surmise that in a place that works steels a lot, somebody making springs would mention to somebody making saws (from the same steel) that a good peening would make saws last longer, because it works for springs.

Thus, I suspect that inducing a residual compressive stress in the surface of a saw blade makes it less prone to cracking since cracks don't start in compressed material. Why call it 'tension' then? Well, I don't know, but perhaps because the idea came from spring-making?

A residual compressive stress can be induced in the surface of a metal by hammer-peening it, by shot-peening it, or by rolling it. Modern spring-steel comes from the mill with a cold-rolled finish (they call it 'temper rolled', which is rather confusing) to give it a good surface finish (the bulk of the reduction from cast ingot is done hot). That will effectively give modern saw makers pre-'tensioned' material to work with. The old makers, who hot-rolled their steel from the ingot, then shaped and toothed the soft steel, then heat-treated, had to 'tension' as a separate operation, either by hammering or rolling.

Well, that's not proof positive, but I offer it as a working hypothesis. It seems to fit the historical facts as far as we know them, and it fits with modern materials science knowledge.

Thoughts?

 

[D_W]

More practically, if anybody cares to measure the thickness of their blades with a micrometer, they'll find that the stiffer ones are the thicker.

So, you've concluded now that there's no such thing as tensioning without actually proving it.

I mentioned earlier that I measured my saw plates for taper and tension (I measured them with calipers at the front top, front bottom, back top and back bottom to get an idea of how they were ground. I did that long ago out of curiosity, though. The floppy saw that I'm comparing to a good rip saw was just as thick as a disston D8, and it was tempered only in one direction (there was more steel left in it).

Nearly every rip saw is about 4 hundredths or slightly more at the tooth line, and the small panel saws are about a hundredth thinner. They vary a lot at the front and top, depending on how much they were ground. The saws with more steel remaining aren't any less stiff in use (they're more stiff, actually, but not because of thickness).

I think you should do a little bit more homework and actual hands on use before you make a definitive conclusion. I guess you wanted to come to this thread and authoritatively state that saw tensioning has no effect and you've ignored everything since then. =D>

 

[Cheshirechappie]

Oh well, can't please all of the people all of the time.

On stiffening hardened and tempered high carbon (spring) steel by hammering, or rolling, or shot-peening it, I stand by what I wrote. It just doesn't happen. Stiffer saws are thicker saws, and as described, it doesn't have to be much thicker to be appreciably stiffer.

You say I wanted to come to this thread and authoritatively state something. Well, I didn't, and I haven't. Virtually from the off, I've said I'm open-minded but sceptical about 'tensioning'. I now have a hypothesis about what tensioning is (so I'm not saying it doesn't exist), but I do state categorically, as I have for some pages, that it isn't some way of stiffening steel by hammering it. I think it's more a way of making saws less liable to crack and break - extend their fatigue life, in technical terms.

The idea occurred whilst I was lying in bed on Saturday night. I was (not intentionally - the mind was just doing it's own thing, as they sometimes do) mulling over the shot-peening Wikipedia reference, and remembered a passage from "The Village Carpenter" by Walter Rose. It's on pages 59 and 60 - "The saw, eventually purchased, was of marvellous quality, a delight to use. But I took it to work on a new house, it was borrowed by the labourers for cutting firewood, and on my return I found it in my basket broken in two." I've never broken a saw, and I don't know of any other instances either.

If they're so hard to break these days, why is it? Maybe because the compressive stress in the surfaces makes crack formation very hard - the same compressive stress that the old blacksmiths used to peen into their springs, and that results from cold rolling or shot peening. Maybe that's why (and how) saws were 'tensioned'.

I've spent those odd moments when I haven't been doing things I ought to be doing since then trying to pick holes in the idea, but I can't. It just fits the known data. That doesn't mean proof, but it does mean it's an idea worth offering as a possible explanation.

 

[D_W]

You have stated something specific. You've specifically said that you can't tension a saw by hammering or rolling it.

So you're making the claim that it was either a sham or someone was misguided (and that includes a gaggle of engineers working at disston, and not in the stone age).

As well as a gaggle of japanese saw makers.

If Lloyds took bets, and you asked if you can make a saw plate stiffer by hammering or rolling it, I know which bet I'd take.

The poster who described two saws here should have an interesting opportunity at this point to take a set of simple calipers and compare the saws he was discussing as he clearly noticed a floppy saw and two stiff ones, just as I have in the past. Do I know for sure that it was tensioning that made them stiffer? No, for all I know, they could've been different hardness.

By the way, the tensioning step isn't described as something that makes a saw more flexible and less apt to break. All decent saws can have their toe turned to their handle unless they are defective or damaged. It makes the saw stiffer and more likely to return to straight.

 

[pedder]

Wow 7 pages and nobody has looked for the hammer marks on saws.

 

[Cheshirechappie]

You have stated something specific. You've specifically said that you can't tension a saw by hammering or rolling it.

That accusation is flat wrong. I have not stated that you can't 'tension' a saw by hammering or rolling it (though earlier in the thread I was pretty sceptical). I have stated - and continue to state - that you can't STIFFEN a saw by hammering or rolling. (You can, however, work harden a piece of soft metal so that it becomes springier and more elastic, but that doesn't actually make it stiffer. You can, to a much lesser degree, induce a bit more work hardening into a piece of hardened and tempered spring steel by hammering it, but again, that won't change it's stiffness.)

That leaves us trying to interpret what all the old sources meant by 'tensioning', the problem being that whilst some explicitly state that it was a process in saw manufacture (though I've yet to see one that says when this step started to be used), they're all frustratingly vague about what it was or did. Some strongly imply that it was carried out during one of the smithing operations, and involved hammering, but that's about as much as they tell us.

The suggestion that it may have been a technique borrowed from spring-making practice, using a process of peening to increase the life of springs, is just that - a suggestion. It isn't proof. However, it fits the metallurgical facts as we understand them in the 21st century, whilst the idea of stiffening a hardened and tempered saw blade by hammering does not.

 

[Rhyolith]

Andy's post has made be confident that my original hypothesis of saw tensioning in panel saws being the same as in bow saws (stretching of the blade/cutting edge) is correct. I am not at all convinced that stiffness is even relevant to saw tensioing, I think its a seperate process or at least not the principle objective of saw tensioners.

Please answer these questions (especailly Ches.):
- Whats better a bow saw with a tensioned or un-tensioned blade?
- Is it possible, by mechanical means such as those in Andy's book, to expand to central area of the saw panel to manipulate the tension of the cutting edge in a panel saw?

 

[MIGNAL]

If you can't reduce a difficult engineering problem to just one A4 sheet of paper you will probably never understand it.
Page 8 and I'm still none the wiser.

 

[Cheshirechappie]

If you can't reduce a difficult engineering problem to just one A4 sheet of paper you will probably never understand it.
Page 8 and I'm still none the wiser.

Sorry probably my fault, using technical terms like 'stress' without explaining them.
materials
Just for the sake of (some attempt at) clarity, I'll try to explain a bit about stress, as used in engineering and materials science world. It's not quite the same thing as the stress all of us encounter in everyday life; though I suppose there are parallels of a sort.

An engineer would define 'stress' as a way of measuring a material's capacity to cope with the loads applied to it, and define the point at which the material can't cope with the loads, and fails. A simple example would be a crane cable holding up a load. The cable would have a tensile stress in it, because it's in tension. The stress is defined as the load applied divided by the cross-sectional area affected by that load. So if the crane cable had a cross sectional area of 1 square inch, and the load was five tons, the stress in the cable would be 5 tons per square inch.

The load applied to the cable would tend to stretch it a little bit, and the amount by which it stretches (the extension) divided by it's original length gives another figure engineers use quite a bit - strain. Up to a point, if you divide the figure for stress by the figure for strain, you end up with a constant, called the Modulus of Elasticity (or sometimes. Young's Modulus). Note the word elasticity - that's because up to a point, many materials behave like a piece of elastic - apply a load, and they deform a bit, but take the load off again, and they return to their original shape.

However, that's only up to a point - after that point, they deform permanently. In other words, they behave plastically. That point is known as the 'yield point'. Keep on applying load, and they keep on deforming permanently - up a point. THAT point is when the material gives up and breaks - the Ultimate point.

Thus, a piece of (say) mild steel subjected to gradually increasing tension load will behave elastically up to it's yield point, then plastically up to the Ultimate Tensile Strength of that grade of steel.

Each material has it's own points for Yield and Ultimate, often quoted by material suppliers.

Just to make life more interesting, you can modify the yield and ultimate for many materials by such actions as heat treatment or cold work. A piece of spring steel in it's annealed - soft - state will have quite a low yield point, and a longish plastic range. However, harden and temper it, and it will have a very long elastic range - a high yield point - but a much shorter plastic range thereafter, but it's Ultimate will be much higher than it was when it was soft. Conversely, a blacksmith or forgemaster can heat the spring steel to a suitably high temperature when it will have a very low yield, but a very long plastic range - so he can do a lot of drastic shaping without causing the steel to fail. Also, if you take your piece of soft spring steel and cold work it (hammer it, bend it, draw it through dies or whatever) you will increase it's yield point by work-hardening it. This is very noticeable with metals like copper and brass. Once it's hard - high yield point, short plastic range - you can soften it again by annealing, thus restoring a low yield point and large plastic range.

Confused yet? You will be!

Stresses are 'tensile' if the material is being stretched, and 'compressive' if it's being squeezed. (There's also shear stress, which happens when the load applied is trying to 'shear' the material - bolts, rivets and shear pins often see a stress of this type.)

Now, let's consider our saw blade. If it's just lying on the bench, it's not subject to any stress. If it's picked up, and the toe end bent round so that the blade takes up a curve, it's effectively acting as a beam (a cantilever, in this case). Beams see both tensile and compressive stresses; if you take a piece of flat steel and bend it into a gentle curve, the outer side of the curve is now a little bit longer than it was when straight, and the inner side a little bit shorter. The outer side is thus in tension, and has a tensile stress, and the inner side in compression, and has a compressive stress. The line through the middle remains the same length, and sees no stress - that's the 'neutral axis'. Thus, our bent saw blade is in tension on the outside of the bend, and in compression on the inside, with stresses to match.

It now gets complicated. So far, we've only considered stresses resulting from externally applied loads or forces. However, there exist another sort of stress known as 'internal stresses', or sometimes, residual stresses - https://en.wikipedia.org/wiki/Residual_stress . They have been known about for quite a long time, but even now are not well understood, perhaps because they are very difficult to measure. They can be a problem, or they can be beneficial. They tend to find an equilibrium within a piece of material, so a piece of steel - such as a sawblade - just sitting on a bench on it's own, can appear stable, and under the influence of no loads (and hence stresses) applied to it. There can, however, be stresses IN the saw blade. If any loads are then applied externally, the stresses resulting from those loads add to the internal stresses.

Most failures of relatively strong materials like spring steels tend to happen when a crack starts in part of a component under tensile stress. The crack runs quite fast, and the component breaks in two. It's much harder to start a crack in part of a component under compressive stress. Try bending a stick of wood, then putting a nick or sawcut in the tensile (stretched) side of the bend - it'll break sooner. Not so if the sawcut is in the compressed side.

Let's now imagine some way in which we could manipulate the internal stresses in the saw blade such that the surface layers were in compression (and to balance the stress distribution, the centre was in tension). When the saw lay on the bench, it would be stable, because no external forces were applied. Now let's pick up the saw and bend the blade into a curve, as before. Now, the outer side, which had a tensile stress in it as soon as the bend started, starts with compressive stress, so as the bend develops, the stress comes from compressive, back to zero, and only then starts to become tensile. Thus, the maximum tensile stress the surface sees is lower than in the saw blade with no internal stresses. Cracks start in materials with tensile stress in them; less tensile stress, less chance of cracks starting.

Way back in the mists of time, blacksmiths discovered that if they peened the tensile side of leaf springs, the springs lasted much longer. They didn't know why, they just knew it worked. It works because peening a surface (or rolling it lightly, or shot-peening it - https://en.wikipedia.org/wiki/Shot_peening ) puts a compressive internal stress in the surface. Thus, the maximum tensile stress the spring surface saw was lower than one not so peened, so cracks were less likely to start, so the spring was less likely to break.

Springs and saws are made from pretty much the same grade of steel (sometimes in the same works, in times gone by). Not hard to see that what works for springs would also apply to saws (which are just wide, flat leaf springs with teeth on one edge).

Thus, I think 'tensioning' is a process that makes sawblades less likely to snap in use, and it's done by peening the surface of the saw blades. Light rolling would work too, as would shot peening. I've no idea why it's called 'tensioning', but as the idea came from spring-making practice, maybe that has something to do with it.

Well - that's the brief :shock: explanation. I don't know if it's the full answer to the question of what 'tensioning' is, but it seems plausible from the materials science standpoint.

Right. I'm off for a rest after that!

 

[Cheshirechappie]

Andy's post has made be confident that my original hypothesis of saw tensioning in panel saws being the same as in bow saws (stretching of the blade/cutting edge) is correct. I am not at all convinced that stiffness is even relevant to saw tensioing, I think its a seperate process or at least not the principle objective of saw tensioners.

Please answer these questions (especailly Ches.):
- Whats better a bow saw with a tensioned or un-tensioned blade?
- Is it possible, by mechanical means such as those in Andy's book, to expand to central area of the saw panel to manipulate the tension of the cutting edge in a panel saw?

The bow saw blade is placed under an external tension, so the 'tension' in the blade is only there if it's stretched in the bow-saw frame. It's not there if the blade is just lying freely on the bench - or if the blade is used without the frame. Handsaws and panel saws are used without any frame stretching them.

If the middle of a hand or panel saw blade is hammered to the point where the metal yields (see previous post - I'm not typing that lot again!), but the metal either side of it is not hammered, you get a bulge developing in the blade. That bulge can be removed by stretching the metal above and below it - the old saw smiths used to do that to get blades cockled in heat treatment flat again.

 

[MIGNAL]

Thanks CC. Much clearer. Now can you explain why Building 7 came down at free fall speed?

 

[Cheshirechappie]

Now can you explain why Building 7 came down at free fall speed?

Not tonight. I've got a blister on the end of my typing finger!

 

[Rhyolith]

Andy's post has made be confident that my original hypothesis of saw tensioning in panel saws being the same as in bow saws (stretching of the blade/cutting edge) is correct. I am not at all convinced that stiffness is even relevant to saw tensioing, I think its a seperate process or at least not the principle objective of saw tensioners.

Please answer these questions (especailly Ches.):
- Whats better a bow saw with a tensioned or un-tensioned blade?
- Is it possible, by mechanical means such as those in Andy's book, to expand to central area of the saw panel to manipulate the tension of the cutting edge in a panel saw?

The bow saw blade is placed under an external tension, so the 'tension' in the blade is only there if it's stretched in the bow-saw frame. It's not there if the blade is just lying freely on the bench - or if the blade is used without the frame. Handsaws and panel saws are used without any frame stretching them.

If the middle of a hand or panel saw blade is hammered to the point where the metal yields (see previous post - I'm not typing that lot again!), but the metal either side of it is not hammered, you get a bulge developing in the blade. That bulge can be removed by stretching the metal above and below it - the old saw smiths used to do that to get blades cockled in heat treatment flat again.

Well I am not convinced. What you merely makes me think its hard to do rather than impossible.

Unless more evidence comes to light, I think expanding the central area of the saw and thus streatching the blade is what tensioning is. If this is what it is then it will make a difference, as evidenced by bow saws.

 

[AndyT]

As an aside, to anyone who's followed CC's essays in this thread, and thinks they would like to get a more scientific understanding of concepts such as stress, strain, elasticity, stiffness, cracking, malleability etc I would recommend a couple of books.

Both by JE Gordon, published by Penguin/Pelican, they are Structures, or Why Things Don't Fall Down, and The New Science of Strong Materials or Why You Don't Fall through the Floor.

Probably in your local library (the links can help you find them) or pennies secondhand.

They cover the fields mentioned and a bit more, illustrated with everyday examples. I find that while I am reading them, they make perfect sense - but I admit that I struggle to reproduce the explanations as clearly as CC has done.

 

[ED65]

I have stated - and continue to state - that you can't STIFFEN a saw by hammering or rolling. (You can, however, work harden a piece of soft metal so that it becomes springier and more elastic, but that doesn't actually make it stiffer. You can, to a much lesser degree, induce a bit more work hardening into a piece of hardened and tempered spring steel by hammering it, but again, that won't change it's stiffness.)

Hang on a sec, isn't harder metal stiffer by definition? I hope this isn't one of those things where the engineering understanding of the word is utterly at odds with how a layman would use it [-o< but I've experienced firsthand that work-hardened steel can bend less easily than the same steel in a fully annealed state, surely that does mean it is stiffer?

 

[D_W]

Harder steel (of the same alloy) is stiffer, yes.