Panel Saw Tensioning

[Cheshirechappie]

Charles - the first link is about the old trick of straightening out a slightly cockled backsaw blade by tapping the back of it's spine. Nothing about hammering the blade itself. The second link, if you read it carefully, is about getting hand saw blades flat and straight, and circular saws slightly dished to allow for thermal expansion of the edge and centrifugal forces at working speed. Neither link, if you read them carefully, makes any reference to the type of 'tensioning' under discussion here.

 

[Trafalgar]

The links are for general consumption. Somebody might find a nugget that strikes a chord, or make a logical inference you've decided you are unwilling to make. Resources are so few and far-between it makes no sense not to put them out there and let participants judge for themselves.

I don't see any of this as an argument per se, but if it is, it's certainly going to be one you'll 'win' by default if you get to make the final judgment about what does and does not apply.

Things that walk and sound like ducks are often actually ducks.

 

[Cheshirechappie]

The links are for general consumption. Somebody might find a nugget that strikes a chord, or make a logical inference you've decided you are unwilling to make. Resources are so few and far-between it makes no sense not to put them out there and let participants judge for themselves.

I don't see any of this as an argument per se, but if it is, it's certainly going to be one you'll 'win' by default if you get to make the final judgment about what does and does not apply.

Things that walk and sound like ducks are often actually ducks.

What sort of duck, though?

The problem with the word 'tension' is that it has a very precise late 20th and early 21st century technical and scientific definition, which didn't really exist in the late 19th century. Consequently, when reading late 19th century texts, one has to try and interpret the meaning the writer was trying to convey. For a start, they probably didn't mean "dramatic tension", and if you assume they mean "the state resulting from the application of a uniaxial force tending to stretch an object" the text often makes no sense at all, because there is no frame or mechanism to apply such a force.

Context, as so often, is all.

 

[Trafalgar]

Probably the physical state of a piece of sheet steel when certain areas are thinner than another.

Personally, I couldn't care less what it's called. My only concern would be the efficacy of producing such a condition in a saw blade.

It's clear, I believe, that the major manufacturers thought there was something to it. I'm happy to give them the benefit of the doubt.

As to the change in meanings of words from the 19th to 20th to 21st centuries -- have fun with that.

 

[Cheshirechappie]

Probably the physical state of a piece of sheet steel when certain areas are thinner than another.

Personally, I couldn't care less what it's called. My only concern would be the efficacy of producing such a condition in a saw blade.

It's clear, I believe, that the major manufacturers thought there was something to it. I'm happy to give them the benefit of the doubt.

As to the change in meanings of words from the 19th to 20th to 21st centuries -- have fun with that.

It's some effect connected with taper grinding, then, perhaps?

Earlier in the thread, D_W told us that whilst he could detect this difference, he thought the rest of us bumbling amateurs possibly couldn't. If you take that literally (not sure that I do, by the way), then the difference it makes is marginal at best.

The links Mignal posted seem to come closest to shedding light on the issue, but even they raise more questions than they answer. So far, there are some strong opinions, some head-scratching, but no solid evidence or proof either way. I don't accept the argument that Disston had a secret process, now forgotten, that could not be replicated in the modern world. If it's been done before, it can be done again. So far, nobody seems to have achieved it; nobody has demonstrated a noticeable difference between a 'tensioned' saw and an identical saw 'untensioned', which leads to some scepticism that it exists at all, or if it did, that it made more than a marginal difference.

Did 'tensioning' just mean the process of getting a sawblade flat and straight? Were Disston very good at marketing kidology in implying that their (granted, very good) saws had a little magic something nobody else knew about?

We don't know. If somebody can answer our questions, please do. Until then, it's just another rather pointless internet discussion bordering on argument, rife with opinion but short on solid evidence, as far as I'm concerned.

 

[MIGNAL]

In other news I used a Stanley speed cut yesterday (on some Oak). Amazing what a cheap bit of steel can do. Made this guy seem like he was cutting at snail speed.

https://www.youtube.com/watch?v=c_KGrltTSZg

 

[Trafalgar]

Chappie, what was it about the shot peening link I posted that didn't convince you that tension and compression forces are present in sheet steel and can be manipulated by striking it?

While shot may be more efficient, what is it about a hammer you think wouldn't accomplish essentially the same thing(s)?

"Shot peening is an important and, for safety reasons, an essential process in many industrial sectors, primarily in the aerospace and automotive industries. Peening dates back to Bronze Age armourers; although in more recent years, this technique has been used by engineers who worked the surface of a component with a ball peen hammer in order to induce internal compressive stress and thus increase service life."

"Shot peening is a cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals and composites. It entails impacting a surface with shot (round metallic, glass, or ceramic particles) with force sufficient to create plastic deformation.[1][2]

It is similar to sandblasting, except that it operates by the mechanism of plasticity rather than abrasion: each particle functions as a ball-peen hammer. In practice, this means that less material is removed by the process, and less dust created."

"Shot peening is often called for in aircraft repairs to relieve tensile stresses built up in the grinding process and replace them with beneficial compressive stresses. Depending on the part geometry, part material, shot material, shot quality, shot intensity, and shot coverage, shot peening can increase fatigue life up to 1000%.[2]

Plastic deformation induces a residual compressive stress in a peened surface, along with tensile stress in the interior. Surface compressive stresses confer resistance to metal fatigue and to some forms of stress corrosion.[1] The tensile stresses deep in the part are not as problematic as tensile stresses on the surface because cracks are less likely to start in the interior."

 

[Trafalgar]

S&J was apparently still hand-tensioning into at least the mid-20th century:

Spear and Jackson, Story of the Saw (1961) (PDF can be found at the Toolemera website).

In Sheffield, at Spear and Jackson's Aetna works,
Spearior quality hand and tenon saws and Mermaid
quality circular saws are all made from steel melted in
electric arc furnaces. The ingots to controlled analyses
and free from impurities are hammered or roll-cogged
to slabs (or 'cheeses', as circular saw slabs are called
in the trade), cross-rolled to plates in a sheet mill and
passed through blanking press or paring shop in an
annealed or soft state, and then go to the particular
production department. The handsaw blank is toothed
by rotary or reciprocating punch, hardened and
tempered, tested, hammered flat and tensioned by
highly skilled smithers, before going forward to be
ground and glazed.

 

[Cheshirechappie]

Shot peening can induce a compressive stress in the surface of a component, which is very useful because cracks can't propagate in a compressed surface. They can in one under tension. Thus, the technique can be used to increase the fatigue life of components, or to relieve fabrication stresses, such as those produced by welding operations.

If something like a saw blade has compressive stresses induced in the surface, they must be balanced by a tensile stress in the core. However, if the component is in equilibrium, the net effect on the whole is zero.

I can find no indication anywhere that shot peening a piece of metal will stiffen it - and believe me, I've looked. That's about an hour of my life I won't get back again!

As to the S&J link, it doesn't say anything about what 'tensioning' is, or what it does to the saw, so it doesn't really inform us any more than any of the other links.

By the way, I've been looking on the interweb for Disston marketing pamphlets and information. The ones I've found so far don't mention the word 'tension' at all in connection with saw blades. Can you, or anybody else, post a link or a photo of the relevant page of any Disston literature that does - it must be out there somewhere.

 

[Cheshirechappie]

Aren't Dissiton still about? I thought they were?

Also I thought Lie-Neilsen claimed to tension saws (have not checked that yet)... i vaguely remember someone i knew having a problem with a small LN tenon saw and it being fixed via tensioning... but I cannot remember the details so don't rely on that account.

Thanks for the link Mignal, will read it tomorrow... bed time for me should have been 40mins ago

Yes - Disston are still about, but they don't make handsaws - http://www.disstonprecision.com/

 

[Rhyolith]

If something like a saw blade has compressive stresses induced in the surface, they must be balanced by a tensile stress in the core. However, if the component is in equilibrium, the net effect on the whole is zero.

This is exactly how I think tensioning works. If the tensile stress on on the cutting edge, then too all instensive purposes its stretched like a bow saw blade. This stretching of the blade in bow saws something of known benefit and without doubt makes the saw cut better, so why wouldn't that work in Handsaws?

 

[D_W]

Earlier in the thread, D_W told us that whilst he could detect this difference, he thought the rest of us bumbling amateurs possibly couldn't. If you take that literally (not sure that I do, by the way), then the difference it makes is marginal at best.

I'm a bumbling amateur, too, but I guess that in the recent past, I've done a fair amount of sawing on rough wood. I was kind of surprised that more people didn't speak up about a higher dollar saw being stiffer without being harder to file (and despite having at least as much of their cross section removed for tapering).

Some of the rest was kayfabe to some extent.

Maybe the selection of saws is different in the UK. The bulk of the $50 clean saws here that were intended to be used on a jobsite by a professional were disston, but there were plenty of cheap saws, too (often branded by disston or some other substandard companies).

The saws with good stiffness are just a bit nicer to use. Less rattling in the cut, fewer surprise (but harmless) binds. I'm not going to make the claim that they kink less because you have to be pretty seriously horsing a saw to kink it.

I think the statement just before here is apt - something causes the saw to be stiffer. I would assume it's tensioning (which is a step that japanese makers still list separately from straightening). It can be noticed, by amateur or whoever else. Maybe it just takes the right kind of work to notice it. I guess if I think back to when I used saws only for crosscutting, I didn't really know what the big deal was about the better saws. If there is marketing to marks, I think it is at the very top of the range where saws had sayings put on them and were given things like rosewood handles.

 

[AndyT]

Way back on page one of this thread, I mentioned a note in an old book about saw tensioning. It was in "Woodwork Tools and How to Use Them" by William Fairham, one in a series of handbooks published by The Woodworker in the 1920s.

It was actually about backsaws, but I think it's applicable to panel saws. It said:

"The sheet of steel which forms the saw blade is hammered so as to make it flat; it is also hammered so that the centre of the blade is slightly expanded, and always pulling at the toothed edge to keep it taut and true. The condition of the saw is, in fact, like a piece of paper as at Fig 20, the toothed edge being stretched tight and the body of the saw loose."

Sorry it's taken me a while to get round to posting this, but I've been away.

 

[Sheffield Tony]

I have to say I share the scepticism mentioned earlier in this thread as to whether a significant amount of tension can be created without the blade becoming dished, poping between bulging out one side or the other. I also wonder how much roller pressure or hammering is needed on a tempered spring steel to stretch it significantly whilst cold ? But then I'm an engineer too :wink:

 

[D_W]

As for the rollers, I think the engineering term would be "many elephants of pressure".

It would be interesting if someone could come up with a picture of the rollers at disston. I'm sure they don't look like skateboard wheels. Even if they had 50 tons of pressure on them each, or more, it would've been trivial for disston to make something like that given the size of their operation.

 

[D_W]

As to the odds of the tension being a real difference (it's possible that my generics were just dogs, and not necessarily lacking tension, but just bad saws - that are floppy because they're lacking ____...I don't know what).

These saws were sold to people in an era where there wasn't a lot of disposable income, in an era when labor was probably fairly cheap compared to tools and equipment, and they were sold in droves at an extra cost. I know how my relatives (who were farmers) lived back then, and they didn't spend extra money on much.

There probably wasn't a bloated tax rate to write off equipment against, either. It sort of reminds me of the double iron discussion - cost a lot of extra money to get one put in a plane, but there are still people who call them a con on the craftsman.

 

[Corneel]

The pressure of a hammer can be very high too, because the area of the impact is small. When you can straighten a saw with a hammer, it should be possible to do the "tensioning" with a hammer too.

 

[worn thumbs]

Way back on page one of this thread, I mentioned a note in an old book about saw tensioning. It was in "Woodwork Tools and How to Use Them" by William Fairham, one in a series of handbooks published by The Woodworker in the 1920s.

It was actually about backsaws, but I think it's applicable to panel saws. It said:

"The sheet of steel which forms the saw blade is hammered so as to make it flat; it is also hammered so that the centre of the blade is slightly expanded, and always pulling at the toothed edge to keep it taut and true. The condition of the saw is, in fact, like a piece of paper as at Fig 20, the toothed edge being stretched tight and the body of the saw loose."

Sorry it's taken me a while to get round to posting this, but I've been away.

Excellent post and it reminds me of the tenon saw I found in a box of old tools I bought at auction in the eighties.It hadn't been of interest but when I had sorted the other items and sold on those I had no use for it languished at the back of the workshop for a while.Finally I picked it up with a view to either binning it or making it usable.After a good scuff with wire wool and WD-40 there were bulges apparent and the whole saw felt floppy.Since I had a block of steel and a hammer (most of us own hammers,don't we?) I had nothing to lose but the time and I spent an hour or so making things less bad.It worked and I was able to set and sharpen the saw.It was even good enough that a good few years later I sold the saw to a carpenter for twice the price I paid for the box of old tools.Could I do it again?Probably and eventually,but it demonstrated to me that tensioning with a hammer works.I think we have arrived at a point where the general principle has come to the surface,even if it seems counter-intuitive.

 

[Sheffield Tony]

Well yes, but people also were impressed by "Electro Boracic steel" (whatever that meant) and corrugated plane soles. I've no doubt various processes were done, and were either thought to, or claimed to, improve the tool. Whether it actually made a difference is harder to tell !

(sorry, got out of sync there - this was in response to D_W's comment that people would pay more for the higher quality "tensioned" saws)

 

[D_W]

The pressure of a hammer can be very high too, because the area of the impact is small. When you can straighten a saw with a hammer, it should be possible to do the "tensioning" with a hammer too.

Definitely. Just less uniformity in results, maybe. It wouldn't take a very large hammer to distort the metal, it doesn't take much to straighten a saw and leave visible burnish marks.