Panel Saw Tensioning

[D_W]

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.

Here's what disston said about saws that were "loose" (hard would be a good term, meaning a layman is not likely to chance into - say - finding some guy to just hammer around on a saw and then make a conclusion that you cant increase stiffness by manipulating the metal).

http://www.disstonianinstitute.com/factorytour.html (halfway down - this all occurs after a blade has been hardened, tempered and ground - as we were told earlier in this thread, all of this stuff is unnecessary)

The next step is that of "Smithing." in this the blades are flattened and made perfectly straight, all inequalities being taken out by the skillful hammering of the mechanics.

The blades are next "Ground" to gauge and to a taper so that the back will be thinner than the cutting edge. The back of the hand saw blade is ground to taper from the teeth to the back and from handle to point, the tooth-edge being of even thickness from end to end. A saw not ground to proper taper cannot be ranked as a high-class tool.

Taper Grinding

Grinding
After being ground they are returned to the smithers for "Looking-over" and preparing for next operation.

Now follows "Tensioning." In this the blades are hammered so that they shall not be too "fast" or too "loose;'' but shall possess the proper tension, spring or character. If the blade is what is termed "fast" the metal is too long on the edge and needs expanding through the centre, or, if too "loose" the metal must be stretched on the edge. A saw not properly tensioned will run out of its course, in other words it will not cut straight and true.

 

[Rhyolith]

Thats still quite vague. Even though it supports my thoughts on what tensioning is, its not enough to "prove" anything definitively.

 

[D_W]

Thats still quite vague. Even though it supports my thoughts on what tensioning is, its not enough to "prove" anything definitively.

No, proof only comes in a craftsman's sense sort of way. That is, until someone has enough skill to actually duplicate what Disston is talking about, we are left to guess one of two ways:
1) the above process either does nothing at all or it doesn't stiffen the tooth line (it's clear that disston doesn't think that tensioning is intended to make the saw more flexible or keep the steel from failing - that is taken care of long before)
2) that the process does something, and then decide what we think it is. Certainly the tensioning is described in some places as ensuring that the saw will return to straight, and in this case, it has to do with tracking. A stiff saw tracks better. If someone doesn't believe that, try making a very tall backsaw with a thin plate. See what happens.

I'm satisfied to prefer the stiffer saws. How they got that way without seeming harder under the file, and while having more of their plate ground off is, apparently, not going to be explained.

I thought at one point I'd like to hammer a floppy saw to see if I could tension it, but I don't know if I have a candidate in my shop or not (I have one or two, but I might rather sell them and get some money for them rather than beating them into valuelessness). At this point, after reading about it, I'm thinking that my odds of actually stiffening a saw as an inexperienced amateur - they're pretty low.

Just because I have not much confidence in my ability, or the modern boutique saw makers' ability to tension a saw as disston describes doesn't mean I think they couldn't do it.

 

[Cheshirechappie]

Here's what disston said about saws that were "loose" (hard would be a good term, meaning a layman is not likely to chance into - say - finding some guy to just hammer around on a saw and then make a conclusion that you cant increase stiffness by manipulating the metal).

*sigh*

I've spent 8 pages saying that you can't make a piece of hardened and tempered spring steel stiffer by hammering it. How much more often will I have to state that before it finally sinks in?

Whatever 'tensioning' is (see rest of thread for discussion) it isn't a way of stiffening saw blades. If you want a stiffer saw, make it from slightly thicker material.

 

[pedder]

Still discussing? Why not look for the hammer marks. Or try.
Prove it. This can't be dissolved with words.

 

[Trafalgar]

Chappie, unless you are a metallurgical engineer (are you?) why should we accept what you say any more than you accept what we say or by extension the links that have been posted?

I'm an accountant by professional training. That doesn't mean I could walk into General Electric tomorrow and take over their general ledger accounting for derivatives, special purpose entities, and/or foreign subsidiaries who use different bases of accounting.

If I recall, David W. is a licensed actuary. I'm certain that he has a broad range of expertise in many areas but surely there are some parts of the profession he would initially be uncomfortable dealing with. Aren't all professions like this? Why is yours any different? Would one hire a bridge engineer to design biomedical devices?

You've assumed, perhaps erroneously, that you are somehow the most qualified participant in the thread. I'd rethink that.

 

[Rhyolith]

I have been emailing tool companies to try and get some more information to work with, this is a semi-useful response from Lie-Nielsen.

“The Disston Saw co was the first maker of what is called Crucible Steel here in the US. They were very secretive about the process that they used to prepare the steel for their saws, and they also were very tightlipped about the process they used for tensioning their saw plates. They did hammer tension the saws, but there is no documentation of what that actually meant. It is far more likely that it was the formula used for the steel and the heat treating process for that steel that gave their saws tension, than that it was the hammering itself. I could be wrong, but I have talked to other makers as well and none of them have been able to get provable results from hammer tensioning. You can hammer out a bow or a kink, but it does not seem to increase or decrease the stiffness or “tension” in the saw.
This was either a form of misdirection on their part or a technique that has been lost to time. I wish that I had a better answer for you, but it is definitely not an old wives tail. Hammer Tensioning was done, it just may not have been done to give the results that we think.”

 

[Trafalgar]

One would have to believe they beat saws with a hammer just for the **** of it, or as previously mentioned it was a marketing ruse practiced by every major saw making firm of the era we're discussing.

 

[Cheshirechappie]

Chappie, unless you are a metallurgical engineer (are you?) why should we accept what you say any more than you accept what we say or by extension the links that have been posted?

I'm an accountant by professional training. That doesn't mean I could walk into General Electric tomorrow and take over their general ledger accounting for derivatives, special purpose entities, and/or foreign subsidiaries who use different bases of accounting.

If I recall, David W. is a licensed actuary. I'm certain that he has a broad range of expertise in many areas but surely there are some parts of the profession he would initially be uncomfortable dealing with. Aren't all professions like this? Why is yours any different? Would one hire a bridge engineer to design biomedical devices?

You've assumed, perhaps erroneously, that you are somehow the most qualified participant in the thread. I'd rethink that.

Charles, I hold a degree in Mechanical Engineering. That involved the study of, among other subjects, Strength of Materials and Metallurgy. I am currently a Corporate Member of the Institution of Mechanical Engineers, and a Chartered Engineer. My professional responsibilities have included, among other things, twelve years of responsibility for the analysis of stress in complex specialist chemical plants, mostly pipework and it's supports, but also on occasion pressure vessels and various ancillary bits and bats. That included ensuring that they new designs were safe for all operating and fault conditions for the design lifetime of the plant, and ensuring that it cope with possible events such as extreme wind, heavy snow load (in the UK? Ha!) and earthquakes (that one was quite interesting), so I do know a bit about stress. In a voluntary capacity (one day a week, basically), I have 25 years of rebuilding and operating full size steam locomotives, which includes a lot of hands-on manipulation of castings, forgings, plate, bar-stock and sheet metal.

So all in all, yes - I do feel that I have some qualification to comment on the stresses in metals, and on their manipulation. Whether I'm the most qualified person commenting on this thread I don't know, but I do feel that I have some relevant specialist knowledge to contribute.

 

[Cheshirechappie]

I have been emailing tool companies to try and get some more information to work with, this is a semi-useful response from Lie-Nielsen.

“The Disston Saw co was the first maker of what is called Crucible Steel here in the US. They were very secretive about the process that they used to prepare the steel for their saws, and they also were very tightlipped about the process they used for tensioning their saw plates. They did hammer tension the saws, but there is no documentation of what that actually meant. It is far more likely that it was the formula used for the steel and the heat treating process for that steel that gave their saws tension, than that it was the hammering itself. I could be wrong, but I have talked to other makers as well and none of them have been able to get provable results from hammer tensioning. You can hammer out a bow or a kink, but it does not seem to increase or decrease the stiffness or “tension” in the saw.
This was either a form of misdirection on their part or a technique that has been lost to time. I wish that I had a better answer for you, but it is definitely not an old wives tail. Hammer Tensioning was done, it just may not have been done to give the results that we think.”

Thanks for posting that, Rhyolith. It's interesting that other people, independent of the discussion on this thread, haven't been able to pin down what 'tensioning' is, either. It's also interesting that Disston's writer was 'secretive' about it - possibly because he didn't really know either.

 

[D_W]

Still discussing? Why not look for the hammer marks. Or try.
Prove it. This can't be dissolved with words.

If the boutique sawmakers haven't been able to figure it out, what chances do we have just beating around saws?

We're not likely to find many saws with hammer marks on them, most of us have saws that were made when straightening was done with rollers, or at the very least, a light dressing of the metal was made after the saw was finished so as to have a bright and uniform plate.

If we go back to when saws would've been tensioned by hand, we don't have any good reason to believe the saws haven't been hammered between then and now, especially since they likely wouldn't have been tensioned as well as a later saw and would probably need to be straightened with a hammer at some point.

What we would like to have is someone comparing saws of similar hardness (where one is known as a second or third line saw, and another - or ten - first line - all from the same maker), and where the tensioned saw has less metal in its cross section, and see if we can find that the better quality saws deflect less under a given weight.

Or we can just use them and know they're stiffer and more likely to remain straight in a cut (that part is pretty easy to notice if someone wants to actually go to the trouble of using a saw for more than cutting off 2x4s)

 

[D_W]

Thanks for posting that, Rhyolith. It's interesting that other people, independent of the discussion on this thread, haven't been able to pin down what 'tensioning' is, either. It's also interesting that Disston's writer was 'secretive' about it - possibly because he didn't really know either.

That's beyond naive. You're talking about print that was made at a time when saw makers were intensely competitive with each other. They're not going to create a 12 step process, show and describe everything so that someone else can just do it, too.

Plus, you assume that just because you and I don't know what a fast or loose saw is, that nobody would've known what it was in 1900 - also very naive. For all we know, it may have been a common term back then.

 

[Cheshirechappie]

Thanks for posting that, Rhyolith. It's interesting that other people, independent of the discussion on this thread, haven't been able to pin down what 'tensioning' is, either. It's also interesting that Disston's writer was 'secretive' about it - possibly because he didn't really know either.

That's beyond naive. You're talking about print that was made at a time when saw makers were intensely competitive with each other. They're not going to create a 12 step process, show and describe everything so that someone else can just do it, too.

Plus, you assume that just because you and I don't know what a fast or loose saw is, that nobody would've known what it was in 1900 - also very naive. For all we know, it may have been a common term back then.

The terms 'fast' and 'loose' are described quite fully - at least, I understand what they mean ('fast' means wavy at the edges but straight and flat down the middle, 'loose' means straight and flat down the edges, but bulging in the middle, both conditions can be cured by stretching the straight bit, thus pulling the wavy bits straight). It's the term 'tensioning' that's frustratingly vague.

 

[pedder]

One the one hand all I have is the writing of a PR manager of a industry corporate group. On the other hand, every engineer, I've spoken to, think it is completly nonsense.

Oh yes and proofs can't be found today, because, they changed the way.

So nobody can describe the procces, no proofs can be found and it can't be done by one of us....

Get a hammer and a floppy saw and try your luck it can make you rich. :lol:

Cheers
Pedder

 

[Trafalgar]

Bob Smalser, in the link already posted, states:

"When a thin blade is struck on an anvil by a convex-faced round hammer, a dimple is created; often so small it can’t be seen by the eye. Steel from the area around the dimple is pulled inwards toward the point of impact, making the steel in the circular area radiating from the dimple stiffer, or “tensioned” on its surface."

If this statement is true, metallurgically, then it would seem that we have our answer. A lot of these little dimples would add up to a saw that had been 'tensioned.'

He goes on to say:

"Hundreds of such hammer blows applied in certain patterns equally to both sides of a handsaw blade can make it stiffer, can true a warped circular sawblade, or can dish a large bandsaw blade to conform to its wheels while at the same time tensioning the cutting edge."

 

[MIGNAL]

Should be easy to prove then, just with the use of a hammer, an anvil and a bit of saw plate.

 

[Trafalgar]

http://sro.sussex.ac.uk/43235/

http://www.scientific.net/KEM.410-411.449

https://www.google.com/search?q=sheet+m ... 3IhAAcM%3A

https://www.google.com/search?q=sheet+m ... CTxW-bM%3A

"The UltraSTEELTM process, developed by Hadley Industries Plc (Hadleys), is a novel surface dimpling process used on steel strip prior to cold roll forming. This dimpling process increases the strength of the final rolled products and enhances other product properties such as fire test performance and screw retention."

C. J. Wang, D. J. Mynors, M. English, "Simulating the UltraSTEELTM Surface Dimpling Process", Key Engineering Materials, Vols. 410-411, pp. 449-456, 2009

 

[Rhyolith]

I think there is yet another divide here: What Disston meant by "saw tensioning" and a number of other techniques that can justifiably be called "saw tensioning". It seems increasingly that whatever Disston did will remain a mystery, which is a shame really.

 

[MIGNAL]

Was this 'tensioning' exclusive to Disston?

 

[Trafalgar]

It wasn't confined to Disston.

See my link earlier in the thread to S&J's 'Story of the Saw.' Spear and Jackson were tensioning their Spearior line by hand hammering AT LEAST as late as the early 1960s, the date of the publication.

They used the same term, tensioning, in the publication and also mentioned that it was done by 'highly skilled smiths.'