Panel Saw Tensioning

[Rhyolith]

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?

 

[Cheshirechappie]

I know of four methods.

Method 1 works for definite kinks on some (not all) saws. Place the sawblade with the peak of the kink against the bench edge, and using the thumbs either side of the kink, bend it out. This can take quite a lot of force, but it's best to start gentle and work up the force scale. I've had success with this method on one saw with a kink resulting from jamming in the cut.

Method 2 is one I've not tried, but is said to work for blades with an even curve in them. First, submerge the blade in boiling water, or pour boiling water over the blade. Then, wearing thick gloves, flex the blade against the curve until the blade comes straight. Allow to cool, check, and repeat if necessary.

Method 3 works for blades with multiple kinks. Using a heavy hammer with a clean, preferably polished, face (any marks on the hammer face will transfer to the blade), place the blade on an anvil concave side of each kink down, hold it tight to the anvil, and using heavy, dead blows, knock the kinks out. Again, start gentle and work up the force scale - too heavy a blow could result in cracking, and there isn't a huge margin between yield point and ultimate strength of the material - not much plastic deformation is possible. I've had some success with this method, but it takes a bit of practice to get a blade really straight.

Method 4 works for blades with slight bulges and gentle bends. Using a hammer with a straight (or cross) pein and a block of hardwood, place the blade concave side down, and tap the hammer such that the pein makes a line of blows across the bend. The idea is to use the give in the hardwood to induce a very slight bend in the blade opposite to the bend to be removed. Continue with lines of blows across the bend until it is eliminated. I've not tried this one, but it should be a little easier than the anvil method. Again, some practice with weight of blow would be required.

There was (is?) another, used to remove bulges. The metal either side of the bulge was hammered to stretch it - note, NOT the bulge itself. I've not tried that one, and I suspect it's a rare fault in saws that have seen service, but would be common during the manufacturing process after heat treatment of the blade.

The key to all of these is careful assessment of exactly where the bends and kinks are. The professionals used to hang the saw up such that the blade hung vertically, and use two wooden straightedges one each side of the blade to find the faults.

Hope that helps a bit!

 

[biskit]

(hammer) (hammer) (hammer)

 

[D_W]

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).

 

[Rhyolith]

Thank you Ches.

I know of the first method, but never heard of the boiling water one before! It seems to me that Methods 3 & 4 require a fair bit of practice to get right, its a another reason to buy old cheap saws to practice on (which i want to do to practice saw sharpening as well). I usaully just cut of the end of bent saws (which leaves me with rather useful stubby panel saws), but it will be good to have a few alternatives.

My friend thinks there is some kind of "trade secret" to re-tensioning the blade of panel saws (Dissitons usually the subject of the conversation), by re-tensioning I think he means stifferning the saw along its back by stressing the metal to strench the cutting part out (imagine like a coping saw). Apparently a similar thing is done with circular saws, but I have no expierence of that.

I am talking about the old Dissiton Panel saws here mostly, as it is those me and my friend would like to restore.

 

[Trafalgar]

http://www.wkfinetools.com/tRestore/saw ... Blade1.asp

 

[Cheshirechappie]

My friend thinks there is some kind of "trade secret" to re-tensioning the blade of panel saws (Dissitons usually the subject of the conversation), by re-tensioning I think he means stifferning the saw along its back by stressing the metal to strench the cutting part out (imagine like a coping saw). Apparently a similar thing is done with circular saws, but I have no expierence of that.

There's been much discussion about 'tension' in handsaws, but I've never seen anything that explains what it is, how it works, or even convinces me that it exists at all. Most of the references seem to suggest that a 'tensioned' handsaw is stiffer than one that has not been 'tensioned', but I don't see how hammering a piece of sheet metal of a given thickness and hardness can make it stiffer.

The reason saws were hammered years ago was to correct distortion that occurred during heat treatment - the process by which soft steel was given more hardness and the springiness needed to avoid their being easily bent or kinked in use. Modern heat treatment methods allow the production of flat steel sheet hardened and tempered to spring grade - and crucially, still flat.

If a piece of soft sheet iron or sheet steel is hammered evenly all over, it will work harden, and go from a state in which it bends or kinks easily to a state in which it will hold it's shape better. Maybe that's the origin of the misconception - back in the 18th and early 19th century, the cheapest grades of saw were made from sheet iron hammered to give them some spring. At least one contemporary writer (Moxon?) advised against purchasing such saws, and stated that the steel ones were much better.

Circular saws are different animals altogether. When spun at full cutting speed, a fair bit of tensile stress develops in the periphery of the saw. (Same thing happens to anything spun - the old cast iron flywheels of mill engines have been known to 'burst' if the engine oversped because the tensile stress in the rim exceeded the cast iron's ultimate tensile stress - with very distressing consequences to anybody in the vicinity.) With a circular saw, the tensile stress developed produces an associated strain. In simple English, the rim stretches. The metal nearer the centre of the saw doesn't stretch as much because the stress isn't as high near the middle, so the only thing the rim can do to relieve the strain is to distort into a sideways wave. The saw thus won't cut straight.

To cure this problem, circular saws, especially large ones, used to be made like very flat cones. As the saw reached working speed, the periphery stretched, pulling the cone flat. Modern saws have small cuts around the periphery that effectively divide the toothed edge into segments, and give a little space into which the metal at the periphery can expand at working stress (thus allowing circular saws to be made from flat stock).

A handsaw does not see such stresses. Indeed, the forces acting on a handsaw cutting edge are very low, especially when the saw is sharp and in good condition - a fact readily proved by the lack of effort required to use a good saw. Using a saw with any force risks a jam and a kinked blade.

It sometimes happens when using a panel, hand or rip saw that the toe end waggles about as the saw is returned for the next cutting stroke. I've had this happen when using a saw horse of less than ideal height - a Workmate, for example. It happens because the body position is not good, and the sawing arm isn't keeping a straight line, so the saw is being pulled just a bit sideways, causing the teeth to rattle against one side of the cut. With a better sawing position that keeps the sawing arm straight with the cut, it doesn't happen so much. I've seen people say that to eliminate saw toe-end waggle, they need a saw with more tension. Wrong - they just need a better sawing position.

If someone can explain what 'tension' is in a handsaw, and how it's produced, and demonstrate the fact, I'll listen. However, until I've seen someone make a saw noticeably stiffer by hammering it, I remain unconvinced that 'tension' in handsaw blades actually means anything other than 'straight'.

 

[AndyT]

I'm away from my books at the moment so can't look it up, but somewhere I recall a diagram in an old book purporting to explain this tension.

It likened the saw plate to a piece of paper hung up by its top edge, with the teeth along that edge. To have any chance of cutting, the fingers holding the paper need to stretch that edge so it goes taut. This is achieved, so the book said, by hammering the metal behind the edge so it is trying to be longer than the edge, with the result that the edge goes stiff.

I'll try to find the source when I can, or maybe someone else will recognise it.

It could be a load of old tosh, though!

(It might have been in a book on tools for woodwork by JC Brough, which I was looking at having watched a recent video review of it by Graham Haydon, but I'm not sure.)

 

[Cheshirechappie]

Indeed, Andy - I'm fairly sure I've read similar here and there, and the authors may well have sincerely believed what they wrote. However, I think it is a load of old tosh.

Firstly, sawblades are substantially thicker (and inherently stiffer) than sheets of paper. Secondly, if part of a sheet of steel is stretched and part not, the shape of that piece of steel must change to accommodate the stretched part. The fact that saw blades stay flat indicates that the strip behind the toothline hasn't stretched at all - the blade would bow if it had.

The hammering process during manufacture is undertaken to change the blade's shape by removing bends, kinks and bulges and make the blade as flat and straight as the smith can manage. Any hammering sufficient to stretch part of a blade that already IS flat must therefore, by extension, remove the flatness created - exactly what is not wanted!

Edit to add - the only way I can think of to make a sawblade stiffer (other than to make it a backsaw) is to make it from thicker material to start with. Stiffness of flat pieces of material is proportional to the cube of their thickness, so not much thicker makes it significantly stiffer. Conversely, not much thinner makes it significantly more flexible.

 

[Rhyolith]

It likened the saw plate to a piece of paper hung up by its top edge, with the teeth along that edge. To have any chance of cutting, the fingers holding the paper need to stretch that edge so it goes taut. This is achieved, so the book said, by hammering the metal behind the edge so it is trying to be longer than the edge, with the result that the edge goes stiff.

Thats exactly what I was trying to state earlier. As thinking about it, it's the only logical way I could think of to make the blade mechanically stiffer... short of some metallurgical magic I don't understand yet I can see it being difficult to achieve (maybe impossible) to tension the blade in this way without cause it to go all over the place (bend), but maybe if the central portion of the blade (the strip between the back and the cutting edge) were not tensioned and the back was this would keep it straight? I am imagining it working like a hacksaw with is sprung back and tort blade, just in one sheet of steel... mechanically I don't see why I would not work.

http://www.wkfinetools.com/tRestore/saw/strSawBlade/strSawBlade1.asp

This seems to treat saw tensioning as a real thing, but I have only read the 1st page so far (slow reader). Thanks for the link Stanford, its more or less exactly what I was after I am very interested to find out whether saw tensioning is myth or reality now though.

Yes, please find that reference when you can Andy. Info on saws is remarkably hard to come by.

Edit: I had a thought, if a panel saw is really tensioned like a hacksaw, then cutting into the the cutting edge of the blade to about half way through (give or take) should cause it to bend upwards of its own accord... If any has a old panel saw they don't mind ruining :?

 

[blackrodd]

When I worked in the saw shop, I was not taught the art of Rolling, hammering or tensioning but did watch when possible, probably when I should have been sharpening.
I have found a link below, which helps show when tension is lost, nearly always by overheating.
The handsaw "flapping" is mainly caused by not allowing the saw to dominate the cut, but trying to speed up the saw, easily done when sawing with both hands, and "slogging" away, the timber closing on the blade,
waxing the blade helped cure this.
In the days before laser welding made cheap TCT blades available, An Untensioned mild steel circular saw blade would not stand the rigours of spinning and sawing in a straight line.
It's heat treated as hard as possible, yet allowing the sharpening by file.
In the mill when the new blade was fitted, the blade was packed each side using the correct wadding and the ever present fag packet unfolded and used.
The fibre packing was about 4 " long and started just after the gullet, and sit in a rebate, about 5/8 deep, but flush just under the bed height.
When too tight the blade would wobble quite soon after start up.
Too slack and the blade would tend to "meander"in the cut, particularly around knots in harder wood.
It was all too easy to burn the saw, when pinching and kickback occurred, sometimes you could get halfway down a
8' or 10' board before trouble ensued.
Then the saw doctor was not a happy bunny, brown and blue hot spots on the blade, had to be hammered and re tensioned, and then the "hotspots"polished with emery tape to hopefully ensure the burn wouldn't catch again.
Regards Rodders

https://www.youtube.com/watch?v=6rLmeZ8CGBQ

 

[Cheshirechappie]

Thanks for that link, Blackrodd - it must be one of very few indeed on the subject of circular saw doctoring!

As I said in my first essay, circular saws and handsaws are very different animals. Circular saws see stresses at working speed that no handsaw will ever see, and have to be set up to cope with those stresses. If you look very carefully at the video, you can see what he's doing - making the saw into the very flat cone I was talking about. When he puts the longer straightedge across the saw, it touches at the periphery, but there's a small gap nearer the centre. You do have to know what you're looking for when you watch, and if you blink you'll miss it - but it's there.

 

[Trafalgar]

Here's a section of an article on the Disstonian Institute web page.

"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.

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. "

Another link that might be helpful: http://www.wkfinetools.com/mLibrary/Gri ... Filing.asp

 

[Cheshirechappie]

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. "

That just means 'flat and straight', doesn't it?

I've seen the terms 'fast' and 'loose' (sometimes 'tight' and 'loose') used in other references, where it means slight humps and bulges. A slight bulge is 'loose', and the metal either side of it is 'tight'. All it means is the correction of any out-of-flatness found after grinding. Just as plane sole castings can distort after machining if they're not annealed or seasoned, steels - especially thin, flat plates - can distort if ground after heat treatment, especially if the heat treatment was of the 'old school' type done in coke hearths without the benefits of modern controls. Slight differences in temperature across a plate when it was quenched would give slight variations in hardness and temper, hence slight stresses set up in the quenched plate, hence the distortion as those stresses are partly released by grinding or surface finishing.

Modern saws made from spring steel of even hardness and temper throughout, supplied by the mill in a flat, smooth condition, just doesn't need the corrective smithing, because it stays flat. A saw bent in service just needs enough hammering to bring it straight enough for service again.

Thanks for the link to 'Grimshaw on Saws'. I bought a copy of that from Abebooks (their 'print on demand' service for old books - doubt I'd find or could afford an original!). There's nothing in it that suggests to me that handsaw hammering was anything other than a way to get them flat and straight.

 

[Trafalgar]

"...but shall possess the proper tension, spring or character..." Seems to indicate something other than just the removal of bulges or making the plate flat.

Could be that modern saw stock is 'better' but the saws don't really seem to be any better (not all that worse, either, I'll grant). Maybe they're better in the sense they're more easily made entirely with machines.

 

[Cheshirechappie]

"...but shall possess the proper tension, spring or character..." Seems to indicate something other than just the removal of bulges or making the plate flat.

Not sure about that. A sawblade is basically a piece of flat spring steel with teeth along one edge. You'd want it to have 'proper tension, spring or character' - in other words, springy, or elastic; having the capacity to return to straight if deflected. The springiness is imparted by the hardening and tempering, the subsequent hammering corrects any distortions arising. Thus, when finished, the sawblade is flat, straight and springy.

 

[Trafalgar]

Bad Axe Tool Works mention the word 'tension' in their entry regarding straightening saw plate:

"The reason so many saws (particularly back saws) have a chip in the handle or a missing horn is because a previous owner dropped it; this most likely put a kink or bow in the sawplate, because the rib of metal (back, or spine) of the saw no longer exerts even pressure across the length of the blade. Acts of careless sawing also result in bows and kinks in the sawplate. Consequently, many old saws present a taper from toe to heel, because to straighten a bow after dropping the saw (and chipping the handle), a previous owner tapped the spine at the toe deeper into the sawplate—an inelegant correction at best. I can remove the back, judiciously hammer the kink out on my saw anvil, replace the back, equalize blade depth the way Disston (and other manufacturers) originally made the saw, and deliver a restored blade to you that cuts faster with a thinner kerf. Same technique applies for handsaws, obviously without the back. Do understand that hammering a sawplate straight often results in a 90% solution. Once badly kinked, a sawplate will always be weaker in that spot, and overhammering will result in a permanent loss of tension. Sometimes, there is a point where you just have to accept less than perfection. For sawplates with a significant buildup of rust, add another $20."

It apparently applies to circular saw blades as well. http://www.carbideprocessors.com/pages/ ... plate.html

"As a final step the plate is examined with a flat edge and hand hammered to correct high and low spots as well as to ensure proper tension in the steel.

A circular saw must be tensioned to operate properly; the larger the saw is, the more important it is to have the proper tension. As the saw comes up to speed, the rim stretches much more than the center of the saw, due to centrifugal force.

An untensioned saw will be loose and floppy at speed and will wander out of the cut and heat up."

 

[Cheshirechappie]

That's Mark saying he can't always get them dead flat, which is perfectly understandable. He flattens maybe one or two a week; the old saw smiths of Sheffield and Disston were doing them 50 hours a week 50 weeks of the year. Also, too much hammering on a bad kink can crack the blade - there's not a huge margin between elastic limit (or perhaps more accurately, something like the proof stress point) and ultimate tensile strength in hardened steels - not much plastic deformation is possible.

We covered circular saws earlier in the thread.

 

[Trafalgar]

I didn't realize that the word tension was synonymous with a flat or out-of-flat condition; one can apparently take their pick. :roll:

I always thought the word 'flat' did a nice job of describing the condition of flatness. And 'not flat,' the converse.

The fact that sheet steel stretches, well all steel stretches just ask a bridge engineer, seems to indicate the presence of tension somewhere in it.

The phrase tensile strength has some meaning in physics.

 

[Trafalgar]

I'm going to post this link one more time in hopes that somebody won't go willy-nilly whacking a kinked saw with a hammer and expect good results. You won't get them. Of this, I'm sure. Otherwise, bowing out of the thread.

http://woodcentral.com/articles/handtoo ... _866.shtml