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Thread: Straightening Bent Handsaw Blades

  1. #1

    Straightening Bent Handsaw Blades



    The best Western and Japanese handsaws are quite thin yet in competent hands won’t kink in a cut, and the spring steel they are made from is tempered to allow sharpening with files. They are usually taper ground in two dimensions, so the blade’s cutting edge is thicker than the back, and both the back and the cutting edge taper from the saw’s heel to the saw’s toe. The thinner the cutting edge, the greater the taper, and the higher the polish, the higher the saw’s quality, as taper results in less set required for the teeth, aided by the steel’s polish that inhibits binding in the cut. All these features allow for a narrower kerf requiring less sawing effort.

    There are two fundamental choices in manufacturing a thin saw that won’t kink. The least expensive choice is to make the saw stiff by using hard steel and disposable blades, because such saws can’t be economically resharpened. The second choice is to temper the saw so that it can be filed, and to stiffen it by tensioning the blade using hammer and anvil.

    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. 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. Truing sawblades are not low-order skills, and the major saw factories and filing shacks of logging camps and commercial sawmills were where you found them. Today it’s largely done on computerized machines, except for hand saws. Here you either find an old, retired saw doctor who worked for a big mill, a Japanese saw maker still tensioning by hand, or are on your own because there are few references. I’m not going to make a saw doctor out of you today. But I can get you started with some basics to practice with on old sawblades.



    This old saw has a 3/8” kink in the area marked in chalk, and before I do anything else to rehabilitate the saw, I’ll remove this kink and true the cutting edge.



    The first step is to remove the handle and bend the blade using your hands in as complete a circle as possible….in both directions. This relieves any recent stress put in the saw, and sometimes makes the existing kink worse or reveals additional problems like bow or twist.



    The tools I’ll use to remove the kink are a steel anvil and two hammers, both heavy and light, both with slightly convex faces. I’ll mark the areas to be struck with chalk and using and oily rag, keep all steel surfaces clean and oiled to prevent marking the blade.



    Removing a kink or bow requires stretching the steel surface on the concave side of the kink, and compressing the surface on the convex side. Before doing either, the saw’s tension at the cutting edge needs to removed or my attempt may make the kink worse. I accomplish this on the concave side by striking along a line running an inch or slightly less upwards from the tooth gullets. Each “X” represents two light hammer hits. I made identical chalk marks on the opposite side of the saw, but I don’t attempt to strike them yet.



    On the identical marks on the convex side of the kink, I’ll accomplish two tasks simultaneously using light hits with the heavy hammer. The heavier hammer strikes will both remove the tension from this side of the saw and straighten out the kink by reversing the conditions that caused it, compressing the near side and stretching the far side of the blade.



    I sight down the cutting edge to insure I removed the kink and repeat the previous steps if necessary. Once the kink is removed, I tension the cutting edge using identical light hammer strikes on both sides of the saw in turn, around ¼” to 3/8” above the gullets, insuring I don’t strike the gullets or teeth.



    When tensioning is complete, the saw should flex in either direction as I did at the beginning, and return to dead straight.

    Continued....
    Last edited by Bob Smalser; 08-11-2007 at 1:28 PM.
    “Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

  2. #2


    Only after the blade is straight and tensioned do I continue with rehabilitation. This saw was habitually filed without jointing, and instead of straight or crown-breasted, the cutting edge resembles the hooked nose of the Wicked Witch of the West. Accordingly, I must stamp in new teeth. And guess what? 7 new teeth per inch on a 26” cutting edge require 182 strikes of the stamping dies, and a major retoothing usually bows the blade. So as soon as the blade comes out of the carrier, I again bend it both ways and strike 182 blows on the convex side just above the gullets with the light hammer.
    Last edited by Bob Smalser; 08-11-2007 at 1:15 PM.
    “Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

  3. #3
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    That's great, Bob. Timing is right on, too. I saw an old D-23 in an antique store this week that was shaped like a Z. It was in bad shape overall, and they wanted $22, so I wouldn't even get it as a trainer, but it's still amazing timing. Thanks for all the time you spend helping us learn.

    Greg

  4. #4
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    I have a real old fine toothed diston that is overall pretty good, but is kinked at the verry end. I will have to give this a try.

  5. #5
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    Bob, as usual very educational as it turns out I need to work on a saw these days. I have a diston cross cut that is giving me trouble. Visually it looks straight but when I pull it back, and if I am moving a little fast, it starts wobbling. I have checked it several times and looks true but the wobble it still there. Although it may have something to do with my faulty technique.
    The means by which an end is reached must exemplify the value of the end itself.

  6. #6
    Quote Originally Posted by Zahid Naqvi View Post
    I have a diston cross cut that is giving me trouble. Visually it looks straight but when I pull it back, and if I am moving a little fast, it starts wobbling. I have checked it several times and looks true but the wobble it still there.
    This little piece is a trial balloon to see if I can satisfactorily explain something significantly more complicated than tuning a block plane.



    It could be technique. Set yourself up to saw exactly like the old timer above from Pete Taran's site. The sawing arm moves in one arc directly in line with the kerf, not two or more that are created by poor body position. And hold the saw very lightly to let the teeth and gravity do more work than your muscles.

    It could also be the saw's tension. Usually if there's a tension problem, the blade is bowed. But not always, and on some saws that look straight, you can stone one side of the teeth to oblivion and the blade still won't track straight in the kerf.

    On yours after bending it back and forth, I'd check to make sure it was still straight. If it was, I'd retension it with 6oz hammer strikes immediately above each gullet on one side and measure the bow I just created by standing the saw on its edge over a length of butcher paper and scribing the bow with a pencil. The side you hit will be the concave side of the bow.

    Then I'd hammer it identically on the other side and see if the bow fully straightened out. If it didn't, the saw needed additional hammering on the convex side.

    If it still didn't cut straight, I'd repeat the procedure on the other side of the saw, using my scribed line as a reference. Each side should bow identically, and if they don't, the side with the deeper bow needs some additional hammering on the convex side.
    Last edited by Bob Smalser; 08-11-2007 at 12:59 PM.
    “Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

  7. #7
    To add to Bob's excellent advice re body position and straight arm movement (which is an arc), twisting the hand while withdrawing the saw is a natural tendency and will also cause the saw to vibrate on the back stroke.

    Typically a slight amount of hand rotation also happens on the forward stroke, but the tension created by the teeth being engaged makes it negligable in its effect--but should be consciously avoided as well as it can make it more difficult to stay on line.

    With the back stroke, the lack of tension (no teeth engaged in cutting per se) the rotation can show up by the teeth rattling against the cut and the top of the blade hitting the slight roughness in the kerf.

    Take care, Mike

  8. #8
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    Excellent article, Bob.
    Where did I put that tape measure...

  9. #9
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    Thank you again Bob!
    Chuck

    When all else fails increase hammer size!
    "You can know what other people know. You can do what other people can do."-Dave Gingery

  10. #10
    Here's a comment from another site that relates to Zahid's question here.

    I have yet to hear of a good reason, other than decorative, for the "horns" at each end of the saw handles.

    These protrubances do severely limit the size of the hand that fits comfortably, unless one is fond of blisters on the sensitive skin between thumb and index finger.

    I have yet to get hold of a handsaw with a truly comfortable handle. Surely, these tools are meant for work and not just for admiring their intricate finish.

    So, anybody knows the reason for the long "horns"/
    It's all in the manner you were trained to hold and use the saw. Previous generations certainly liked them, or they wouldn't be there.

    The giant sawmakers from 1860 to 1940... Disston, Atkins, Simonds, Bishop, Spear and Jackson....were in hard competition with each other making the type and style of saws that were demanded by the professional tradesmen who bought them, not the other way around. And just like with all the other makers of professional-grade hand tools sold during the era of some of the finest craftsmanship in furniture, homes and yachts our countries have ever seen. They competed with each other to meet the demand, not determine it.



    Here's why this handle style was demanded. What I often see today is a hard, 4-fingered grip more suited for a framing hammer than a handsaw. This picture from the Vintage Saws website of a mature carpenter who likely used handsaws every working day of his life reveals all the features of efficient sawing:

    1) His body position both takes advantage of gravity and allows his sawing arm to swing in a single arc directly in line with the kerf, instead of two or more arcs both up and down and sideways that cause the saw to wobble. A death grip on the saw handle alone will result in multiple sawing arcs, and a wobbly sawblade.

    2) His grip on the saw is three-fingered and very light, using his index finger to aid alignment of his forearm with the kerf. Here you can see the benefit of the handle's horns for kerf alignment in combination with a soft grip. The web and heel of the hand unconsciously feel the horns and use them to aid keeping the saw plumb with the kerf.

    3) In efficient use of his body after decades of daily practice, you can see he's letting the saw's teeth and gravity do more work than his forearm and shoulder muscles.



    4) Cuts in boatbuilding are usually more difficult because neither the workpiece nor the worker standing on a scaffold are plumb and level like our carpenter cutting on sawhorses. That's why youngsters in that trade were assigned thick saws building shelves to master the saw there before tackling more difficult cuts with more fragile and expensive saws.





    Saw handles without a top horn were either flooring saws designed for surface penetration or larger saws used to fall or buck 25 or more inches of green log where two hands were required, gravity wasn't always in the user's favor, precision was less important and the muscles did as much or more work as the saw's teeth. Sandvik and other continental European sawmakers also made handsaws without horned handles, although the tradition there was largely frame saws, and there's no shortage of Disstons and others with the top horn cut off, which is easy enough to do if you don't like them.
    Last edited by Bob Smalser; 08-12-2007 at 8:56 PM.
    “Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

  11. #11
    Thank you very much Bob. I'm just starting to do some rehabs of 2 old Disston saws I found in my fathers garage. I also want to attempt my hand at making a dovetail saw per the Norse Woodsmith's site (and if any of you have not checked that out, all I can say is wow! LINK)

    Much appreciated.

    Michael

  12. #12
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    Thanks for making these fading skills available for those of us who are interested in them even though they aren't a necessary part of our livelyhood Bob. Even if we aren't saw doctors, at least we can save a few saws that would otherwise be decoration.

  13. #13
    Quote Originally Posted by Michael Fross View Post

    Much appreciated.
    This just a small trial balloon to see if I can satisfactorily explain something significantly more complicated than tuning a block plane. This will eventually be a major magazine article on advanced saw filing to go a step beyond all the good basic filing primers out there.

    A good filer can crown or change the crown of the cutting edge, make the teeth taller and the gullets deeper to clear damp sawdust better, and even change the fleam or cutting edge bevel at the heel to make the saw start easier. These are features than can only be done by hand.

    Filing with a 60-degree triangular file used at a 22 to 45-degree slope from vertical instead of straight across like the filing machines do produces taller teeth with longer cutting edges along with deeper gullets to better clear sawdust.

    A crown-breasted cutting edge:



    12-pt crosscut sloped gullets with no set, for hardwoods:



    8-pt crosscut sloped gullets with light set, for softwoods:



    5-pt rip sloped gullets with full set complete with raindrop, which is why I often phosphate blue the blades:



    My focus is on boatbuilders who as you can see have no choice about hand saws because of odd angles and 500lb workpieces.
    “Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

  14. #14
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    How many of you guys out there still using hand saws, work in a button down dress shirt and tie!
    Life's too short to use old sandpaper.

  15. #15
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    Thanks for the generosity with your knowledge, Bob. As I've looked more into hand saws and read about horns, I've developed a question. Do the horns help with the proper technique? The saws at the Norse Woodsmith's site had handles with horns that looked longer than usual. You mentioned that some types of saws didn't have horns on top or at all. Does the type of saw, back saw, rip, panel, etc., have anything to do with the size and/or style of horns?

    Just got my curiosity up. Thanks.

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