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Thread: How to straighten bent saw blades

  1. #1
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    How to straighten bent saw blades

    If you have a backsaw with a wavy blade,it is because the blade has slipped in the back. Clamp the leading edge of the blade in a vise,and tap the front edge of the saw's back so as to stretch the blade,and it will be jerked straight. THIS DOES NOT INCLUDE KINKS where the blade actually has a crease in it. The correct way to straighten the cutting edge of a back saw whose blade has developed a smooth curve,is this: Clamp the tip of the saw's back in a vise with the blade upside down. Adjust a crescent wrench to fit the back. Sighting down the blade,twist the back vertically. That is,hold the crescent wrench vertical with the back clamped in its jaws,and move the far end of the wrench's handle sideways to twist the saw back as if you were making a twist drill out of it (hopefully not!). Most people think only of bending the back to straighten the cutting edge,but then you have a bent back.

    To straighten a curved crosscut or rip saw: You must pour boiling hot water on the blade,and instantly bend the curve out of it. You can bend a cold blade all day long,and not get it straight-IF it is made of GOOD spring steel. Most commercial saws aren't made of good spring steel,and have fairly soft blades that you might bend cold. The antique Disston saws,and saws of other good makers were made of much better steel than now.

    There are basically 3 grades of spring steel: 1070,which has .70% carbon; 1080,which has .80% carbon,and 1095,at .95% carbon. Only the 1095 spring steel is hard and durable to make a real good saw,but it costs more,so manufacturers don't use it because most people don't know any better these days. 1095 is about 52 rockwell hardness,much better than the lesser grades. This is why the old timers said a good saw could cut nails. I don't recommend this,of course,but this is why the good old saws are better. In the old days,when hand tools were the tools used,craftsmen knew better,and makers had to meet their requirements to sell tools.

  2. #2
    Quote Originally Posted by george wilson View Post
    There are basically 3 grades of spring steel: 1070,which has .70% carbon; 1080,which has .80% carbon,and 1095,at .95% carbon. Only the 1095 spring steel is hard and durable to make a real good saw,but it costs more,so manufacturers don't use it because most people don't know any better these days. 1095 is about 52 rockwell hardness,much better than the lesser grades. This is why the old timers said a good saw could cut nails. I don't recommend this,of course,but this is why the good old saws are better. In the old days,when hand tools were the tools used,craftsmen knew better,and makers had to meet their requirements to sell tools.
    This is not true, and in fact almost all modern saw makers use 1095.

    The exception being the folks that make those PAX/LYNX/GARLICK saws, which I consider to be marginally acceptable as a saw, those seem to use 1080 (but I don't own one so can't verify).

    Everyone I know uses 1095 spring steel.
    --
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  3. #3
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    I made hundreds of saws while toolmaker at Colonial Williamsburg. Also was responsible for sharpening the older saws that used to be there. The other saws were much softer,and easier to file than the 1095 ones I and my journeyman made. I know that the high priced makers like LN,and other makers of expensive saws use 1095.That's one reason they cost more. Yes,Garlick,and the other saws you mention,are soft as butter.The only way to gauge what a saw is made (on a home shop basis,which includes most of the members here)of is to have a known sample of 1095,and compare how it files with others. I can tell you that a piece of 1095 for one of my crosscut size saws cost over $22.00 for the blank piece. I seriously doubt that the saws you mention even use 1080.
    Last edited by Dave Anderson NH; 02-01-2009 at 8:44 PM. Reason: violation of TOS

  4. #4
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    George - Got any tips on how to planish out a kink or a crease in a saw? I think there's probably more than a few of us that have a few of those around and would love to know how to fix them.

  5. #5
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    Read the thread "I'm curious and need a clue." Or,something like that. There is a well photographed dissertation on the subject,David.

  6. #6
    Thanks for the idea of using boiling water on the saw. I'm assuming that a propane torch would accomplish the same. Also--have you any thoughts on re-tensioning or tensioning (i.e. is it necessary--how do you do it??)
    thanks
    Glen

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    Glen,unless you are really good with a torch,I encourage you to NOT try to use a torch. If you draw the temper,it's gone. This is why I mentioned using boiling water. First,a blade is thin,and with a torch,it would be nearly impossible to heat a large area all at once. The water would instantly heat a large area. Then,you must bend the saw quickly.It will cool quickly.

    The trouble with thin sheet like saws,is that they will suddenly get way too hot,before you can control it. I might recommend a heat gun,with great caution. I would recommend using it on a low setting,and practicing on a sheet of steel about the size of the saw,and certainly close to the same thickness as possible. Heat guns can put out 1000 Degrees,they claim for some models. Don't get the nozzle too close to the saw,as the temperature RAPIDLY escalates. I use a heat gun to soften edge bindings I'd be fitting to guitars. They can catch fire quickly. At least,they put out a fairly large swath of heat,which is what you need. Remember,if you go past blue,you are finito!!! It takes very special furnaces to harden and temper sheet steel. I doubt you could get the saw made right again.
    Last edited by george wilson; 02-02-2009 at 10:05 PM.

  8. #8
    Geroge,

    Yeah, I guess I should have thought about the risk of screwing up the temper by overheating--good point. I'll try the boiling water on a gently curving Disston D-12. I've been trying to hammer along the convex side to tension the curve out of the blade but only had limited success.

    I have a beautiful old Spear and Jackson rip saw with split nuts that has a bow and a twist near the toe. Do you think boiling water would work on the twist?? Any suggestions for technique--I think I'd just clamp the untwisted section of the toe in the vise and twist it back holding the heel of the saw. Does this sound about right??

    thanks
    Glen

  9. #9
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    Two questions have been asked. First,I hope you have not already made the Disston un repairable by hammering on the blade. That induces lord knows what new stresses in the steel. This is not an attack on you.Don't take it as such. It's just fact. About the Spear and Jackson. That saw might be fairly valuable if it is old. Had a collector offer me $75.00 for what I thought was a well worn,rusty saw back in the 70's. When you clamp it in a vise of metal,the vise will instantly cool the blade locally,and you could break it from twisting it.I'd just put on some gloves that hot water can't penetrate,like cotton ones dipped in rubber,and bend the blade freehand. Hot water will go right through ordinary gloves,and burn you well before you can get them off !!! If you use a vise,use wooden jaws only. Maybe the twist is too close to the end to bend by hand alone. You could saw a slit in a piece of strong hardwood,and use it as a wrench. I think that's the best idea since you might want to be outside when pouring hot water over the saw.

    I'll tell you what will get REAL hot,but won't be too hot and ruin your saw. Recommend a hot plate out of doors,PLEASE. A deep pot of simmering vegetable oil will be at more like 500 degrees. That will be hot enough for any bending. Use the wooden wrench. The blade was tempered at about 750 degrees. You won't be going over that. I warn you,I used to make varnish and it is very easy for the oil to suddenly boil up and over the top of the container. Then,you have a 4' fireball,and an embarrassing black mushroom cloud. Never,never try this indoors,and KEEP YOURSELF AWAY from the area above the top of the pot. I used to use sawed off mapp gas tanks with a cup of oil in the bottom,to try to get more warning time. The oil could still boil up before I could get it off the fire. I learned what I could get away with,but it takes repeated trials,and experience from those trials.

    French fries are cooked in oil everyday,but they have good control of the temperature.
    Last edited by george wilson; 02-03-2009 at 12:54 PM.

  10. #10
    This is great info George! I used a heat gun on the Spear and Jackson today with pretty good results. I think I may try the oven to get the blade hotter in a controlled way. Unfortunately, I have damaged a saw or two with the hammering method!

    I've read and re-read Bob Smalser's article on straightening handsaws with hammer blows. He speaks alot of tensioning/untensioning the blade but the process still baffles me. I can't argue with his or others success doing it--but I've never been able to replicate it. I take it from what you have said that you don't "re-tension" the blade. Any comments on that methodology?

    I think I'll try to re-habilitate a couple of the hammered blades by bending the saw in both directions to hopefully remove some of the tension and then pop them in the oven and follow the method you have suggested.

    Who knows, maybe I'll make some fries in the oven at the same time!

    Any other suggestions??

    thanks
    Glen

  11. #11
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    Glen I am very glad that I have been of some benefit to you. Saving a good tool is always a very beneficial,and cost effective thing to do. Plus,you may have saved a valuable tool for posterity. Good work,and the oven is a great idea,too.I learned a trick from you on that one. Home ovens can be 75 degrees off,so don't crank it up too high. 400 is most likely plenty,and hotter than water.

  12. #12

    Twist and shout!

    I've been able to straighten 4 old saw blades using the "Easy-Bake" oven method. Its worked like a charm on bows and minor kinks. My new problem is a blade with a twist in the middle. The toe and the heel are coplanar but in the middle is a twist that so far has stymied my best efforts. I've used the oven trick and tried to work out the twist from the heel end, to remove the twist at the heel end and then re-work the toe of the saw back so that it is again coplanar with the heel. Each time it seems to "undo" the corrections. I just finished clamping the heel and the toe to adjacent, (and level), wood workbenches leaving the center twisted part suspended. Using a heat gun I've heated the center and then used a slotted board to twist the center--still no joy--I've got TWIST and I want to SHOUT!

    Any ideas??
    Thanks
    Glen

  13. #13
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    Glen WHAT TYPE SAW are you trying to get the twist out of the middle of? I hope it's a back saw. I can tell you how to do that.

    If it is a crosscut type,the metal MAY have become stretched causing the permanent bulge. Really,the way to fix a bulge that was from stretching the sheet steel at the cutting edge,is to hammer carefully the blade that is flat,and between the bulge and the back of the saw. BUT,you have to know what you're doing,or you will make things worse.

    Years ago,I found out that the silversmiths in the Williamsburg silversmith shop did not make trays because they got the trays all distorted when forming up the edges,and the flat area beyond. Someone mentioned it,and I knew what to do,because I worked a lot with the behind the scenes silver manufacturing facility. Professional jealousy prevented communication about things like this. It was not the fault of the manufacturing facility,who had all been properly trained in England.

    The secret was: Clamp the warped tray flat with 1/4" steel plates sawed to shape,one on each side. Use heavy duty steel "C" clamps. Heat the whole thing red hot with very large gas/air torches. Let cool overnight. The metal was stress relieved,and when cool was beautifully flat. Then,a gentle upward crown might be hammered into the flat tray,or leave it flat.

    On this basis,I recommend WITH CAUTION,the same type of thing,EXCEPT,do not exceed the tempering temperature of the original saw making. I have warned you that kitchen ovens can run 75 degrees out either way. If there is no other way to save the saw ONLY,clamp the blade between 1/4" steel plates with unpainted heavy duty steel "C" clamps. I mean THICK frame clamps like those that cost $75.00 -Hargreaves,really good ones like machinists or professional welders use. Thinner ones will bend when hot,and let the blade loose. Try heating the clamps good and hot with the heat gun before using the oven.Tighten them if the heat enlarges the clamp frames,loosening them. Don't get carried away with heat,you could draw the saw temper.

    Put the blade without handle on,of course, clamped with 2 clamps,one on each side of the bulge into the oven. The steel plates must cover the full width of the blade,and go well beyond the limits of the bulge. My oven will take 30" diagonally. Lay the saw on edge,teeth up,in the oven. Place DRY bricks under the handles of the clamps. Those heavy clamps must be supported. Their weight may warp the blade when it is hot. Turn the oven on to 500 degrees.Even if the oven gets to 575 degrees,it is not up to ruining the saw. Spring temper can be 650 to 750 degrees.. Make sure those clamps are tight. Use a cheater bar!! Let the whole thing bake for 3 hours. It will take a while to get all the plates and clamps up to temp. Doing it longer won't hurt. I hope your oven can take it. They get hotter than that on automatic clean,I believe.

    I say teeth up because the red hot coils might overheat the teeth and ruin them. Let the whole thing cool down till it is at room temp. Hopefully the maximum heat we dare use will stress relieve the bulge,and push it back flat.

    P.S.: do not put the steel plates over the actual teeth. Put them right up to their gullets. Don't want to heat the set out of them. Now,if this works,and some other part of the blade warps,you can deal with the other warps with the heat gun. I wouldn't put the whole blade back into the oven,just in case the bulge pops out again. Any other warps you incur won't be from metal stretching,so by now with your experience,you can flatten them easily.

    IF after all this writing,your saw is a backsaw,read my first postings on how to jerk the kink straight,print this post,and keep it for reference. LET ME know how you do!!! I can't guarantee this as we can't go red hot,but I think it will work.
    Last edited by george wilson; 02-09-2009 at 11:13 PM.

  14. #14
    George, thanks for the quick response. The saw in question is another beautiful old Spear and Jackson split nut rip saw that I have never been able to use. I'd hoped that your heating method would work for both of these old ladies. (I love the hang angle of these old saws and the lower placement of the handle on the saw plate--not to mention the handle shape!) The cross cut worked perfectly with your heating technique--but not this one.

    Your comments confirm my fear that the issue is stretched metal as opposed to a simple bow or kink. That also explains why my previous efforts kept "undoing" themselves on subsequent reheating.

    I'm going to have to see about scavaging some steel plates--I don't have anything that size. I'm not worried about the oven--but I'm going to have to figure out clamping and plate options...

    I'll let you know how it works--I'd love to have both of these saws in user condition--but I may have to settle for good looks with this one...

    Whether I get this one right or not--you've taught me alot and helped resurrect some great saws!

    Thanks again
    Glen

    ps I saw your saw on that Woodwright episode--what a beauty! I liked the look of the blade left blue--I think I'll try that myself

  15. #15
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    Quote Originally Posted by george wilson View Post
    If you have a backsaw with a wavy blade,it is because the blade has slipped in the back. Clamp the leading edge of the blade in a vise,and tap the front edge of the saw's back so as to stretch the blade,and it will be jerked straight.
    Thanks for the tip. My Disston #4 had a bit of wave and the blade would "oil can" back and forth 1/8" or so. It's an old one (shown below) with a fairly thin plate and the wave made cutting a clean line a bit problematic. I tried your method above, but likely misunderstood and it got worse . I turned the saw over and rapped the tip of the back (end opposite handle) on the benchtop to try and drive the front of the blade deeper in the back. It apparently worked as the blade is now straight as an arrow and the toothed area seems to be in a pretty good state of tension.

    It's wood dust. Saw dust would suggest a problem.

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