How to straighten bent saw blades

[george wilson]

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.

[Alan DuBoff]

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.

[george wilson]

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.

[David Keller NC]

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.

[george wilson]

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

[Glen Evans]

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

[george wilson]

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.

[Glen Evans]

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

[george wilson]

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.

[Glen Evans]

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

[george wilson]

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.

[Rob Luter]

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.

[george wilson]

Glad it worked,Rob,you are lucky that the saw blade was loose enough at the front end to slip there the most. Had it slipped in the center,the blade would have ended up more wavy. Why don't you run a thin stream of Loctite down the juncture of blade and back? If it was prone to slipping,it could slip again.

[Rob Luter]

I may try that, or perhaps some CA glue. I wonder if that would prevent me from "adjusting" it again in the future if I need to? What's odd is that there are no perceptable "witness lines" that would indicate anything moved. I know it wouldn't need to move more than a few thou' to cause ripple, but I expected I'd see it.

[Joe Cunningham]

Thanks for sharing your expertise. I have a Disston backsaw with a slight wave in it, but I've not tried straightening it yet out of fear of making it worse, so it sits unused. Maybe I'll take a look this evening.