Backsaw blade straightening

Re: Backsaw blade straightening

Ted Shuck, Centennial, CO

>I've had some success straightening backsaw blades.

There are two general classes of problems. One, if the blade has sharp bends in it, these need to be planished (hammered) to straighten the blade. The blade may need to be removed from the back if it is in bad shape. Two, if there is just a gentle bow or wave in the blade, it can be caused by three possible problems.

1) The blade is not under tension from the back. Think of the back as a long leaf spring which should keep the blade under tension at all times. If the back is pushing on the ends of the blade, though, it will cause the blade to bow. This can be corrected by putting the toe of the blade on a piece of scrap and tapping the toe of the back with a mallet. Don't whack it hard. All you want to do is drive the blade into the back an infinitesmal part of an inch so that it puts the blade back under tension.

2) The back is bent into a wave. You can test this by placing a straightedge along the blade right next to the back. There should be no gaps. If there are, put the back in your vise, with some padding to prevent scratches, and bend the back straight. I have found saws that have a straight line of teeth, but a bow near the back. These will start a cut fine, but will not cut straight for very far at all. It can be very frustrating to sharpen and set a saw like this and not be able to figure out why it won't cut straight for more than half an inch! DAMHIKT!

3) The back is twisted. If the back is straight and the blade is under tension from the back, a twist in the back can still cause the edge of the blade to bow. Put the back in your padded vise and use a large crescent wrench to grip the blade and twist it to remove the bow. This may require gripping the blade near the heel in the vise and twisting it near the middle, then gripping it in the vise at a different spot and twisting it some place else to remove all the bow.

Through a combination of these methods, I have managed to straighten all the saws I have come across.

Regards,

Ted

Re: Backsaw blade straightening

Mark Kuzee

>Thanks Ted,

Your sytem worked well. I did drive the "toe" end of the back down about 1/8" and the balde improved dramatically. Not perfect but good enough for wood.

Maybe this resolves the old question about why backsaws are "tapered". Maybe they were "tuned" depending on the blade condition.

Thanks Again

Mark

Re: Backsaw blade straightening

Ted Shuck, Centennial, CO

>There has been a discussion of "tapered" backsaws on Old Tools recently. No clear consensus on this, but I am a believer that they did not come tapered. I have had many saws come to me tapered, but they now are not. It is usually not too difficult to pry the toe of the back up to get the blade square, then tap it down a few thousandths to restore the tension on the blade. If you drop your saw much, you may find that this taper is enhanced by trips to the floor. 8*).

Ted

Questions

Adam Cherubini

>I don't understand the relationship between how much blade is in the spine and tension in the blade. Come to think of it, I don't exactly understand tension in a flat plate either.

For the original post, I've had limited success straightening saws with light taps of a planishing hammer. For larger curves, I've simply bent the saw by hand, but that doesn't work for a back saw. Todd has said that its good to roll the saw over the horn of an anvil or some such. I really don't know what I'm actually doing when I do this (am I workhardening the saw for example, yielding the metal (beyond its elastic limit???)

I'd appreciate an education on this matter.

Adam

Re: Questions

Ted Shuck, Centennial, CO

>The reason that I have some experience in straightening backsaws is that I have acquired most of my from that big electronic auction house and they are generally not straight when they arrive. Out if necessity, I have had to learn something about how to deal with this. I got some information from Pete Taran over on Old Tools and picked up some more through experimentation with saws that he probably would not be bothered to touch.

Just so we can have a frame of reference, consider the saw blade and back in the vertical plane. Consider the back as a leaf spring in this plane. If you anchor the heel, out to the middle of the back, then push down on the toe of the back, it will have a tendency to spring up. If you pull up on the toe, there will be a resulting downward resisting force from the back.

Although the blade is gripped along the entire back, consider only three points, heel, middle, and toe. If the blade is gripped at these three points and the back is sprung so that it is pulling up in the middle and pushing down at the heel and toe, the blade will give to relieve this pressure. The only way the blade can give is to bend along the bottom edge, where the teeth are. This is analogous to pushing on the ends of a thin steel plate, it bends in the middle. If you reverse the spring tension on the back so that it is pushing down in the middle and pulling up on the toe and heel, this will tension the bottom edge of the saw blade and result in a straighter edge. The edge may not be completely straight due to other factors.

A backsaw is very different from a regular handsaw. The backsaw blade is usually much thinner and relies on the back for rigidity. The blade should be flat and straight before being inserted into the back. The back exerts enough control over the blade that the blade will not remain flat and straight if the back forces it into another direction. I am sure that most of these saws came from the factories with flat straight blades, but through years of use and abuse they have been distorted so that some corrections are in order.

I hope this answers your questions.

Best Regards,

Ted

Re: Questions

Todd Hughes

>I agree with Ted on his methods, is about Just how I straighten backsaws.I normaly just tap the back down or up till you get the blade straight and never gave it to much thought as to why it worked. I have had a few saws where I had to take the blade out of the back and clean it but this is pretty rare. I once had a very bad bent Backsaw where I had to take the blade out of the back straighten it like a bench saw and then put it back in the back but I think that was unusual. Most times like Ted says by repositioning the back you can fix the blade.Once you learn how to straighten saw blades you never have to worry about buying a bent saw ....Todd