I get a lot of questions about straightening saws. I’ll be the first one to admit that hammer straightening a bent hand saw is as much art as it is skill. Do I think anyone can learn to do it? Absolutely! But it does take a bit of practice and the ability to get over the fear of permanently ruining a saw (it’s likely that you will ruin several during your education).
Backsaws are a bit easier though. Most backsaws that I’ve worked on have not needed to be hammered. This is because the spine typically prevents the blade of a backsaw from being kinked to the point of needing to be hammered.
A backsaw may seem more complicated to straighten, however, because they typically don’t require hammering, they’re actually easier to straighten than an unbacked saw in my opinion. The trick is to take it one step at a time in order to isolate the reason for the bend and only fix the true cause. In a backsaw the wave could be caused by a few conditions. The blade itself could be bent, the blade could be buckled inside the spine and held by the tension of the spine, the spine itself could be bent, the spine itself could be twisted, the spine could be applying uneven pressure to the sides of the blade, or it could be a combination of all of these things.
I will caution you not to just tap the spine lower on the blade to try and fix the problem. While this can sometimes work, it is a somewhat inelegant bandaid type fix that often results in the spine being knocked all the way down at the toe and pivoting up off of the blade at the heel, giving what is actually a rectangular blade the appearance of a canted blade but lacking the support of the spine along the full length of the blade that a true canted blade would have. See the link for the podcast below if you aren’t exactly sure what I’m talking about.
So with that said, here’s how I approach straightening a backsaw, in this order.
- Remove the handle. Clamp the heel of the saw, right under the spine, very tightly in a metal working/machinist’s vise, with smooth, flat jaws that won’t mar the saw blade. Using a hammer and a scrap block of hardwood as a “drift”, rap the back of the spine (don’t hit the blade) in an attempt to slide it forward on the blade. Sometimes the saw was dropped on its toe, moving the spine rearward or moving the blade rearward. In either case, the drop causes the blade to buckle inside of the spine. Being under tension, the spine will hold the buckle and not allow it to straighten out on its own. “Sliding” the spine forward can stretch things back to straight if this is the case.
- If #1 doesn’t solve the problem, remove the spine. I did a podcast on this if you’re unsure how to do so. With the spine removed, check to see if the blade itself is straight or bent without the tension of the spine. If the blade is not straight, it will need to be hammered straight before reinstalling the spine. If you’re not comfortable doing this, it might be a good time to employ the services of your nearest saw smith. If the blade is straight (or after you have straightened it), check the spine to see if it is straight. If the spine is straight, reinstall the blade per the podcast. If the spine is not straight, it will need to be straightened prior to reinstalling the blade. This is done by a combination of light hammering, pressing in a vise, and hand bending until it looks straight. Once the blade & spine are straight, reinstall the blade into the spine per the podcast. Then go back and do #1 again. Reinstalling the spine can put a small buckle in the blade so it will need “stretching” again per #1. This will also allow you to reposition the spine if you didn’t get it installed exactly in the right spot.
- If #1 and #2 do not make things straight, then the spine is either twisted or pinching the sides of the blade unevenly (or a combination of both). The fix for both of these issues is the same, so it’s really not necessary to identify the exact culprit. To fix these issues, I put the assembled spine/blade back into the metalworking vise, held by the toe of the spine, with the teeth facing up. Sight down the toothline and see where the wave is. Using a pair of adjustable wrenches, you need to gently apply twisting pressure to the spine opposite the direction of the wave, to line things back up straight again. This is easily over done, so go easy. You will probably have to apply the opposing twists at various places along the spine with the wrenches held at different points and varying separations between them in order to straighten things back up. There’s no real science to it. It’s kind of a feel and eye thing. Just take your time and go easy with the twisting and you’ll be fine. It will quickly become apparent where you need to place the wrenches along the spine and where you need to twist in order to even out the pressure. Just make sure to always use two wrenches to do the twisting. The vise is to hold things only. Don’t twist with one wrench against the pressure of the vise. Always use two wrenches lightly twisting in opposing directions.
Follow those three steps and you should be able to get things all straightened out. Just take it slow and don’t force things. If the metal in the blade gets stretched too much, it can’t be repaired, so it’s better to go slow and take longer to get things straightened out than to try and do it quickly by hammering or twisting harder. That almost always results in making things worse.
Once I get my computer issues sorted out, I do plan to do a podcast on this. Until I can do so, hopefully this post will help.