When to retension a brass saw back?

[Joseph Thomas]

I recently purchased an old 10" tenon saw with a folded brass back. In most respects it's in great shape, but the sawback has slipped 3/8" down the front of the plate so that it's no longer parallel to the teeth. It appears to be in the correct position at the heel of the saw, clamping the top 1/4" of the plate.

I came across this article describing a technique for correcting the problem with a crowbar and vise. Has anyone tried this technique? Did it work well? I'm undecided about trying it because the saw plate appears to be quite straight ("if it isn't broken, don't fix it...") and will probably work as-is. On the other hand, not having tuned up one of these older saws before, I'm not sure if there are benefits to having the back properly aligned (maybe it's not actually a very risky fix?). Any advice would be much appreciated.

[Mike Allen1010]

Sorry I don’t have a good answer about “fixing“ the issue you described. However, as long as the salt plate is solidly attached to the saw back with no movement, I think you’re right to say the first principle is “if it ain’t broke don’t fix it“.

In my experience it’s not uncommon for the width of the Saw plate below the back to be narrower at the toe then at the heel. I believe some vintage saw maker/woodworkers preferred this set up. I believe some folks would say this might have some advantages in giving better invisibility to start saw on the layout line, although I’m not exactly sure how that would work. Perhaps Ron Bontz or Pete Taran will comment.

Cheers, Mike

[Warren Mickley]

Just 150 years ago back saws were routinely made with a taper. This gives more room at the heel for the handle and is firmer at the toe than a straight saw.

[Rob Paul]

I have "quite a few" old backsaws, and often move the back to re-align with the toothline, or remove the back entirely in order to clean the rust off the plate. I just clamp the blade in the vise, and use a hardwood punch and hammer to move the back. Usually they move quite easily, and the saw looks much better with the back aligned. Wavy blades can often be straightened by adjusting the back, and I have never made blade straightness worse by my adjustments. (Tapered plates seem rare. I haven't yet found an obviously tapered blade, even on my early 1800s backsaws, although I have seen pictures of tapered plates)

[Ron Bontz]

Just 150 years ago back saws were routinely made with a taper. This gives more room at the heel for the handle and is firmer at the toe than a straight saw.

+1 to what Warren said, although most of the vintage saws I have seen were not actually canted on purpose. The backs were simply knocked down in the front, purposely or accidentally, giving the appearance of being canted.
A) The taller the saw plate, the more flex is possible, thickness dependent. ( As Warren mentioned ) The appearance of stiffness of the plate is dependent on the ratio of height to thickness. The handle helps to some extent.
B) Saw backs of any kind have the sole purpose of keeping the plate/ tooth line straight/ ridged.
C) A folded brass back may not have as much spring force as a steel folded back. The less spring force the easier the plate can deform due to compression forces from use.
D) Personally, If the tooth line is straight, I would just leave it that way, unless there is some aesthetic reason you want to change it.
Best wishes.