How to Retension a Traditional Backsaw
Have you ever picked up a great-looking Disston No. 4 on eBay, only to discover upon taking delivery that the plate was bowed, or presented an S-curve along the toothline? Or perhaps the plate was jammed all the way up into the toe end of the sawback?
The real question is, can you make all these fixes yourself easily, and with a minimum of tools? Absolutely, you can. This article will show you how to stiffen an out-of-shape toothline, because all of the scenarios above (with the exception of sharpening) share back problems. Not of the sacroiliac variety, but because of unequal clamping tension the traditional folded sawback exerts along the spine of the sawplate.
So follow these simple procedures as you scroll down, and I’ll show you how to slap your vintage backsaw back into shape like a drill sergeant snapping a buck private to attention.
Understand the purpose of the traditional folded sawback, and how it works: The purpose of a sawback in general is to stiffen the thin metal sawplate of a backsaw, which is significantly thinner than a handsaw plate. As such, backsaw technology has remained essentially unchanged over the centuries until very recently.
Today, many sawmakers mill a slot in brass bar stock and epoxy and/or pin the plate in place, usually with some added compression to lock it in for good. While this is a perfectly serviceable method that keeps production costs down, you will see in the paragraphs ahead why the traditional folded back remains a superior design, extending the life of the saw and enabling the sawyer to retension the plate at will.
Historically, traditional backsaws were made with folded brass backs, until Henry Disston began fabricating folded backs out of carbon steel in the mid-1800's. Why steel? It's been my experience here at Bad Axe whether working on vintage saws or building new saws that steel is more resilient than brass, and takes a corrosion-resistant finish. And in keeping with Disston and other 19th Century American sawmakers, we like protecting our steel backs with a firearms-grade finish.
Mechanically-speaking, a folded sawback is like a long binder clip, designed to clamp the spine of a sawplate from heel to toe quite tightly, yet able to slip under duress, and this is the meat of the discussion. Why is this important?
Think of the folded back as a dynamic safety feature, akin to how a mountaineering rope dynamically stretches when a climber falls, rather than jerking his guts out if using a static rope. So it follows that if you drop the saw, or torque it through inexpert saw technique, better to have the plate slip a little inside the folded back to avoid kinking the plate.This ability to shift under duress also allows one to use a dead-blow mallet to equalize the clamping tension along the spine, thus keeping the plate straight and true, even many decades after the saw's fabrication. Here's how to do exactly that:
Conceptually, what you’re going to do is LIGHTLY tap the toe end of the back, followed by another light tap on the heel end of the back with a dead-blow mallet. These alternating taps promote a teeter-totter effect as the back slips up and down at the toe and heel end of the plates. To do this, just use the following procedure:
• Hold the saw upright on a jointed flat surface (a jointed board that won’t rock on your workbench works just fine).
• With your left hand, tightly pinch the bottom edge of the back where the plate meets it about 3” behind the toe end of the saw. It’s important to feel both components simultaneously where you're pinching them with thumb and forefinger, so you can feel the back shift at the spine as you tap it.
• Using a dead-blow mallet, LIGHTLY tap the toe ahead of your hand. You should feel the back shift minutely where it meets the sawplate (hence, the reason you’re pinching it with thumb and forefinger). It should only shift by about 1/32—any more than that is overkill, so easy does it.
- (NOTE: If you're working with a really old saw, where patina (rust) has locked the back securely in place, consider squirting some WD40 or Break Free along the underside of the back, and let it sit overnight.)
• Lightly tap the heel end of the back. Again, you should feel the back shift downward onto the plate about 1/32” where you’re pinching.
• Upend the saw, and site down the toothline; determine whether the S-curve or bow still exists. Chances are, it has straightened out and gone away. If a bit of a bow remains, then the sawback might well be bent—best to let a Saw Doc straighten that out.
• If much of the S-curve has straightened out, but now you have a slight wiggle toward the heel end of the toothline, then that means you probably tapped the heel too hard; what has happened is that the back has torqued pressure in the area where the fasteners bind the handle to the plate, and that pressure can be relieved with one final, very light tap on the toe end—again, with just enough force to feel the plate/back assembly shift downward ever so slightly.
But what if you tapped the sawback in a far more robust fashion than what was warranted? Or—perhaps you have a vintage saw where the toe end of the sawback is crammed down onto the spine of the plate as far as it can go. How do you equalize a consistent depth of plate under the back in accordance with original factory specifications?
First of all, understand that traditional folded sawbacks are intended to pinch not more than only 3/16 to a ¼” of real estate along the spine, though the sawback may be as much as an inch tall. The sawback provides mass—and was never intended to have the spine of the plate abutted against the inside of the fold—otherwise, how could you adjust the clamping tension?
But your sawback is crammed all the way down, a common site when checking out vintage saws on eBay--the severe cant at the toe was not an intentional design created by the sawmaker. The cant exists because the saw was dropped. This is an easy fix too.
See how I’ve secured the plate/back assembly in some leather-lined angle iron, which is in turn clamped by my vise? Note how I also leave about 1/8” of space at the toe end of the sawback (the heel end rests positively onto the surface of the angle iron). This allows you to to slide your crowbar (with a scrap of leather in the fork) underneath the tip of the sawback. Push in until you can’t go any farther, and the leading edge of the sawplate is inside the fork. Then, gently push the crowbar upward, until the toe end of the sawback rises while the sawplate remains stationary in the angle iron. Again, don’t overdo it, or else you’ll deform the underside of the sawback.
Reposition the plate and back assembly in the angle iron, until once again the toe end of the sawback is about an eighth of an inch up from the surface with the heel end seated. Lift the toe end of the back some more with your crowbar. You may need to repeat this procedure again. Unless you have a severely canted plate with the back shoved down onto the plate as far as it can go, all it should take with this procedure is about two maneuvers to equalize the plate/back assembly back to slightly above its original factory setting.
But what if you hit your sawback too hard on the heel end, and now there’s no way you can reattach your handle, because the back disallows clearance to pass the fasteners back through the holes in the plate? Yet another easy fix. Just release your vise again and reverse the plate/back assembly in your angle iron. As you have done already at the toe end, this time leave 1/8” clearance beneath the heel end of the back, while the toe end rests positively on the angle iron. The sawplate will disallow the fork of your crowbar to gain a purchase, so this time use a stout screwdriver to engage the underside of your sawback, and pry it up. Easy does it here.
Reattach your handle before closing up the remaining air-gap where the underside of the sawback seats on the floor of the handle mortise receiving the plate/back assembly. Use a ruler while making a judgment call on attaining equalized plate depth below the back, based on the final seating of the back in the mortise. If you want a cant along the plate, simply tap the toe end of the back about 1/8" more into the fold.
These procedures are simple and direct—and go a long way in demystifying how to keep your vintage or traditionally-made backsaw straight and true. It is a simple tool meant to be disassembled, cleaned,retensioned and tuned up in this fashion, akin to various methods you’d employ to clean and tune up a hand plane.In closing, the traditional backsaw has stayed the course over the centuries with 400-year-old technology that simply works. Though it clamps onto the sawplate with an iron grip, the traditional, folded sawback can still slip under duress, a dynamic advantage that allows one to easily retension the plate to straighten out kinks or bows as they develop over time through accidents or abuse. Cheers! ~ Mark