Inserting the Blade Into the Back
Here is the meat of most of what is needed to make back saws - and what I found the least information on. With hindsight - I guess that could be because it really is a very simple concept, and I was thinking entirely too hard about it. That happens to me a lot...
In any case, I think it will interest some if I explain some of what I did to arrive at my final, chosen method. It might not, either....
Different Methods to Choose From
These makers mostly have one thing in common - they make the backs for their saws by milling a slot out of a single chunk of brass using a slitting saw in a milling machine of some sort. Most epoxy their blades into place, and some even pin together separate pieces of brass to make the back.
Digging out my out my old back saws - I confirmed this was the way they had done it:
Top row - a 10" Jackson, then a 12" and a 10" Disston. The bottom row is two "Warranted Superior" nameless saws. They are all steel-backed saws, not brass - but all are folded over the blade, not machined. My guess is that you'll find most 100 year old back saws you find will be of the folded variety. I've known others who have repaired bent old backsaws by removing the spine to straighten it - so it stands to reason none of them glued or pinned into place.
Even having decided on which basic philosophy to follow, there were still questions on how to proceed. Looking at my back saws - I could see differences with how they were done. Some were done by pinching the blade by forming the back into an "ankh" or elliptical shape, such as with the 2nd Disston above, or as I show in the first diagram below (slightly exaggerated for clarity):
The second method , shown at right, is to flatten the entire back over the blade. There was also the question of whether to flatten the back over the blade, or to flatten the back first and insert the blade into it. While the real reason for the back is to stiffen the blade so it stays straight - there's no reason that either wouldn't work, so long as it adhered itself to the blade well.
" 1. File a small chamfer on the inside edges of the back-to-be. As in, the two edges that will be closest to the saw blade.
2. Bend the back
3. Crimp it shut sans blade. As in all the way shut, as in gee, there's
no gap left.
4. Mount blade in saw vise with ~3/4" of blade exposed.
5. Tap back onto exposed 3/4" of blade (this is where that chamfer from
step #1 comes into play) using small ball pein hammer."
"We're talking about a *gentle* installation of the back onto the blade. I used a 4 oz ball pein and couldn't see any damage when the blade was installed. Furthermore, I filed the top of my back square after the installation. As far as the "pry open" concern, I found that when the crimped back was forced open 0.020-0.040" (the blade thickness) it served more as a spring to hold the blade. My assumption here is that the bending of the back cold works the brass, thus increasing its stiffness and strength. Prying the back open a mere 0.020" sets up a residual stress that serves to hold the blade more tightly. If you do it this way try removing the back along with a back that you hammered onto a blade for comparison. I found it was noticeably more difficult to remove the "pre-crimped" back. Give it a shot."
I honestly think that without being able to be right there beside each other to see exactly what the other was up to, variations in how each of us did it could have produced different results. I think that's important to point out here - if you, the reader, think that there must be better ways - you could well be correct - so experiment. But, since I'm writing this, and you're not -
I found I had more success with hammering the back onto the blade, and then hammering it as flat as possible. At first, using too light of a hammer combined with my inexperience led me to hammer the brass too hard, leaving hammer marks that were difficult - nay impossible - to get out completely without thinning the back too much. A 4 lb. hammer, bought at Harbor Freight for about $6 on sale, did the trick much better. An anvil completed the package, giving me something with some weight to work the metal against:
I had to be very careful that I brought the head of the hammer straight down on the brass, centering it right at the top of the back at the center of the radius of the curve, just like you see in the photo above. The mass of the large hammer made this easier, allowing me to lift it only about 4" or so above the brass, and bring it straight down. I worked the entire length of the back without the blade installed at about one inch increments, from front to back, flipping it over working the other side - then starting over again until the back just starts to close up, like in the photo below:
Notice the opening is straight for it's entire length... If you aren't careful, its easy to introduce a bend into the brass that can be difficult at best to remove. It's best to try and minimize this by working slowly, keeping the bend as uniform as possible.
Next, I place the blade into my vise (with some plywood cauls to help prevent marring the sides of the bade), and slide the back onto the blade. It's tight enough that it requires a small hammer to tap it all the way home (the teeth in the blade shown were just left over from the saw that I got it from so please ignore them - I later removed these teeth to file new ones, much finer):
Which reminds me - it's best to have done the majority of the cleaning of the blade before you get to this stage, as it will be more difficult to clean the blade with the back installed. Did I mention that before? Must be important...