Yeah, I know. My mind jumps from thought to thought lke a frog, from lilly pad to lilly pad. Forever questioning conventional thoughts, while struggling to keep the OCD balanced with the ADD. :)
So I will post a few random thoughts from time to time. But beware. I am not a "Blogger". Or a "Writer" in the least.
Just some thoughts about saw spines.
Originally written in 2014/2015
I was making a reproduction Richardson Bros. Tenon saw some time back. You know. The type with the saw screw going through the spine and handle that often split along the grain. But I had made a new plate for the original and noticed how difficult it was to "re tension" a saw plate and still maintain the alignment of the spine to the screw that needed to go through the wood handle and spine. It occurred to me this was, in fact, a flaw in design. A slotted back, being made to seat all the way down and slotted straight, would have been better. But they didn't have those then. The folded backs, which may lose some of their compression force over time tend to allow the plate to move up and down as well as back and forth under careless use and dropping. Or rather the back can be bumped up and down, etc.. This can and does create a moment about the axis of the screw in that Richardson saw and ultimately causes a stress crack in the wooden handle at that point. Cantilever effect on the screw and wood. Despite marketing hype, I do not see this as an advantage from the stand point of the physics involved. But rather a disadvantage, or flaw in design. Otherwise, we would never have to " re tension" a saw plate and we would not have saw plates become distorted/ ruined from too much friction and kinking. (under setting, overzealous or careless users, etc. ) Obviously a saw back that does not have the proper amount of spring force is more likely to allow this to occur. Unfortunately, folded backs have been the standard from the times of hammered backs. Mostly due to the universal fit of the backs on various thicknesses of saw plates. ( One size nearly fits all ) I have personally taken a steel folded back off of a 0.042” thick plate and tensioned it onto a 0.015” plate. The back worked but not as well as one meant for that thin of steel. I should note one should be careful not to compare a slotted brass back to a folded Steel back. It is a bit like apples to oranges. A properly made steel folded back may or may not, be heat treated and tempered to have a better spring tension. Unfortunately, folded backs depend entirely on the spring force located at the top of the back. ( That arched part ) Mr. Diston understood the folded back, but I doubt Henry Disston had the technology to make slotted backs with a high level of accuracy on such a massive scale as well. He did, however, also have the knowledge and understanding of the importance of the ratio between plate thickness and plate height. Something the "Thin is in" crowd pushing the thin plate saws seem to have forgotten about. Let's not forget. Steel is sold by the pound. Thin steel is less steel and less cost to make a saw. :) But I digress.
Unlike others, however, I am NOT saying one type of back is superior to the other. But rather to point out there are pros
and cons to all types of saw backs. Folded, slotted, laminated, or molded.
Since saw backs, folded, slotted, or laminated do very little to anything at all for the strength of the saw plate vertically, this leaves the back/ spine of the saw to do nothing more than maintain the saw plate straight, horizontally. (To "tension" the plate). As eluded to earlier, the strength of the saw plate vertically is primarily derived from the thickness of the plate and handle.
In reality, the point of contact of a folded back is a small line running along the horizontal axis of the plate. Perhaps
1/16th" or less. Dependent on the maker. Not all folded backs were/ are made equal with the same exact shape or thickness of steel. So they function more like a two sided clip board clip than
anything else. If you place a wrinkled piece of paper in a clip board it still has wrinkles in it unless you remove them by pushing the wrinkles out and allowing the compression of the clip on the
clip board to then hold the paper flat. Those wrinkles radiate outward, like frozen ripples in a pond from a rock, to the rest of the paper/ plate. (Tin can effect ) So we play with the folded back
tapping it front then back alternatively to "push" out the wrinkles ( equalizing the external forces applied by the compression of the back ), leaving the paper clip ( folded back) to keep it
straight at that point. Like those clip board clips fastened together or a spring clothes pin.
A slotted back, on the other hand, has more contact with the plate, up to 3/8", at least on my saws. It affects the alignment if not slotted correctly, but does not play a significant role in vertical stability. Only alignment. Unfortunately everything on a slotted back saw has to line up near perfectly or the plate will have that tale tell curve in it. Assuming the handle was slotted straight in the first place. This is, in part, due to the increased point of contact between the plate and spine. A down side to slotted backs. They do not, however, create a tin can effect from unequal lateral forces being applied to the plate unless you hammer them or compress them unevenly. Also, slotted backs do not have the universal fit of a folded back and require being slotted for a particular thickness plate. Another reason folded backs are more economical. Not to mention brass is more expensive than steel. So no clip board affect. Slotted backs are required to be glued, compressed, or pinned to remain in place and prevent the plate from distorting under the force of use etc..
So now that I have stated much that you undoubtedly already knew, more to the point. What causes the plate to be pushed back in
the first place?
The simple answer is the compression force being applied to the toe end of the plate during the forward stroke. The same reason hand saw plates kink with a careless user being too aggressive at the beginning of the stroke. But hand saws, being thicker of course, do not have backs to prevent the kinking. So now we are back to square one. Hmmmmm.
Another question... Why do Japanese saws not kink? Easy answer is because they are always under tension in use. The point of contact between the saw and the handle is a mechanical bond. Not glued, compressed, or pinched.
On every western saw spine/ back, again, folded, slotted, etc. There is a certain amount of compression force being exerted on the toe end of the plate, initially. The plate, not having any real mechanical means of fastening to the more rigid spine, can and will eventually "give" being pushed backwards along that horizontal axis until it reaches a point where that force no longer exceeds the resistant forces of the spine compression. This can and will eventually distort the plate, unless it is, again "Re-tensioned"
As the stroke of the western saw is advanced forward, the point of compression shifts toward the heel of the plate with
the stroke, placing the area toward the toe in tension. A see saw, sort of speak. But if the toe end does not spring back completely to its original position, as at least one maker has suggested
happens ,again, the plate will eventually deform and the heat of friction will continue to deform the plate to a point of being unusable. Nothing more than the heat of friction and simple
So in my naive and intuitive way I was simply wondering, why we don't create a mechanical fastening system at the toe end of the plate/ spine to prevent the plate from slipping in the first place. The spine is, after all, just a horizontal continuation of the saw handle. Is it not? We do not need to take out the plate from the spine to sharpen the teeth and once the plate is wore out, (not in our life time most likely ), we simply need to remove the set screw, or if we use a more permanent pin, drill out the pin in the toe end and replace it when the new plate is set into place. Or perhaps just use a "Chicago Screw" as the pin, but aesthetics is important to some and this requires more time. A folded back, unfortunately does not always tension a plate in the exact same spot, so is not suitable for the set screw. It is left to depend entirely on the force of friction to stay in place. This same principle can be applied to a Richardson Brothers saw. You know, the one with the screw going through the wood and spine. Using a slotted back we could simply pin the toe end of the plate to the spine. The heel of the spine is already pinned. Or one could add a set screw in the plate/ spine inside of the handle mortise unseen. Basically creating a zero force system. The same system that has been used in building trestle bridges, buildings etc. for hundreds of years with the forces canceling each other out.
So I did indeed place a set screw into the toe end of the spine/ plate of the Richardson Brothers sash saw, pinning the plate both heel and toe in a compressed spine. The end result?
I have played with this saw cutting tenons and waiting to see if there was any distortion of the plate, etc. Nope, nada, zero. The plate remained straight and I don't have to "re tension " the plate or worry about the cantilever effect of the spine coming loose at the toe of the plate causing the handle to be cracked.
My final take/ opinion on this little experiment is as follows:
Ultimately it comes down to the saw maker and the personal preference of the user and maker. I hoped to offer both at some point. At least that was the plan. I currently have folded backs I have made in my own designed presses. These backs are made of 260 brass, copper, mild 1018 steel, 4130 alloy (preferred ), 316 stainless, ( preferred ), and 304 stainless. I have had some failures, and some success. It has been an in shop trial and error learning process. To what end???? Who knows?