From: Stephen LaMantia
To: Multiple recipients of list <ools@
Subject: Re: yet more saw sharpening...
Tim Swihart wrote:
It's part of the "Handbook for Lumbermen" from Disston (published about 1902...when this stuff made a big difference in a worker's daily life).
I've got a copy of something similar; probably a later version of the same publication: the Disston _Lumberman Handbook_, dated April 1919.
I got mine from the EAIA book list
Mine's a copy of something Mike Davies found tucked away in the bowels of the St. Louis public library.
Their explanation runs from page 129 to page 145. It covers the differences between a rip and a crosscut (best explanation I've seen and I've seen a bunch), ...
They did explain this quite well. I've seen it explained well in other works, too -- after all, operationally the mechanism of cutting in either case, rip or cutoff, isn't all that complicated and geometrically it's a function of only two angles, the rake angle (pitch) and the bevel (or fleam) angle. Still, the clarity of the Disston explanation puts it up there with the best.
... and most of the other articles I've read are actually teaching how to prep a saw for crosscutting softwood without bothering to point out that you'd get better performance if you change the fleam (bevel) on the BACK of the tooth.
Now *this* is something I'd wish Disston had been clearer on. I also have a copy of an old Disston catalog, called the _Disston Saw, Tool, and File Book_, published in 1921. Both this catalog and the 1919 _Lumberman Handbook_ discuss this notion, so they apparently felt it was quite important. (In talking about putting less bevel on the back of the teeth, the 1921 catalog is quite explicit about the importance: "The proper amount of bevel to give the teeth is very important, for if there is too much bevel the points will score so deeply that the fibres severed form the main body will not crumble out as cut, but must be removed by continued rasping.")
Oddly, though, for something they deemed so important, they sure didn't take too much care to elaborate. The 1921 description has less detail than the 1919 LH (i.e. the _Lumberman Handbook_) description, but I find even the 1919 LH account dearly lacking in any good detail.
Specifically, I wish Disston had explained how to *achieve* this smaller tooth-back bevel when filing. Still, my own deduction is that without calling them such, what they're describing is an effect of filing "sloping gullets". (I'll explain in a moment how I reached that conclusion.)
Here's my thinking on this. First of all, an obvious question: What difference does the angle of the fleam on the *back* of a tooth make? The answer is that, in and of itself, it makes no difference at all, actually. How could it? It's on the *back* of the tooth.
But it *does* make a difference in the *cross-thickness* angle of the tooth. (I guess you could also call this the "across-the-toothtop angle". The terminology to describe the elements of saw-tooth geometry seem to be less than thorough, I'm afraid; sorry to be struggling to make up my own terms.)
This across-the-toothtop angle is the angle that you'd see looking straight into the toothline of the saw, peering straight down the length of the saw, as though the teeth were all coming right at you; it's the angle of the edge that goes from the *point* of the tooth on one side of the blade across and down to the top of the smaller triangle formed on the other side of the blade. On alternate teeth this across-the-top angle slopes downward to the left, then to the right, then to the left, then to the right, and so on.
When using a crosscut saw, the points of the teeth are what score the parallel cuts that define the borders of the kerf. But it's the edge across the top of the tooth -- formed by the intersection between the front and back surface of the tooth -- that then crumbles out the little pieces of severed wood fibers between these parallel cuts. If this edge runs too steeply from one side of the tooth to the other, then the points score the wood very deeply but the fibers are rasped upon rather than being easily popped out. According to Disston, this is suitable for softwoods or for where fast work is desired and a messy cut is tolerable. (I have to admit here that I'm a little bit fuzzy here on the exact complete reason behind this effect, though.)
Conversely, if the edge across the top of the tooth isn't so steep, then the points don't score as deeply, so the fibers crumble out more cleanly although more slowly. Again according to Disston, this configuration is suitable to hardwoods or where a cleaner cut is desired.
So, for finer work you want the slope of the edge across the top of the teeth to be gentler. And even without understanding how the befuddling three-dimensional geometry of it all works out, to go with Disston if you want a shallower cross-tooth slope you should then put less fleam on the back of the tooth. In other words, less fleam on the back of a tooth causes the slope of the cross-tooth edge to be less.
Okay, let's accept that prescription. But *how* can you, the person standing at the saw with the file in your hand, do that? After all, if you adjust the angle of the saw file so that it's more perpendicular with the side of the saw -- and hence giving less of a fleam angle to the back of the tooth -- wouldn't that also affect the fleam angle being filed onto the *front* of the adjacent tooth, as well?
This isn't a trick question. The answer's simple and straight- forward: yes, it would.
So what's the technique to use? Well, for some strange reason, Disston doesn't explain. Not in words. But a detail in the three illustrations provided in the _Lumberman Handbook_ gives just enough of a clue, in my opinion, to reveal how to achieve a smaller fleam on the back of a tooth yet keep the fleam on the front of the tooth unaltered.
The three illustrations, each respectively showing less and less bevel on the tooth back, are simple line drawings with a skosh of old-time pattern-shading added. Each drawing shows a saw blade face- on, with the saw file resting in a gullet. In each drawing, the handle of the file is angled to the right, and there is a shadow of the file cast across the width of the saw blade, downward and to the left. The angle of this shadow would indicate a light source above and to the right of the file. It's easy to overlook these shadows, because they could be seen to be simply a "decoration" added by the illustrator just to add a feel of three-dimensionality to the pictures, the same way some video games nowadays are paying great attention to shading and shadows in order to make the images more realistic. (Anyone seen Sega Saturn or Sony Playstation yet? Real scary.)
However, there's a difference between the three file shadows in the respective drawings. The angle in which they cast down across the sawblade is closer to vertical in the second drawing than in the first, and even more vertical in the third than in the second. Assuming the illustrator wasn't just being sloppy, the only way this could occur would be if the file handle was being tipped more downward toward the floor in each subsequent drawing. Voila, sloping gullets!
A sloping gullet is where the bottom angle of the gullet doesn't pass perpendicularly across the thickness of the saw, but instead it ... well, it slopes. The bottom of the gullet is higher on one side of the saw than on the other.
I've seen them mentioned by only two saw-sharpening writers. One is Harold Payson, and he is in fact where I get the name "sloping gullet" from. Payson wrote the saw-sharpening article in FWW on Bench Tools, which was a reprint from the January 1988 FWW (#68). He's also written a saw-sharpening book, _Setting and Sharpening Hand and Power Saws_, one of the more detailed I've seen. (I've heard he's also written an article in Fine Homebuilding, although I've not seen that.) According to Payson, the purpose behind sloping gullets is that they add extra volume to the gullets, which allows them to chamber more sawdust out of the cut. As far as I remember, this is the *only* reason he cites.
The other saw-sharpening writer I've seen that mentioned sloping gullets, although not by that name (or by *any* name, for that matter), is Leonard Lee in his sharpening book. That's a great book overall, but to me, Lee is pretty puzzling -- no, make that *downright mystifying* -- on sloping gullets. He mentions that by sloping your gullets by the same angle you're using for tooth bevel, you can retain a gullet angle of 60 degrees (counteracting the effect that a non-zero tooth bevel angle would have in making the gullet angles greater than 60). Yet, he doesn't offer any explanation at all as to *why* you'd prefer to keep the gullet angle at 60. In fact, he then goes on to mention that by letting the gullet angle go higher than 60, you actually can strengthen the tooth, because by the rule of supplementary angles this also increases the included angle of the tooth tip. Man! So Lee explains how to do something, but doesn't mention any reason why you should, but does mention a reason why you shouldn't. Damn it, why'd he even bother bringing it up in the first place, then? I don't know; it beats the hell out of *me*.
Oops, okay, I'll settle down now. Didn't see that one coming. Anyway, my point was just that Lee *mentions* sloping gullets, not that he says anything remotely useful about them.
So, just those two guys. And in a "secret" sort of way, but with a shadowy clue given, in the Disston _Lumberman Handbook_.
What's the conclusion of all this? (It must surely be over 140 lines by now.) Well, I don't know. It's 2am now, and if I ever had a point to all of the above, I've forgotten it by now.
I wish the same effort that's given to describing how a plane iron cuts -- i.e., the phenomenon of chip formation and wood deformation as described in Lee and Hoadley, for example -- would also be given to describing the underlying first-principle mechanisms of how a saw cuts. The "saw-science" literature does take things to a certain level of detail, but it seems to stop too short of giving a full understanding. While prescriptive, it isn't fully explanatory. Sadly, it sometimes seems to me that while the techniques are being passed along, the underpinnings of rationale are not, and we have to keep going off into ancient texts praying for clues.
Okay, enough serious.
They also include a humorous (I was amused) bit on badly filed saws...mine came out FAR better than these, but I've seen some that came close to what they're griping about. :-)
In both the 1921 catalog and the LH it looks like somebody at Disston took wicked and wonderful delight in showing caricatures of badly- filed saws. You're right, Tim, someone there had an excellent sense of humor. I laughed out loud when under a drawing of saw teeth of varying length, gullet depth, and angles randomly going every which way like the Grand Tetons painted by a drunk Picasso, I read, "Fig. 13 is a representation of some of the saws we have seen; there are entirely too many such now in use;, and we have no doubt their owners are shortening their lives in the use of them as well as those of the saws." (Try to write something like that in a manual today, though, and someone will probably sue you for traumatizing them.)
My opinions are my own.
My opinions are long. ;-)
Okay, let's get *ALL* you saw-sharpening folks into this. There's still a whole bunch of demystifying that needs to be done.
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