Sloped Gullets: The Finer Points of Sharpening
I find it hard, even after many years, to carry on a conversation about "sloping gullets" with at least a little chuckle. Not at how it refers to a certain method of sharpening a hand saw mind you - its more that every time I hear the phrase "sloping gullet", I can't help but think of some sort of deformed fish...
That, and when researching the origins of this icthyological pursuit, I ran across a reference to a photo of a WWII era front-line French infantry "installation" (actually a shack the infantrymen had set up as a bar) called "L'Auberge des Gosiers en Pente" - or "The Inn of the Sloping Gullets" - that is to say, always thirsty... (from "The French in love and war: popular culture in the era of the World Wars" By Charles Rearick)
When sharpening a saw, there are several angles you are concerned with. The terms associated with these angles which are most important to this conversation include rake, fleam, and of course - slope (as shown in the graphic above -you can click on any of the images to see a larger, clearer version). I'll try not to go into too heavy technical detail on saw sharpening as that's another subject, and it's been well covered by others... as well as myself.
What I would like to do is discuss just what sloping gullets can - and cannot - do for you. I've seen several discussions on the web that indicate there is some confusion as just how it is they work....
First, I'd like to dispel a myth, one that inspired this writing - and that is that you can have sloping gullets on a rip-filed saw. That is simply not the case. Here is a saw (pictured at right) that was presented to me, the sender claiming it was filed rip with sloping gullets:
Looking closely at the teeth you'll see they really are not filed rip at all - the effect of sloping gullets has essentially made them all filed to what I would call an aggressive crosscut pattern - and they are no longer "rip" teeth.
Ok, perhaps a quick review is in order of just what the term "rip" filed means.
Looking at a comparison of rip and cross-cut filed teeth on the left, you can see that the rip teeth are flat across the top (to chisel through the wood with the grain), vs. the crosscut teeth that all come to a point (to slice through the wood at a perpendicular angle to the grain). If you look at the photo of the saw above, it's easy to see that the teeth resemble the crosscut profile at left far more than the rip profile.
So, how is that then? It's because the act of filing using a slope on the file files a point on the teeth. Think about it - the upward angle of the file will naturally sharpen the tooth to a point... To say it another way , the very act of sloping the file while sharpening is to file the saw to a crosscut profile.
To be fair, there are some instances where you do put a slight slope on rip teeth - but it's 5 degrees or less, and then you get what my dad would have called a "modified" rip profile, which is one that you use where you have some wild grain... but I'm getting ahead of myself, I'll touch more on that in a later post.
Also - for purposes of clarity - fleam (for the purposed of this post) is the angle produced on the tooth of the saw and can be produced by angling the file either to the left or right, or by sloping the file up or down (sloped gullets). Just so I keep them straight, when I refer to fleam (or flat fleam) in this conversation I will only be referring to the angling of the file (while sharpening) from left to right; I will use the term slope when referring to the angle of the file in the up/down direction. In reality, it is all fleam in the end... or should I say sawdust in the wind?
The exact origins of the sloped gullet are lost to history, though perhaps we can deduce a few things about it's beginnings. The term "sloped gullets" itself I think is a more recent moniker, having been coined - or at least more commonly used - in the last several years than previously.
It's my belief that the sloped gullet was the first iteration of the cross-cut saw, as many (very - like 1700's or so and older) if not most old saws were commonly filed rip... some experimenting sawyer back in the day discovered that the addition of an angle - or slope - to the filing made saws perform better when sawing across the grain. Further experimentation - in my opinion - then led to the development of fleam and eventually the crosscut saw.
In "The Art of Saw-Filing: Scientifically Treated and Explained on Philosophical Principles" By Henry Wells Holly, first published in 1864 or so, sharpening saws is gone into great detail, though exact rake angles are not discussed. The illustration at left (illustrations from this book were later used by the Disston company in their reference manuals) clearly shows an approach that uses a sloped gullet in combination with an angle to the file to produce fleam. So the practice was well established by that time.
Yet not twenty years later (1880) Robert Grimshaw's seminal treatise on saws, Saws: The History, Development, Action, Classification, and Comparisons of Saws of All Kinds (aka Grimshaw on Saws) doesn't show or refer to a sloped gullet at all when referencing hand saws of the kind we are discussing; some slope is obvious in the diagrams of pruning saws and logging saws, so he was obviously aware of it . But none when referring to a hand saw destined for dried hardwood.
Now granted, Grimshaw's book tends more towards the industrial than Holly's, and therein may lay our explanation of what happened in those twenty years. Industrialization..
Saw-making had grown along with the industrial revolution, and benefitted greatly at the hands of the very revolution that would one day spell it's near doom. One of the advents of this was the development of saw filers - and they don't lend themselves well to sloping gullets My hypothesis is it's possible that the flat-angle fleam we all know and love grew out of this mechanization.
On the retail end, while most saws came flat fleamed, there were a few higher end saws back in the day that were sharpened with slope. A case in point woul be the Disston Acme line of saws - that came filed from the factory with factory with sloped gullets. In the case of the acme saws, they also came without set... The Disston Acme saws were a special case, in my opinion. First, he steel was extra hard, so did not lend itself to being set. But because it was extra hard, the steel could hold a sharper edge... Next the plate was heavily tapered to make up the difference for having no set. The extra depth of the sloped gullet could only help. With a regular saw, the set of the saw would make up the difference easily. So to me, the Acme was a saw specially made that benefitted from the sloped filing, and outside of the scope of this writing - which I'm aiming more towards the regular user with a regular saw. The great majority of saws sold after 1880 or so were filed with a flat fleam from the factory.
So - what are the effective differences between a sloped gullet and a flat-filed fleam? On the top of the illustration at left is a fleam-filed saw. The bottom illustrates a sloped-gullet saw filing. The most noticeable difference is the depth of the vally on the side being filed - it's obviously deeper on the sloped saw.
As far as the flat-fleam filing goes, once you reach a certain angle it starts to become exponentially more difficult to sharpen simply because of the angle of the file in relation to the saw plate. Adding slope allows you compensate for that difficulty and achieve a greater fleam angle when used in concert with a standard fleam angle attack.
Because of it's angle, an additional benefit to the sloping gullet is that it also makes that slightly larger valley between teeth to carry and the theory is this will evacuate more sawdust out of the cut.
The end result is that it allows for a sharper tooth that cuts more cleanly and one that runs cleaner because of how it can handle sawdust more efficiently. At least in theory, anyway...
In my own experience - the people I've known that used logging saws almost always used sloped gullets. Likely - I think - it's due to their experiences sharpening their larger saws. Larger toothed saws seem to benefit more from the sharper angle than smaller toothed saws in my experience. Most old-school carpenters and cabinet makers that I knew used the flat fleam method of sharpening.
The first thing one needs to understand is about just how fine of a point you can put on a hand saw. This relates to all styles of sharpening...The steel in a hand saw is much softer than the steel of say, a chisel. It simply will not hold as fine of a point as a chisel will. The very tip of the points when filed to greater angles will fail quicker, hence the saw will dull more quickly, and require that the saw be sharpened much, much more often. Or, optionally not to use sharp angles, just use slope for the sake of using slope - perhaps to take advantage of the deeper gullet. More on that in a bit.
Such sharp angles are probably OK, if you sharpen your own saws and are good enough at it, but not if you are going to send the thing out to be sharpened that often, you'd go broke quickly. Which leads me to my second consideration - I think it depends a lot on the skill of the saw filer. Just above is a photo of some sloped gullets as filed by Mark Harrell of Bad-Axe Toolworks. Masterful work! There's no doubt he can make them work ... Now I'm no where near as accomplished at filing as Mark is, but still - I'm not a complete slouch. Most of the time anyway...
I decided to do a little experimenting of my own. I took a 20", eleven point Richardson panel saw and a 28" seven point Disston D-8 (pictured below) and filed them each with sloping gullets. Performance-wise, the end result was the saws worked as good or perhaps even better than any previous results I've had. It did seem the seven point saw had the most distinct improvement. Of course, that may have been "an undigested bit of beef, a blot of mustard, a crumb of cheese, a fragment of an underdone potato..." (from Dickens - A Christmas Carol). I can't say with authority it was the case.
The first issue I had was with setting up and maintaining the correct angle, all while keeping the teeth consistently sized. It's not that the angle was so difficult to figure out, but it did require a bit of practice - I had do an extra pass over the saw (one each one, none-the-less) to re-set the angle so it was appropriate after deciding I the results I was getting were not satisfactory. It worked - and I suspect with practice it would come easier, as is usually the case. It's not like I've had the greatest amount of practice lately either - so I'll admit my skills have gotten like my saws - a bit rusty.
The consistently sized teeth were more troublesome. The deeper gullet on one side sets up a bit of an optical illusion, making every other tooth look larger than the one next to it. I've gotten fairly good at shaping teeth by sight over the years - but when shaping the sloped gullets I found myself bending over to look at the saw straight on to make sure that I was keeping the teeth consistently sized, and even to go through re-shaping the teeth on the larger saw one time to fix where I screwed it up. Again, I suspect practice would remedy the problems I had - but it did take me quite a bit of work to get it right the first time. Not to mention using up an entire file just re-shaping that 7 point animal - there was a lot of metal to remove to convert it to a sloping gullet! The next time it gets sharpened that won't be the case, of course...
My verdict? I think that a sloping gullet can produce a sharper crosscut saw. The caveat is that it requires a skilled filer to do it. For 99% of the users out there, who are averse to sharpening their own saws to begin with or simply don't have the time to practice the craft daily as was done back in the day, the extra angles introduced and the difficulty of maintaining the consistent tooth sizes simply isn't worth the effort. I was able to sharpen my saws with a sloped gullet, and perhaps did notice a slight improvement in their performance - but not enough of a difference that I could say with authority that it was entirely due to the sloped gullet. It might have simply been the fact it was freshly sharpened. It's hard to quantify these things in the real world when writing - but for my next sharpening I will be returning to the simpler flat fleam, if that says anything. It's just easier.
bibliography and special thanks:
The Art of Saw-Filing: Scientifically Treated and Explained on Philosophical Principles By Henry Wells Holly
First published in 1864, still in publication
Available at Google books here:
You can download a PDF copy from Norse Woodsmith here:
Grimshaw on Saws
Available on Google books here:
One of the best charts I've seen to explain the angles of saw sharpening is on Joel's blog at toolsforworkingwood.com:
Mike Wenzloff also goes through a great discussion on fleam here:
Brent Beach has done an exhaustive study on sloping gullets here that you should take the time to review:
Erik von Sneidern's Disstonian Institute - a great resourse for all things Disston, including the Acme line of saws;
And finally, special thanks to Marv Werner, Mike Wenzloff, Mark Harrell, Andrew Lunn, and all the other saw-makers out there with whom I've traded emails. Thanks, guys.
This article is subject to revision... as I'm still always learning things.