If you read most descriptions of how to sharpen saws written since 1920, you would know to place the saw in a saw vice with the teeth vertical, then file with the file horizontal. This drawing is from the 1926 Disston booklet on saw filing.
Disston saw filing advice in 1876 was quite different. This 1876 drawing clearly shows a saw vise that was designed to tilt. The ball joint allowed you to angle the saw plate to the vertical. You then worked with the file horizontal.
How did something that was common knowledge in the late 1800's disappear from the literature without a trace?
Why did they slope the saw when filing back then? Why did they stop?
This page is an ongoing examination of sloping gullets, in history, in theory, and in practise.
My Exposure
My early exposure to sloping gullets occurred on the old tools list. Look in the Old tools archive for message 3722 (Stephen LaMantia) for the first use of the term. Steve was looking at drawings in old Disston booklets and wondering what they meant. Bob Brode looked at the pictures discussed below and asked about them in message 46256, in July 1998. I suspect the lack of web pages back then severely constrained the debate. No one else had the booklets and it was much harder to send pictures around back then (especially on the old tools list which still does not allow attachments). No one had personal web pages!The typical advice back then was to provide such a drawing and then tell the woodworker to file the teeth to look like the drawing. That was it - when you are done the teeth should look like this. No mention of technique at all.
This page is an exploration of the issues involved. I began with some drawings in the Disston Lumberman's Handbook - 1916 or so. Others thought that these drawings might have been to scale and indicated practical teeth filings. Sadly, after considerable time spent scanning, enlarging, and counting pixels in these diagrams, I have concluded that they can show us nothing about tooth shapes - they are simply not accurate drawings.
I then spent some time rediscovering high school geometry and worked out the geometric rules that govern tooth shape. For any combination of the angles involved, these formulae define the tooth shape. I have included a number of drawings of tooth shapes and will add more as time goes on. For each combination of angles I have included face and front profiles.
I am interested in input on any part of these web pages. My contact information is at the bottom.
Why does it matter
Before spending a lot of time on sloping gullets, we have to understand why we would bother.The problem sloping gullets is attempting to solve is the tooth shape problem introduced when we increase the bevel angle. When the bevel angle is zero, the included angle in the tooth and the included angle in the gullet are both equal to the angle in the file - 60 degrees.
Unfortunately, a saw with bevel angle zero does not work for cross cutting - it tears rather than cuts the wood fibres. The solution to the tearing problem is to increase the bevel angle. As you increase the bevel angle, the leading edge of the tooth gets sharper and begins to slice the fibres. Unfortunately, as you increase the bevel angle you get a shorter tooth. The relation between bevel angle and tooth height is covered later on in this page.
Sloping of gullets, working with the file handle dropped down toward the face of the blade, can reduce or eliminate the effects of increasing the bevel. When the slope is equal to the bevel, the file again creates a 60 degree gullet and tooth height exceeds that in the no slope case.
The main point of sloping gullets is then to recover tooth height lost with increasing bevel angle. A secondary goal is the ability to do special filings, with different front and back bevels as well as unusual combinations of front and back rake angles that could not be achieved with a triangular file with no gullet slope. A short discussion of these special filings appears at the end of this page.
Joseph Baron, from the Oldtools list, suggested that sloping the gullet may be a natural way to file a cross-cut saw if you are changing the bevel angle. If you are increasing the bevel, the file will touch only the tops of the teeth at first. With little support at the start, it is very difficult to maintain the correct rake angle (unless of course you use a jig that maintains the rake angle). By increasing the slope until the file makes full contact with the teeth on both faces, you can only really maintain the existing rake angle. (I have not actually tried this, but it makes sense.)
Angles Names
Terminology is confusing, with different writers using different words to refer to the gullet angle. I will simply call it the slope.There are 3 angles involved.
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The rake is the angle of the front of the file face from the perpendicular. Changing the rake involves tilting the file face. Rip saws have rake near zero while cross-cut saws have rake up to 30 degrees. (Some authors define rake as the angle of the tooth face, not the file face. For them a rip saw has rake near 90 degrees.)
-
The bevel is the angle created by tilting the file handle toward the saw handle. Rip saws have bevel near zero (almost no tilt of the file handle back toward the saw handle), while cross-cuts have bevel up to 45 degrees. (Also called the fleam.) (Some authors define the bevel angle as the included angle of the bevel in the tooth, rather than the angle of the file.)
- The slope is the angle created by lowering the file handle from the horizontal. Most saws today are filed with no slope (saw filing machines seem to only be able to file with zero slope).
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As we shall see later on, defining these three angles in terms of the orientation of the file could be confusing. For example, depending on the rake, bevel, and slope, the rake as measured on the tooth need not equal the rake as measured on the file producing the tooth.
This astonishing result goes unmentioned in the descriptions of saw filing. |
The slope is defined as the angle the file handle drops down from the horizontal. You could file this way, by dropping the file handle by the slope angle.
There is a second way of getting a sloping gullet - you can tilt the saw and keep the file horizontal. Most metal saw vices have the ability to tilt to allow for sloping gullets. Here is an illustration from the Disston Price-List 1876 (Roger Smith reprint 1994).
This illustration shows a fairly large bevel and slope angle - perhaps 30 degrees or more. It also makes it clear that filing with non-zero slope was standard practice back in the 1870s. Why is it not done, not even known today?
Unlike the saw vice in this drawing, some saw vice designs only permit a tilt in one direction - away from the filer. These vices clamp to the bench top with a screw from below. Part of the vice is below the bench top. This part collides with the clamp part, preventing tilt back toward the filer. This means you can only file up the gullet, not down the gullet. Since the down part of the gullet is always pointing back toward the handle, this means the file handle must always point back toward the handle during filing with sloping gullets (using these saw vices).
Some Drawings of Saw Teeth
The Disston Lumberman Handbook has a number of drawings that clearly show sloping gullets. These drawings look pretty precise, as if they intended them to be exact copies of the teeth they describe.I have scanned these pictures, then enlarged them in a graphics program and counted pixels to determine original sizes of teeth, and various angles.
It turns out, unfortunately, that the drawings are not to scale. Pixel counting in enlargements of the drawings produce front and back rake angles that are inconsistent with the stated bevel angles. Some drawings show sloping gullets - the gullet bases alternate up and down - but there is no mention of sloping gullets in the text. Some drawings show teeth profiles that can only be achieved with sloping gullets, but the drawings do not show sloping gullets.
I suspect that by 1918, when this revision of the original 1907 Handbook was published, sloping gullets had gone out of fashion. Some references were edited out, others did not get removed. The result is confusing and misleading.
I have retained the material here for others interested in historical saw tooth drawings.
H. W. Holly - The Art of Saw Filing
Drawings of saw teeth have appeared in various books on saw sharpening and in trade catalogues. Recently, Gary Roberts put a scan of a very early book The Art of Saw Filing by H. W. Holly on his Toolemera web site. The book appears to be an 1882 printing of a book written in 1851 (perhaps 1854).This book contains the earliest drawings of saw teeth I have encountered. It appears to be the model for most subsequent books, including the Disston Lumberman's Catalog.
This is Holly's drawing of a cross-cut saw for use on soft woods. Holly never mentions angles in his book - he shows a picture and tells people to file their saws to look like the picture! If people had no access to small protractors or other ways of measuring angles, this probably makes sense.
The front and back bevels appear to the be same, and the gullets line up. The rake appears to be a little less than 30 degrees.
This is Holly's drawing of a cross-cut saw for medium wood. The only difference in saws for soft and medium woods appears to be in the number of teeth per inch -- he recommends 6 teeth for soft woods, 8 teeth per inch for medium woods. The gullet bottoms again lie on a straight line.
This is Holly's drawing of a cross-cut saw for hard wood. Again he changes teeth per inch - now recommending 10 teeth per inch.
The big difference now is that he recommends that "the back of the tooth be filed square".
He also says that a saw filed this way "will mitre soft or medium wood well".
The gullet bottoms are no longer on a straight line. This filing is achieved, in part, by sloping the gullet. Holly gives no indication of this - he says nothing about gullets in relation to hand saws. Was the technique so well known that it need not be mentioned?
This is Holly's recommendation for a back saw for use on soft woods - 10 teeth per inch.
He says it has the same bevel as figure 12. It sure does not look like it to me - the teeth look taller.
This saw has quite a pronounced bevel. As the number of teeth increase, the size of the tooth gets very small if you file the teeth with no slope and large bevel. Usually the teeth look quite squat. These teeth look quite tall. It could well be that Holly meant people to use a slope when filing these teeth. However, if that is so then the drawing should show zig zag gullet bottoms.
This is Holly's drawing of a back saw filed for use with hard woods - 12 teeth per inch.
Again, he recommends that the tooth back be filed square. Again, the gullet bottoms zig zag.
A mitre saw - a saw used in a mitre box - is a back saw. Mitre saws are often filed as peg teeth - same filing on the front and back of the tooth so they can cut in both directions. A saw filed square on the back would not present the same knife shape on the back stroke. I wonder if this matters.
You may be confused by these drawings. They seem to be less than clear to me. No angles involved. No explantion of why things are done this way. Just a few drawings that are easily misunderstood.Were individual workmen sharpening their own saws back then? Were they sending them out to shops that jealously guarded the secrets of their craft. Were the actual techniques passed on from filer to filer as part of the apprenticeship experience?
Front Views
The following three drawing from Holly give a little more detail on the teeth shapes for the various types of wood. Holly's innovation was to show the teeth in both side and front views. The front view is important for his explanation of how a saw works. From Holly:
| "Now the sharper each tooth is -- that is the more bevel on the point -- the deeper it will cut; but it must not cut any deeper than will crumble out across to the point of the other tooth. This is the difference between saws for soft or hard wood; if a saw for hard wood is too much bevel on the point, it will score deeper into the wood than it can carry out the chip, so that it will keep moving up and down in the same scores, and not accomplish anything." |
Sloping the file, and hence sloping the gullet, is the only way to achieve the latter two filings in figs. 14 and 16 above if you use triangular files.
Soft Wood
From Holly:
|
a shows the position of the file for a cross-cut saw for soft wood, such as pine, bass wood, cedar, etc.
b Shows the shape of the tooth, and c the bevel of the point, consequent on the position of the file and bevel of the back of the tooth. |
Looking at b: the front bevel is wider than the back bevel. You cannot achieve this filing if the file is in contact with both teeth at the same time. You can do it by adding a little slope, which allows you to file the fronts on one pass and the backs on a second pass. This should be clear in the drawing - the gullet bottoms should zigzag, but they do not.
Looking at c: [the way the wood sees the tooth] the tooth is tapered sharply to the tip. The tip will slice deeply into the wood. When used with soft wood the chips will be easily pushed sideways by the alternating teeth, break off, and be carried away in the gullet.
Medium Wood
From Holly:
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a shows the position of the file for saws for medium wood, such as chestnut, bay-wood, black walnut, cherry, etc.;
b shows the shape of the tooth, and c the bevel of the point. |
Looking at b: the bevel on the back of the tooth is about half as wide as the bevel on the front.
Looking at c: the front profile is not as sharply pointed.
Comparing Figs 7 and 8, it appears in Fig 8 that the slope of the file is larger. He is using sloping gullets, so the gullet bottoms should zigzag, but again they do not.
Hard Wood
From Holly:
|
a shows the position of the file for saws for hard wood, such as hickory, ash, oak, maple, beech, etc.;
b shows the shape of the tooth, and c the bevel of the point. |
Looking at b: the back of the tooth now as no bevel.
Looking at c: the tip is quite blunt.
Again, comparing these last 3 Figs, the file appears to have more slope in Figs 8 and 9 than in Fig 7. In this drawing the zigzag of the gullet bottoms is quite clear.
According to Holly, this flatter front profile means that the chip is broken off by the action of successive teeth. With hard woods you need a fairly blunt front profile or the tip goes so deep that there is so large a chip that it does not break off.
Fleam-Tooth Filing
A distraction possibly, but included because I cannot understand the drawing.
Holly calls this a Fleam-Tooth Saw. He says: "This saw is filed extremely beveling, so much so, that the saw must be laid down flat to be filed." Notice as well that this is the only case in which you file down the gullet. If you did this filing up the gullet your hand or the file handle would hit the saw spine.
It appears that the slope is almost 90 degrees. This is the most extreme case of sloped gullets. Holly does not say where you would use a fleam-toothed saw.
Filing Angles
While Holly was reluctant to specify angles, later authors were not.Hodgson on Saws
Another early saw filing manual, Fred T. Hodgson wrote a book Hand Saws ... published in 1909 but copyright 1888, copies some of the Holly diagrams but includes some angles for cross cut saws:
| wood type | rake | bevel | |
|---|---|---|---|
| soft wood | 20d to 25d | 40d to 45d | |
| hard wood | 5d to 10d | 20d to 25d |
He adds a few of his own drawings, but those drawings are impossible.
Charles L. Johnson
Charles L. Johnson produced a small pamphlet in 1907 which he called "THE CARE, SELECTION / AND / PROPER FITTING / OF HAND SAWS / A COMPLETE TREATISE / FULLY ILLUSTRATED / BY / Charles L Johnson / SAW EXPERT".For regular hand saws, he gives at least 8 different filings (10d means 10 degrees):
| Use | Rake | Bevel | Points | |
|---|---|---|---|---|
| Fine Cut-off | 10d | 30d | 10-11-12 | |
| Coarse Cut-off | 15d | 40d | 7- 8- 9 | |
| Buck Saw | 20d | 30d | 5 1/2- 6- 7 | |
| Rip Saw | 3d to 7d | 2d to 5d | 5 1/2- 6- 7 | |
| Heavy Ripping | 3d | 2d | 5-1/2 pt | |
| Light Ripping | 5d | 5d | 6 pt | |
| Long Miter | 6d to 7d | 5d to 10d | 5 1/2- 6- 7 | |
| Short Miter | 10d | 10d to 20d | 7-8-9-10-11-12 |
A Buck Saw is a saw for bucking up fire wood - cut quality not an issue, cut speed more important.
A Long Mitre is a cut angled to the grain, but closer to a rip cut.
A Short Mitre is a cut angled to the grain, but closer to a cross cut.
Johnson got around the problem of people not having protractors by including a number of celluloid protractors with his filing manual. The protractors were marked with the various angles. Although copies of the pamphlet survive, I have not seen a protractor.
Johnson says that the file should be level at all times - without saying it, he denies the possibility of sloped gullets. His pamphlet was written at about the time that the Disston books no longer appeared to be suggesting people use sloped gullets.
I use Johnson's Light Rip angles, but find that his recommended bevels for crosscuts are too big.
Leonard Lee
As usual, Leonard Lee's book A Complete Guide to Sharpening includes an excellent section of saw filing.His directions for filing cross cut handsaws are contradictory on the topic of slope. In the instructions he says: "With your file aligned with a bevel guide, drop the handle the same number of degrees as the amount of bevel you are using ...". However, in a sidebar on he says: "In practical terms, this is not worth the effort."
His recommendations for filing angles:
| Saw type | Rake | Bevel | Slope | |
|---|---|---|---|---|
| Cross-cut | 15 - 30d | 10 - 20d | 10 - 20d | |
| Rip | 0 | 0 | 0 |
Picture of Sloping Gullets
I bought this saw - a Disston 112 - at auction at the Whippletree Auction north of Victoria, in April 2005, with 3 other saws. All three saws looked fairly well sharpened.
I tried this saw out but found that it cut very poorly. It worked, but slowly.
The image is magnified 10 times.
The lower red line is the line of the bottoms of one set of gullets, the upper red line shows the bottoms of the other set of gullets.
This zig zag gullet bottoms is a sure sign of sloping gullets.
Looking closely at the teeth set away, the front face of the tooth appears to have bevel 0 while the back of the tooth has bevel around 15 degrees. The rake appears to be around 15 degrees, perhaps a little more.
The problem with this saw is that in filing the gullets, the filer hit the fronts of half of the teeth with the file -- blunting half the teeth. I don't think a hand filer would make this mistake and not notice. I suspect this saw was machine filed using a bad setup in an attempt to achieve a complex filing without using any slope.
Theoretical Analysis
As mentioned above, we slope gullets not because the base of the gullet is involved in the cutting action, but because it allows us to have different tooth shapes when using triangular files.This section develops the geometry of the gullet and teeth, as a function of rake, bevel, and slope. It is the combination of certain bevels and slopes that produces the desired tooth shape.
Bevel angle and Cutting Action
For rip saws, a bevel angle near zero is fine. This produces a tooth with a leading edge that acts like a plane blade. The tooth slices along the fibres, removing shavings. [This is the standard explanation, but who puts a 60 degree included angle on a plane blade?]For cross-cut saws a plane does not work. If you use a plane across the grain, it tears the fibres on either side of the blade. It is also a lot of work because you are tearing rather than cutting. Because it tears the wood fibres, the plane cutting across the grain leaves a poor edge. A rip saw cutting across the grain produces the same result.
Planes use nickers for cross grain work. The nickers slice the grain ahead and on either side of the plane iron, which is then able to lift the shaving. Two man cross-cut saws use a combination of nicker-shaped slicing teeth and chisel-shaped planing teeth (called rakers). Handsaws typically settle for a single type of filing - bevelled teeth that slice across the fibres, although some people file a few teeth as rakers even for handsaws.
How are handsaws able to use only slicing teeth, with no raker type teeth? Surely, slicing the fibres is only half the work - the saw must also lift the resulting wood chips out of the work. It turns out that there are two actions. First, the tip slices the fibres. Second, the bevelled face below the tip pushes the resulting chip to the side. As successive teeth slice deeper into the fibres, the alternating bevels push the fibres back and forth, breaking them away from the work.
What tooth shape achieves these two goals - slicing the fibres and pushing the sliced fibres to break them off - most effectively?
A very fine leading edge (a large bevel angle) cuts best, but wears quickly and does not push enough. If you cut too deep before breaking the fibres off, the saw could hang up, simply riding on the sliced but not broken off fibres between the teeth. A very blunt leading edge (a small bevel) slices poorly, so does not go deep enough into the wood to provide any fibres to dislodge.
The best bevel angles are a compromise between slicing and pushing actions.
Bevel Angle and Tooth Shape
In addition to the benefit of sharper cutting edge, increasing the bevel changes the shape of the tooth. As the bevel angle increases from zero with constant file rake angle, the tooth rake angle increases.
This drawing shows a series of tooth profiles, all based on a file rake angle of 20 degrees, with increasing bevel angles. (The front of the tooth faces left.)
- green - bevel 0
- yellow - bevel 10
- blue - bevel 20
- mauve - bevel 30
- red - bevel 45
The tooth rake on the green tooth is 20 degrees, on the red tooth it is 27.2 degrees.
The preceding drawing is of an ideal tooth - one with zero thickness. In an actual tooth, these shapes correspond to the shape of the tooth half way across the blade. This corresponds to the file rotating about the midpoint of the gullet as the bevel angle increases.
For a saw of non-zero thickness, the shape of the tooth, which we normally think of as the shape of the big side of the tooth, changes in a different way. As the file rotates about the middle of the gullet, the base of the tooth widens. The height of the tip decreases more slowly since it is actually not as far along the file as the middle of the tooth thickness.
- green - bevel 0
- yellow - bevel 10
- blue - bevel 20
- mauve - bevel 30
- red - bevel 45
Bevel angle and Rake angle
In the above drawing, the rake angle of the file remained constant at 20 degrees, but the rake angle of the front of the tooth changed from 20 degrees to 27.2 degrees as the bevel changed from 0 degrees to 45 degrees.The tooth rake angle depends on both the file rake angle and the bevel angle, according to the following formulae:
| Effective front rake = atan(tan(file rake)/cos(bevel)) | Effective back rake = atan(tan(60-file rake)/cos(bevel)) |
Filing technique and Rake angles
Imagine you set the file rake with the bevel angle zero (with the file in a gullet, bevel angle 0, set the rake using the file teeth as the horizontal). Imagine then increasing the bevel angle but not changing file rake. As the bevel angle increases, the actual angle put on the front of the tooth, the tooth rake, increases according to the above formulae.Some people put a block of wood on the file tip, with the block positioned so that the file has the correct rake when the block is horizontal. If you use this method, be sure you set the rake angle with the file at the desired bevel angle.
Instead of setting the rake first and holding it constant, you could set the bevel first, then set the rake using a protractor along the face of the saw blade. In this case, the tooth rake would be as you set it, the file rake somewhat less. People who use my jig use the rake angle on the slider to set the file rake with the file at the bevel angle. This produces the expected tooth rake angle.
Slope and Tooth Shape
As you increase the slope from zero (either by dropping the file handle while keeping the saw blade vertical, or by sloping the saw blade and keeping the file horizontal), the angle at the base of the gullet decreases (both the front and back rake angles increase). If you lower the file handle all the way, the file no longer makes any gullet at all and the gullet angle has decreased to zero.The formulae that relate rake, bevel and slope are similar to those above. These formulae let us calculate tooth rake for a variety of bevels and slopes. In particular, they confirm the general statement that you can regain the 60 degree gullet angle by having the slope equal to the bevel.
Here is drawing of tooth shape, for rake 20 and bevel 45, as the slope increases from 0 to 45 degrees. It reverses the drawing of tooth shape with increasing bevel shown above.
- green - slope 0
- yellow - slope 10
- blue - slope 20
- mauve - slope 30
- red - slope 45
Complex Tooth Shapes
The above discussion assumes that the front and back bevels are equal. This is a basic requirement of filing, where the file is seated in the base of the gullet and files both sides of the gullet at the same time.Non-zero gullet slopes allow the filer to produce different front and back bevels. For example, with a 15 degree rake and a 30 degree bevel with slope 0, the gullet angle is 66.3 degrees. By increasing the slope to 30 degrees, the required gullet angle is only 60 degrees. This means we can file the saw with slope 0, then go back and refile the backs of the teeth with slope 30 and work just the back of the tooth at a steeper bevel angle. The more of the back of the tooth we remove, the wider the gullet is and the more room we have to reduce the bevel angle even more.
Scale Drawings
These scale drawings assume a 9ppi saw with thickness 0.40".The first few are standard filings with slope 0. The last is a filing with slope 45 degrees.
In all drawings, there are two views of the tooth: side view, showing the bevels, and front view, showing the tooth profile from the point of view of the wood.
In the face view the front of the tooth is flooded with a blue colour, and a line is drawn between the tip at the front and back to show the intersection of the front and back bevels. In crosscut saws, the chip is pushed sideways by this bevel, so the angle is important.
In the front view, the angle of the bevel intersection line shows degree to which the bevel pushes the chip sideways.
Rip saw filing, rake 5 degrees, bevel 5 degrees
My standard filing for rip saws. The non-zero rake helps with starting the cut. The small bevel provides for a bit of slicing action on the outside at the tip, but relies most on the planing action of the bevel intersection.The included angle at the tip in the face drawing is 60.1 degrees (the angle you see looking at the side of the tooth), the front profile is 81.0 degrees (the angle you see looking at the front of the tooth).
Mitre saw filing, 15 degree rake, 30 degree bevels
By Mitre Saw I mean a saw used in a Mitre Box. While such a saw can be expected to cut at angles other than 90 degrees, most of the cuts I make are 90 degree cuts.I remove all set from mitre saws. With no set and this filing I get a very smooth cut face that usually needs no planing.
The included angle at the tip in the face drawing is 66.0 degrees, the front profile is 44.8 degrees. According to Holly, this very sharp tooth is better suited to soft woods (where it can dislodge the chips in the kerf) than for hardwoods.
Mitre saw filing, 15 degree rake, 30 degree front bevel, 0 degree back bevel
Without worrying about how to do this filing, look what removing the back bevel does to the front profile. The cutting action at the tip is the same - the front bevel is still 30 degrees. The chip ejection action must be much different though, given the much steeper angle of the bevel intersection line.The included angle at the tip in the face drawing is 66.0 degrees, in the front profile is 68.3 degrees. According to Holly, this filing is required for hardwoods.
Mitre saw filing, 10 degree rake, 40 degree bevels, 45 degree slope
I filed a saw using this filing, my first with sloping gullets.The included angle at the tip in the face drawing is 57.2 degrees, in the front profile is 31.0 degrees.
An included angle of less than 60 degrees is possible because the slope is greater than the bevel.
The saw did not work well at all - it kept catching, cut very slowly. I thought at the time it was a problem with the filing angles - that they just did not produce a saw that cut well across grain. Looking at this diagram and at the saw itself, I think the problem was with the filer, not the filing angles.
It appears I did not file the fronts of the teeth enough from one side of the saw, leaving a much flatter angle in the front view.
This failure points out two filing challenges. First, when changing the filing angles drastically, expect to have to file the entire saw several times to get a good result - the first filing will always be pretty poor.
Second, filing with large bevel angles is difficult. At least from one side the filer is jammed between the file and the saw, with little room to work. As well, it is hard to see the tooth from this angle. Large bevel angles might have some advantages in use, but are difficult to achieve.
However it is possible that the real problem was that the front profile was too sharp! Since I was sawing hardwoods, the saw could have been sliding back and forth in the slots cut by the tips, unable to push the chips sideways enough to break them off.
Crosscut, rake 15 degrees, front bevel 20 degrees, back bevel 20 degrees
A comparison of a basic crosscut profile, with and without slope. This filing has no slope.The included angle at the tip in the face drawing is 62.7 degrees, in the front profile is 56.0 degrees.
Crosscut, rake 15 degrees, front bevel 20 degrees, back bevel 20 degrees, slope 20
A comparison of a basic crosscut profile, with and without slope. The difference in front profile angle is too small to be significant. The difference in tooth height is also small (7% greater for the 20 degree slope).The included angle at the tip in the face drawing is 60.0 degrees, in the front profile is 54.9 degrees.
To Slope or not to Slope?
There are a number of cases of interest, two of which are most interesting. First, simply sloping to increase the gullet size. Second, sloping to allow different front and back bevels.
Gullet Size
Sloping the gullet increases tooth height, which increase gullet size (by the same proportion) and hence the volume of sawdust the gullet can hold. You can saw a given board thickness with a finer saw (more points per inch) by using sloping gullets. Or, you can use a saw with more points per inch. [Recall the huge gullets on two man cross-cut saws, meant to work on very large trees.]If you want to saw thick boards with fine saws you should probably try sloping gullets. Often tenon saws have very fine teeth. When filed at the larger bevel angles the teeth and the gullets become quite small. A sloping gullet on such a saw would probably help.
However, two other options may make more sense. First, use a smaller file. The smaller the file the finer the corner and thus the sharper the gullet base. On fine toothed saws this could make an important difference.
Second, use a saw with fewer teeth per inch. Fewer teeth produce larger gullets. Fewer teeth do not necessarily mean a rougher surface, provided you take care with the setting and filing.
Tooth Front Profile
The last two diagrams above are conventional filings - a triangular file working front and back bevels at the same time. The only difference is the slope - one with no slope, one with 20 degree slope. The front profiles are: 56.0 degrees with slope 0, 54.9 degrees with slope 20. The difference in tooth front profile does not warrant the extra effort of sloping the gullets.You can produce a more radical change in the tooth front profile as follows. Using the standard triangular file, file the saw with no slope. Then go back and using slope equal to the bevel angle, file just the backs of the teeth to remove some or all of the bevel. As shown in earlier drawings, you can increase the front profile angle from 44.8 degrees to 68.3 degrees by reducing the bevel on the back of the tooth from 30 degrees to 0 degrees.
Other Files
The above discussion assumes you are using only triangular files. You can easily create special filings without sloping gullets if you use a file with a different profile. For example, cant files have two 30 degree corners, one 120 degree corner. Using the sharper corners will allow you to easily file a different angle on the tooth back without resorting to sloping gullets. The instructions in older Disston Catalogs for resharpening saws with complex filings do mention cant files.In both cases (triangular file, sloped gullets versus cant file) you must file the front bevel on one pass (actually one pass each way, file each tooth once), then the back bevel on a second pass, so there is no effort advantage to either file.
Conclusions
This page was begun as an attempt to understand why saws were filed with sloping gullets at one time. Taller teeth and the corresponding larger gullets are one possibility, complex filings with different front and back bevels are another. Both make some sense. The largest effect appears to be in increased tooth front profile angle possible with complex filing that may help with hard woods.Further Study
The new questions then are:- Are extreme bevel angles useful? Given the difficulty in getting a good filing at 40 degrees, and the expected reduction in tooth durability, is it still worthwhile to use these higher bevel angles?
- Are complex filings useful? Does the difference in front profile included angle make much difference in saw performance (in softwoods versus hardwoods)?
- How would you actually go about doing a complex filing? What sequence of filing angles on the fronts and backs of the teeth is required to produce one of these complex filings?