This series of drawings taken from the Disston Lumberman Handbook, 1917 edition, are provided as examples of saw teeth filed using sloping gullets.
The text of that book makes no mention of sloping the gullets. It would however be impossible to achieve the tooth shapes in these drawings without using sloping gullets.
I hoped when I started scanning and examining these drawings that they would be accurate enough to lead to some firm conclusions on what combinations of rake, bevel, and slope Disston favoured. Unfortunately, that turned out to not be the case. The drawings and commentary are retained here in case they may be useful to someone else.
If you discover something in these drawings that leads you to some positive conclusions about filing angles, please let me know.
I have scanned these pictures, then enlarged them in a graphics program and counted pixels to determine original sizes of teeth, and various angles.
I assume that if all gullets line up the saw was filed with no slope to the gullets. This assumption is valid for most of the drawings (except the No. 120 ACME). In many drawings the gullets do not line up. The variation, converted to inches and compared to the thickness of an average saw, allows determination of the gullet slope.
A geometric analysis of angles produced by a triangular file, based on rake and bevel angles, described below, shows that a specific rake angle always produces the same tooth shape (angle in the gullet, which is equal to the angle at the tip), provided the gullets are flat. The bevel angle does not affect the tooth profile.
Here is a drawing (Fig. 3, page 155) they describe as a "five-point cross-cut ... bevel about 45 degrees ... with no pitch at all."
This is enlarged to show the precision of the drawing. Notice that the bottoms of the gullets all fall on a line.
I emphasized the word "pitch" above because I believe it refers to what I am calling slope. Unfortunately the word pitch in the context of hand saws is not defined in the book. They use the same word for rake when talking about bandsaw blades, then for tooth spacing when talking about circular saws, then for the angle of the saw handle with respect to the teeth for hand saws. In some places in the handsaw discussion, they clearly use it to mean rake.
Here is a second drawing (Fig. 8, page 157). They describe this as a "six-point ... with a medium amount of bevel on the front face of the tooth and none on the back."
Notice that the gullets are not aligned in this picture. Counting pixels to measure the difference in gullet depth between the front and back of the tooth, and assuming a blade thickness of 0.04", we get a gullet slope of 45 degrees!
Pixel counting to measure the tooth height and the location of the tooth tip relative to the tooth bases, gives a front rake angle of 26.3 degrees, a back rake angle of 32.8 degrees, for an included angle of 59.1 degrees.
Pixel counting to measure the width of the bevel, and assuming a blade thickness of 0.04" gives a front bevel angle of 41.5 degrees. If accurate (and I doubt that bevels are accurately represented), it is interesting that they consider this a medium bevel.
Without the sloping gullet, the included angle for a 26 degree rake and 41 degree bevel is over 74 degrees. Sloping the gullet has allowed them to have a smaller included angle of only 59 degrees.
They continue with a series of pictures of cross-cut saws in which they keep the rake and bevel the same, but vary the back bevel. This is only possible, assuming you use a standard triangular file, if you vary the slope.
The text says that the next three drawings all have front bevel of 45 degrees, and all have the same rake. The text makes no mention of changing the slope from one saw to the next, but they must to achieve the observed included angles and back bevels. The drawings also clearly show variation of gullet base.
Can we learn anything about gullet slope from these drawings? Unfortunately, I think the answer is no.
The third drawing (Fig. 10, page 158) is a "five-and-a-half-point cross-cut saw showing the same amount of fleam front and back". The gullets line up, suggesting the gullet slope is zero.
Pixel counting yields a front rake of 23.6, a back rake of 46.5, and an included angle of 70.1 degrees.
If the rakes are accurately depicted and the gullet slope is zero, the bevel angle must be 35 degrees, not 45 degrees as stated in the text.
They include a second small drawing of a single tooth, with side and front views.
Pixel counting here gives: 26.5 front rake, 43.6 back rake, 70.1 included angle.
Based on the statement in the text that the bevel is 45 degrees, this combination of rakes is not possible. Assuming the front rake is correct, the back rake must be just over 50 degrees, for a total included angle of 78 degrees.
The fourth drawing (Fig. 11, page 158) is a "seven-point saw ... less fleam on the back".
Pixel counting gives front rake 25.7, back rake 35.4, gullet slope 30 degrees, and bevel 14 degrees.
The measured bevel angle contradicts the text (45 degrees), so the drawing must be wrong.
Pixel counting on the small drawing of a single tooth, with side and front views, gives: 23.5 front rake, 41.5 back rake, 65 degree included angle.
There is no mention of sloping the gullet, and no indication of it in the small picture (the gullet base on the left and right line up), but a sloping gullet is required to achieve these rake angles.
Assuming 45 degree front bevel and 23.5 degree front rake, a 35 degree slope is needed to achieve this included angle.
The fifth drawing (Fig. 12, page 159) is a "eight points to the inch. This
saw has no fleam on back".
Pixel counting gives front rake 22.6, back rake 45, gullet slope 23 degrees.
With those rakes, we would normally expect a bevel angle of around 31 degrees. The text suggests that all three saws have bevel 45 degrees on the front.
Pixel counting on the small drawing of a single tooth, with side and front views, gives: 21.8 front rake, 41.8 back rake, 63.8 degree included angle.
With level gullets, a front rake of 22 and bevel of 45 degrees would correspond to a back rake of 53 degrees. With a slope of 41 degrees, this filing is just possible.
The small drawing does not show the sloping gullet though, since the gullet bases line up.
|"are made especially hard and will not admit of setting, but being made thinner at the back, when properly filed, will cut clean and sweet. Teeth such as shown in Fig. 17 are used principally on Back Saws and smooth cutting hand saws. To maintain the original shape of those teeth use our cant safe back file."|
Disston only advertises a few saws that should not be set, so this comment is a little confusing. Are they suggesting that their backsaws are tapered teeth to spine? Do they mean that you should not put any set on back saws? There is little information in this book directly pertaining to back saws.
The sixth drawing (Fig. 14, page 161) is an "eleven-point saw suitable for the finer
kinds of work on dry softwoods".
Pixel counting gives front rake 26.5, back rake 26.5, gullet slope 23 degrees.
Having an included angle of 53 degrees should be impossible with a 60 degree file. By sloping the gullet, it is possible.
The seventh drawing (Fig. 15, page 161) is an eleven-point saw, no back bevel, suitable for the finer kinds of work on medium hardwoods. This is the kind of saw you might use in a mitre box. It might be interesting to try to duplicate these angles.
Pixel counting gives front rake 31.25, back rake 41.8, gullet slope 33 degrees.
Having an included angle of 73 degrees, would without the sloping gullet, imply a bevel angle of 39 degrees.
The eighth drawing (Fig. 16, page 161) is a thirteen-point saw, no back bevel, suitable for fine work on the very hardest woods. They say it is dressed like Figure 14, which is interesting. That saw has both front and back bevel, but also has an included angle of less than 60 degrees.
Pixel counting gives front rake 20.1, back rake 31.8, gullet slope 21 degrees.
Having an included angle of 51.9 degrees would be impossible without the sloping gullet, if you use a triangular file.
The ninth drawing (Fig. 17, page 161) is a sixteen-point saw, suitable for fine work on the very hardest woods.
Pixel counting gives front rake 20.3, back rake 20.3, level gullet.
Having an included angle of 40.6 degrees is impossible without a sloping gullet, if you use a triangular file. They mention that this saw must be filed with the Disston "cant safe back file".
The tenth drawing (Top figure, page 170) is a No. 120 ACME SAW. This saw is designed to be used without set. I am assuming the front of the tooth is to the left in this picture (it has been to the left so far). This could be wrong. Why would they want such a steep back rake?
Pixel counting gives front rake 32, back rake 15, level gullet.
Having an included angle of 47 degrees is impossible without a sloping gullet if you use a triangular file. They mention that this saw must be filed with the Disston "cant safe back file".
They include some specific sharpening information for this saw. In particular - "the saw should be placed in a clamp that is set at an angle of about 45 degrees". This seems to mean that the saw should have a gullet slope of 45 degrees. The picture, which they emphasize is "Showing actual size and shape of cross-cutting teeth, six points to the inch", shows no indication of sloping gullets.
Here is a picture, supplied by Jim Thompson, of the teeth of his 12 point Acme 120 crosscut saw. Notice the alternating positions of the gullet bases, always produced by sloped gullets but not shown in the Disston drawing. I have applied a sharpening filter to the original.
Close examination (aided by a little imagination) reveals that the teeth have no back bevel (the front of the teeth is to the right in this picture).
Even cursory examination reveals that this saw needs to be resharpened (and the sharpening filter won't help). With the hard teeth and complex filing, it could well be that no one would sharpen this saw so it did not get sharpened and did not get used.
My ACME 120 rip saw has 3 broken teeth - someone was sure they could sharpen and set the saw.
Jim assures me that he can take better pictures, and will as soon as he finds time to read the 200 page manual that comes with his camera.
Disston changed the name to D 120 in the 1928 catalogue. They also changed the shape of the teeth, although they do not say in what way. The saws looks much the same but has more wheat carving and nickel plated screws!