This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Any exhaustive attempt to deal with the considerations which present themselves to one who enters upon the question, what under all the varying conditions of the problems should be the form and set of a saw-tooth, would require more experimental knowledge and patient research than the subject seems to have received. There are more than 100 different forms of teeth. Sheffield and London do not agree upon the shape of the handle. The Eastern hemisphere and the Western do not agree whether sawing should be an act of tension or one of thrust.
The quantity of timber cut down in America must have led to investigations with respect to saws such as the requirements of this country were not likely to call forth. Hence we have very much to learn from the Americans on this point.
As it seems most judicious to investigate the principles by considering a large and heavy tool, perhaps it may be well to examine the largest handicraft saw. This (Fig. 301) is a "one-man saw" 4 ft. long, by Disston, Philadelphia. Long as the blade is, it is not too long. The travel is near, but still, within the limit of a man's arm. To enter the wood, the teeth at the extreme end are used. These are strong, but of the form generally met with in the largest of our own cross-cut saws. The acting teeth are of an M shape, with a gullet or space between them. The angle at which the teeth are sharpened is very acute; the consequence of this and of their form is, that they cut smoothly as a sharp knife would do; indeed, much as a surgeon's lancet would. Some teeth are formed on the principle of the surgeon's lancet, and these are called "fleam" teeth. The spaces between the M's in the "one-man saw" are "gums" for the reception and removal of the pieces cut out of the separated fibre. In the particular case before us, the M is 3/4 in. broad and 3/4 in. deep; the upright legs of the M are sharpened from within, the V of the M is sharpened on both sides. The legs are "set" to one side and the V to the other side.
Thus arranged, the saw cuts equally in tension and in thrust, and the debris is brought out freely at each end. The M tooth for this double-cutting results from an observation on two carefully-toothed short cross-cut elementary saws, where it will be noticed that the form of tooth to cut both ways, resulting from the combination, is M. The set of this large "one-man saw" is worthy of notice. An inspection of the cutting points will show that each point is diverted from the plane of the saw blades not more than about 1/32 in. When the object of "set" is considered, it will be allowed that so little is sufficient.
The annexed diagrams (Fig. 302) of teeth of certain cross-cut saws used in America may illustrate the present subject. A single tooth will in some instances be observed between the M teeth: this is a "clearance" tooth, and is generally shorter than the cutting tooth. Sometimes it is hooked, as may be seen in c; in such case it is shorter by 1/32 in. than the cutting teeth, and acts the part of a plane iron by cutting out the pieces of fibre separated by the other or cutting teeth, which cutting teeth under these circumstances are lancet-like sharpened to very thin edges.
That the "set" of the teeth should be uniform in the length of the saw follows from a moment's reflection upon the object of this set. If one tooth projects beyond the line of the others, that tooth will clearly scratch the wood, and therefore leave a roughness on the plank. As more than its share of work is then allotted to it, the keenness of edge soon leaves it, and thus increases the labour of the sawyer. The American contrivance for securing a uniformity in the set of the teeth is the " side-file." The three set screws determine the elevation of the file above the face, and the travel of the short length of fine cut file reduces all excessive "sets" to a uniform "set" through the entire length of the saw. The "crotch punch" is also an American contrivance for obtaining a clearance set out of a spreading of the thick steel of the saw by an ingeniously formed angular punch.
It is occasionally required to saw certain cuts to the same depth, as, for instance, in the making of tenons. The saw to which the term " tenon " is applied is more suited for cabinet than for carpenters' work. However, an ordinary saw may be provided with a gauge which can be adjusted so as to secure a uniform depth in any number of cuts, and in this respect it is even superior to a tenon-saw, and may be suggestive to some whose labours might be facilitated by the adoption of such a contrivance.
The rip-saw considered as a cutting tool, may bo likened to a compound chisel, and the form of teeth which would operate with the least application of power would be the same as that of a mortising chisel; but knots and hard wood are conditions which call for rigid teeth rendering the chisel form impracticable, except for sawing clear lumber, and with a high degree of skill in filing and setting. The limit of endurance of such steel as must be employed for saws, will not admit of pointed teeth; these will break in cutting through knots and hard wood, and no form of saw-teeth which permits their points to crumble and break should be adopted. In actual practice, with the skilled filer there is a tendency to create pointed saw-teeth, and when there is a want of skill in the filer the tendency is the other way, and teeth unnecessarily blunt are common. " The action of a saw when ripping or cutting with the fibres of the wood is entirely different from that when cross-cutting or severing the fibres of the wood transversely; the shape of the teeth and the method of sharpening should therefore differ.
In the case of a ripsaw, the action of the saw is chiefly splitting, the teeth acting like a scries of small wedges driven into and separating the longitudinal fibres of the wood; whilst with cross-cutting saws, the fibre of the wood has to be severed across the grain : it is comparatively unyielding, the teeth of the saw meet with much more resistance, and it is found necessary to make the teeth more upright and more acute or lancet-shaped than for cutting with the grain. The faces of the teeth should be sharpened to a keen edge, and for hard wood filed well back, so that in work they may have a direct cutting action, similar to a number of knives. Care should also be taken that the teeth are made of sufficient depth to afford a free clearance for the sawdust. This is an important point too with rip-saws. The teeth should also be equal in length; if not, the longest teeth get the most work, and the cutting power of the saw is much lessened. The length of the teeth should depend on the nature of the wood being sawn: for sawing sappy or fibrous woods, long, sharp, teeth are necessary, arranged with ample throat space for sawdust clearance; care must be taken, however, that the teeth are not too long, or they will be found to spring and buckle in work.
In sawing resinous woods, such as pitch pine, the teeth of the saw should have a considerably coarser set and space than for hard woods. It will also be found advisable - especially with circular saws - to lubricate the blades well, as the resinous matter is thus more easily got rid of. In sawing hard woods, either with reciprocating or circular saws, the feed should be not more than one-half as fast as for soft wood, the saw should contain more teeth, which should be made considerably shorter than those used for soft wood, roughly speaking, about 1/4; it is impossible, however, to make a fixed rule, owing to the great variety of woods and their different hardnesses; the length of teeth which may be found to suit one wood well may in another case require to be increased or decreased. In cutting woods which are much given to hang and clog the saw-teeth, increment teeth may be used with advantage; these are arranged with fine teeth at the point of the saw, which gradually get coarser till the heel of the saw is reached; thus the fine teeth commence the cut and the coarser ones finish it, obviating in a great degree the splintering and tearing of the wood caused by coarse teeth striking the wood at the commencement of the cut.
As regards the angles of the teeth best adapted for cutting soft or hard woods no absolute rule can be laid down. The following may be modified according to circumstances. If a line be drawn through the points of the teeth, the angle formed by the face of the tooth with this line should be : For cutting soft woods, about 65o-70°; for cutting hard wood, about 80°-85°. The angle formed by the face and top of the tooth should be about 45°-50°for soft wood, and 65°-70° for hard. The angle of the tooth found best for cutting soft woods is much more acute than for hard. Terms used in describing the parts of a saw are: - "Space" : the distance from tooth to tooth measured at the points. " Pitch " or " rate " : the angle of the face of the tooth up which the shaving ascends, and not the interval between the teeth, as with the threads of a screw. "Gullet" or "throat" : the depth of the tooth from the point to the root. "Gauge" : the thickness of the saw, generally measured by the wire gauge. "Set": the amount of inclination given to the saw-teeth in either direction to effect a clearance of the sawdust. " Points " : small teeth are reckoned by the number of teeth points to the inch.