The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking. Paul N. Hasluck
Fig. 215.—Saw Kerf.
ACTION OF CROSS-CUTTING SAW.
Taking the ordinary cross-cutting single-handed saw, the forward thrust is intended to separate the fibres, by removing a small piece by two parallel cuts. For example, if O O (Fig. 216) be a fibre, then the action of the saw must be to cut clean out the piece a b, so making a space (a b) wider than the steel of which the saw is made. The cleaner the parallel cuts a d, b c are, the better. Now this clean cut is to be made by the tooth advancing towards the fibre. If the tooth come on in axe fashion, there is a direct thrust of a sharp edge; and though a wedge-like action may be the best for separating fibre adhering to fibre, it is quite out of place in the cross-cutting of a, single fibre, in which cohesion has to be destroyed. There must be a cutting action, that is, the drawing of an edge across the mark for separation; besides this drawing action there also is pressure. Now, in soft timber, and with a saw having teeth only moderately sharp, this pressure tends to force the fibres into closer contact, to squeeze them amongst each other, to solidify the timber, and increase the difficulty in cutting. Two actions are here, pressure and thrust The pressure must be very light indeed; if otherwise, the point of the tooth will gather up more fibres than the strength of the workman can separate; indeed, as a rule in the cross-cutting of broad timber, and when all the saw teeth are in action, pressure is not required, the average weight of the saw-blade sufficing for the picking up of the fibres. Whatever may be the form of the teeth, the small piece a, b, c, d (Fig. 216), has to be removed so as to leave the ends from which it is taken as smooth and clean-cut as possible; therefore the cutting edge must be on the outside of the tooth. This being so, it follows that the act of severing a fibre will be attended with compression, and thus condensed it will be forced up into the space between the teeth. This space must be so formed as to allow the condensed piece to drop freely away as soon as the tooth passes from the timber, or the saw will become choked, and its proper action cease. In large saws this is provided for in the shape of what in America are called “gums” and in England “throats,” in which the teeth may be said to be set. Saws cannot work easily unless as much care is bestowed upon the throats or gums as is given to the teeth.
Fig. 216.—Wood Fibre Divided by Saw.
Fig. 217.—Hand Saw Handle.
Fig. 218.—Hand Saw Handle.
Fig. 219.—Hand Saw Handle.
Fig. 220.—Hand Saw Handle.
Fig. 221.—Hand Saw Handle.
THICKNESS AND STRENGTH OF SAW BLADES.
A saw must in one dimension (that of thickness of blade at least) be very thin, and that part cannot be strengthened by means of ribs. When a strengthening bar is introduced at the back as in tenon saws (see Fig. 244, p. 66), the depth of cut is limited. If a light saw blade be pressed against an object or hooked on one, then tension causes this straight blade to be more and more strengthened. On the other hand, if the saw blade be pressed forward by thrust, the weakness of the blade is apparent from the bending. Now, formed as saw teeth are, either to cut in both directions or in the forward direction only, there is always one direction in which the work to be done is accomplished by a thrust upon this thin metal. Clearly the metal will bend. If, however, the teeth cut in the direction of tension only, the work tends to preserve the straightness of blade, and upon this an important quality and use of the tool depend. That this tension system can be efficient with a very narrow blade is clear from the extensive use of band saws. There is, however, a property in the breadth of the blade which applies equally to the tension and thrust systems—it is the guide principle. The breadth of the blade operates by touching the sides of the gateway opened by the teeth. When it is desired to dispense with a straight guide for sawing purposes, it is done by narrowing the blade as in lock saws, keyhole saws, tension frame saws, etc.
Fig. 222.—Hand Saw Handle.
Fig. 223.—Three Rip Saw Teeth Points to Inch.
Fig. 224.—Three and a half Rip Saw Teeth Points to Inch.
Fig. 225.—Four Rip Saw Teeth Points to Inch.
Fig. 226.—Four and a half Rip Saw Teeth Points to Inch.
POSITION AND SHAPE OF SAW HANDLES.
In thrust saws the hand and arm of the workman occupy a definite position, and the line of pressure on the saw is thus very much determined by the inclination of the handle to the line of teeth prolonged backwards. If the handle be placed at such an angle that a large part of the resolved thrust be perpendicular to the line of teeth, then the bite may be greater than the other resolved portion of the power can overcome. At another angle the bite may be very little, and although the saw thus constructed would move easily it would work sweetly, but slowly. The construction is suitable for saws with fine teeth and for clear cuttings. With regard to single-handed saws only, whatever may be the other conditions required, most handles have curved hook projections, shown in Figs. 217 to 224, p. 61, which show a variety of shapes in hand saw handles. These projections are connected with the pressure of the sawyer on the teeth. If, in sawing, the hand bears upon the upper hook, then an increased pressure is given to the forward teeth; if upon the lower hook the pressure on the saw teeth is released, and there is consequent ease in sawing. The angle at which direct thrust ought to act upon the line of teeth in the saws is obviously very different. Each material may be said to have its own proper angle, and in some saws provision is made by two set screws opposite the hooks for varying the intersection of the line of thrust with the line of teeth. In the biggest one-man saw used in handicraft—a tool quite 4 ft. long—the upper hook of the handle is wanting (see Fig. 222), because under any circumstances the weight of the saw is more than sufficient, and therefore it is not requisite that any resolved portion of the workman’s energy should be compounded with this But the lower hook must be retained, that the weight of the saw may be taken from the work. For these reasons the line