Modern Machine-Shop Practice Part 204

Fig. 2751.

Fig. 2752.

Fig. 2753.

Fig. 2754.

Fig. 2755.

Fig. 2756.]

In Fig. 2750, for example, the grain of the wood runs lengthwise and the throat, or front face of each tooth, is hooking or hooked, so that the cutting edge will cut through the fibres at their ends before it is attempted to remove them from the main body of the wood. Suppose, for example, that the saw shown in Fig. 2750 was put into a piece of timber and a tooth pressed hard enough on the wood to leave a mark, and this mark would appear as in Fig. 2751 at E, extending across a width equal to the full width of the saw tooth. It would do this because the front face or throat B and the back face A are both at a right angle to the saw length as is denoted by the dotted lines. As the grain is supposed in Fig. 2751 to run lengthways of the timber, clearly the fibre between the indentation E and the saw slot is severed and would be removed as the tooth pa.s.sed farther down through the wood, the action of first severing the fibre at its end and then removing it being carried on by each tooth.

In Fig. 2752 is shown a cross-cut saw in action upon a piece of wood in which the grain or fibre runs across the timber, and in this case the teeth require to be shaped to cut on each side of the saw instead of directly in front of it, because in that way only can the ends of the wood fibre be severed before it is dislodged from its place.

To enable the cross-cut saw to accomplish this, one tooth cuts on one side of the saw slot and the next tooth on the other, as at A and B in Fig. 2751, from which it will be seen that as the grain runs lengthways of the timber, the fibres between the lines A and B will be severed at their ends by the extreme edges of the teeth before the thicker part of the tooth reaches them to remove them.

The necessity for this action may be plainly perceived if we apply the rip saw for cross-cutting and the cross-cut saw for ripping. Suppose, for example, we place the saw shown in Fig. 2750 to cut across the grain of the piece of timber, and as its tooth met the wood it would indent it as at G, Fig. 2751, and as this is in line with the grain, the tooth would wedge in the piece and the piece cut could not be dislodged without first tearing the fibres apart at each end. Or suppose we take the cross-cut saw and apply it for ripping (as cutting lengthways of the grain is called) and if we indented the surface with a single tooth it would leave a mark as at F, Fig. 2751, which is lengthways of the fibre, so that the tooth would here again wedge between the fibres and not cut them. The next tooth would make a mark parallel to F, but on the other side of the saw slot or kerf as it is called, still leaving the fibre unsevered at its ends where it should be severed first.

In order that the saw may not rub against the sides of the slot or kerf, and thus be hard to move or drive, it is necessary that the kerf be wider than the thickness of the saw blade, and to accomplish this the teeth are bent sideways, each alternate tooth being bent in an opposite direction, as shown in the front view of the teeth in Fig. 2753. This bending is called the set of the saw, and should be sufficient to make the kerf about two-thirds wider than the thickness of the saw blade.

While preserving the feature of severing the fibre before attempting to dislodge it from its place, we may at the same time give the teeth of rip saws more or less sharpness by fleaming their faces.

In Fig. 2754, for example, the throat face is filed square across or at a right angle to the length of the saw, but the back face A is at an angle, making the points of the teeth sharper, and therefore enabling them to cut more freely. The result of this fleam would be that the tooth, instead of cutting equal and level all the way across as in Fig.

2751 at E, would cut at the corner first and only across its full width as it entered deeper into the wood; we have, in fact, placed the leading part of the cutting edge more at the extreme point and less in front of the tooth.

In Fig. 2755 the throat or front face of the saw is given fleam, as shown by the line B, which is not at a right angle to the saw length, and as a result the cutting edge is carried still more advanced at the point and more towards the side of the tooth and we have, therefore, to a certain extent, qualified it as a cross-cut saw.

We might give the face B so much angle as to carry the leading part of the cutting edge to the side of the saw, thus giving it the characteristics of a cross cut.

In Fig. 2756, both the throat face B and the back face A are given fleam, making the points extremely sharp, and showing the leading part of the cutting edge towards the side, the corner leading still more.

[Ill.u.s.tration: Fig. 2757.]

In Fig. 2757 we have two saws R and S, the latter having fleam on the front and the former on the back face of the tooth, the amount or degree of fleam being equal.

[Ill.u.s.tration: Fig. 2758.]

In Fig. 2758 we have indentations of their teeth. The teeth of S would leave a mark as at E F, and R would leave a mark as at G H. The side cut F being more than the side cut G, and the front cut E being at a less angle to F than the front cut H to G, it follows that the saw S would be the best, provided the grain of the wood ran diagonally as shown, not only because it has more side and less front cut, but also because its cutting edge is keener on the side, as is seen on comparing the lines P and A in Fig. 2757.

[Ill.u.s.tration: Fig. 2759.]

If we give fleam to both faces we alter the indentation, as denoted in Fig. 2759, in which E F represents the line of tooth cut when one face has fleam, and G H the line of tooth cut when both faces are fleamed, the shape of the actual saw cut being shown at J.

Obviously the fleam makes the points weak, but this in coa.r.s.e saws may be partially remedied by shaping the teeth as in Fig. 2760. Fleam on the front face or throat of the tooth has the effect of preserving its set, the pressure of the cut being as shown by the arrows in Fig. 2753.

It is evident that the finer the point of the tooth the sooner it will become dulled, and that the harder the timber the more quickly the tooth will become dull. So soon as this occurs the teeth refuse to cut freely, and the extra pressure on them acts to spring them upward and to take off the set. It is obvious that for soft wood the teeth may be given fleam on both faces, and that the front face should have some fleam, even for the hardest of wood, whether the back face has fleam or not.

Also, that in proportion as the grain of the wood runs more across the saw kerf than in line with it the teeth should be filed to cut on the side, and the hook of the front face may be lessened, while _vice-versa_, in proportion as the grain of the wood runs parallel with the kerf, the tooth may have hook and fleam on the back face with a slight fleam on the front one.

[Ill.u.s.tration: Fig. 2760.]

[Ill.u.s.tration: Fig. 2761.]

GAUGES.--Of gauges for marking on the work lines parallel to its edges there are several kinds, a common form being represented in Fig. 2761, in which the block that slides against the edge of the work is secured by a set-screw.

A better method, however, is to use a key set at a right angle to the stem, so that the head may be tightened or loosened by striking it, as if it were a hammer, against anything that may happen to lie on the bench, hence the gauge may be set and adjusted with one hand while the other is holding the work, as is often necessary when marking small work. The marking point should be a piece of steel wire fitted tightly in the stem, the protruding part being ground or tiled to a wedge, with the two facets slightly rounding, and whose broad faces stand at a right angle to the stem of the gauge, the point or edge only projecting sufficiently to produce a line clear enough to work by; otherwise it will not be suitable for accurate work.

[Ill.u.s.tration: Fig. 2762.]

The mortice gauge, Fig. 2762, is similar to the above as regards the stem and sliding piece, but it is provided with two marking points, their distance apart being adjustable. The head screw works in bra.s.s nuts.

[Ill.u.s.tration: Fig. 2763.]

For lines that are to be marked more than about ten inches from the edge of the work a broader base is necessary to the head or block, which may be shaped as shown in Fig. 2763.

[Ill.u.s.tration: Fig. 2764.]

The lines drawn upon pattern work require to be very fine, and for this purpose the cutting scriber, Fig. 2764, is employed. The end A is bevelled off on both sides like a skew chisel. The end B is ground to a fine point and both ends are oilstoned. The point end is for drawing lines with the grain, and the knife end for lines across the grain of the wood. The wooden handle is to afford a firm grip.

[Ill.u.s.tration: Fig. 2765.]

In Fig. 2765 we have the cutting gauge, in which a steel cutter takes the place of the marking point, being wedged in position. It is employed to cut thin strips of wood, that is to say, of thicknesses up to about a quarter of an inch. The cutter point should be tempered to a dark straw color.

The princ.i.p.al forms of joints employed by the pattern-maker are as follows:--

[Ill.u.s.tration: Fig. 2766.]

Fig. 2766 represents the mortice and tenon, the thickness of the tenon being one-third that at C, which leaves a thickness at E and D equal to that of the tenon. When the mortice is away from the end of the work the breadth B of the tenon is made less than the breadth F of the work so as to leave stuff at A to strengthen the mortised piece. To make this joint the two pieces, having been planed or otherwise made to size as required, are marked for the position and length of the mortice in one case, and for the length of the tenon in the other; both pieces are now gauged with a mortice gauge, both being marked alike; and then from the face side we mark a tenon or mortice of the dimensions required.

[Ill.u.s.tration: Fig. 2767.]

If the stuff is broad two or more tenons and mortices may be given, as shown in Fig. 2767.

[Ill.u.s.tration: Fig. 2768.]

To lock the tenon in the mortice two methods may be employed. In the first and preferable one the mortice is tapered, as in Fig. 2768, and the two wedges are inserted and driven home. In the second the tenon is provided with saw cuts to receive the wedges.

[Ill.u.s.tration: Fig. 2769.]

A very superior method of jointing is the dovetail, shown in Fig. 2769, which is serviceable for connecting the ends and sides of a box, or any article in that form. The strength of the corner formed in this way is only limited by that of the material itself; therefore it should be preferred when available in making standard patterns, or for work too thin to admit nails or screws; the corner formed by this joint is not limited to 90 or a square, so called, but may form any angle. Nor is it imperative that the sides or ends of the box or other article be parallel. They may incline towards one another like a pyramid; a mill hopper is a familiar example of this. If it be required to dovetail a box together, get out four pieces for the sides and ends, to be of the full length and width respectively of the box outside. They are to be planed all over, not omitting the ends. The gauge, that is already set to the thickness of the stuff, must now be run along the ends, marking a line on both sides of each piece. Then mark and cut out the pins as on the piece A; the dovetail openings in B are traced from the pins in A.

The pieces having been tried and found to go together are finally brought into contact and held in their places with glue.

[Ill.u.s.tration: Fig. 2770.]

Fig. 2770 is a mitre joint, the only one serviceable to mouldings, pipes, and other curved pieces. It is not a strong form of joint, and is only used where the preceding kinds are inapplicable. It is made with glue, the pieces having been previously sized; and as an additional precaution, if the work will admit, nails, brads, or screws are inserted at right angles to one another.

[Ill.u.s.tration: Fig. 2771.]