Valuable and beautiful woods are seldom used in the solid state for decorative furniture, but are cut into veneers or thin plates, to be glued upon fabrics made of less expensive woods, an art successfully practised by the Romans, as formerly adverted to (Vol. i., page 64). Until of late years the cutting of veneers was generally accomplished, either at the saw-pit with very thin plates strained in the ordinary pit-saw frame, (see Vol. ii., page 703), or by the cabinet-maker with the smaller frame-saw, (page 726). In this latter mode, which is still much practised on the continent, the wood is fixed perpendicularly, and the saw is also guided by two men. Expert pit-sawyers could cut six veneers out of each inch of wood, and cabinet-makers seven or eight from smaller pieces, but the difficulty of these methods rapidly increases with the size of the veneers.
Small veneers for the backs of brushes and other works, have been split or planed from small pieces squared to the respective sizes. Pine, willow, and other woods, are planed into thick continuous shavings called scale-boards, for making hat and bonnet boxes (Vol. ii., p. 504). And of late years oak, when softened by steaming, has been split into staves for casks (foot-note, Vol. i., page 32). All these processes are accomplished without waste of the materials, but they are only applicable to pieces of limited dimensions.
* See the original paper, Trans. Soc. of Arts, Vol. xlvii., pp. 121-7. In the year 1883, Mr. Samuel Hamilton took out a patent for "certain machinery for sawing, boring, and manufacturing wood for various purposes, such as bevilled timber for ship-building, tenon cheeks, felloes of wheels, the circular rails of chair backs, choir legs, and other works of the same description, either square on the face, or bevilled to any required angle, or in any required radius or diameter of a circle."
The specification is Tory complex, but it may be said briefly, that the felloes are cut by a vertical reciprocating saw worked by a crank, and the edge of the work is guided either by a fixed circular fence, or by radius bars; for bevilled works the table of a similar machine is tilted to any angle. For other classes of work, the saw-frame is jointed, and may be brought down by a swing-frame in the arc of a circle, to penetrate to any assigned depth. The work is grasped by numerous arrangements of parts, that hold any successive number of pieces exactly in the same position. - See Newton's London Journal and Repertory, etc., Vol. vii, p. 1.
In 1806, Mr. Brunei took out a patent for splitting veneers, of considerable size, by means of a horizontal knife, the length of which exceeded the length of the block to be converted. The knife was composed of several pieces of steel, placed exactly in a line on their lower surfaces, but with edges faintly rounded and very keen. The compound knife received a short reciprocating or sawing action, and the block of mahogany or other wood was carried slowly sideways, and beneath the knife by a strong screw slide, worked with a spoke wheel, like that by which a ship is steered. After one veneer had been cut off, and the log brought back again to its first position, it was raised in exact parallelism, by a system of two right and two left-handed screws at the four angles of the frame, which were simultaneously moved with one winch-handle, by aid of appropriate mechanism.*
This machine for cutting or splitting wood into veneers, the precursor of the segment veneer-saw, is said to have answered moderately well with straight-grained and pliant woods, such as Honduras mahogany, but there were serious objections to its use for woods of irregular, harsh, and brittle grain, such as rosewood; as the veneer curled up considerably on removal, and the wood if harsh and brittle had a disposition to split and become pervious to the glue.† This is to be regretted, as the splitting-machine converted the whole of the wood into veneer without waste, whereas the veneer-saw, on the average, cuts one-third of the wood into saw-dust.
As already explained, the ordinary circular saw will not, in general, serve for work exceeding in thickness about one-third the diameter of the saw, and the larger the saw, the thicker it is required to be, to give a proportionate degree of stability. These two conditions, joined to the impracticability of obtaining plates of steel exceeding some 4 or 5 feet diameter, limit the application of the circular saw under ordinary circumstances.
* Set the drawing and description in the Rep. of Arts for 1810, Vol. xvi., p. 257.
† The Russian machine for cutting the entire tree into one spiral veneer, Vol. i., p. 154,) seems open to the same objection in regard to brittle woods, neither does it expose the most ornamental section of the tree.
But when this instrument is employed for veneers, advantage is taken of the pliancy of the thin leaf or veneer, and the saw is consequently made thick and strong towards the center, to give it the required stability, but towards the edge it is thinned away almost to a feather edge, as at s s, in the diagram, fig. 802. Therefore the solid block of wood or ivory w, which is unyielding, can pass along the parallel guide g, and across the flat face of the saws s s, whilst the thin pliant veneer v, separates so much as to form an opening that admits the wedge-formed edge of the blade, and the veneer proceeds along the conical back of the saw without fracture or interruption; circumstances that would be impracticable were both parts of the material when sawn, alike unyielding.
In the small application of this principle, as for sawing blocks of ivory into leaves for miniatures, and small square pieces of wood into veneers for brushes and small works, the veneer-saw is made as a single plate of steel, from 6 to 36 inches diameter. In the large application of the principle, as for cutting logs of square or round wood into veneers, the saw is composed of many segments or plates, and commonly varies from about 5 to 18 feet diameter. But as the segment-saws are occasionally made as small as 20 inches diameter, the two kinds constitute an unbroken series, and their principal applications will now be described, beginning with the smallest.
The single-plate veneer-saw (described in section 2 of the table, on page 784), is thick and parallel at the center for about one-half its diameter, the edge is ground away, as a cone, almost to a feather edge; at other times the edge is thin, and nearly parallel for about an inch, and is then gradually coned, making the section somewhat concave. The edge is required to run exceedingly true, and the teeth must be sharp and very faintly set.