The same edge may be obtained by a blade with a single chamfer the flat side of which is placed in either of the dotted positions of fig. 326. The first, or b, is that previously in common use in the ordinary moulding planes for mahogany, and c is almost the position of the bed for the iron of the mitre-plane, also pre-viously common: in all three planes, the ultimate angle of the face of the cutter is just 60 degreee from the horizontal.
Fig. 328 represents the mouth of the mitre plane full size, and fig. 329 the entire instrument one-eighth size. The stock is much less in height than in ordinary planes, and the iron lies at an angle of of about 25°, and is sharpened at about the ordinary angle of 35°, making a total elevation of 60°, which, together with the delicate metallic mouth, render the absence of the top iron unimportant, even when the plane is used lengthways of the fibres, although its ostensible purpose is to plane obliquely across their ends, as in the formation of mitre joints.
* See Transartions of the Society of Arts. 1825, vol. xliii. p. 85.
In all ordinary planes the mouth gets wider as the iron is ground away, because of the unequal thickness or taper form of the blade as seen at c, fig. 327. In the mitre plane this is avoided by placing the chamfer upwards, now therefore the position of the blade is determined by its broad flat face which rests on the bed of the instrument d, and maintains one constant position as regards the mouth, uninfluenced by the gradual loss of thickness in the iron.
The smoothing and trying planes are also made with metal soles, and with single irons of ordinary angles, as one great purpose of the top iron is to compensate for the enlargement of the mouth of the plane by wear, this defect is almost expunged from those with iron soles, and which are gradually becoming common, both with single and with double irons. See Appendix, Note A.H., page 978.
Some variation is made in the angles at which plane irons are inserted in their stocks. The spokeshave is the lowest of the series, and commences with the small inclination of 25 to 30 degrees; and the general angles, and purposes of ordinary planes, are nearly as follows. Common pitch, or 45 degrees from the horizontal line is used for all the bench planes for deal, and similar soft woods. York pitch, or 50 degrees from the horizontal, for the bench planes for mahogany, wainscot, and hard or stringy woods. Middle pitch, or 55 degrees, for moulding planes for deal, and smoothing planes for mahogany, and similar woods. Half pitch, or 60 degrees, for moulding planes for mahogany, and woods difficult to work, of which bird's-eye maple is considered one of the worst.
Boxwood, and other close hard woods, may be smoothly scraped, if not cut. in any direction of the grain, when the angle constituting the pitch entirely disappears; or with a common smoothing-plane, in which the cutter is perpendicular, or even leans slightly forward; this tool is called a scraping plane, and is used for scraping the ivory keys of piano-fortes, and works inlaid with ivory, brass, and hardwoods; this is quite analogous to the process of turning the hardwoods.
The cabinet-maker also employs a scraping-plane, with a perpendicular iron, which is grooved on the face, to present a series of fine teeth instead of a continuous edge; this, which is Bailed a toothing plane, is employed for roughing and scratching veneers, and the surfaces to which they are to be attached, to make a tooth for the better hold of the glue.
The smith's-plaue for brass, iron, and steel, fig. 380, has likewise a perpendicular cutter, ground to 70 or 80 degrees; it is adjusted by a vertical screw, and the wedge is replaced by an end screw and block, as shown in the figure, which is one-eighth In the planes with vertical irons, the necessity for the narrow mouth ceases; and in the smith's plane some of the irons, or more properly cutters, are also grooved on the faces, by which their edges are virtually divided into several narrow pieces; this enables the instrument to be more easily employed in roughing-out works, by abstracting so much of the width of the iron, and by giving it a greater degree of penetration, but the finishing is done with smooth-edged cutters, and those not exceeding from five-eighths of an inch to one inch wide
It is well known that most pieces of wood will plane better from the one end than from the other, and that when such pieces are turned over, they must be changed end for end likewise; the necessity for this will immediately appear, if we consider the shade-lines under the plane-irons a, b, fig. 331, to represent the natural fibres of the wood, which are rarely parallel with tthe face of the work. The plane a, working with the grain, would cut smoothly, as it would rather press down the fibres than otherwise; whereas b would work against the grain, or would meet the fibres cropping out, and be liable to tear them up.
It was explained in Chap. IV., Vol. I., that the handsome characters of showy woods, greatly depend on all kinds of irregularities in the fibres: so that the conditions a and b, fig.331,continually occur in the same piece of wood, and in which we can therefore scarcely produce one straight and smooth cut in any direction. Even the most experienced workman will apply the smoothing-plane at various angles across the different parts of such wood according to his judgment; in extreme cases, where the wood is very curly, knotty, and cross-grained, the plane can scarcely be used at all, and such pieces are finished with the steel scraper. This simple tool was originally a piece of broken window-glass, and such it still remains in the hands of some of the gun-stock makers; but as the cabinet-maker requires the rectilinear edge, he employs a thin piece of saw-plate, which is represented black and highly magnified at s, fig. 331. The edge is first sharpened at right angles upon the oilstone, and it is then mostly burnished, either square or at a small angle, so as to throw up a trifling burr, or wire-edge. The scraper is held on the wood at about 60°, and as the minute edge takes a much slighter hold, it may be used where planes cannot be well applied. The scraper does not work so smoothly as a plane in perfect order upon ordinary wood, and as its edge is rougher and less keen, it drags up some of the fibres, and leaves a minute roughness, interspersed with a few longer fibres.