CHAPTER XL. CUTTERS. THE action of revolving cutters, such as those used in planing and moulding machines, is similar to that of circular saws. For planing soft wood the bevel of the cutting edge of the iron should be more extended than when used for hard wood. About 25° to the face of the iron is the best angle, whilst for hard woods about 40° is found most suitable. They are occasionally worked at a more acute angle than this, but in working very hard woods the edges of the cutters are more liable to break. Cross-cutting cutters, such as those used in tenoning machines, should be arranged to work diagonally to the grain of the wood. An angle of about 15 degrees to the axis is usually found suitable for soft wood, as it is found the nearer they act with the fibre of the wood the smoother the work. Cutters for tenoning machines are made by some engineers slightly helical. We think, however, that anything gained in this manner is more than lost in the extra trouble involved in keeping them in order, as well as increased first cost. For planing wide surfaces M. Arbey, of Paris, has for some years used the spiral or twisted irons patented by Mareschal and Godeau. The advantage claimed for this form of knife is, that the pitch of the knives is so arranged that the end of one comes opposite to the beginning of the other, thus giving a continuous cut during the whole revolution of the cutter block. As part only of the whole length of the knives strikes the wood at the same time, the jar or vibration is thus considerably lessened; and, as they always present the same cutting angle to the wood, cross-grained and knotty stuff can be worked. The knives used are very light, being from one to two millimetres in thickness. They are, however, more difficult to manage than straight knives, and require considerable skill and care in keeping them in order; hence their very partial adoption. Where this form of knife is adopted an arrangement is generally made to sharpen them in their places on the cutter blocks by means of a revolving emery wheel. Whatever form of knife is used, the steel employed in its manufacture should be of the highest obtainable quality. We prefer cutters made of wrought iron faced with steel to those of solid steel, as being easier to work and less liable to fracture. In establishments where a large variety of woods are worked it is advisable to have several sets of knives ground to the various bevels found best suited to the work. Much has been written as regards tempering cutting tools. No absolute rules, we think, can be laid down, at any rate as regards wood-cutting tools; it simply resolves itself into a matter of practical experience. For working soft woods with knives of an acute bevel a light straw-colour temper is suitable, whilst for harder woods, where the bevel of the knives used is made more obtuse, the temper should be made slightly harder in proportion. Cutters should always be ground with a double bevel, leaving at the cutting edge, say, about in. to be whetted with a stone to a keen edge by hand. Several varieties of stone are suited to this purpose, but we have found nothing better than a good Turkey stone. All plane irons above 12 inches long should be ground in a sliding frame, fitted with adjustments for any desired bevel, as it is impossible to keep long irons, such as those used in panel-planing or trying-up machines, true with hand grinding against an ordinary fixed rest. In panel-planing or other machines for working wood of considerable width, instead of using two long planing irons extending the whole width of the machine, eight short ones should be arranged in succession, two on each of the four sides of the cutter blocks; this plan, although causing a little more trouble in adjusting the irons, does away with the difficulties often experienced in keeping irons of very great width in a satisfactory condition. In constructing moulding irons, a plan often pursued, but still essentially wrong, is to cut the shape of the required moulding on the edge of the steel and grind a bevel backwards from it. The result is the exact profile of the moulding is constantly liable to be altered when sharpening. In place of this the form of moulding should be always milled into the face of the cutter itself, as it thus, if sharpened to the proper bevel, retains its true form. A few years since a somewhat novel system of steel cutters for working wood was patented by M. Guilliet Perreau, of Auxerre, France. He claimed that in his cutters, when the profile of the moulding was once formed, it was unalterable, no matter how badly the sharpening might be done; that they made a cleaner cut and were less liable to accident. We have seen this form of cutter in opera tion in various machines and giving very satisfactory results. They are made from one piece of steel, in form something like a deep saucer; the periphery is shaped to the profile of the desired moulding, and has several openings, which are sharpened towards the centre and present as many cutting edges to the wood. These cutters can be modified in shape for tenoning and other operations; they possess several features of value, but, unless manufactured on a considerable scale and with special appliances, their first cost would be considerably in excess of the ordinary form. Fixed cutters for planing machines should be fitted with back irons, and the cutting edge arranged at a slightly oblique angle to the wood, as the shock on the knife is thus received gradually. This also applies to veneer-slicing machines when a sliding cutter-block is used, except in some kinds of wood where it is found necessary to cut the fibres of the wood the whole width 'of the board or block at the same moment. In establishments where a considerable number of moulding irons are in use, some half-dozen stones of fine grit should be mounted and turned up to fit the rounds and hollows of the most usual form of irons, which can thus be sharpened without the aid of hand filing, which is an expensive method and deteriorates considerably the cutting power of the steel, from the constant softening and hardening processes through which it has to go. In surfacing and squaring-up machines, where cylindrical gouges are used, especial care must be taken that they are set at the most suitable angle required by the nature of the wood, and that the temper of the |