One of the best-known manufacturers of emery-wheels classifies them according to their coarseness of grain and hardness. Class one, which is coarse and hard, is suitable for edging cast iron or steel, for taking gates and sprees from castings, and for general rough grinding. Class two, which is of medium coarseness and is hard, is about the same as class one, but may also be used to excellent advantage for chipping, moulding-knives, and lathe-tools, and for gumming saws. Class three, medium grain and soft, is suitable for grinding brass and for surface-work on steel and on cast and wrought iron. If used for edging, it will cut fast and freely, and will also wear away rapidly. Class four, which is hard and of medium coarseness of grain, is for grinding not only upon light work, but also upon all work which will not affect the shape of the wheel. Class five is used for grinding upon brass or other soft metals, for polishing fine surface work on iron and steel, and for sharpening tools. An extra class, good for "gumming" saws, is moderately coarse, soft and of open texture, and grinds freely, while it generates but little heat. The ordinary emery wheel frame, which so effectually supplanted the grindstone in most of its uses, has in the performance of good work been superseded by the univer sal grinding machine.
The Universal (p7. 19, fic. 9) has importantly modi fied machine-shop practice, as by grinding it readily gives an accuracy in finish previously considered commercially impracticable, and, moreover, demonstrates that most conical and cylindrical surfaces, soft or hard, can be better and more economically finished by grinding than by any other process. In many kinds of work it takes the place of a lathe, which it supplements by finishing other work previously roughed out. It does both internal and external grinding-, straight or taper, and finishes spin dles. arbors, either straight or angular cutters, reamers, jewellers' rolls, hardenod boxes, and standard plugs and rings. The sliding table (A) rests upon the bed (13) and carries the swivel-table (C). Thus, for grind lug tapers, the line of centres of the head- and foot-stock can be set at any angle with the sliding table without throwing the head-and-foot stock spindle out of line. The table (C) is provided with an adjusting screw (D) and a scale showing the taper both in degrees and in inches per foot.
l'he wheel-bed (E) is mounted upon a knee (not shown in illustration), and 11laV he set at any angle from o° to 9o° on either side of the line at right angles with the sliding table. The semi-circumference at the lower edge of the wheel-bed is graduated in degrees. The wheel-slide rests on the wheel-bed. The table may be fed and reversed automatically or by hand. The cross-feed is operated by hand. The head-stock fr) is attached to a basc-plate (0) bolted to the swivel-table (C), and is made to swivel about a centre-pin. The circumference of the swivel-table at its lower edge is graduated to degrees. A friction-brake enables the driving-drum
to he stopped almost instantly. The swivel-table can be moved to either side of its central position to grind tapers from o to 2 inches per foot and from o° to ro° in angular measurement. For grinding work on the face plate or chuck the head-stock can be set at any angle within the whole circle. Work can be revolved upon dead centres or upon one dead and one live centre. Two tapers can be ground, either external or internal, without changing any of the settings. Ample provision is made for wet and turn-bench is the most primitive ma chine-tool for producing articles of circular cross-sections by removing shavings or turnings from the exterior of the partlyfinished piece with a non-rotating tool while the work rotates. The principles of design, con struction, and'operation of the turn-bench and of the plain continuously rotating foot-lathe with simple tool-rest have been described at length under the head of wood-working machines (p. SS), the former being illus trated ill Figure i (pl. 2o) and the latter in Figure 2. In metal-working the tool requires firmer holding and guidance and the machine a slower rotative speed for a given diameter of the piece to be worked, although the speed varies with the hardness and other physical characteristics of the metal, \vitt' the width and the depth of the cut, and with other conditions familiar to the expert machinist.
for for turning and boring metals in a lathe are practically the same as those required in a reciprocating metal-planing machine. They have more obtuse angles than wood-working tools, and are made heavier not only that they may have sufficient strength, but also that they may carry off the heat engendered by the work of cutting. This heat is considerable under many working conditions, notwithstanding the use of oil, water, and various solutions applied at the point of cutting to lessen friction and at the same time to cool the work by convection. A description of the hundred or more varieties of lathe-tools for roughing, finishing, screw-cutting, boring, and chasing the different grades of wrought and cast iron, steel, brass, etc., would require a treatise in itself; we shall, therefore, briefly describe a few of the more usual and important tools, after noting the general principles governing their form and use. They require that for a given acuteness the strength of the cut ting-point or edge be kept as great as possible by its lower face (that along which the chip or shaving does not pass or the face next the newly-cut portion of the circumference), being kept, at the point of cutting, at the maximum possible angle with the tangent to the work, leaving merely the necessary clearance between the work and the tool-face, and keenness being obtained by inclining the cutting-face as little as strength will per mit to the same tangent.