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small, as then, the expansion it acquires when at work, from the heat and friction, will enlarge the edge just sufficiently to bring the saw into a uniform state of tension. Otherwise, if before the saw is set to work the edge is fully large enough, when expanded by the heat it is likely to become loose on the edge, and to vibrate from side to side, without proper stability; so as to produce a wide irregular cut, and make a flanking whiplike noise, arising from the violent vibration of the buckled parts of the plate, in passing through the saw kerf; the sides of the wood will then exhibit ridges like the ripple marks on the sandy shore.

In hammering all plates, preference should in the like manner be given to keeping the edge rather small or stiff, to serve as a margin or frame to the more loose parts within; it gives a degree of stability somewhat as if the object had a thickened rim, and when a rim really exists, the process of flattening is comparatively easy.

If by undue stretching, the edge is made too loose, the whole piece is flaccid and very mobile; and we seem to lose the governing power, or those retaining points by which the changes of the plate are both influenced and rendered apparent; the edge should be therefore always kept somewhat tight, by being proportionally less hammered, especially as it more easily admits of expansion than the inner part.

As a general rule, it may be said that every part of the plate which is straight and tense, whilst others are curved and flaccid, denotes that each straight part is under restraint; and that its straightness is due to its being, as it were, stretched either lengthways or around its edges, by the other parts which are too loose, and therefore arched, and also strong. In such cases, the straight lines require to be extended in length, to allow sufficient room for the curves to expand to their proportional sizes. This refers not only to small local errors towards the inner part of the plate, as explained by diagram, fig. 288; but should the one edge of a plate be tolerably straight, whilst the opposite is quite loose and flaccid, the rule also applies with equal truth, and the straighter side must be hammered; in this case the curved side is as it were a great bulge cut in two parts.

Should a circular saw have a sudden dent, such as at g, fig. 290, standing the reverse way, and which may result from its

having rested upon a small lump of coke whilst in the fire, the first blows will be given on the hollow side, between the lines i i, to lessen the abruptness of the margin by stretching it to the dotted curve, and then it will be driven downwards by violent blows, to form a part of the general sweep or concavity; a little time is gained by these driving blows, over the mode of stretching by the hammer.

The foregoing descriptions have all referred to solid blows, upon the face of the hard anvil, but to expedite the process, recurrence is often had to blocking, which is only one application amongst many others of a wooden anvil or block with a narrow flat-faced hammer, such as fig. 263. In this case the blows are to a certain extent hollow, as the wood immediately beneath the hammer-face yields to the blow, whereas the margin around the same does not. Such blows are therefore hollow, and bend with very little stretching.

The blocking is considerably employed in saw-making, after the loose parts have been entirely removed, as the hollow blows correct any slight errors of figure, by bending alone, and with little risk of stretching the plates, if the work be delicately performed.

Towards the conclusion however, each of the different modes of work is required to be used in combination, as the true condition of the plate is only the exact balancing of all the forces, or of the tension of the several parts; and it constantly happens that attention to one error causes a partial change and fluctuation throughout the whole. It therefore requires great tact to know when to leave the anvil for the block, and when to return to the anvil, and so on alternately; and also which side of the plate should be upwards for the time, which particular points should be struck, and the required force of the blows.

Of course, within certain limits, a thick plate is easier to hammer than a thin one, as the latter is difficult from its excessive mobility; also a soft plate of iron is more difficult than a hard plate of steel, although the latter requires more blows to produce the same effect; but when the works are very thick they become laborious, and the difficulty always increases with the size of the plate.

Those who may desire to practise this art should therefore commence with a plate some 4, 6, or 8 inches square, and moderately stout, and subsequently proceed to pieces larger and

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FLATTENING THIN PLATES.

thinner. They will also find some advantage in raising the anvil to within about a foot of the eye, as the alterations can be then more easily seen whilst the work lies on the anvil, and the effect of any predetermined blows can be better watched. One other observance is essential, namely, patience; as although the process is thoroughly reducible to system, and no blow should be struck in vain, the beginner will frequently find it necessary to pause, examine, and consider, especially as the errors decrease. Whereas the accustomed eye will follow the fluctuations of the plate almost without intermission of the blows, and will also accomplish the task with the fewest possible number of blows, which is the great desideratum.

Indeed it may happen from hammering some parts of a plate excessively and improperly, that it is rendered so hard and rigid, as to make its correction very tedious, or indeed nearly impossible without previous annealing, as it might burst or crack from the extension being carried beyond the safe limit of malleability. As in raised works, the annealing is mostly done by a gentle red heat; but in hardened steel plates, a slight increase of temperature barely sufficient to discolour the plate, will make a perceptible difference; and this latter process is always the last step in making a saw, in order to restore, by a gentle heat, the proper elasticity which has been mysteriously lost in the grinding polishing and hammering required in its manufacture.

CHAPTER XX.

PROCESSES DEPENDENT ON DUCTILITY.

SECT. I.-DRAWING WIRES, ETC.

THE ductility of many of the metals and alloys, or the quality which allows them to be drawn into wire, is applied to a variety of curious uses in the manufacturing arts, and the process may be viewed as the sequel to the use of grooved and figured rollers; but the ductile metals submit to this various degrees of perfection.

process with

In drawing wire, the metal is first prepared to the cylindrical form, either directly by casting, or between rollers with semicircular grooves; and the process is completed by pulling the metal through a series of holes gradually less and less, made in a metallic plate, by which it becomes gradually reduced in size, and elongated; but as in rolling, the process of annealing must be resorted to at proper intervals.

In general, the draw-plates are made of hardened steel, and they are formed upon the same principle, whether for round, square, or complex sections, either solid as wires, or hollow as tubes; the substance of the metal is partly kept back, as in a wave, by a narrow ridge within the draw-plate, acting as a burnisher.

The section of the holes is explained by fig. 60, p. 173, which represents one of the jewel draw-plates patented by Mr. Brockedon, for gold, silver, and other fine wires; but the plates are generally made of hardened steel, or else of alloys of partly similar nature, which allow the holes to be contracted and repaired, by closing them with blows of a pointed hammer or punch around the hole.

The holes for round wires are sometimes ground out from both sides upon the same brass cone or grinder, the sides of which vary from about 10 to 30 degrees, according to the metal to be drawn; for the sake of strength the ridge is always nearer to the side on which the metal enters, and the sharp edge is

also removed, either by wriggling the plate upon the grinder in order to round the inside, or in any other manner*.

The end of the wire is pointed to enable it to be passed through the hole, and it is then caught by a pair of nippers, themselves at the extremity either of a chain, rope, toothed rack, or screw, by which the wire is drawn through by rectilinear motion. The nippers or dogs resemble very strong carpenters' pincers or pliers, the handles of which diverge at an angle; they are sometimes closed by a sliding ring at the end of the strap or chain, which slides down the handles of the nippers; there are some other modifications, all acting upon the same principle, of compressing the nippers the more forcibly upon the wire the greater the draught. It requires a proportionally strong support to resist the strain; and to avoid the fracture of the hardened steel drawplate, it is usually placed against a strong perforated plate of wrought-iron. In manufactories where large quantities of wire are made, the wire is more usually attached to the circumference of a reel, which is made to revolve by steam or other power.

It is frequently necessary to anneal the wire, although no general rule can be stated in respect to its recurrence; and before resuming the drawing process, the wire is invariably immersed in some acid liquor or pickle, to remove the slight coating of oxide, which would otherwise rapidly destroy the plates, (as many of these metallic oxides are actually used as polishing materials,) and in general some lubricating matter is applied to reduce the friction, as beer-grounds, starch-water or oil; and for gold and silver, wax is generally used.

Most of the wire is drawn upon reels, and is therefore met with in circular coils, and it is necessary, in almost every case, to straighten it before use. The soft or annealed wires, such as the copper wire used for bell-hanging, the soft iron binding-wire used in soldering, and others, are stretched and straightened by fixing the one end, and pulling the other with a pair of pliers; or short pieces of soft wire may be straightened by rolling them between two flat boards t.

* Sometimes the plate is made with three cones instead of two; the third cone is exaggerated in fig. 302, p. 429, which represents the arrangement for drawing tubes, the central cone being just equal to the wave, or the quantity the metal is reduced. Under any circumstances all the keen edges are removed, as they would tear instead of compress the material.

† Soft steel wire for making needles is straightened by rolling or rubbing: it is