The punches used in fly-presses do not differ materially from those already described, but it appears needful to commence this section, with some explanation of the principal modifications of the press itself. The fly-press is a most useful machine, which, independently of the punch or dies wherewith it is used, may be considered as a means of giving a hard, unerring, perpendicular blow, as if with a powerful well-directed hammer. The precision of the blow is attained by the slide whereby the punch is guided, the force of the blow by the heavy revolving fly attached to the screw of the press. When the machine is used, the fly is put in rapid motion, and then suddenly arrested by the dies or cutters coming in contact with the substance submitted to their action. The entire momentum of the fly, directed by the agency of the screw, is therefore instantaneously expended on the work to be punched or stamped, and the reaction is frequently such as to make the screw recoil to nearly its first position.

The bare enumeration of the multitude of articles that are partially or wholly produced in fly-presses, would extend to considerable length, as this powerful and rapid auxiliary is not only employed in punching holes, and cutting out numerous articles from sheets of metal and other materials, but also in moulding, stamping, bending or raising thin metals into a variety of shapes, and likewise in impressing others with devices as in medals and coins.

Fig. 952 represents a fly-press of the ordinary construction, that is used for cutting out works, and is thence called a cutting press, in contradistinction to the stamping or coining presses. It will be seen the body of the press, which is very strong, is fixed upon a bed or base that is at right angles to the screw, the latter is very coarse in its pitch, and has a double or triple square thread, the rise of which is from about one to six inches in every revolution. The nut of the screw is mostly of gun-metal, and fixed in the upper part or head of the press. The top of the screw is square or hexagonal, and carries a lever of wrought iron, terminating in two solid cast iron balls, that constitute the fly, and from the lever the additional piece h, descends to the level of the dies to serve as the handle, so that the left hand may be used in applying the material to be punched, whilst the right hand of the operator is employed in working the press.

The screw is generally attached to a square bar called the follower, which fits accurately in a corresponding aperture, and is strictly in a line with the screw; and to the follower is attached the punch shown detached at a. The punch is sometimes fitted into a nearly cylindrical hole, and retained by a transverse pin or a side screw, but more generally the die is screwed into the follower, like the chucks of some turning lathes; the bed or bottom die c, which is made strictly parallel, rests on the base of the press, and is retained in position by the four screws, that pass through the four blocks called dogs; these screws, which point a little downwards, allow the die to be accurately adjusted, so that the punch may descend into it without catching at any part, and thereby inflicting an injury to the tools.

Fig. 952

Section III Punches Used In Fly Presses And Miscel 200265

The piece b, which rests nearly in contact with the die, is called the puller off; it is perforated, to allow free passage to the punch; when the latter rises, it carries up with it for a short distance the perforated sheet of metal that has been punched through, but which is held back by the puller off, whilst the punch continuing its ascent rises above the puller off, and leaves behind the sheet of metal so released; the sheet is again placed in position whilst another piece is punched out, and so on continually.

Before proceeding to speak of some of the works produced in stamping presses, it is proposed to describe some of the points of difference met with in fly-presses.

The body of a cutting press is in general made with one arm, as represented in fig. 952, because the sheet of metal can be more freely applied to the die, but stamping and coining presses, which are used for pieces that have been previously cut out, require greater strength, and have two arms, or are made somewhat as a strong lofty bridge with the screw in the center.

The fly of the press is frequently made as a heavy wheel, which may be more massive and is less dangerous to bystanders than the lever and balls, and in large presses there are two, three, or four handles fixed to the rim, as many men then run round with the fly, and let go when the blow is struck.

Fly-presses are variously worked by steam power; thus in the Royal Mint the twelve presses for cutting out the blanks or disks for coin, are arranged in a circle around a heavy fly-wheel, which revolves horizontally by means of the steam-engine. The wheel has one projecting tooth or cam, which catches successively the twelve radial levers fixed in the screws of the presses, to cut the blanks, and twelve springs immediately return the several levers to their first positions, ready for the next passage of the cam on the wheel.

The fly and screw are also worked by power, in some cases by an eccentric or crank movement fixed at a distance, a long connecting rod then unites the crank to an arm of the wheel, or to a straight lever, and gives it a reciprocating movement.

At other times, in place of the crank motion are ingeniously substituted a piston and cylinder worked after the manner of an oscillating steam-engine, if we imagine the boiler to be superseded by a large chamber, exhausted by the steam-engine nearly to a vacuum, thus constituting an air engine, the one side of the piston being opened for a period to the exhausted chamber, whilst the other receives the full pressure of the atmosphere. This mode is adopted in several Mints, constructed by Mr. Hague, of London, for foreign countries, and the author believes it is also employed for the stamping or coining presses of our national Mint.*