The frog, then, (so called because it seems to leap when the Machine works) must now be adverted to: it consists of an under mass, formed prismatically to fit exactly the slide n q, cut out of the porte-outil N. This mass is capped by a thickness of steel, which completes the passage for the rack n w, and offers, besides, a compartment for the punch-clams o, and another (x) for a wooden or steel bridge, being a portion of a cylinder, so formed, as to support the engraved cylinder after the stress of the impression is passed, and thus to equalize the depth of the engraving. The compartment for the punch-clams at o, is terminated to the right hand by an obtuse angle near x, which serves as a centre, when, by proper fixing screws in the rim near o, it is found necessary to place the punch a little awry. The other properties of this frog will easily be supposed by my mechanical readers.
We come, then, to it’s motion in the slide. p r shews a wheel, running loosely on the axis of the porte-outil; and having fixed to it a concentric rim r, with three or four waves in it’s circumference. Further, above s, is seen a lever, turning on a pin in the stud s, and pressing against the right-hand end of the rack w, when driven to the left by the waves p r, &c. This rack is cut into ratchet teeth as at w, in which enters the catch o, as impelled by a proper spring acting on it, (but not seen in the figure.) As long then, as the waved wheel p r can turn, with the porte-outil N, this last described mechanism does nothing: but when p r is stopped, it begins to work usefully; for the lever s then rides on the waves p r, and presses the rack w against the spring n, so that the catch o, takes into some new tooth; by which means, when the spring n unbends (by the sinking of the lever s into any wave p) the frog is itself carried toward the right hand—which is the effect intended. But, in fine, how is this wheel p r stopped and set agoing a propos? [Fig. 5] will shew this, with the aid of a little imagination—since our [fig. 5] is a kind of transparency rather than a regular view. The wheel m, is a crown wheel, near which the wheel p r ([fig. 3]) turns, having a spiral g on it’s hither surface, which runs between the teeth of the wheel m and turns it one tooth, in each of it’s own revolutions: But when, after a given number of these turns, the end of the spiral g meets with a large tooth on m, it lodges on it, and stops the motion of the wheel p, and then the aforesaid waves r perform the task of driving the rack w backward; after which the spring n changes the place of the frog, so as to make another line of impressions round the cylinder. It remains then, only to be explained, how this stoppage is itself stopped; which is thus: to the porte-outil is fastened, near g, a small arm, which turns with it, and which in [fig. 5] the dot t represents. This arm, therefore, drives back the beak t, (connected with the spring v) at every revolution of the porte-outil, thereby working the small catch that hangs to that beak. This catch, therefore, slides on the edge of the crown wheel m, but produces no effect, until it finds there, one small notch, so placed as to be acted on by the catch when this disengagement is wanted—and, then, this motion jogs forward the crown wheel m just enough to take the large tooth out of the way—when the spiral g begins to move through the common teeth of m, and thus ceases to act on the rack till the large tooth again comes to stop the wheel p, and recommence the rack’s motions. And thus is the place of action of the punch changed after any number of it’s contacts with the cylinder—that number being doubled or trebled—or more—when necessary, by increasing accordingly the number of common teeth in the crown wheel m, before a large tooth occurs.
A few practical remarks on this mode of engraving may here be added with advantage. Theoretically speaking, the punch should form a portion of a cylinder, of equal radius with the porte-outil wheel, taken at it’s pitch line. But through the relative weakness of some mandrels, a certain spring takes place, which requires the punches to be more curved than that wheel, and even considerably so. This also depends on the size of the punch, and the fullness of the pattern. In a word, it depends likewise on the method of employing the Machine—whether with few passages, and considerable pressure, or with light pressure, and many swift passages:—The latter System is in my opinion much the best; since it brings the practice nearer to the theory of this Machine. If, indeed, the cylinders and mandrels of Calico Printers, had been originally made thicker, and thus strong enough to bear the pressure without sensible deflexion, this would have been, from the first, a perfect process: and the nearer these objects are brought to this state of inflexibility, the nearer will it’s effects approach to perfection; for in all other respects it works with admirable precision.
I may just add, that the facility with which the revolutions of this Machine are counted, has induced some persons to dispense with the rack movement: but for small patterns with numerous impressions, it is doubtless better to use it—especially when employing the rapid and light pressures just alluded to; and these will become additionally interesting when the punches themselves acquire a more exact form—which is the object of the third [Punch Machine], still remaining to be described.
It is not superfluous to add, that this Engraving Machine is dangerous to the persons employed—and should therefore be guarded behind, by a fence-bar, to prevent the hands or clothes from being drawn in.
OF
A HORIZONTAL WATER WHEEL,
Probably the best of the impulsive kind.
