Fig. 313.
Fig. 314.
Fig. 315.
If one considers the movement of the piece (of work) when the wheel M is concentric with the mandrel, it will be perceived that although it makes two revolutions upon its axis, yet inasmuch as it has no eccentricity it will describe no particular curve or figure; but if an eccentricity of three divisions is given to it, two buckles or loops will result as in [Fig. 315]. Before cutting the material, however, approach the tool as near the work as possible, and putting the lathe in motion, observe whether the buckle[23] passes too near the centre or too far from it, and also how near it goes to the circumference. If another change wheel with forty teeth instead of sixty is substituted, the slide C, being adjusted accordingly, three loops will be described (forty being one-third of one hundred and twenty), but it is always necessary before actually cutting the material, to try whether the buckles will pass near to the centre without going beyond it. The result of the latter movement will be shown presently, as it entirely alters the appearance of the pattern. The divisions commence on the arc at the left hand, the index resting at 0° when the plate is against the stop L, and the screw of the chuck concentric with the mandrel. The preceding figure of three loops will become similar to [Fig. 317], retaining the same wheels and some degree of eccentricity, but by means of the slide rest moving the tool towards the circumference, so that the buckles overlap the centre. The effect thus produced is, that of a set of three curvilineal triangles of which the apex of one falls upon the base of the next. The use of this chuck is stated to require upon the part of the operator more care than any other, as regards the derangement of work or tool in the least during the operation, as, if either is once moved in the least out of position, it will be found next to impossible to strike the line again, owing to the peculiar nature of the curve, for although the tool may be replaced upon any one part of the line already cut with the intention of deepening—it is by no means certain that it will trace the same curve again. This curve, says Bergeron does not produce an agreeable effect on the cover of a box, unless it is very finely cut, the tool, therefore, should be very sharp in the angle and very keen. Bergeron specially mentions this in reference to filling the cuts with thin strips of horn or shell, a method of inlaid ornamentation not much known or admired in the present day, but to which allusion may probably be made again in this series. To form the second set of loops, which are parallel with the first in these designs, it is only necessary to use the leading screw of the slide rest, to move the tool nearer to or further from the centre, while the eccentricity and the arrangement of change wheels remain as before. It is scarcely necessary to detail the formation of the larger series of four loops and upwards, as these are simply the result of different sized change wheels: the following principles, however, by which the buckling of the several loops is controlled or prevented, may perhaps be serviceable. "When, for example an eccentricity of sixteen divisions is used—if the tool is placed at a distance of two such divisions from the centre of the piece, a line only will be produced of as many curves as the wheel or pinion D would produce buckles. If the tool is moved further from the centre by a quarter division, the angles (connecting the curves) will be more defined, but still no buckles will be made. A little further movement of the tool will produce very small buckles which will thus gradually increase as the tool is set further and farther from the centre—until at last when the curves pass beyond the centre, the result is arrived at already shown in [Fig. 317]. Another form of this chuck is shown in [318], in which, instead of the iron plate being pivotted for the purpose of eccentricity upon the axis of the wheel E, a parallel slide motion is given to the main wheel by guide bars, as in the eccentric and oval chucks. This form is figured in "Lardner's Cabinet Cyclopædia." The large front wheel carrying the screw for chucks is pivotted to the slide C, and protected by a plate D which nearly covers it. The wheel L, is arranged to follow this slide, so as to remain in gear with the large wheel without leaving the fixed wheel or ring on the face of the poppet. In both patterns of this chuck the front wheel is used as a division plate, being moved in either direction as many cogs as desired to produce interlacing of the looped designs. It is better, however, to add a racked division plate and tangent screw, as in the eccentric chuck to act as one piece with the chuck screw, and with the latter turning on a conical pin in the centre of the large wheel underneath. The above apparatus requires to be used with a slow motion owing to the complication of parts, and the whole ought to be so well constructed, that the various wheels revolve with perfect smoothness and without shake or noise.
[23] The word buckle is used to signify the small loops—not the large curves.
