Machine for Local Corrections.
The mirror a, is carried by an iron chuck or table b, covered with a triple fold of blanket, and is prevented from slipping off by four cleets c c′. The vertical shaft d passes through a worm-wheel e, the endless screw of which f, is driven by a band g, from the primary shaft h. At i is the band-wheel for connection to the foot-power. At one end of the primary shaft is firmly fixed the cogwheel k, which drives the crank-shaft l. Attached to the horizontal part of l, is the crank-pin m. The two bolts n n′ move in a slot, so that the crank-pin may be set at any distance from 0 to 2 inches, out of line with l. Above, the crank-pin carries one end of the bar o, the other end passing through an elliptical hole in the oak-block p. Down the middle of the bar runs a long slot, through which the screw-pin q passes, and which permits q to be brought over any zone from the centre to the edge of the mirror a. It is retained by the bolts r r′, which are tapped into s. The local polisher is seen at t. The curve which the centre of the local polisher describes upon the face of the mirror, varies with the adjustments. Fig. 26 is a reduction from one traced by the machine, the overlapping being seen on the left side. The mirror is not tightly confined by the cleets c c′, for that would certainly injure the figure, but performs a slow motion of rotation, so that in no two successive strokes are the same parts of the edge pressed against them.
Fig. 26.
Hypocycloidal Curve.
The local polishers are made of lead, alloyed with a small proportion of antimony, and are 8, 6, and 4 inches in diameter, respectively. The largest and smallest are most used, the former on account of its size polishing most quickly, but the latter giving the truest surface. The rosin that covers them is just indentable by the thumb nail, and is arranged in a novel manner. The leaden basis, as seen at t, Fig. [25], is perforated in many places with holes, which permit evaporation, serve for the introduction of water where needed, and allow the rosin to spread freely. Grooves are made from one aperture to another, and the rosin thus divided into irregular portions. The effects of the production of heat are in this way avoided.
The mirror may be ground and fined on this machine, in the same manner as on that described at page [21], or it may be ground with a small tool 8 inches in diameter, as recently suggested by M. Foucault, the results in the latter case being just as good a surface of revolution as in the former. It is best polished with the 8 inch, and a moderate pressure may be given by the screw q, if the pitch is not too soft. This, however, tends to leave an excavated place at the centre of the mirror, the size depending on the stroke of the crank m, which should be about 2 inches. The pin q ought to be half way from the centre to the edge of the mirror, but must be occasionally moved right or left an inch along the slot. When the surface is approaching a perfect polish, the warmed 4 inch polisher must be put in the place of the 8 inch. The pin q must be set exactly half-way between the centre and edge of the mirror, and the crank must have a stroke of two inches radius. The polisher then just goes up to the centre of the glass surface with one edge, and to the periphery with the other, while the outer excursion of the inner edge and inner excursion of the outer edge meet, and neutralize one another at a midway point. Wherever the edge of a polisher changes direction many times in succession, on a surface, a zone is sure to form, unless avoided in this manner. All the foregoing description is for a 15 1/2 inch mirror.
By this system of local polishing the difficulties of heat, distribution of polishing powders, irregular contact of the rosin, &c. that render the attainment of a fine figure so uncertain usually, entirely disappear. A spherical surface is produced as above described, and afterwards by moving q towards the edge, and at the same time increasing the stroke, it is converted into a paraboloid. The fleecy appearance spoken of on a former page is not perceived, and the surface is good almost up to the extreme edge.