GRINDING, the reducing hard substances to fine powders.
Method of GRINDING optic glasses. Mr Huygens directs, in general, to make the breadth of the concave tool, plate, dish, or form, in which an object-glass must be ground, almost three times the breadth of the glass. Though in another place he speaks of grinding a glass whose focal distance was 200 feet, and breadth 8½ inches, in a plate only fifteen inches broad. But for eye-glasses, and others of lesser spheres, the tools must be broader in proportion to the breadth of these glasses, to afford room enough for the motion of the hand in polishing. Mr Huygens made his tools of copper, or of cast brass, which, for fear they should change their figure by bending, can hardly be cast too thick: however, he found by experience, that a tool fourteen inches broad, and half an inch thick, was strong enough for the forming glasses to a sphere of thirty-six feet diameter; when the tool was strongly cemented upon a cylindrical stone an inch thick, with hard cement made of pitch and ashes.
In order to make moulds for casting such tools as are pretty much concave, he directs, that wooden patterns should be turned in a lathe, a little thicker and broader than the tools themselves; but for tools that belong to spheres above twenty or thirty feet diameter, he says it is sufficient to make use of flat boards turned circular to the breadth and thickness required. When the plates are cast, they must be turned in a lathe exactly to the concavity required; and for this purpose it is requisite to make a couple of brass gages in the manner following, according to the directions of Mr Molynex.
Take a wooden pole, a little longer than the radius of the spherical surface of the glass to be formed; and through the ends of it strike two small steel points, at a distance from each other equal to the radius of the sphere intended; and by one of the points hang up the pole against a wall, so that this upper point may have a circular motion in a hole or socket made of brass or iron, fixt firmly to the wall. Then take two equal plates of brass or copper, well hammered and smoothed, whose length is somewhat more than the breadth of the tool of cast brass, whose thickness may be about a tenth or a twelfth of an inch, and whose breadth may be two or three inches. Then having fastened these plates flat against the wall in a horizontal position, with the moveable point in the pole, strike a true arch upon each of them. Then file away the brass on one side exactly to the arch struck, so as to make one of the brass edges convex, and the other concave; and to make the arches correspond more exactly, fix one of the plates flat upon a table, and grind the other against it with emery. These are the gages to be made use of in turning the brass tools exactly to the sphere required.