American cinematographer (Nov1933)

266 American Cinematographer • November 1933 At top Fig. 15. At bottom Fig. 17, a semi- automatic Grinding Machine. The Production of Lenses (Ciasswork).— In order to il- lustrate both the primitive methods of making lenses and the mechanical engineering methods, I will describe the processes in sequence, beginning with the raw glass. A feature which distinguishes the working of glass from the working of metals is the fact that cold glass has no plastic phase. If strained beyond its elastic limit it rup- tures immediately. Its elastic limit and ultimate stress are coincident. And that fact makes it impossible to cut glass in the sense in which we cut metals, for since there can be no plastic deformation of the chip it is impossible for the edge of a cutting tool to penetrate glass, as it does in metal, to determine the exact surface of separation of the chip from the work. What we do in shaping glass, when “cutting” it with diamonds or grinding it with abrasive grains, is very largely to follow the process used by prehis- * Reprint of address delivered to The Institution of Mechanical Engineers. Mechanica toric men who “knapped” flints and broke conchoidal flakes from the surfaces of these stones. The first step in making a lens element from a plate of glass is to form the lens blank as a disk. And the primi- tive way of doing this is indicated in Fig. 8. It is to cut a square of glass with a glazier’s diamond, then with the soft-iron pincers called “shanks” to break away the cor- ners of the square until the disk is roughly circular, and finally to smooth the rough edges by grinding them by hand, as shown in Fig. 9, on a cast-iron lap fed with wet abra- sive such as coarse emery or carborundum. Over forty years ago I investigated systematically the problem of sawing glass by means of sheet metal saws, formed as circular disks and as tubes, whose edges are charged with diamond dust. The variables investigated were: (a) the metal for the saw blade, (b) its physical condition, (c) its thickness, (d) the type and source of diamond, (e) the size of dia- mond particles and how to procure these, (f) the way of securing the diamond particles in the edge of the saw, (g) the linear speed of cutting, (h) the rate of penetra- tion of the saw, (i) the manner of controlling the rate of penetration, (j) the coolant, (k) getting rid of detritus. Fig. 10 shows as a result tubular saws whose annular upper edges are charged with diamond dust. The open joint helps the escape of detritus. The metal is of nearly pure iron, known as Lapidary plate. The wire winding of the tube to its chuck was adopted after trying more elabo- rate means. The diamond is held in the saw edge by first notching the edge with a chisel as a file is cut, filling the notches with diamond dust and oil as a paste, and closing Ihe notches, to trap the dust, by rolling with a grooved roller. Fig. 1 1 shows the automatic notch-cutting machine. Fig. 12 shows part of a machine which cuts glass disks with these cutters, driving them under rigid control at 850 linear feet per minute, feeding a coolant continuously through the spindle into the cutter and controlling the rate of penetra- tion permissively with a predetermined maximum value. Fig. 13 shows a slab of glass and disks cut from it. These are produced in a fraction of the time taken by the primitive method. They are truly circular, they can easily be held to uniform diameter within 0-005 inch, and there is great economy of material. Fig. 14 contrasts the primitive waste with the scientific economy. From the same amount of material thirty-two poor and costly disks are produced by the primitive method, and forty-four good disks are made in a fraction of the time by the scientific method. And yet how often do we en- counter the fallacy that to make or do anything, however indifferently, by primitive methods is better than to do it well and more abundantly by mechanical aid! The next step in making a lens from the disk of glass is to roughly grind its two faces to the requisite curvatures and with their common axis sufficiently central in the disk. Fig. 1 5 shows the primitive method which is still used in many works. A cast-iron tool, a counterpart of the surface