Glass, stone, plastic
Grinding of glass
Oldest archaeological finds go back to the later Stone Age, about 7000 B.C. The origin of glass was in the countries of the Near East. The oldest glass making is closely connected with the pottery. During baking of pottery the glass was found to add a coloured glaze by mistake so the by-product was produced rather coincidentally.
Only about 1500 B.C. they managed to produce glass independently of ceramic base and to form it into single objects.
The grinding process
Formed bubbles of glass are placed on a slowly rotating disc and are scored by means of a diamond. Through light hitting the upper part of the formed bubble of glass breaks. The glass with a slight edge of break, the so-called bezel, which subsequently is ground straight, for example in a tray, remains. Abrasive grains of SiC are loosely spread on a steel disc and sprinkled with water. The glasses are pressed with the bezel on the steel disc and are ground.
By gently holding the glasses at an oblique angle a surface is ground along the outside of the bezel.
The time period for this method is large and, in addition, loose abrasive grains are washed over the edge of the disc without being completely worn-out.
Many factories, however, did not want to give up this method and the existing machines. The solution was a waterproof disc of abrasive cloth on which abrasive grains of SiC were fixed by means of resin bond. This measure guaranteed a smaller expenditure of time and the maximum utilisation of the abrasive grains. In order to optimize the set-up times as well, they used discs with abrasive grain on both sides of the disc (double sided discs).
An improved method is grinding with endless abrasive belts on relatively simple belt grinding machines for glass with horizontal and vertical abrasive belt guide. The cutting speed is 8 – 12 m/s. A permanent supply of water prevents the development of higher temperatures which could lead to bursting of the glass. The abrasive belt normally glides over a contact disc of sponge rubber. Hereby also thin-walled glasses can be ground without breaking. The grit size 180 is mostly used for rough grinding and grit 240 for finishing (KLINGSPOR: CS 320 Y / CS 321 X). In a third grinding process the rough ground glass edge is polished with a cork belt (KLINGSPOR: CS 322 X).