ISO 12844 pdf download.Raw optical glass — Grindability with diamond pellets — Test method and classification
1 Scope
This International Standard specifies a test method for determining the grindability of raw optical glass with diamond pellets, and defines a classification of optical glasses according to the grindability determined by this method.
2 Terms and definitions
For the purposes of this International Standard, the following term and definition applies. 2.1 grindability volume removed from a glass specimen when it is ground with a diamond pellet tool for a fixed period of time, relative to that removed from a standard reference glass
3 Principle
The grindability of any glass type is compared to a standard reference glass type and classified. Twenty samples of the unclassified glass and twenty samples of the standard reference dense barium crown glass are first prepared using aluminium oxide and then ground using a standard diamond pellet tool and water coolant for a period of 30 s. The samples are weighed to determine the mean mass of material removed, taking into account the density of the glass to determine the volume reduction. The grindability class of the unknown glass is established according to the classification table given in this International Standard.
4 Reagents and materials
4.1 Standard diamond pellet, 1 0 mm in diameter and 3 mm thick. The pellet shall be produced by the hot pressing technique using diamond of particle size 5 ?m to 1 2 ?m and a copper-tin alloy consisting of 80 % copper (Cu) and 20 % tin (Sn). The concentration shall be 1 5, corresponding to 0,66 carat/cm 3 (= 0,1 32 g/cm 3 ). In order to obtain reproducible grindability test results, it is considered necessary that all laboratories use only standard diamond pellets from the same producer. 1 ) 4.2 Dressing tool, of cast iron, about 280 mm in diameter. 4.3 Abrasion blocking tool as used in 6.3, of flat cast iron, about 250 mm in diameter. 4.4 Abrasive grains, of fused aluminium oxide with a mean grain size of 1 1 ,5 ?m to 1 4,5 µm.
5 Apparatus
Usual laboratory equipment, together with the following. 5.1 Grinding test apparatus (see Figure 1 ), consisting essentially of: 5.1.1 Diamond pellet tool (see Figure 2), having a diameter of 1 1 5 mm and constructed of cast iron. The tool shall rotate horizontally at a constant speed. The flat tool surface shall contain diamond pellets (4.1 ) in the approximate pattern shown in Figure 2. After adhering the diamond pellets to the cast iron plate, apply the following abrading procedure for finishing the diamond pellet tool: a) rotate the dressing tool (4.2) with a rotational frequency of 60 r/min to 1 00 r/min; b) keep the centre of the diamond pellet tool at a distance of 80 mm from that of the dressing tool without any swing; c) apply a load of approximately 69 N to the diamond pellet tool; d) dress the pellet tool for as long as needed to obtain a flatness across annulus, as measured with a spherometer, of  0,005 mm. 5.1.2 Supporting rod, to maintain the specimen at a fixed position and apply a constant load F. The specimen holder shall have an inside diameter of (35,8 ) mm 0 0,05  . 5.1.3 Device for supplying coolant. 5.2 Balance , capable of weighing to an accuracy of  1 mg. 5.3 Desiccator , containing a 2:1 mixture of silica gel (for H 2 O absorption) and soda-lime (a mixture of CaO and Na 2 O, for CO 2 absorption, with indicator for regeneration).
6 Specimens
6.1 Shape The reference specimens and the test specimens shall be discs of diameter 35,7 mm ? 0,05 mm with a thickness of 4 mm to 8 mm. All specimens shall be prepared without chamfering. 6.2 Number For a test, a set of 20 reference specimens and a set of 20 test specimens are required. A test quantity of 20 pieces is used to establish consistent process performance. 6.3 Preparation Using wax, cement onto the abrasion blocking tool (4.3) the 20 specimens to be used in one test. Repeat the same procedure for the 20 reference specimens. Carry out the abrasion by applying first the coarser abrasive and then the finer abrasive (4.4 and 4.5) to the specimens with a rotational frequency of about 50 r/min and a load of approximately 29 N. Stop the grinding when the roughness is between 6 µm to 8 µm for at least 98 % of the surface.