Overview
ISO 12844:1999 defines a standardized laboratory test method to determine the grindability of raw optical glass using diamond pellets, and provides a six‑level classification (HG 1–HG 6). Grindability is expressed relative to a reference dense barium crown glass by measuring the volume of material removed during a fixed grinding operation. This standard is intended to give reproducible, comparable grindability data for optical glass production, quality control and material selection.
Key topics and requirements
- Test principle: Compare volume removed from test glass to that removed from a standard reference glass under fixed conditions. Grindability HG = 100 × (mean volume removed from test glass / mean volume removed from reference glass), where volume = mass loss ÷ density.
- Specimens: Discs, diameter 35.7 mm ± 0.05 mm, thickness 4–8 mm. Tests use sets of 20 specimens; mass measurements are carried out on specimens numbered 16–20 (five specimens per set).
- Reference material: Fine annealed dense barium crown glass (composition and density specified; density = 3.58 g/cm3).
- Grinding apparatus and consumables:
- Diamond pellet tool (cast iron plate with bonded diamond pellets), tool diameter 115 mm.
- Standard diamond pellets: 10 mm × 3 mm, hot‑pressed with diamond particle size 5–12 µm; pellet concentration specified in the standard.
- Dressing tool, abrasion blocking tool, and fused aluminium oxide abrasives (specified FEPA grain sizes).
- Water coolant (flow rate 2.5 L/min) and isopropanol (propan‑2‑ol) for cleaning.
- Test parameters:
- Diamond tool speed: 1 200 r/min.
- Specimen load: approximately 49 N.
- Grinding time: 30 s ± 1 s.
- Tool dressing and specimen surface preparation procedures are specified to ensure reproducibility.
- Classification:
- HG 1 ≤ 30
- HG 2 > 30 to ≤ 60
- HG 3 > 60 to ≤ 90
- HG 4 > 90 to ≤ 120
- HG 5 > 120 to ≤ 150
- HG 6 > 150
- Test report: Must include reference to ISO 12844, specimen IDs and densities, mean mass reductions, calculated grindability and HG class, and any anomalies.
Applications and users
- Optical glass manufacturers evaluating machining performance and process yield.
- Quality control laboratories verifying lot‑to‑lot consistency and acceptance criteria.
- Optical component producers (lenses, prisms, windows) selecting glass grades based on machinability.
- Materials scientists and R&D teams assessing new glass compositions for grinding behavior.
- Suppliers and procurement teams specifying grindability classes in purchase and technical datasheets.
Related standards
- References in ISO 12844 include FEPA Standard 43 (coated abrasive grain specifications) and other ISO test standards cited in the bibliography (e.g., ISO 6508‑1, ISO 7438). ISO/TC 172 (Optics and optical instruments) is the responsible technical committee.
Keywords: ISO 12844, grindability, raw optical glass, diamond pellets, grindability test, optical glass classification, HG class, optical material testing.