Is there a specific reason most CMMs ride on black diabase tables, aside from mass? I’ve always heard it’s about low thermal expansion and low porosity at the 20°C reference, but I can’t find who first codified black granite for grade AA plates/CMM bases — ASME or DIN — does anyone know the origin?
, I’m pretty sure the “black” convention traces back to the old U.S. GGG‑P‑463c granite surface plate spec. > the 20°C reference, but I can’t find who first codified black granite for — that was GGG‑P‑463c, later superseded by ASME B89.3.7; DIN 876 sticks to flatness grades, not color. Black diabase stuck because of low CTE (about 3.5 µin/in/°F at 20 °C), low porosity, and good damping, but it’s not mandatory if your cert meets the numbers.
Not codified as “black”: ASME B89.3.7 and DIN 876/ISO 8512 set flatness/material quality at ISO 1 (20 °C), not color; black diabase won out because low‑quartz, low‑porosity blocks lap hard and hold about 6–7 µm/m·K CTE. If you want a citation trail, @aaron_j92, skim B89.3.7 and early Zeiss/SIP brochures — you’ll see “black granite” as a sourcing choice (Jinan/Sweden/Zimbabwe), not a requirement.