By imc
Published: February 18th, 2009

The surface finish for very smooth molds is typically described by such terms as:-

SPI-A1, SPI-A2, SPI-B1, SPI-C1etc. .    Ra numbers appear to have been attributed to these finishes in industry but the SPI master referenceis an ABS plastic plate without traceability to ISO, ANSI, MIL or any definition of finish


Can anyone explain this and who assigned the Ra numbers to the commonly used SPI finishes ?



Hi IMC, I believe the Ra


I believe the Ra numbers are best used as guidelines when the proper polishing steps are used. In other words, if you are bringing up your steel to an SPI A-3 (15micron diamond buff), you need to step through finer and finer stones and paper as preparation. Bear in mind the grits used in the stones, paper and diamond are really ranges as well and the better quality stones, paper and diamond will have tighter tolerances. Where the Ra can be ignored is when there is a need or call for a diamond finish but a "dirty diamond" might work. This is where the polisher knows he can get away with skipping some otherwise needed prep steps and still make everything work. You will save money on polishing but not get a true A-3. The Ra numbers on a "dirty diamond A-3" won't match a true A-3 because you get a polished, scratched surface. I can't say who came up with the Ra numbers but I want to believe if any good polishing house with decent, fresh materials brought up a surface to an A-3, you'd get, on average, in this case a 2-3 micron Ra. Hope this helps!

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