Surface Plate Calibration and Conditioning
Why Hacker News recommends it
Cited in threads on flatness and precision as a glimpse into a niche metrology profession.
What it is
A look at how granite surface plates are inspected, calibrated and reconditioned using an autocollimator, a repeat meter and diamond-charged cast iron laps. Measurements are at the level of millionths of an inch.
Recommended by 1 HN commenters across 1 different months, 2017 to 2020.
What Hacker News said
Recommends 205guy · 2020-04 · on “Hand Scraping a Truly Flat Plane (1908) [pdf]”
The YouTube channel oxtoolco has a video showing lapping of surface plates. Not the same as hand scraping, but the principle of using a reference surface is similar, and the master plate looks like the illustrations in this 100-year old document. Plus I think with modern synthetic diamond abrasives, you can directly lap the plates against each other. Lapping starts at 17:40 (they do surface measurements first): [this video] I am no machinist, but I like the glimpse into the hobby/profession shown on channels like this and others that comments have mentioned. In the video, the channel owner hires a company to service his stone regency plates, so there is an expert who comes and uses specialized to
Cites NegativeK · 2017-03 · on “Mirror grinding”
Hand techniques are still used for reference surfaces in machining. Granite surface plates (highly accurate planes) are hand lapped[1], and metal bearing surfaces on machine tools and cast iron reference tools are sometimes hand scraped[2]. [1] On location calibration and lapping of granite surface plates: [this video] [2] Hand scraping (effectively chiseling) of reference cast iron surfaces: [LINK]
Cites blhack · 2018-11 · on “International System of Units overhauled in historic vote”
What applications are you talking about? Just for a general example of the sort of thing I'm talking about: [this video] They're resurfacing a surface plate, which has to be very flat. If you listen to the exchanges between them, everything is being done in millionths of an inch.