The scraping process
1.A surface plate provides a verified level surface as a standard. It is a fieldstone with a small coefficient of expansion. Metal surfaces are processed using this surface plate as a standard. The metal surface has already been machined, and is flat to a certain degree, but the precision of the flatness is improved through scraping.
2.An orange pigment is spread onto the reference surface of the surface plate.
3.The metal surface to be scraped is brought into contact with the reference surface of the surface plate. The metal piece is then slowly rotated.
4.Pigment is thus transferred to high spots on the metal surface.
5.A scraper (a special bladed tool used in scraping) is used to scrape off the parts on the metal surface where the pigment has adhered.
6.Steps two to five are repeated until the entire surface of the metal piece is covered with pigment. The end result is a flat surface with micrometer precision.