(Hand) scraping refers to https://www.youtube.com/watch?v=REeGn4hN1Bg https://www.youtube.com/watch?v=REeGn4hN1Bg . With two surfaces you can still have some b

(Hand) scraping refers to https://www.youtube.com/watch?v=REeGn4hN1Bg https://www.youtube.com/watch?v=REeGn4hN1Bg . With two surfaces you can still have some bias (convexity/concavity) even though they appear to be relatively flat, you need three surfaces for absolute flatness. See the Whithworth Method described earlier in this topic. If you're interested in this sort of stuff and want a video introduction, see: https://www.youtube.com/watch?v=gNRnrn5DE58 https://www.youtube.com/watch?v=gNRnrn5DE58