Make a near-perfect geometric something, and then make it fit through a something-shaped hole in a plate with no light showing. The wavelength of blue light is about 0.000,017 inches (450 nanometers). This would be a hard project with a surface grinder. You have a file.
Filing a Cube or Sphere
Filing a cube, and related projects. Single objects only (not "object-and-plate").
Square End and Plate
Henry Royce (of Roll-Royce fame) is said to have done this as a part of his apprenticeship. It was also apparently done with both a square end and a triangular end.
Hexagon-with-Keyways and Plate
Cube and Plate
A square end has four rotational positions. A cube has three axes, each with four rotational positions. This project, which may be the apotheosis of the apprentices' project, is well attested from the British Royal Small Arms Factory.
The epigraph to this page, by Richard McCarty, is from 2010-04-02 posting to a forum on the "Practical Machinist" website, at: http://www.practicalmachinist.com/vb/antique-machinery-history/filing-flat-201685/
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