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3D3DElectrochemically machined washer.
It may not look very impressive, but I'm quite pleased with this steel washer. The depression at the top was supposed to be a sharply-machined peace sign, created by electrochemical machining. Instead it's a blurry mess, but the point is that it's etched several millimeters deep into solid steel, and that was done using a cheap tin earring (shown under tin) as the machining bit.
The method is called electrochemical machining, and it's described in my July, 2009 Popular Science column.
Source: Theodore Gray
Contributor: Theodore Gray
Acquired: 28 June, 2009
Text Updated: 29 October, 2009
Price: Donated
Size: 2.5"
Purity: >95%