Cool story from the MAKE Flickr pool: Alex French designed and carefully machined a torsion-set engagement ring to propose to his longtime sweetheart, Colleen. He studied up, tooled up with a Sherline 4000A hobby lathe/mill/drill press combo, then turned, bored, and milled the ring before setting the stone, himself, all while working secretly in a spare bedroom in their shared home.

The ring started as a 2″ long piece of 1″ diameter rod of 6Al-4V Grade 5 ELI titanium. Ultimate tensile strength: 125,000 psi. Yield strengh: 115,000 psi. 6% Aluminum, 4% Vanadium, bio-compatible. ELI is “extra low interstitial”, less than 0.14% iron and

The stone is held in place by the torsion of the band, which was very carefully engineered:

I turned to Machinery’s Handbook for structural analysis, based on looking at the tension setting as a single loop of a torsion spring…If I’m correct, 1 pound of force at the stone setting deforms the ring about 0.007 inches. The opening was milled to dimensions that creatae about 2 pounds of force on the ring. It would take about four pounds of force to open the ring enough for the stone to slide out radially, or less to slide out along the ring finger axis.

See the work-in-progress shots, and read the whole story, in Alex’s Flickr set.