Machining a Titanium Engagement Ring

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.

3 Responses to Machining a Titanium Engagement Ring

Anonymous on December 12th, 2011 at 6:18 am said:

Great project. It’s awesome that you did this. I have to say that personally I’d be petrified of that stone being held in by nothing but tension, but since you felt good about it, then it must be solid enough.

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Anonymous on December 12th, 2011 at 8:33 am said:

Any chump with a trust fund, fat job, or credit card can buy a beautiful ring, but it takes quite a lot of character to put your time, talent, heart and soul into the engagement gift of a lifetime! Congratulations Alex!

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Jason Deck on December 24th, 2011 at 11:18 am said:

I applaud the level of effort that went into the calculation for the dimensions, but many professional jewelers who make tension set rings just eyeball the dimensions. One of the reasons (among several) that the commercial tension set rings are “complicated and gaudy” is that the ring needs to be rather beefy to hold the stone for long term use. An experienced jeweler won’t use sterling silver, for example, because it’s not sufficiently strong and wear resistant enough. I have no experience with a titanium tension set, but I expect that it will last a very long time, and loosing the stone won’t be an issue under normal wearing conditions.

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