Iron Eyewear Hinge Options

Thickness is 1mm for each "stamp".

Corrosion resistance is a big design condition as eyeglass frames are subjected to sweat. Staining of the skin from corrosion contact and allergic reactions (nickle metal) must be avoided. Frame weight also is a constraint. That is why molded plastics, gold, titanium, stainless steel are most commonly used. Iron or common steel are poor choices for frame material.

The material will be acetate; my question pertains to the hinge alone, for which major manufacturers often use nickel-copper alloys.

Anyone with an idea on material or fabrication for my purposes?

I would consider a high strength titanium alloy, for example 6Al4V. Fabrication will be a challenge, but it's strong, won't corrode and is medically inert.

Thanks for the response. This seems to be a good resource on both the ease and difficulty of machining 6Al4V:

http://bbs.homeshopmachinist.net/showthread.php?t=37013

The main issue is tapping the alloy, but it looks workable.

Would such a strong metal be bendable enough even at a thickness of 1mm? Please recall that the hinge must be fit into place in the precut impressions in the frame material.

Tapping these metals isn't much fun, but they make taps designed for this purpose and they work well. When you ask whether the piece can be bent, that will depend on the alloy and heat treat you select. For bending, you will want to trade off some strength for greater plastic range.

Cold working stainless steel selectively hardens and strengthens bent areas while leaving other areas flexible. The more machinable alloys tend to be less stainless, but thermal-diffusion galvanizing produces a completely stainless jet-black finish and stress relieves the work-hardened areas.

Thanks for the feedback. Since it sounds like casting has been ruled out. I would like to keep fabrication as simple and cost-effective as possible.

With machining a non-black metal I would have to then send it to be coated, which is an addition and costly step considering the low production volume. Fermtosecond laser blasting sure is tempting though!

These are a niche item, so I may stick with machining iron, perhaps including a bit of steel wool in the packaging (!) or attempt to work the Ti alloy mentioned above, and available here in black:

http://bjtx.en.alibaba.com/product/313268555-210001143/Ti_6AL_4V_Gr5_titanium_black_bars.html

I assume this is just a surface coating, though, yes?

Any closing thoughts?

No machined metals are naturally black. Making iron or steel look black always requires a coating of some kind. Originally, iron was blackened by coating the part with oil and baking it on, forming a hard thin layer of carbon and rust, which will soak up a fresh coat of oil and inhibit further rust formation. Iron frying pans are still blackened this way, but various chemical blacking treatments are generally used for machine parts. In general, blacking only prevents rust in combination with oil or wax. Thermal-diffusion galvanizing stainless steel is an exception.