I just received an old Sovos proprietary lockring for their old flip flob hub, which are hard to find. My mother in law says that the machinists in her union can fabricate these in a short run, they just need to know what kind of metal to make it out of. I assume steel is the way to go, but what kind? Should it flex at all, or should it be rigid?

I'm not a machinist, but it shouldn't flex. what are dura-ace lockrings made of?

steel

definitely steel but what is unique about the sovos ?

i'd think the important things to know would be thread pitch and diameter.. they'd have the tools to measure but will they have the cutting tools to match ?

From Sheldon Brown's site. Basically, the early lock ring is larger than normal, and no one makes them anymore. The one I have now has been passed around several Sovos hub owners, so I figured it'd be a good idea to have some made.

As far as them having the equipment, she works for the International Machinist and Aerospace Workers Union, so I'm quite sure they'll have the equipment to make a few. But I don't think they have too many fixed gear cyclists. My first thought was make it out of hardened steel, but I wanted to hear what others say first. Especially if I end up distributing these and somebody has an accident because it was made of the wrong material.

I know that they're made of steel, but bike frames are made of steel too. there are many grades of steel, some are better than others at specific jobs. I wish I knew who to ask, though; I don't think that the particular steel used is listed anywhere.

hardened tool steel is a pretty safe bet. you could also ask the machinists what they think.

If you can cut it with a file it's probably not hardened. If I was making it for myself I would try some 4140, there's usually some laying around the shop.

since you are going custom how about a nice lockring of 316 or 18-8 stainless steel ?

can't go wrong with that..
anyway, i'm pretty sure any grade of steel (mild or otherwise) would be perfectly fine.
lockrings are not hardened; they don't really see any wear.. they are basically just big fasteners