This a problem I have every three months or so. I am unable to resist buying a lens where the flange is lost.
I have a quantity of reserve flanges - both imperial and metric - and I often get lucky! I have also used the method of solder spacers on the inside and cutting a thread using the harder thread on the lens.
In connection with my lastest flangeless lens "scoop", I couldn't find a suitable brass lens to fit - or modify. Just a late aluminium flange, with about 0.5mm clearance and a very different thread pitch. I don't think an aluminium alloy would accept the solder I have.
I don't like the idea of a permanent glueing of a flange to an historic lens, so I thought it might be possible to use a flat, flexible, material that would "enter" the two threads and provide a secure fit - without being permanent.
So I made a first attempt with black electrical tape, which is always handy to repair cracking lens boards and mahogany plate holders. In this example, I used two layers - cut into 4 pieces in order to place them reasonably precise on top of the inside flange thread. The tape was bent back where I screwed the lens thread in. Obviously this has to be done carefully, ensuring that the flange is equi-distant from the mating edge.
I was able to screw completely in with a medium amount of resistance and the lens was rock solid. The important thing to ensure that the lens could be removed easily and, hopefully, that the system would survive for reuse! It passed this test - see the last photo. Whether it would survive a dozen ins and outs, I do not know! I think it is very dependent of the number of layers of tape necessary to ensure a close fit. There is no reason to remove the bent back tape as this probably gives the tape stability for multi "screwings".
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