I have got a Sigma 300 mm F2,8 lens from a customer, which has some dirt in the inside. A lens element was dirty or some fungus on the surface. (As a supplementary information I was told, that on the lens somebody made a “screw-exchange”. Ok.)
Not a big deal.
When I wanted to open the screws which hold the barrels on the back side, I have seen, that the 3 screws are of 3 different types. Hmm.
Unfortunately, the head of the screw was not too strong. After a few trials broke the head – without any movement of the thread.
And after this I observed, that the screws were glued into their places. Beautiful.
As at this moment I had to take out them anyway, started a fight against the glue. After approx. 20 minutes one screw was out – and another has no more slot in the had. And I had less with one expensive screwdriver bit, which also has broken during the operation.
Now there was only one way: to drill out the head without PH slot. No space – and no possibility to stop the metal particles in falling in the inside of the lens.
After another 20 minutes the screw was away, and the inside of the lens full of debris. But I could separate the barrel.
Another surprize: under this other 3 screws were glued too. Hrrrrr.
The broken screw, the drilled screw and the normal thread.
As next I had to eliminate the debris. At first I tried it from the front. It’s huge front lens – and no visible method for disassembling.
I tried to rotate the inner ring in the front of the front lens – but was stuck.
And then came the secret weapon: a little bit of alcohol (in my case methanol) between the internal and external thread (has a pitch of 0,75mm).
This helps always. (You can use it by stuck filters for example – and any liquid could work, vodka or whiskey, whatever). The ring came out, the front lens came out – but no further possibility. At this moment I had to clean the lens, and to put it back.
In such cases with big and heavy lenses I always use my home-made suction tool.
To eliminate the broken screws I use my brand new small (Chinese) milling machine. At first milling the screw with a tungsten-carbide end milling cutter, after that making a centre of the drilling, then the drilling itself. A spiral drill bit would never stay in the middle of a M2,5 screw, without a centering bore.
After this I drilled the holes 15 mm deep – and I cutted ca. 12 mm long thread instead of the original 6-7 mm.
In the meantime I cleaned the lens internally and externally, mounted together the parts. The “dirty” lens element has not been cleaned as it should be – but even after cleaning with white spirit, alcohol, methanol etc., etc. remained on it a bigger spot, which was visible only after breathing on the glass.
Here are the screws which I user (with Torx-slot,to be able to take them out later), and the mounted barrel. By the way: I always lock important screws. But always with a material which makes possible the disassembly later. Like Loctite 243 for example.
(“LOCTITE® 243™ is designed for the locking and sealing of threaded fasteners which require normal disassembly with standard hand tools. The product cures when confined in the absence of air between close fitting metal surfaces and prevents loosening and leakage from shock and vibration”)
After assembling, cleaning etc the lens, I went on the internet, just to see, approximately how much is the value of that lens.
My dear god…. Approx 3000 euros.
I have never drilled a hole in a piece of metal with that value. Great feeling!