About six weeks ago I started posting about how to dismantle a Cooke IIA lens. When this lens arrived it was completely seized. Various posts found here and elsewhere talked about this problem and how to clean out the gunk preventing the aperture mechanism from working. This proved more difficult than I expected, no amount of cleaning fluid would help. Nearly everything I read warned emphatically against pulling the lens apart to get access to the blades. I continued to work on freeing up the aperture with disastrous consequences when trying to remove the lens flange with force because it too was stuck firmly to the lens. Without knowing it the aperture moved beyond its threshold bending the blades badly. Now having no other choice I had to dismantle it.
This is the horrible depressing mess, I was thinking therapy, out of a brown bottle, when I realised what had happened.
After removing the two minute screws on the outer barrel, one acted as a pin for the aperture mechanism, I could remove the aperture from the rest of the lens. The iris blades were badly bent making the lens completely inoperable, but because I was desperate to use the lens I set about looking in cyber world for advice on its repair. Next I had to remove the blades out of the barrel, again this proved more difficult than expected, nothing would move. I'm not sure what this lens had been through with its previous owner/owners but everything on it apart from the lens elements was firmly jammed on it and there were no signs of serious damage. I used various cleaner fluids recommended on the net, what eventually worked was Loctite Freeze & Release, amazing stuff, this finally freed up the ring that held the iris blades in their place, and the flange which refused to budge with anything else finally came off without a whimper.
Next step was repairing each damaged iris blade. Once again nothing on large format forums or you tube to help me here. There is plenty in dismantling and reassembling 35mm lenses but nothing on large format so I took another direction and went to metal work forums. This was a good decision, a number of threads were found explaining how to flatten out sheet metal, it is not as straight forward as you would think. Metal that is bent must be bent the reverse way to eliminate any damage, well you also have to work it over and over to get a good flat finish. Nothing I found related to metal anywhere near as thin as an iris blade which discounted heating it until it was glowing orange then hitting it with a hammer hard then following the same steps until it was flat again, but it gave me enough insight to come up with a process which went like this. Straighten the blade as best I could using my fingers. Then using a piece of tuble stainless steel I rolled it over the damaged part of the blade over a sponge like surface that allowed the metal to flex enough to counter the bend in it, that is bending it beyond the initial bend. Turn it over and repeat until the kink had more or less gone. But now you had a blade that had curled up at the ends from the pressure of the tube rolling over it on a soft surface. Placing it on a flat porcelain surface I rolled it again and again, turning it over occasionally to offset any flexing the other way until the blade was as flat and smooth as I could get it. This I repeated over a week for each damaged blade which was the majority of them, it was repetitious repetitive work, those blades are fine and fragile.
Putting each blade into its place had been said to be monumentally difficult and to be avoided by various forum sources, it certainly looked daunting with twelve very frail blades to be carefully placed individually around the confined space of the inside of a barrel. There was one piece of advice I did find on the re-assembling of a 35mm lens that was useful, make sure the seated end with the node that slots into a hole, sits under the free end of the blade of the one put in beside it, this got more difficult near the finish when the holes for the nodes were covered completely by the installed blades. The last four required much delicate lifting of seated blades, and careful pushing around the perimeter under four and five other blades without dislodging them into place the new blade around to its hole, I should have taken a shot to show how difficult this was. The other advice I received from a large format user proving exceptionally important was to install the holding ring sitting on top of the blades, with the blades at fully open position. Putting the ring not quite on the blades I lined up each blade node with the notch in the aperture ring for them and then pushed the ring fully down into place, this held them in place. You can see this clearly in the last photo.
Here are the blades seated with their nodes placed in the holes on the underside (not visible here), you see these as little round brass dots around the aperture blades if you look from the back of your lens. The nodes facing you are to be placed into the slots in the aperture ring.
Next the ring that sits on the seated blades is placed inside the barrel but not on the blades. The nodes on each blade have to be lined up with the notches in this ring before it can be placed onto them.
The ring has now been positioned so each node could be lined up with the notches in the ring. Next the ring was pushed down so the nodes went into the notches to hold them in place. This is the aperture ring that moves the iris blades so they open and shut.
Finally the very fine holding ring is screwed down inside the barrel to hold everything in place. It is sitting beside the barrel on the photo below.
Everything looks ok, but the aperture ring is still stiff and difficult to turn, I'm thinking I need to lubricate the aperture ring to reduce the friction, that is the next and should be the easier part of this repair, and better be the last #@!*&% thing to do.
This was my very first foray into repairing a lens, the experience was insightful into the design of a lens, they're not that complex really but there are design elements that require specialised tools or intimate knowledge to do a job quickly and expertly. My job is not perfect, it should be good enough though to get it back to full working condition once I work out why the aperture ring is stiff.
Thanks to Jim Galli for guiding me along the way.
BC
Reply With Quote
Bookmarks