Sigma 500 F4.5 APO clean aperture

The aperture of my Sigma 500 F4.5 APO moves slowly, so I guess that the aperture blades are oily although no oil on the blades is visible. I want to remove the aperture for cleaning. The question is, how to disemble the lens until I can remove the aperure. The lens is the old non-EX version like shown here:
http://www.dyxum.com/lenses/Sigma-500-F4.5-APO_lens360.html

I have started to disemble the lens from the backside. I have removed the mount, but behind I can not find any screws that are obviously the rigt ones to remove the next lens section. Does anybody know how to go on? I have searched the internet but haven't found any useful hints.

Thank you, Tim. I am not sure about all the locations you mentioned. Here is an image where I have assigned numbers to the different lens sections. The silde in filter holder is on the right edge of section 1 and the focussing scale in section 2 just below the number 2.


Image 1 Overview



Image 2 View from behind after removing the mount

Great, those 3 screws are located exactly like on my lens! I have now released already two of the three screws but then my tiny cross type screwdriver gave up. I have to buy a new one tomorrow to continue.

Try to unscrew the entire element set. The set appears to be a fixed cylinder. it should expose the aperture blades.

Hi travelshots, sorry just seen this and had been looking out for it. It looks like you have got a long way.

Not a lot of advice about the screws, except dont heat them and do try penetrating oil and leaving them in a warmish room for a week or so and then turning them just a 16th of a turn clockwise before anticlockwise (the heads are not usually so damaged for clockwise turning and this may break the oxide build up that is causing them to seize).

Also the problem with these screws is is worse because of the aluminum oxide build. normally when a screw seizes it is possible to remove the head and as the remainder is no longer under tension they come out by turning the shank with a pair of long nose pliers. In this case there is a very good chance this will not work as the oxide may be on the thread its self.

If the above dosent work a risky process is taking a dremel and cleaning up the slot a little or making it so as a streight edge screwdriver will fit, but even then there is a good chance that the weakened head will break when being unscrewed.

Hand threadding on these size srewes is also a pain and it is awkward just getting hold of a tap that will cut the thread deep enough in the hole even if you get the sets of 3 (which is often a necessity).

Thanks for showing the pictures and I keep my fingers crossed to hear more.

Tim

thanks for your continued support, Tim. I had no time to gon on with work on the lens during the past two weeks, but your post motivated me do the next little step. I removed the 4 screws that fixed the cover to the next lens section (section 2). Now it is easier to access the screws with the rotten heads. After closing all openings of the lens, I have put e little MOS 2 oil around that srews. Now it has to react for a while. Afterwards, I will try to hammer a slot in the damaged screw heads.

A few imges:





http://www.sonyuserforum.de/galerie/data/media/6/Sigma500Zerlegung_031acrkl.jpg

We were once told in school that if it is aluminum and steel that the aluminum should be heated as it has a greater thermal conductivity and coeficient of expansion, and that if you couldnt do it that way round you shouldnt do it. But i was also just thinking that if you were to heat the barrel up it would damage the ZEN coating pretty quickly and would may just make everything else worse so wouldnt reccomend it for any lens.


I am not sure i would hammer a slot into the screw, but i guess it could be done very carefully. and perhaps it could be used tapping the phillips screwdriver into the original slot to reform it a little? The thing that really knackers those tiny screws is if the circumference of the head is cut they tear appart easily on turning. How much pressure can you get on the screw driver? If you get someone else to help you it can often be sucessful even on partially dammage screws. Make sure the lens isnt going to fall to the side under pressure (fasten it or get someone strong to hold it. Then push down with the screwdriver so as you can use it to support your weight, a straight locked arm with your hand directly under your should. In this position you should not be able to turn the screwdriver so you have to ask someone to use a pair of mullgrips to turn the screwdriver when you are pushing it down. Try a couple of degrees clockwise if it doesnt come off immediatly, and then anticlockwise again. It may sound daft but it is a technique that has saved me a number of times.

Tim

Edited by tpetpe - 03 July 2012 at 21:30