HELP! My Nokton fell apart! (Sort of.)

I had one screw break as I was unscrewing to take the mount off to send to John Milch for coding. I emailed Stephen to get one and he told me to get one from a camera shop. Seeing he is the Guy for Voigtlander and has a camera shop (I sent him a M4P for a CLA once), I thought it was rude. I emailed Don Goldberg and he sent me one , no charge, so don't hold your breath waiting for Sephen to respond

By the way, it was a black one, not chrome
 
Disclaimer; I am not an engineer so I don't know how to calculate the stress on those screws but, I am a machinist with 23 years in the trade and those screws look very small for the weight of the lens. I don't mean to criticize Cosina but it looks like it could have been designed a little better.

I would caution against loctite or other thread lockers. If there is no loctite then the broken screw can probably be rotated out using a needle. If there is a thread locking compound it gets more complicated. In that case heat, perhaps from the tip of a small soldering iron, could loosen the locking compound and the the broken stubs unscrewed.

Good Luck. Joe
 
Another thought. If someone figures out the size and pitch of the screw would they please post it here. Owners of this lens may want to buy a small quantity of these screws and replace them every six months or a year as a precaution.

Crack propagation in steel takes time and replacing the screws every so often might save a lens from a bad fall. Joe
 
Frequently replacing screws is likely to make matters worse. Meticulous discipline when assembling heavy lenses to mounts seems to be a requirement, as the screws themselves are not at fault!

Standard 8.8 class steel (the one most screws you can buy at tool stores are made from) has a strength of 64kg/mm². Assuming five 2mm screws, each capable of 128kg (the threadless core of a M2 screw is approximately 2mm²), we arrive at 600kg - that mount ought to be fit for lifting a small car. Even mild steel (or V2 stainless) would still have half to 3/4 that strength. And M2 is on the tiny side for a mount screw, I'd expect M2.5 to M3.2 (and possibly stronger steel), so the above figures are already pessimistic.

It obviously is not a matter of the screws, but of them having been damaged in manufacturing or maintenance. Screws of the above size are rated for 1-2Nm of torque (already easy to exceed), while medium strength (blue) Loctite is supposed to require around four Nm to break loose in a 2x3mm thread - so chances are that the screws literally get screwed when untightening them a single time. Memo to self: Thread holes ought to be cleaned and fresh screws used whenever replacing a screw on a mount (and there should be no or only the lightest variety of screw locker applied).
 
Your calculations may be correct but they ignore some important realities. Were these screws carefully made and of the proper steel? What is the thread profile of the screw. If the threading method leaves a sharp V at the bottom of the thread that becomes a place for a crack to start. Were the threads cut or rolled? Proper torque is essential and may be one of the chief causes of failure. Crack propagation in steel is progressive so replacing old screws with new ones that have no cracks can be preventative maintenance. Threads like these should be able to be removed and replaced many times before wear on the internal threads becomes a problem. Five of these screws will not hold a small car. I doubt they would hold your weight. Joe
 
lewis44 said:
I had one screw break as I was unscrewing to take the mount off to send to John Milch for coding. I emailed Stephen to get one and he told me to get one from a camera shop. Seeing he is the Guy for Voigtlander and has a camera shop (I sent him a M4P for a CLA once), I thought it was rude. I emailed Don Goldberg and he sent me one , no charge, so don't hold your breath waiting for Sephen to respond
Click to expand...

I received a reply from Stephen, but since I'd already found a source for the screws from Voigtlander HK, it was moot. If the HK thing doesn't work out, I will contact Stephen again, or maybe just ask him for extra screws.
 
I used the Minolta "replacement" screws today, and the lens stayed intact. Still a bit nervous to walk around with it, but I guess in time the trauma of a lens snapping off with wear out!

I may take the suggestion of just replacing these screws every five years (I've had this lens for seven, so I'm using this as the basis of the MTF) as a precaution.

@noimmunity, how long did you have your lens before your screws snapped?
 
UPDATE: The screws snapped again two weeks ago, and I put in new replacements. As I have started to lose confidence in using this lens often, I have decided to "semi-retire" the Nokton and use it for special outings only.

I really think that the extra weight of the chrome version is the culprit. As there are only five screws (my Leica lenses have six), I think the weight of the glass and chrome put too much stress on the screws. Maybe I need titanium screws - until then, the Nokton will be getting a lot less exercise. :-(

The good news is that I've replaced it with a 35 'Lux FLE. 🙂
 
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