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06-25-2013, 10:09 AM   #1
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Suggestions to stop microfissure in filter thread

About 6 weeks ago, I got a SMC Pentax-A 4.0/35-70 from eBay. A quick check didn't show anything wrong with it.

But now, when I wanted to use it, it was not easy to fix a hood. Inspection with a magnifying glass showed there is a microfissure in the filter thread, which already has expanded slightly to the distance setting ring.

So it probably was dropped some time in the past.

I made tests for sharpness/resolution over the zoom range and all apertures at near infinity, and IQ is everywhere very near to prime quality, and in places it is absolutely prime quality (with a 10MP sensor). A rough check in the macro range also showed excellent results. So the elements and groups are still where they belong to.

Of course, after 6 weeks I cannot return it. My question is about the best way to prevent the fissure to further expand. I hesitate to use it with any filter or a hood (even a flexible one), as a bump to that hood could make things worse than without a hood. Obviously, fixing a hood with some glue could solve the problem. But then it could not easily (or not at all) be replaced if needed, and any future servicing of the lens could be very difficult.

If anyone has experienced the same problem, and tried to solve it (either successfully or ending in desaster), I would be interested to hear.

06-25-2013, 10:28 AM   #2
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My thought would be to use something like JB Weld epoxy or the sort of resins used to repair cracks in windshield glass. The question is if enough of the material could be forced into the fissure to form a strong bond. You could always leave a "patch" of epoxy on the surface as long as it clears the focusing/zoom collar. Glass repair resins use UV light to cure and you may not be able to get enough light in to cure the resin.
06-25-2013, 10:57 AM   #3
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Drill a small circular hole at each end of the crack. Use a very sharp bit, preferably carbide, with a very high rotational speed, and a slow advance speed to make the hole smooth. A mill or drill press will keep the hole straight, which is best. The radius of the hole is much larger than the crack tip, therefore the stress concentration is lower. This should prevent further propogation. Since this area is not normally loaded, this fix should work for an extended period of time (but not forever). I would probably also epoxy it, just to add a little bit of strength back.

If you just epoxy it, it will continue to propagate.

They use this technique to slow crack propogation in bridges, aircraft components, etc. usually in those applications, you want to fill the hole with something, preferably a bolt or a rivet, but in this case epoxy should suffice. This is usually considered a temporary fix, and there is increased failure risk compared to the original state, but the risk is reduced compared to not addressing the crack at all.
06-25-2013, 11:54 AM   #4
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Original Poster
I have not finally decided yet, but I tend to screw in a metal step-up ring 58->62mm, together with some super glue which works with most plastics as well as with metal. I think this should prevent putting any stress to the damaged part when screwing in a hood. Using a hood sized for a longer lens should care for the difference in diameter.

I just had a look at my step-up rings (not a few, as collected over a period of 40 years), but there isn't a 58-62mm. As it seems I now have to order one all the way from China, I will stay open for better suggestions for roughly the next 5-6 weeks.

Thanks to all trying to help.

By the way, after the forced tests I really think this lens is a keeper.
I also own the SMC-A 3.5-4.5/35-70 and the SMC-F 3.5-4.5/35-70. They both are much smaller and lighter, but IQ of this lens seems to be better. And, as said before, it seems also to be very good in the macro (=close distance) range.

EDIT:
Just to show what I am speaking about. The cracks which you see were made visible by using a 4.6VA 60 bulb LED service light at a special angle, and can barely be seen at normal lighting. It also seems there is now a narrow gap which could not be seen before, probably caused by me when I scewed in a hood.

@Kozlok
I guess the holes could help if that area was not to be touched again. But I really want to use a hood. Screwing it in, or even touching it, will cause a tension where the holes may not help a lot.

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Last edited by RKKS08; 06-25-2013 at 01:02 PM.
06-25-2013, 03:01 PM   #5
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QuoteOriginally posted by Kozlok Quote
they use this technique to slow crack propagation in aircraft components
I hope these bits are for ground use only and not up in the air.
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