How to repair damaged threads on microscope objective?

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Chris S.
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How to repair damaged threads on microscope objective?

Post by Chris S. »

I recently purchased an M Plan 40x ELWD which, though cosmetically rough, seems OK optically. However, the threads are messed up in spots, as can be seen in the images below.

This objective will mount--though roughly--on my Leica microscope turret (and in one of the pictures below, you'll see the metal shavings from the turret nestled among the objective threads--ouch.) The objective will not thread on my bellows adapter, which is more finely machined.

Rather than return this lens (the bad threads were not disclosed by the seller), I'd rather fix and use it, if this can reasonably be done. Any advice on fixing these threads?

My thought is to try and get my hands on an RMS-threaded die and carefully run it over the threads a few times. Have never done that, though--can a more experienced person tell me if it's as simple as that?

I don't imagine that an RMS die is easy to find. We have a shop in town that specializes in dies, and I have a diagram of the RMS mount to show them. But if anybody has a known source for an RMS die, I'd appreciate hearing it.

Incidentally, since this is my first post to this forum, let me express huge thanks to those whose posts I've been studying voraciously. I've long been shooting with Nikon micro lenses, more recently with a reversed Nikon EL 50 f/2.8, and had started experimenting with microscope objectives on the bellows. The objectives I already had (Leica and Bausch & Lomb) were giving less than great results. So based on the information from this forum, I've been putting together a working set of Nikon M and N Plans, and reworking my movement rig. I can't imagine how many hours of experimentation this forum has saved me--and I hope I can eventually add something of value myself.

Pictures of the lens and threads below. Additional images are very welcome in this thread.

Thanks!

--Chris

The lens--most thread damage is not visible in this frontal view.
Image

Close up (some stacking) of damaged threads on left side of lens. Note the metal shavings from my Leica turret.
Image

Closeup (some stacking) of damaged threads on right side of lens.
Image
Last edited by Chris S. on Thu Apr 23, 2009 8:24 am, edited 1 time in total.

rjlittlefield
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Post by rjlittlefield »

Chris S, welcome aboard! :D

It is sad to see a fine objective banged up like this. On the other hand, it's just the threads.

Yes, if you can lay hands on an RMS die, then fixing this problem should be as easy as you hope. But I don't know offhand where to get one of those. That is a question that I would probably ask the Yahoo Microscope Group.

If this were my own lens, I would approach it with a sharp scalpel and a procedure that looks very much like shaving wood, but performed on brass under a 30X stereo microscope. But if that idea hasn't already occurred to you, it's probably not a good one. The process is undignified at best, and dangerous at worst.

--Rik

g4lab
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Post by g4lab »

I don't think there is such a thing as an RMS threaded die. I think microscope objective threads are cut or chased on a lathe. That would be the way to clean up these threads too.

Find a really good machinist who has a good benchtop tool room lathe.
He could then get a v tool centered in a good part of the thread and cut away the banged up parts. You could do it the way Rik suggests too but I think it would take more than a scalpel. Maybe a moto tool or dental drill with a tiny burr. Just remove the damaged threads.

But doing it in a lathe is probably "the right way to do it". (turning the chuck by hand and advancing the tool by half a thousandth each time. Then back it up and turn the chuck backwards to
move the cutting carriage back. Then advance the tool another half thousandth.

But If you just bought this and the seller didn't disclose it I would send it back. Unless it was a complete steal.

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Post by rjlittlefield »

g4lab wrote:You could do it the way Rik suggests too but I think it would take more than a scalpel. Maybe a moto tool or dental drill with a tiny burr.
Wow, now you're making me nervous! I wouldn't feel at all comfortable getting near those threads with my moto tool and tiniest burr.

On the other hand, the nice sharp Swiss Army Knife I used to check the concept before posting worked fine. :wink:

Thin shavings, that's the trick.

The lathe would work OK too. But even though I have one of those, I think I'd go the hand-shaving route. There's less chance of catastrophic error.

About the die, you could be right. RMS taps are easy to find, for example HERE at Thorlabs (for $72.10). But they don't have a matching die, or at least I couldn't find one with their search engine.

--Rik

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Post by Chris S. »

Thanks, Rik and g4lab, for your helpful responses.

Ugh, though. I've never been succcessful at carving wood or even soap--I understand the "thin shavings" bit, but practicing it has proven beyond my abilities.

G4lab's advice about a good machinist, and what he should do, sounds like a first-class approach--but finding such a person and getting that done is probably more than this lens is worth. On the other hand, shipping two ways eats up a third of what I paid for the lens, so shipping it back seems a bit wasteful. Yes, I could perhaps ask the seller to eat the shipping fees, but my guess is that he sold it in good faith, and I have much more interest in taking pictures than arguing about who gets stuck with the shipping bill.

Like Rik, I saw the Thorlabs RMS taps and their absence of RMS dies. Seemed strange, but g4lab's explanation would shed light on it. Little do I know of such things, but I thought a die could be easily made, if the die maker had a spec to work from--and the RMS spec is certainly available. Am I wrong, or is this something that would likely be very expensive?

Thanks again, both, for your helpful advice. Much appreciated.

--Chris

elf
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Re: How to repair damaged threads on microscope objective?

Post by elf »

I'd be more inclined to use a small file to clean up the threads. Usually I'd say use a 60 degree triangular file, but these threads are probably 55 degrees. A set of needle or riffler files will most likely have one with at least one edge smaller than 55 degrees. If not, then using a 60 degree file only on the damaged areas won't adversely affect the holding power of the threads.

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Post by rjlittlefield »

File -- sure, that'll work. Safe, easy, fairly cheap even to buy the tool.
[custom die]... is this something that would likely be very expensive?
Any custom tool will be very expensive.

--Rik

augusthouse
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Post by augusthouse »

Chris,

If intending to dedicate this objective for use with a DSLR on a bellows/extension, then one possible solution, for consideration, may be the item pictured below.

It is an extender tube for microscope objectives. It has RMS internal and RMS external threads.

If the thread of the 40x can be successfully mounted to this type of extender/adapter, then it may solve the problem of future mounts/dismounts ( because you would be using the external RMS thread of the adapter rather than the damaged thread of the objective).

Image

This approach does not specifically answer your question; but it may be an applicable workaround.

I found the image above via a Google Image Search for 'extender tube for microscope objective'.

One type of 'extender' may be found here:

http://rolynoptics.thomasnet.com/viewit ... &forward=1

Craig
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Post by Chris S. »

Elf, your approach seems very sensible, and I hadn't known the terms "riffler" and "needle" for files. Now that I do, I see a wide variety available at reasonable prices. But you've got me thinking--I have a friend who is a bench jeweler, and who would likely have a selection of these files and experience at using them. Think I'll give her a call.

Craig, I'd thought about mounting the objective in an adapter, but hadn't found anything as simple and elegant as you did. Looks great. Your search term "Extender tube" works much better than "RMS-RMS" adapter--variations on which I'd been trying

I'll probably try cleaning up the threads, and then if they don't work with perfect smoothness, put the lens in an extender tube.

Thanks!

--Chris

g4lab
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Post by g4lab »

I like the idea of an RMS adapter , one made out of aluminum. Just hog the objective into it and leave it there permanently. A jeweler would not be a bad idea either. They are used to handling delicate things carefully even if this would not be the usual item.

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Post by augusthouse »

Here is another example of an RMS threaded extension Adapter.

Microscope Objective Lens Adjustable Extension Adapter.

I purchased one of these a long time ago.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0188876555


Craig
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Post by Chris S. »

An afterword for this thread, in case anyone ever has the same issue:

I did take the lens to my jeweler friend, who cleaned up the threads in a few minutes and didn't seem to find it difficult. She started with a "watch knife" that I understand is normally used to open watch cases. Pretty soon she went to her kitchen and grabbed a very sharp table knife, after which I heard her say, "Ahah--just what I needed."

So it went very much as some suggested here. Anyway, problem solved without too much trouble. Thanks to all to helped.

Also, my search for an RMS die turned up--as predicted--nothing. I was given a $150 seat-of-the-pants estimate to make one--too much for this event, but perhaps worth it if someone had a lot of abused objective threads to clean up.

--Chris

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Post by rjlittlefield »

Engaging a jeweler was a great idea -- thanks for posting that out for the rest of us.

Glad to hear all went well repairing the lens. Now of course we're eagerly waiting to see some pictures! :D

--Rik

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Post by Harold Gough »

This reminds me of when one plug thread in the engine block of my car was damaged and I could not fully screw home the spark plug. My brother, who was in the trade, aligned an old plug carefully and used a spanner with good leverage to force the plug to act as a die. The result worked well for several years.

If such a repair can withstand thousands of small explosions a second it would have no problem holding a lens.

What I am saying is that e.g. losing some metal from the lens thread, such that it no longer looks so good, is less important than restoring function.

Happily, your problem was solved rather more elegantly than mine.

Harold
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Post by ChrisR »

Er, Artful Bodger here.

Not finding am RMS adapter cheap and easy to get hold of I cut a hole in a plastic microscope lens case - the ones where you screw the lens into the cap.
With its base flattened a bit, and stuck to a body cap with a hole also cut in it, I'm sorted until I find something better. One of those would be fine for a permanent fixture for a lens like this.

I must have "repaired" hundreds of threads. For one like this I'd simply use a needle file with one face ground off, at an angle and size which will go into the threadform.
The RMS one doesn't count as small, if you've ever worked on clocks and watches. :wink:
For a brass thread, I just use a penknife. Not to cut with, but to push the brass back into shape. Put the edge into the base of the thread, and then er, sort of roll the knife out of the thread while twisting it against the side you want to reshape. If it's too resistant, change the angle of the blade and you'll be scraping brass away.
The thread shown would take a couple of minutes.
As has been said, if you end up with a little thead ridge missing, it won't matter at all.


If you have a loose thread, use plumber's ptfe tape (usually white). It can be easiest to start off with the width cut (scissors) to what you want, probably 1/8 inch, or 3mm, for a uscope RMS. Stretch it over the male thread and it'll grip. Force the tape into the thread with a thumbnail.
You need to wind it the right way - if you hold the male thread in your left hand, pointing to the right, wind away from you (on top)with the tape in your right hand.
Start from the root of the thread, holding with your left thumb, which will be at the left, and stretch it slightly as you go off the end of the thread on the right. Trim with a scalpel or fine scissors.

Use the "water" tape, the gas one is too thick.
- unless you need to make up a thickness, like a couple of millimeters, which is pretty easy too. As you turn the lot into a female thread, it'll make a ptfe male thread, which you'll find OK for light lenses and will work for several screwings in and out.

I've probably made it all sound complicated. it isn't , but I suppose it gets easier after a few practise runs.

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