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Thread: How I did it: new balsam for a sick RR

  1. #21

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    Re: How I did it: new balsam for a sick RR

    Thanks Graham! I played with a needle too - to drag the bubbles to the surface or to the edge of the clump. The problem was that there was always one that I didn't spot. You only have about a minute before the two faces to be joined start to cool down to a temperature making the join difficult. I don't think the curing time is very critical - I haven't noticed any discolouration - even after 4 hours cooking.

  2. #22

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    Re: How I did it: new balsam for a sick RR

    HEATING THE LENSES,JOINING AND FIXING THE TWO LENSES.

    Originally the two component lenses were warmed up and baked on an iron worktable with a gas burner underneath. I recommend use of the kitchen oven - with the folowing stages:

    - warm lenses up in the oven (ten minutes).
    - remove the lenses and check surfaces.
    - apply balsam and join the lenses.
    - check depth, approximate allignment and absence of air bubbles/foreign objects.
    - tie the lens together with dental floss.
    - replace in oven and bake at 50-100 deg.
    - remove and reassess allignment.


    The second and third steps have to be done before the lens sufaces cool down too much (less than about 50 degrees?). This is especialy true of the bi-convex flint which has only about 25% of the mass of the crown concave/convex. So it absolutely necessary to have all materials ready at the work place. In particular, and open container of balsam, lots of loose paper towels to act as an insulating device and for removing excess balsam from fingers. In addition a few sheets of optical cleaning paper and a cut length of about 50cm of dental floss.

    Photo 1 shows the layout for working tray which is put into and extracted from the oven. The metal tray will help keep a stable temperature. The newspaper is to protect from soiling the tray and impact damage if you are unlucky in your handling. The concave is placed flat (check this is exactly horizontal in the tray - both in the oven and the "workplace"). The flint biconvex is placed with the joining surface up and with a cushion of paper underneath so the hot edges can grasped without a struggle.

    So now it is easy. Place the tray in the hot oven - wait 10 minutes for everything to warm up. Remove the tray. Check for foreign objects on the two surfaces and wipe off. These surfaces appear to remain clean in the oven environment.
    Then apply/melt down a blob of balsam as show in the earlier section as near the middle as possible. Size? about 2/3 pea size. Be prepared for a violent (but non-hazardous) smell. Lift the biconvex by the edge, reverse it and place carefully down in the right place. Holding it completely parallel to the lens with the balsam seems to work best. The gravity will enough to force the balsam up to edge (and beyond). Do not worry about alignment at this stage. Before the outside surfaces get covered with surplus balsam, check for the absence of air bubbles/foreign objects. If you must take it up remember it is still warm and the flint really floats about so keep it horizontal.

    Once the cementing has been judge OK you can regulate the allignment. Before the cement begins to set you have to ensure that the two lenses are as close together as possible. Old timers with their gas heated tables used a cork for this. A couple of thumbs placed both sides of the centre with gentle pressure should achieve this. Then the 3 dimensional alignment has to checked. This can be difficult as the similar indexs makes optical reference point hard to find. Fortunately, lens makers designed their cementing surfaces to facilitate accurate positioning.

    As the lens assembly cools and the cement thickens it becomes possible it wrap the lens in dental floss. This material looks like it was made for this very purpose. It doesn't slip of the round corners. cuts into the excess, hardening, balsam and increased tension in the later "baking". Photo 2 shows the captive lens.

    When you are satisfied, the tray is replaced in the oven (remember to keep it horizontal) for an hour or so at between 60 and 100.

    Next time it will be cleaning and optical checks of the alignment.

  3. #23

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    Re: How I did it: new balsam for a sick RR

    FINAL STAGE: CLEANING, REMOUNTING IN LENS CELL AND CHECKING ALIGNMENT


    The tray is allowed to cool down. The assembly will look very messy - with surplus solidified balsam, stuck shreads of paper and the floss threads. Cut the threads through on the ground glass side. Pull off carefully. I was always been sick of the smell of xylene by this stage and have user lesser potent solvents to scrape and rub off the surplus balsam. The mechanical activity is unavoidable unless the original dosage of balsam was exactly right. I left some balsam on the side edges to provide a cushion for the fit in the brass cell. The brass edging on the cell was pushed down with a hardwood edge. The brass edge tends to "bounce back" so this has to be repeated a few times until the lens cannot turn in the mount or rattle. Some dulling black enamel will have fallen off so a touch up with a paint brush will be necessary and may even help the physical seal.

    The next section, I think anyway, is correct so I would grateful if someone could quickly step in if I have overlooked/misunderstood something.

    If you have two symmetrical cells - and only one has been redone - then you can use that as a benchmark for the "new" cell. This has the rather tedious consequence that if the "old" cell turns out to be faulty - you have a new rebalsaming job in front of you! This is not just a theoretical problem - lenses can and do "slip" in relation to each other without there being any sign of balsam deteriorations!

    The first test is the seen reflections from a pencil of light from a torch (or low power laser?). In this case, the purpose is not to identify how the doublet is built up but to confirm that all the reflections can be made to stand in a straight line "hidding behind each" by altering the angle of the lens. You could check the "old" lens too at the same time.

    If you get past this test then there is a more demanding test. Mount the old lens alone the barrel - about halfway in, with no play in the threads - but still with ease of rotation. Mount the objective on any camera with an open back and set-up for and measurement of focal length on an interior pale wall for a bright clearly defined small bright object (sun/lamp). The use of the open back/wall rather than ground glass is because the focal length gets pretty big for single cells. Lock down the camera and tripod and check that the front and rear (the wall, in this case) are exactly parallel. Make a pencil mark aroung the focussed bright spot, which should be well away from the centre of the image. Ask permission from the head of the household first. Rotate the barrel in the mounting flange and see if the position changes through the 360 degree turn. Repeat the experiment with the "new" lens. Movement away from the original position during the turn is not a good sign - especialy if there is difference between the old and new lens. Use the other end of the barrel if there is difference in thread sizes for the two cells. This could also be caused by imperfect set-up or that the threads/mounting are not as they should be. I don't think there is any aberation can cause this "wandering" image through a 360 degree turn. Please let me know if there is.

    Do not despair if things havn't worked out to well! It may be possible to make adjustments if the cement isn't to hard. The internal cement will still be comparatively soft. The ancient method was to true up the lens on a lathe at this stage.

    I'll come back again ( unless told not too! ) with some additional illustrations of two other kinds of cell mounts:

  4. #24

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    Re: How I did it: new balsam for a sick RR

    For the record - here are photos of the completed cell, comparison with the original rear cell and the Suter objective. As good as new perhaps - certainly worth the 25 investment in buying the aplanat originally.

  5. #25
    ARS KC2UU
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    Re: How I did it: new balsam for a sick RR

    Quote Originally Posted by Steven Tribe View Post
    I am not sure how easily methylene chloride (CH2Cl2) is available anymore. It used to be the active component in really effective paint removers. Perhaps "wicked" was an understatement. The commericial product was rather gel like so there was time enough to place a dollop in a glass container (not plastic here) close the lid and retreat quickly inside.
    Must still be available someplace. My facility still has several mid-size storage tanks holding it. My first experience with it was in early 1970s when I worked as a clinical manufacturing technician in the pharmaceutical business. It was the solvent of choice for film-coating pharmaceutical tablets. Wicked is indeed not the word to describe a work environment with many hundreds, perhaps thousands of ppm methylene chloride vapor in the breathing zone. I can say from experience that one runs in, holds their breath as long as possible, makes adjustments, and runs back out of the room. And the vapor will make every sweating pore in your body itch like you've been dipped in acid.

    Very pleasant stuff... but a surprisingly agreeable odor in very low concentration.

    Regards. Bob G.
    All natural images are analog. But the retina converts them to digital on their way to the brain.

  6. #26

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    Re: How I did it: new balsam for a sick RR

    Great set of posts! I always enjoy reading about how to repair things. It is good to hear about what went wrong as well.

  7. #27

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    Re: How I did it: new balsam for a sick RR

    Steven, you're wonderful!

    Thankyou so much for this. I've been struggling to recement an eyepiece doublet.

    My first try (after cleaning all the old cement off with acetone) was to pop the elements in a frying pan on low heat on the stovetop, and put a little bit of Canada balsam on the concave surface to melt (my balsam came as solid chunks).

    Unfortunately I was unable to get the bubbles out. I had a go at leaving it on the stovetop to cook the bubbles out, but the balsam overheated and went yellow.

    So then I tried dissolving a chunk of balsam in acetone, and putting a drop in the middle, then putting the other lens on top. This was much the same - still bubbles and voids.

    Finally, thanks to your writeup, I tried preheating the elements in the oven to around 80 degrees, then putting a drop of my dissolved balsam on the concave element. This time the balsam flowed really nicely. It's in the oven at the moment baking for an hour at 80 degrees, but so far it looks like there are no bubbles

    Again, thanks a million for the writeup.

  8. #28

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    Re: How I did it: new balsam for a sick RR

    Quote Originally Posted by rguinter View Post
    Must still be available someplace. My facility still has several mid-size storage tanks holding it. My first experience with it was in early 1970s when I worked as a clinical manufacturing technician in the pharmaceutical business.

    Regards. Bob G.
    In the past methylene chloride was used as a solvent to remove caffeine from coffee; supposedly the residual solvent was low enough to render the decaffeinated coffee safe to drink (not for me, however).

    This is a great thread. What do you think about asking for a new Forum that would cover more technical matters like camera and bellows construction, lens recementing, shutter CLA, etc.?
    They are ill discoverers that think there is no land, when they can see nothing but sea.
    -Francis Bacon

  9. #29

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    Re: How I did it: new balsam for a sick RR

    I proposed a new section last December!

    -to include project descriptions with place for others experience.
    -to include gift of materials ( I will have a lot of small pieces of cuban mahogany to dispose of later this year) for repair.

    I think bellows construction is quite well covered already. I personally think that only a few shutter designs are suitable for the average person to meddle with!

    I am starting on a method for the production of Waterhouse stops suitable for easily available tools which I will post sometime soon. I will add some material in this thread very soon with experience with other achromates. It seems that about 7 days xylene soaking for separation applies to just about all large lenses ( 50mm).

  10. #30
    jim landecker JimL's Avatar
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    Re: How I did it: new balsam for a sick RR

    Quote Originally Posted by Steven Tribe View Post
    I am not sure how easily methylene chloride (CH2Cl2) is available anymore. It used to be the active component in really effective paint removers. Perhaps "wicked" was an understatement. The commericial product was rather gel like so there was time enough to place a dollop in a glass container (not plastic here) close the lid and retreat quickly inside.
    Methylene chloride is readily available at plastics suppliers, as it's used as a solvent for joining plastic materials such as styrene and acrylic.

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