Cementing doublets

[suzyj]

This is starting to drive me a little nuts.

I have a lens that separated. It's the eyepiece lens for one of my older cameras. I successfully completed the separation without damaging either element, and then cleaned off the cement with acetone. So now I have to lovely lenses, that show gorgeous fringing when I put them together.

So next I bought some Canada balsam on the net. It came in solid form - big chunks of lovely amber coloured stuff.

My first attempt was to clean everything carefully, then heat the lenses, with a little chunk of the Canada balsam on the concave surface. When that melted, I dropped the other lens on top, carefully aligned it, and left it to cool. I ended up with little air bubbles trapped between the elements.

So I separated them again (much easier, as the cement hadn't properly set), cleaned it, and tried again. This time I noticed the Canada balsam bubbled a little as it melted. I figured the bubbling was water in the balsam boiling off. Once it had settled down, I placed the convex lens on. Still bubbles. I noticed they were appearing from nowhere while the lenses were still hot.

So I kept them warm for a while in the hope that the bubbles (which I presume are water) worked their way out. But the longer I left it, the more yellow it went - clearly I'm burning the balsam somehow. I pulled them apart again and cleaned them.

For try three, I dissolved a chunk of balsam in acetone. I put a drop on the concave surface, and dropped the other lens on top. It looked beautiful.

So I left it be for a few hours. Sure enough, when I returned there were bubbles.

So what am I doing wrong? Has anyone successfully done this? The UV curing lens cement from Edmund optics is looking more attractive all the time.

 

[Ian Grant]

There's a poster on the LFPinfo forum who has just been giving details of how he's re-cemented a lens. Have a look here.

Ian

 

[John Koehrer]

The technique isn't all that hard. Put A(1) drop of the cement on the concave element. Gently place the convex lens in place and spread the drop of glue around by moving it in a circular motion until the entire surface is coated. It takes surprisingly little and your problem may be too much cement.
I've always used a UV curing cement available in hardware stores but it's less forgiving than the Balsam.
I haven't found a solvent for it. Haven't looked too hard though.
Balsam can be separated by soaking in acetone

 

[Ian Grant]

I haven't found a solvent for it. Haven't looked too hard though.

Paint stripper, I believe that works

Ian

 

[suzyj]

Yay! Finally got it right.

The key was preheating the elements in the oven.

The cementing process I used is as follows:

First, clean the elements thoroughly.

Next, dissolve a chunk of Canada balsam in acetone, to make a nice, thick, gooey mixture.

Put the elements in the oven at 80 degrees for a quarter hour to preheat. Take them out still on the oven tray and check that they're still clean.

Put a little drop of dissolved balsam in the middle of the concave element - a little bit goes a seriously long way.

Put the convex element on top carefully, and press in place to squeeze most of the cement out the sides (I used tweezers to hold it, and a cork to press, as it was still pretty hot).

Check that it's aligned and that there are no bubbles.

Finally, put it back in the oven and cook it at 80 degrees for an hour.

 

[unclemack]

Tried GlassBond (in the UK), which is uv cured, on one of my own lenses when it first became available here.
Not sure when exactly but feels like 20 years ago... and always used it from then on. The lens is still perfect.
You do need to take care using this stuff though and plan a strategy for keeping the squeezed-out excess from the outside surfaces - after washing I used to apply a couple of coats of artist's masking fluid which forms an easily-peelable layer and protects the surfaces from damage as well as contamination.
Washing consisted of detergent, tap water, then distilled water baths and a filtered air line for drying.
I had to wash every other glass surface too - customers look harder at a just-repaired lens than at any other time it seems - and attribute any blemish or dust to the work just done!
You could just wipe them with a clean microfibre cloth though...
I really see no need to use Canada balsam today when a superior product is available.

 

[Nicholas Lindan]

Thanks for the information.

Too many threads dissolve into a mess of divergent suggestions and tangential topics and the OP never comes back with the final resolution to the original problem.

 

[Rick A]

Yay! Finally got it right.

The key was preheating the elements in the oven.

The cementing process I used is as follows:

First, clean the elements thoroughly.

Next, dissolve a chunk of Canada balsam in acetone, to make a nice, thick, gooey mixture.

Put the elements in the oven at 80 degrees for a quarter hour to preheat. Take them out still on the oven tray and check that they're still clean.

Put a little drop of dissolved balsam in the middle of the concave element - a little bit goes a seriously long way.

Put the convex element on top carefully, and press in place to squeeze most of the cement out the sides (I used tweezers to hold it, and a cork to press, as it was still pretty hot).

Check that it's aligned and that there are no bubbles.

Finally, put it back in the oven and cook it at 80 degrees for an hour.

Was that 80f or 80c for the oven temp?

Rick

 

[suzyj]

Was that 80f or 80c for the oven temp?

Rick

Celcius.

 

[John Koehrer]

Paint stripper, I believe that works

Ian

I'll give it a whack, Acetone doesn't touch it.

 

[benOM]

Paint stripper based on methylene chloride is the one to use, I too used glass bond for my lenses and it works well.

 

[Grytpype]

Better stock up on methylene chloride paint stripper, benOM. Our beloved masters at the EU are about to ban it!

Steve.

 

[John Hermanson]

I'm a little late for this but I use Loctite Clear Glass Adhesive. Position lenses, clamp very lightly, put in direct sunlight (cures in a few minutes.) John

 

[Fotoguy20d]

Nice thread and I have nothing to add but have a tangential question. I have a ca1884-1902 (based on the company name) 8x10 Gundlach triple convertible lens - the front element is beautiful, the back has an amazing amount of separation - more than I've ever seen before. Sadly, the lens is held into the brass barrel by a peened over edge. How do I remove that or work around it?

Thanks,
Dan

 

[unclemack]

Hi Dan, when the cemented elements can't be accessed from the other end the only option I've used with success is to cut away the peened-over part and later refix the cell with epoxy. This was always on modern aluminium mounts though where the edge is thinned to almost nothing - any tool inserted between the metal & glass just breaks the alloy into chips.
People have had some success heating the cell it seems but I was never able to make Canada balsam reflow in this way.
Others have cited old papers recommending a lathe and a hardwood (ironwood maybe?) tool for the job. I would rotate the chuck or faceplate by hand if I was trying it.
If the edge is of sufficient thickness and you don't have a lathe you could try a simple version of engineers' v-blocks to let you rotate the barrel in place.
Two holes in a bench-top, distance apart less than barrel diameter, two tightly-fitting pegs or bars in the holes. Barrel sits on bench with peened element uppermost and is rotated against the two pegs by hand. If properly done this can be extremely accurate.
Then all you have to do is bring the tool to the work under control - a cross-vise on a fixed angle-block works well enough.
A dremel tool with tiny saw was how I used to cut away the peened edge - I fixed the dremel in the cross-vise with the above arrangement and it makes for a very quick, neat job.
A cross-vise can travel in two directions and has a simple micrometer drive for both - in case you were wondering!
Familiarity with all the tools involved is probably a good idea for this kind of work though...