Delaminated objective - how NOT to repair it!

[MicroBob]

Hi together,
I recently bought a Zeiss West Neofluar 6,3:1 objective. It turned up and I didn't look closer - delaminated back element, right through. I got my money back and could keep the objective. I don't like to throw things away without at least trying to repair them. So I turned and milled a tool to open the screw ring at the back that sat very deep in a ring slot. I got the delaminated lens group out, turned a bit of the barrel away to be able to press the lenses (3) out in warm condition. This worked fine. Then I took a tungsten carbide scriber and scratched signs into the circulference of the lenses to get them back again in the right order. WRONG! The middle lens apparently is from a glass that splits easily and took my invitiation to crack right through along every line I scribed. Outer lenses ok, middle one cracked! So next time pencil lines will have to do.

What I was able to learn:
- The lens groups are held in the barrel with a grey silicone like glue, probably fairly heat resistant. This glue has to be overcome to diassemble the unit.
- Some lenses (probabalby the name giving fluorite) split easily

and all this for free!

Bob

[Element 56]

Hi Bob,

Obviously (now) you made a stress riser when you scratched the glass. I've read that fluorite is soft and easily damaged so I'm always very conscious of this when working on lenses. I was not aware that it was so brittle but thank you for the warning! I was just reading something about Zeiss lenses and that delamination is a common issue with them. I think the article was talking about camera lenses so I guess it's something to be conscious of with all their lenses.

Thank you far sharing your experience!

Kirby

[BramHuntingNematodes]

Thanks MB All us amateurs have to destroy something delicate and precious at least once. It's for the good of the community.

[Element 56]

Actually Bob the article I read was about microscope objectives.
http://www.microscopy-uk.org.uk/mag/art ... delam.html

The lens groups are held in the barrel with a grey silicone like glue, probably fairly heat resistant. This glue has to be overcome to diassemble the unit.

Did you mechanically remove this silicon like glue? I am currently working on my second Nikon Fluor and the adhesive they use is super hard. I was not able to soften it chemically so I've been removing it via mechanical means. There's a seal around the outside lower lens that I would call the main seal and there's 4 wholes on the side of the lower lens group that are filled with the same epoxy. This adhesive both retains the lens group and keeps the oil out (not so good at keeping the oil out!). On the first Nikon I took apart I just scraped it with a brass graver. On the one I'm working on now I used a wood stick in my micromotor. This is a very fast and efficient way to remove the epoxy. It's easy to control and reduces the chance of slipping with the graver. I just use the stick from a cotton swab and turn it to a point against a piece of abrasive cloth. To make it cut better I put a faucet on the point.

Kirby

[MicroBob]

Hi Kirby,
good idea with the motorized tooth pick!

In my case the lens group was fully seated in the barrel, but only the last element was glued in with the grey elastic glue. I turned of a bit of the barrel so the glue had less surface to grip. Then I placed the barrel with lenses in a fitting milling machine collet, heated the lens-less part of the barrel a bit with a propane torch and pressed out the elements with a bit of tissue and a wood dowel. If I hadn't cracked the lens I might have brought this lens back together with ok function.

Bob

[Element 56]

Bob,

I wonder if heating the lens played a factor in it breaking. I know from experience that abnormal temperature variations can crack lenses. I've broken glass with heat and over cooling. I'm speculating but heating it could have annealed or hardened the glass making more prone to breakage.

My shop can get a little cold in the winter and one day when it was particularly cold out I broke a lens while cleaning it. My cleaning solution was colder than normal and as soon as I touched the glass with a saturated swab it cracked. In this case there could have been a stress riser that I didn't see but regardless from now on I make sure it's at least room temperature before cleaning anything.

Kirby

[Hobbyst46]

Hi Bob,
I think that, as Kirby suggests, a combination of thermal and mechanical stresses. Since you scratched the circumference, it can be compared to cutting a glass tube. When I scratch the tube by means of a triangular needle file or a glass cutter, it does not break by itself, and I must apply pulling and pressing to complete the cutting into two parts. Professional glass blowers scratch the tube, then touch it with the red-hot end of a glass rod, to complete the fracture.
Admittedly, your case differs in that three glass elements were scratched together, so mechanical stresses were applied on all...

[MicroBob]

The cracking happenen when the lens group was at room temperature for a while. But it is possible that there were residual stresses from the former heating. The heating also softened the lens cements and the inner lens might have been partly attatched to one or both outer lenses. But I think it was mostly the stress risers in a delicate lens.

[75RR]

What I was able to learn:
- The lens groups are held in the barrel with a grey silicone like glue, probably fairly heat resistant. This glue has to be overcome to diassemble the unit.
- Some lenses (probabalby the name giving fluorite) split easily

Thanks for allowing us to learn from your mistake ;)

What cement/glue were you planning on using?

[MicroBob]

I wasn't sure. My preferred use for the objective would have been epi fluorescence. Here a low autofluorescence of the glue would be good. I would have preferred to be able to unglue it again to correct errors. I have canadabalsam and LOCA TP 2500, the first easier to resolve, the seceond with less autofluorescence. So I would have had to decide...

[Rossf]

MicroBob-hero of microbe hunters forum-ruining objectives so we don’t have to! I salute you sir!
Regards ross

[Element 56]

Bob,
In case you missed this article I thought you would enjoy reading it.
The beginning is relevant to your experience. It was posted in another thread by MichaelG.

http://www.kowa-prominar.com/product/cristal/index.html

Kirby

[alandavison]

Fluorite has "perfect" cleavage planes. Any form of pressure on the side of a crystal will make it part. The lenses are almost certainly made with the strongest face on the ground (concave or convex) surface. This would prevent the cleavage planes pulling apart. I have made a cabochon from fluorite, a mineral that occurs close to us.

[BramHuntingNematodes]

yes but as I recall the optical axis is not normal to the cleavage plane. I think this one of the reasons grinding lenses out of it is particularly difficult.

[BramHuntingNematodes]

yes but as I recall the optical axis is not normal to the cleavage plane. I think this one of the reasons grinding lenses out of it is particularly difficult.

wait no that was for calcite crystals when making nomarski prisms

[Scarodactyl]

Yeah, that would be calcite. Fluorite is isometric so it doesn't have an optic axis.