PG Optical dividing head

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 15/07/2021 11:52:33:

I've never tried anything like this either, but my Microscope hobby suggests Canada Balsam substitute rather than the original Balsam. […]

Unless they have changed the original [Northern Biological Supplies] formulation; I would strongly advise against using Numount … I have seen numerous specimens ruined by its use to make slides.

After a time [a few months, or perhaps years] it tends to crystallise …with optically disastrous effect.

Note: This may be attributable to the use of some particular solvents, but I haven’t tested.

MichaelG.

.

Edit: __ This may be of interest:

https://www.natsca.org/sites/default/files/publications/The%20Biology%20Curator%20Issue%2010%20supp.pdf

Edited By Michael Gilligan on 15/07/2021 17:59:47

[William SParticipant @williams]

This is the problimatic lens: 

As can be seen Its not clear like the other 2 lenses(below) the thing I cant work out is how they come out of the brass body, the front looks to be machined to that shape and the back doesn't look to unscrew, what's peoples opinions?

I hope these pictures explain.

The previous thoughts are much appreciated, that link to the graticules optics Michael is very handy, they are actually local to me so I may be seeking there expertise on the damaged Minutes scale. 

William 

Edited By William S on 15/07/2021 20:00:57

[Robert Atkinson 2Participant @robertatkinson2]

The fitting behind the lens normally unscrews in microscope objectives like that. It may not be obvious.

Robert G8RPI

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for posting the photos, William

The good news is that it looks like the lens probably has a standard RMS objective mounting thread, so substitution shouldn't be a major problem … although 1/2” x14 is not a common power.

The group in the Brass cone may well have been located [at the small end] by ‘rubbing’ [a.k.a.burnishing] … like a jeweller mounts a cabochon stone: in which case dis-assembly and re-assembly might be tricky.

Edit: as Robert mentions, the ‘cup’ at the large end could be threaded [inspect carefully]

Baker was an excellent British microscope maker, ultimately subsumed into the Vickers brand.

… I will have a look around.

MichaelG.

Edited By Michael Gilligan on 15/07/2021 20:57:31

[Michael GilliganParticipant @michaelgilligan61133]

No luck with the 1/2” x14 yet … but this microscope uses what appear to be contemporaneous objectives:

**LINK**

http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artapr04/pjbaker.html

MichaelG.

[Clive HartlandParticipant @clivehartland94829]

I agree the lenses may well have been burnished in place, hard to get out but not impossible.

[Roger HartParticipant @rogerhart88496]

Mmm, wasn't expecting a microscope objective. This article may encourage you.

http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artapr05/dwobjective.html

A human put it together, I am not convinced the lens are swaged in, it is a fairly low power objective and the front lens looks set back in the cell. So should come apart. Otherwise a close enough substitute will only alter the focus a little bit.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by William S on 15/07/2021 19:46:29:

[…]

This is the problimatic lens:

.

Roger,

I am perfectly happy to be wrong … but these two photos appear [to me] to show a burnished edge separate to the conical brass cell.

Unfortunately, I have yet to find any detail about Baker’s mid 20th Century assembly techniques.

Eagerly awaiting more revelations from William

MichaelG.

[Roger HartParticipant @rogerhart88496]

I fear you may be right MichaelG. Certainly looks as if front lens is either swaged in or up against a thin collar.

I have had a look at my own small selection and they look as if they come apart from the back – not swaged. But I am not about to test this….

A lot hangs on whether the inside thread and cone is a single piece or two – does the internal threading extend down to or beyond the internal cone. How was it made – one piece or two? William said it does not look to unscrew so will a soak in solvent and a bit of pressure from the front reveal anything other than a cracked lens…?

Otherwise I would be looking for a substitute until the rest of the kit is back in order.

[Robert Atkinson 2Participant @robertatkinson2]

One has to ask why you would swage the doublet in when you already have an internal thread?

It does save one turned and threaded part but adds an operation with some risk of damaging the doublet.

A semi-destructive method to tell if the inner part is threaded or solid is to cut into the side with a thin slitting saw (or abrasive disc on a dremel type too)l. Stop and inspect frequently to see if there is any sign of a thread. The slit will be covered on re-assembly so won't affeact performance or appearance. If it is threaded and tight, extending the cut to the edge may make it easier to unscrew.

Robert G8RPI.

Edited By Robert Atkinson 2 on 16/07/2021 16:08:21

[William SParticipant @williams]

It’s looking more to me like the lens is burnished in

The thread is undercut, the picture above shows it just about. The part that would usually unscrew has a perfect flat face where I would expect to see the start of the thread. It does seem a bit odd to put a thread in but not put it all the way down to hold the lens in.

I hope that picture explain why I think it was manufactured like it appears to be, screwing it on a mount would also keep the lens face square.

What would be the process to “unburnish” the lens?
Or leave it and source a replacement, would a slightly different magnification affect much?, there must of been a reason for the uncommon magnification.

William

[Roger HartParticipant @rogerhart88496]

Difficult. Just push it out.

A look at some YouTubes – replacing a watch jewel – may help. Essentially the idea is precisely controlled and evenly distributed force – a thou at a time. If I were to try this I think a hard plastic cup with spigot held in lathe chuck and a shaped brass pusher with paper washer in the tailstock. Warm the cell up to expand the brass and soften any cement and apply some force and rotate the cell whilst under force to distribute the push evenly.

I would aim to put most of the force on the outer part of the lens. That looks the tricky bit. Other people may have better ideas. The watchmakers have some fancy expanding tube tools to re-open the burnished cell once you get the lens out.

Experience is directly proportional to the amount of expensive equipment ruined.

[William SParticipant @williams]

Well a stroke of luck following Rogers advice:

Well good for the lens not so good for my finger, I decided after pressing the doublet out I would press my finger in to the threaded end of the body, a nice gouge and a flap of skin is what happened!

I turned up this plastic holder to support the brass body (it is tapered inside!) the brass body was a good fit.

This was the set up in the lathe, using the lever operated tailstock to gently press the doublet out, It might of just been glued in as it was rather sudden when it broke free. Also I can't see any evidence on the brass body inside of it ever being burnished in, although I cant be certain, its probably so small!

This is the little brass pusher I lined it with electrical tape, after turning the front face convex so just the very outside was pushing on the glass lens.

I am partway though cleaning these 2 up with meths, they are both glass and coming up rather well, I am now looking in to reassembly, how best is it to proceed in your opinions?

Thanks again for all the information that has been given thus far and I look forward to hearing more.

William

[Michael GilliganParticipant @michaelgilligan61133]

Great result, William … sorry you had to suffer for your Art

Traditional bond would surely be Canada Balsam [which has the great advantage of being easy to remove if things go wrong] but a modern alternative, which I think Clive mentioned, would be LOCA [as used for mobile ‘phone screens]

MichaelG.

.

Big question … are the two elements exactly the same diameter ?

Hopefully they are, because otherwise you will need to centralise them in the lathe

No, I’ve never done it, and the idea scares me somewhat !

Edited By Michael Gilligan on 20/07/2021 23:58:30

[Roger HartParticipant @rogerhart88496]

Top man, respect. Sorry about the finger…

From what I remember from the ATM books, a small amount of warm balsam was applied, the lens put together with no bubbles etc and then put into a brass ring to keep them centred. A small weight applied and left to cool/set. Usually the lens are same size, just a bit of tube – not too tight – is needed.

The tricky bit with a UV setting adhesive is making sure the assembly does not get stuck permanently in the line-up cell, that any smears can be removed and that UV light can get to it so it goes off. A few experiments and a deep ponder…. Thousands of these must be made every day, so it can't be that difficult if only we knew how it was done.

A quick trawl of the web found suitable glues but the stuff on removing/cleanup was a bit iffy – maybe others know more.

[Clive HartlandParticipant @clivehartland94829]

Any excess adheisve of both types I always trimmed off with a sharp razor edge. A hard setting adhesive is not good as it will set up stresses in the optics and can cause cracking or distortion..

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Roger Hart on 21/07/2021 06:59:19:

[…]

From what I remember from the ATM books, a small amount of warm balsam was applied, the lens put together with no bubbles etc and then put into a brass ring to keep them centred. A small weight applied and left to cool/set. Usually the lens are same size, just a bit of tube – not too tight – is needed.

[…]

.

Ah but … in the best work, the lens elements are centred and joined whilst slightly oversize, and then the edges ground.

Modern tools have largely automated this process, but mid 20th Century it was very skilled hand-work.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

I don’t think Baker had one of these : **LINK**

https://youtu.be/cl6K-EXPILo

or these: **LINK**

https://trioptics.com/products/opticentric-lens-centration-and-alignment/

MichaelG.

[William SParticipant @williams]

Well:

(quite a difficult thing to photograph)

So in the end I went the LOCA route because I'm a cheapskate!, (it was extremely simple aswell) I also didn't want a huge great quantity left over.

I went with one of the many cheap kits available on eBay, The UV torch was immediately dismantled and jerry rigged to my bench to power supply, the 3 AAA I scrounged only managed to barely illuminate 3 LEDS anyway. This also enabled me to knock up a quick tripod to hold it about 20mm off the bench.

So I experimented with some 1981NOS glass microscope slides(well that's what the boxed wrapped in cellophane said)

The result was a success and gave me confidence to risk the original! It went very well, As can be seen I just used the original brass cell as the line-up ring, the doublet was a relatively good fit anyway (they were the same dia, the lenses Michael) It also meant I didn't have to worry about peening it back it as the glue seems to be holding them in. I cleaned them, a couple of drops of LOCA, place together, wriggled them about to spread the adhesive and dropped it in the cell, lightly pressed it home with my thumb and placed under the UV light. left for about 5 mins, come back to one optically clear doublet.

Reassembly went well, I screwed the brass cell Doublet fully home as it was that way upon the initial dismantle, the glass ring was installed and then the "first" lens was adjusted until the 0.2mm markings were in focus, then the black tube was rotated to get the markings in the right place on the minutes scale, the Baker object mounting thread is off centre to the black tube. The mirrors were not disturbed when I dismantled to preserve the original setting positions, I glad I did this as I would not know where to start to get these back in adjustment!

I then following Clive's advice, "clocked" the glass ring by having it clamped very loosely just pushed it about until the markings all the way around were in the same place on the minutes scale.

Well there we go, I am now going to try and look in to rectifying the minutes scale, Graticules optics is who I will try first, Now that I know it does actually work. And see about a permanent light unit.

[Michael GilliganParticipant @michaelgilligan61133]

Well done, Sir !!

MichaelG.

[William SParticipant @williams]

This is a sales brochure for a 3 second version, but its exactly the same mechanically as my 6 second version, its quite interesting reading, it explains the operation better than I have in previous posts! the Internal diagram especially, this I acquired last week from eBay (again!) It has already provided the rather useful info about the lamp ,6v 18watt.

The Projectorscope 10/250 is actually something my work has on the floor under a bench in the grinding department, it hasn't been used in the 3 years since I started my apprenticeship. Hmm do I have a use for it?!

William

[Robert Atkinson 2Participant @robertatkinson2]

Very nice work.

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