Holding glass lens for grinding

Hi everybody - I’ve been looking on t’internet without much success how to mount or hold a small glass glass lens with the view to grinding the outside diameter to the size I require. I’ve seen clockmakers use melted pitch for mounting brass components. I’ve consider hot melt glue and various grades of Locite but de-mounting may be a problem with the temperature that Locite breaks down.
The over way might be to carefully clamp the lens using a rubber pad mounting in a live centre. Needless to say I haven’t tried anything yet. The last thing I want is the the lens de-mounts while grinding for obvious reasons. Any other advice or suggestions please?

Hi I think that the supporting it on a shaped support that is slightly smaller than the finnished size and a rubber nose on a revolving centre ( you can get them with interchangeable noses ) is the way to go but never having done this maybe someone can suggest a better method. Can you give a size and picture to help.

David

The traditional glue for optical glass is balsam ( Canada Balsam ). There are probably more modern synthetic materials but I am not familiar with any of these.

Balsam was used because of its similar properties ( clarity and refractive index ) to optical glass. It can also be removed without damage to the surface of the glass.

Microscopists will be familiar with the use of balsam in the preparation of mineralogical slides and for sealing the edges of biological slides used in permanent collections. It is also used in the art world in the preparation of varnishes.

Pero

during my apprenticeship as a scientific instrument maker (1950s) ,the company did look into making their own lenses,and aquired a lot of the polishing equipment but did not proceed further, I was told at the time that the glass was secured to the polishing blocks with pitch,it apparently a filthy industry with lots of abrasive covering the equipment, I certainly would never attempt to polish glass in my lathe, Some years later while visiting a plastics moulding supplier,they also had another business of making lenses and it was an opportunity to see all the polishing equipment working exactly as my first employer had described.

With apologies to Pero ...

Canada Balsam is indeed very suitable for joining lenses in a group: but it is not appropriate for Johnboy25’s job.

The traditional mounting ‘pitch’ is more like a mixture of Wax and Shellac ... a very important characteristic being that [whilst warm] it allows the rotating lens to be centred by applying pressure.

MichaelG.

How much do you want to reduce diameter by?.....If it's not much and you just want to make it fit a mounting then it might be better/easier/quicker to do it by hand .......find a round tube or object of the approx diameter to require, place it over the lens and mark with a black felt pen....then hold the lens in you r fingers or in a cloth and carefully grind down to the line using either a (good) grinding wheel or by working on a flat stone.

And is it important to have the centre of the lens in the centre?

Regards Ian

Hi folks - sorry I haven’t been able to get online since putting this new thread up.

Thank you all for your comments & suggestions The lens in question isn’t for it’s optical properties, it’s for a plano convex lens of the type that fits into a railway lamp. In this case a 1.5” scale loco lamp with LED & battery inside.

I’ve seen pitch(?) Being used years ago but didn’t know anything about this method - hence my question. In the light of day I’ll read though the replies once more.

Thank again - you can always rely on this forum to get guidance in the right direction. 👍

Edited By Johnboy25 on 27/04/2020 00:13:34

Silicon sealer such as Silastic?

Hot melt glue makes a good alternative to pitch.

Robert G8RPI.

Exactly what Michael says.

If you are worried about temperature melting the adhesive, then you are planning to be far too agressive with the grinding!

Neil

The times I have watched lenses and the like being ground they have been done using plenty of coolant which prevents the fixing medium not to melt. So if you are able to flood cool then something like hotmelt glue would be suitable

Just for interest, what diameter to what diameter? If a lot to come off, a copper tube with grit on the edge as a tube cutter or Silicon carbide (Wet and dry) paper if not a lot, suitably lubricated in both cases.

Regards Ian

The books Amateur Telescope Making published by Gall Inglis covered this, can't remember which volume.

The idea was to chuck a brass tube, turn true to a convex or concave chamfer to suit lens then stick the lens on with a beeswax/rosin mixture applied hot. You centred the lens by some optical trickery then let it cool down. Next a sheet of brass made into a trough and grinding grit/water and slowly take off the high spots until lens was round and to the right diameter.

I have done this for a clock glass but held an emery stick in the tool holder (paper washers). The trick was not to go too quick and generous water.

All to be done on your best Cowells lathe......

I should think any adhesive that was gooey enough not to let the lens slide away would be OK.

Ingals ATM vol 3 page 35 >

I have them in pdf if you need them.

Edited By john carruthers on 27/04/2020 18:37:01

What about something like this

Johnboy, do you now have the answer you needed?

if not, let me know and I will have a word with a friend who used to grind lenses professionally and see if he can give you a steer.

John