Thinking aloud... reduce diameter of glass filter

I have a 49mm deep red filter (for accentuating IR but still allowing some red through, equivalent to Wratten 25a or 26).

I need a 48mm one. A 49-48 adaptor is no good as the O/D must not exceed 2"

I also have a 48mm skylight and little or no use for it at the moment.

If I can remove the red glass (easy) and reduce its diameter by 1mm I can out it in the 48mm frame. As I have a 48-49 adaptor I can still use it for its original purpose.

So the thinking aloud bit - use something like hotmelt to glue the disk to a mandrel and use something like a coarse diamond slip to take 0.5mm off it all round?

Or fit it on a pivot and use the diamond wheel on my T&C grinder?

Or something else? Will a TCT tool take a 1-thou shave off glass? I'm patient enough to take 25 cuts, but I don't suppose it will.

Could I use a mounted diamond dresser to take shaves off the edge?

Is this a stupid idea...?

Neil

Probably worth talking to SRB-Photographic, see what options they have. They do empty filter rings, but 48 is non standard although may not be to them.

Howard

.

Either of the above

... Or consider a variation: Dremel tool mounted in lathe toolpost, using a diamond disc.

MichaelG.

Grind it off very carefully and slowly as if you were smoothing out a telescope mirror edge that was cut from plate.

There should be some details about how that is done around on the web usually using grit or stone but I don't know what grade.

Also possible to cut circles with notched end brass tube but I doubt if that would be any good for this. The glass is usually laminated in more glass or plaster to stop chips from forming. This how 2ndry mirrors can be cut.

No guarantee given or implied. Glass can chip ever so easily.

John

-

Edited By Ajohnw on 26/08/2016 19:21:47

48mm is standard for telescope filters, GOK where I got a 48m Skylight from but i can use its frame. It's just making the filter I already have fit in it. Standard reason - I can do this for nowt when it's about £20 to get a s/h one from Japan or the USA.

hmm...

hackaday.com/2014/12/04/cutting-glass-with-cnc/

I may be able to use a TCT glass drill as a cutter.

People have ground glass with standard diamond angle grinder discs Neil. Also grinding wheels. A diamond lap stick should do it with the disc rotating.

Usually done wet as breathing in glass dust is not a good idea at all.

It will pay to laminate it on both sides some way or the other as it will result in sharp edges that chip ever so easily pr make sure you keep a bit of a chamfer on both edges.

John

-

Thanks John.

I may try this over the weekend.

Neil

Have you got a wet diamond tile cutter, just rotate it against the diamond edge of that. If I can do thick glass tiel with them your thin filter will be a doddle.

Probably easiest to glue it to a 48mm MDF disc and use that as a template to rub against teh body of the cutting disc

Proper opticians grind round lenses to fit the frames, so taking 0.5mm off all round should be a doddle. I'd ask one of them to do it for a small fee

.

Duncan,

I agree with you 100% ... But ^^^

MichaelG.

I have indeed, another good idea.

Neil

In the glass shop I'd have taken it off on the 'wheel' (a 0000 alox, slow, wet wheel) used for polishing and arrissing edges.
It could be done by hand with some 800 wet'n'dry on a block, or a diamond hone?
The linisher or belt sander would be a bit too fierce.
Beware of surface scratches.

Neil Are you sure it's glass ? A lot of filters are hard resin

The tile cutter would be a good option or a dremel with a diamond disc, would be my choice. I have smoothed the ends of cut test tubes this way.

Perhaps a visit to Poundland before you start might be a good idea too.

They sell phone/tablet screen protectors, which cut to a couple of circles might protect both sides of your filter from scratches due to the glass/diamond dust.

Having played with glass in several ways I think John makes a good point there - slow wet and fine. The problem with a coarse abrasive is that this alone may chip the glass. This is why instruction on making telescope mirror usually mention retaining a chamfer on the edge. Even so 'hit sometimes happens. Working a thin edge is likely to be worse. A coarser high speed lap / grinding thingy may be ok if it's applied precisely with very closely controlled cuts.

People in the past have just used ordinary medium rather than fine sharpening stones for working telescope mirror edges and for applying a chamfer - that does need some care but is working around rather than across it which helps a lot. I'd guess that this sort of approach would be ok for Neil as long as he doesn't rush it.

John

-

I went to look at a job many years ago and one of the things they use to make were glass discs for part of their products. They cut the glass into squares first then "knibbled" them into (very) rough rounds. They were then stacked into a column using bees wax to stick them together. The columns of glass where then mounted between centres on the lathe and turned true. Heating the turned column melted the wax separating it into individual discs. I've no idea what type of tool they use though.

Hi Neil,

I had same requirement with a filter and took it to my local Specsavers and asked if they would reduce by one mm. They did it same day ,cost £5.00

Friction clamped between headstock and tailstock, rotate and attack with Silicon Carbide paper used wet.

Regards Ian.