Thinking aloud… reduce diameter of glass filter

This topic has 27 replies, 15 voices, and was last updated 29 August 2016 at 14:05 by Nick Hulme.

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26 August 2016 at 19:05 #24701
Neil Wyatt
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@neilwyatt
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26 August 2016 at 19:05 #252397
Neil Wyatt
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@neilwyatt
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
26 August 2016 at 19:13 #252401
HOWARDT
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@howardt
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
26 August 2016 at 19:20 #252404
Michael Gilligan
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@michaelgilligan61133
Posted by Neil Wyatt on 26/08/2016 19:05:48:

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?

.

Either of the above

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

MichaelG.
26 August 2016 at 19:21 #252405
Ajohnw
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@ajohnw51620
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

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Edited By Ajohnw on 26/08/2016 19:21:47
26 August 2016 at 19:22 #252407
Neil Wyatt
Moderator
@neilwyatt
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.
26 August 2016 at 19:25 #252408
Neil Wyatt
Moderator
@neilwyatt
hmm…

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

I may be able to use a TCT glass drill as a cutter.
26 August 2016 at 19:47 #252415
Ajohnw
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@ajohnw51620
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

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26 August 2016 at 20:02 #252422
Neil Wyatt
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@neilwyatt
Thanks John.

I may try this over the weekend.

Neil
26 August 2016 at 20:04 #252424
JasonB
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@jasonb
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
26 August 2016 at 20:09 #252426
duncan webster 1
Participant
@duncanwebster1
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
26 August 2016 at 20:14 #252429
Michael Gilligan
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@michaelgilligan61133
Posted by Neil Wyatt on 26/08/2016 19:22:55:

Standard reason – I can do this for nowt

.

Duncan,

I agree with you 100% … But ^^^

MichaelG.
26 August 2016 at 21:00 #252438
Neil Wyatt
Moderator
@neilwyatt
Posted by JasonB on 26/08/2016 20:04:30:

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

I have indeed, another good idea.

Neil
27 August 2016 at 08:15 #252463
john carruthers
Participant
@johncarruthers46255
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.
27 August 2016 at 08:47 #252468
roy entwistle
Participant
@royentwistle24699
Neil Are you sure it's glass ? A lot of filters are hard resin
27 August 2016 at 09:08 #252470
Howi
Participant
@howi
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.
27 August 2016 at 09:24 #252473
peak4
Participant
@peak4
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.
27 August 2016 at 10:53 #252499
Ajohnw
Participant
@ajohnw51620
Posted by john carruthers on 27/08/2016 08:15:21:

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.

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

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27 August 2016 at 11:06 #252503
Vic
Participant
@vic
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.
27 August 2016 at 11:21 #252508
KEITH BEAUMONT
Participant
@keithbeaumont45476
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
27 August 2016 at 13:02 #252527
Circlip
Participant
@circlip
Friction clamped between headstock and tailstock, rotate and attack with Silicon Carbide paper used wet.

Regards Ian.
27 August 2016 at 13:31 #252530
MW
Participant
@mw27036
I think it's doable but the finish could be rather poor, and you have to ask yourself if you want to risk this damage to the filter.

Michael W
27 August 2016 at 16:15 #252555
Neil Wyatt
Moderator
@neilwyatt
Posted by KEITH BEAUMONT on 27/08/2016 11:21:45:

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

Thanks Keith!

Neil
27 August 2016 at 16:17 #252556
Neil Wyatt
Moderator
@neilwyatt
Posted by peak4 on 27/08/2016 09:24:19:

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.

Good thinking.
27 August 2016 at 16:18 #252557
Neil Wyatt
Moderator
@neilwyatt
Posted by Michael Walters on 27/08/2016 13:31:17:

I think it's doable but the finish could be rather poor, and you have to ask yourself if you want to risk this damage to the filter.

Michael W

I have little to lose!
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