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Cutting Small Discs out of Glass

Making a meter glass

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Georgineer15/11/2020 16:10:54
524 forum posts
31 photos

The thread on cutting microscope slides has made interesting reading, and has prompted me to tackle a small project I've been putting off. I have a small meter with a broken glass of 46 mm diameter, and I want to make a new one. I have looked at the standard method of cutting glass circles using dividers with a rubber suction cup, but they are much too large. There are also various proprietary gadgets which cost money and look fiddly to use.

It occurs to me that it should be possible to mount a piece of glass to the lathe face plate with double-sided tape, or some other adhesive. Then an ordinary glass cutter could be mounted in the top slide and brought up until it touches the glass. A single turn of the chuck by hand should scribe the line, then the circle could be broken out in the ordinary way.

Would this work? Should I mount the glass on thin foam to give a little bit of give to maintain the pressure of the cutter? Is there an easier way? Any hints on how best to break the circle out after it's scribed?

I know one could do the job by mounting a grinding wheel on the toolpost, but that seems like more work and more mess, neither of which attracts me.

George B.

Cabinet Enforcer15/11/2020 16:21:37
107 forum posts
3 photos

cousinsuk sell mineral glass rounds intended for watch faces, less than two quid each. I would not muck about making one.

link to the 1.5mm thick ones, other thickness' available.

**LINK**

OldMetaller15/11/2020 16:24:00
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193 forum posts
24 photos

I don't know if this is any help but, when I want small circles of glass to make, ie, spectacle glasses in miniature locomotives, I use watch glasses, available from jewellers' merchants. They are quite inexpensive.

John.

old mart15/11/2020 16:24:18
3349 forum posts
208 photos

I have watched an expert scoring the circle with a diamond and then adding lots of radial scores. He then used some special pliers to snap off a bit at a time. The Maun pliers with the parallel jaws were the nearest that I could think of. Then he finished the edge with a linisher. I noticed that he touched on to the belt with the axis of the glass in line with the belt direction, and in the bit that is between the front roller and the top plate. This minimises the shock caused by the joint in the belt. Of course, he made it look easy.

Neil Wyatt15/11/2020 17:20:39
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18777 forum posts
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One way is to use a rotating tube of soft metal (e.g. copper) and carborundum paste, which embeds in the metal and cuts slowly and gently. Cut for a while, lift and repeat.

Neil

Brian Sweeting15/11/2020 18:07:36
453 forum posts
1 photos

You can also get hole saws with diamond grit edges, not sure if any sizes would be suitable.

Georgineer15/11/2020 19:44:57
524 forum posts
31 photos

Thanks for the ideas so far. I hadn't thought of the watch glass route which I might follow, as they are available in 1.2 mm steps up to 50 mm.

If one needs a larger glass, however, there seems to be a much smaller range of diameters available in flat clock glasses.

Neil and Brian, I hadn't thought of rotating tubes and hole saws but I lack suitable tubes or saws, besides which any non-standard size would involve considerable work and possibly expense.

I'm still minded to try my approach, partly for the experience and partly to save money, since I already have double-sided tape and a glass cutter. So, does anybody have any suggestions they would like to offer? For instance, how much pressure should I impose on the glass with the cutter?

George B.

Andy_G15/11/2020 21:41:30
135 forum posts
Posted by Georgineer on 15/11/2020 16:10:54:

It occurs to me that it should be possible to mount a piece of glass to the lathe face plate with double-sided tape, or some other adhesive. Then an ordinary glass cutter could be mounted in the top slide and brought up until it touches the glass. A single turn of the chuck by hand should scribe the line, then the circle could be broken out in the ordinary way.

It can work - the same principle is used to cut out circles commercially. You need to start the break from the outer edge of the glass sheet and get this to run into the circle. It's likely to take quite a bit of practice.

For small diameters, the circles are broken out thermally - using cold metal cylinders roughly the same size as the circle which are placed in liquid nitrogen, then onto the glass (after scoring)

You might get somewhere using a metal cylinder from the deep freeze to get the circle to break out, but I still think you'd need to break the outer square of glass to release it.

Unless it's for entertainment value I would go to core drilling one out (very cheap diamond core drills are available on Amazon, etc. which will work fine, but *must* be used wet). Or trepanning one with a metal lap and loose carborundum powder - anything will do for a one-off: I used a boot polish tin to trepan out a secondary mirror for a telescope. Needs a fairly slow drive though (I wouldn't dream of doing it by hand).

Or just buy one!

 

(Edit - you need just enough pressure on the cutter for it to create a vent in the glass - it will vary with the type of cutter. You will need to take into account some movement in the glass surface - i.e. the cutter cannot be rigidly mounted - it needs to float.)

Edited By Andy Gray 3 on 15/11/2020 21:43:45

Georgineer15/11/2020 21:47:39
524 forum posts
31 photos

Thanks, Andy. More food for thought there!

George B.

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