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Glasspusher

Making a 12 inch Mirror

138 posts in this topic

I would like to use this thread to describe the making of a 12 inch F5 plate glass mirror, although the techniques described can be used for mirrors up to and including 14 inches in diameter. My target audience is first time mirror makers and those who have some experience and might like to tackle a bigger project. Tips for keeping the costs down will be suggested. OK, here we go. First thing to do is to grind a bevel on either side of the blank, this can be done with an oil stone (carborundum stone), as shown in the first picture below, keep the stone wet when using it. The bevel needs to be no less than 3mm in width and must be renewed if it is ground away. If the edge of the mirror was left unbevelled the sharp edge could easily chip and result in unsightly ‘oysters’ on the edge of the mirror. Next thing to do is to grind the back of the mirror flat. Plate glass is pretty flat to begin with so this should not take too long; about 1 hour should do for a 12 inch mirror using 120 grit, this stage is particularly important with low expansion glass blanks which often have an uneven surface. The grinding tool can be a barbell weight, use the type with a raised rim round the edge, for a 12 inch mirror a 6 inch diameter tool is fine. It is best to grind off the central raised area with an angle grinder as shown in the second picture. A ‘workmate’ DIY stand can be used for a grinding table as shown in the third photograph below. A bag of sand helps to keep the arrangement stable. The fourth picture shows the stroke used to grind the back of the mirror. With the centre of the tool at 50% the distance from the centre to the edge of the mirror blank, the tool is moved back and forth with a slight overhang over the mirror blanks edge. Traditionally mirrors are ground and polished on a barrel so that you can walk all the way round the job for an even grind. The corner of a table or ‘workmate’ can be used, just walk as far either way as you can! More to follow.

John

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Hya GP, Although I'm unlikely to try , I am rather interested. Can I ask do you put the loose 120grit directly onto the surface under the tool?

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John,

It's very likely I'll try - I have a couple of blanks waiting...

Thanks very much for posting this. I look forward to the rest of the tutorial. :D

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This is most excellent. I'm glad to see another mirror making thread. It will give me more advice for when I can start my own 14" mirror.

Andy

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Thanks for the comments so far guys. With the edge beveled and the back ground flat it’s time to hog out the curve on the front surface of the glass. A blank with a pre-generated curve is very useful as it saves a lot of time at this stage. You can hog in the curve with a glass tool the same size as the mirror blank but this is an expensive way to go and takes longer than the method I will describe. I recommend that the curve is hogged out using the same tool as used to flatten the back, a 6 inch diameter barbell weight. Course abrasive will speed up the process, 80 grit or coarser is ideal. The stroke used to create the curve is a centre through centre stroke with minimal overhang of the tool at the end of each stroke, again walk round the work stand as far one way as you can and then the other way. After one hours work you can check the depth of the curve using a set of feeler gauges or a twist drill of known diameter. A 12 inch F5 mirror has a curve depth (sagitta) of .15 inches or 3.8mm. Here is a useful tool to work out any sagitta:

http://www.atmsite.org/contrib/Prewitt/sagitta/

Hogging out the curve with a sub diameter tool will not produce a spherical curve in the mirror, the curve will be deeper in the centre than at the edge. It is probably best to go a little deeper than the calculated sagitta as bringing the mirror back to spherical will reduce the depth of the curve in the centre. OK a little confession; I have made too many mirrors by hand to enjoy the donkey work so I am using a machine for the grinding. Hand work is generally quicker than machine work and has the advantage of introducing randomness into the stroke which is helpful.

John

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I tried making a 6" mirror when I was a teenager but I could not get the polishing right - I kept picking up nasty scratches. Eventually I gave up. Every once in a while I think of having another go...

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Those scratches could well have been the result of pitch that was too hard, more about that later!

John

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...The stroke used to create the curve is a centre through centre stroke with minimal overhang of the tool at the end of each stroke...

Can you describe this technique further, please?

Edited by Beulah

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Pushing too hard? Sounds like the teenagery thing i night have been doing...

EDIT: I see you meant a pitch that was too hard - no idea, I used what the astro club gave me...

Edited by Ags

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John, I think that you are assuming we all have some experience in this field. I'd like to know more details please. For example, I would have thought a rotary motion whilst grinding would produce a more even result to the shaping and flattening of the glass, but I get the impression that a linear motion is used....

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Thanks for the comments. In an effort to keep things brief I have not said that the tool needs to be rotated with the hands as it is pushed back and forth as you walk from one side of the workmate to the other. Sorry about that.

If anyone would like any points clarifying please ask and I will do my best, my intention is to get people making mirrors, not to put them off!!!

John

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I'd love to make a mirror, hense my wishing to know exactly what's involved. Being a mechanical sort, I need as much information as possible before leaping in.

For instance, to create the 'sagitta' (I take it this is the depth of the parabola??), doesn't the tool need to 'swing' from a focal point???

Also, would a potters wheel be of any use??

Edited by yeti monster

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Hi Yeti,

The sagitta is the depth of the curve, during grinding we are aiming for spherical curve on the mirror, the parabolic surface is 'added' after the mirror is fully polished during the process of 'figuring'. Think of the mirror surface as part of a large balloon. If the curve created in the glass was extended in all directions it would form a sphere. When we test the mirror we do so at the radius (or centre) of curvature, the focal point is halfway between the mirror and the radius of curvature.

Yes, I think a potters wheel could be utilised for mirror work, depends on the speed though.

John

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Cheers John, I was about to ask about how to produce a parabolic shape... but I'll let you get to that point in due course.

So, to produce a sherical mirror, can you illustrate the set up? And what grinding medium, and indeed what quality of glass blank? furthermore, where would one obtain glass blanks?

Hope not too many Qs, and if I'm preventing you from staying on track I'll keep quiet for a while and allow you to proceed......

Does look very satesfying though..........

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Hi Yeti,

To produce a spherical mirror it is easiest to make a tool the same size as the mirror blank and grind with a progressively finer series of abrasives. I will be describing this in due course. Ordinary plate glass is quite good enough for mirrors upto about 14 inches in diameter, bigger than this and the thickness becomes a problem (plate glass is only available in thicknesses upto 25mm). A good source for blanks and grinding materials is telescope kits, build your own. newtonian , dobsonian

Don't worry about too many questions, the whole point of starting the thread was to deal with questions about mirror making.

John

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Once the curve in the mirror blank is at the correct depth (or a little deeper as mentioned previously), the next thing is to make a full sized tool using cement and tiles. This will be used for the rest of the grinding process. Begin by making a cardboard strip about 25mm wider than the thickness of the mirror blank, if the blank is 25mm in thickness the strip will be about 50mm wide. It should be long enough to wrap round the mirror to form a damn. Cover the strip with parcel tape to make it water proof. Next place the mirror blank on some greaseproof paper and draw round it with a pencil. Cut out the circle, this will be placed over the mirror to prevent the cement from sticking to the surface of the glass. Now wrap the cardboard strip round the mirror and tape it in place, put the paper circle inside. A mould has been created into which a creamy mixture of pure cement and water can be poured. Best to mix up the cement and water outdoors, a powered mixer is a great help but not essential. Pour the thoroughly mixed cement into the mould, level off the top (best done on a level surface) with a straight wooden stick and leave to cure.

Please ask if any further explanation needed!

John

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You've embarked on a great project John, and will be avidly followed by a great many members. Those who are interestd in making a mirror, and also those who are just curious as to how it is done.

I can almost guarantee that this thread will break the record for the most posts.

I hope the continuity of the process can be maintained, but it may be difficult due to the varying levels of progress made.

I wish you well with this, and I sincerely hope there are many home made mirrors at the climax of your Tutorials.

Ron.:):icon_salut:.

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Am sat on the edge of my seat...am so excited! :)

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Would be so cool to have made your own mirror!!! Pity I have the DIY skills of a bag of hedgehogs.....

Still, an excellent thread :)

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OK Coco and Yeti, time for the next thrilling episode!

Having made the concrete base and allowed it to fully cure the next step is to cover this with tiles. Any ceramic wall and/or floor tiles should be OK; they should be about 40-50mm square and are often referred to as mosaic tiles and are sold in sheets. I have used smaller ones about 25mm square with smaller tools. You will also need some fiberglass resin and hardener to stick the tiles onto the concrete base. A container to mix the resin, a stirrer and some latex gloves will also be required. Sometimes the tiles come on a backing sheet; they will need removing from this sheet by pealing them off. I like to start by laying out the tiles on the concrete base to make sure that I have enough. Some tiles are cut in half to fill in the edges. Best to work in a shed or garage when doing this, the resin does smell a bit! Thoroughly mix up the resin and hardener and coat the entire surface and edge with the resin using a brush. If the resin layer is thick enough start adding the tiles in the centre and work out to the edges, if not add more resin. Ensure there is plenty of resin between the tile and the cement base. Work fairly quickly as the resin will begin to set in a few minutes. Leave a space of about 3-4 mm between each tile, this is not critical. Try to avoid having a tile whose centre is coincident with the centre of the cement disk. Don’t worry if drops of resin end up on the tiles, this same be removed with a chisel after it has set. Leave the whole thing to fully cure. You now have a convex tool whose radius of curvature is the same as the mirror; this will assist in the next grinding stage. Please ask if any of this is not clear.

John

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Great thread....I love the idea of making a mirror like this!

Is there any advantage to using smaller or broken tile pieces to match the curvature?

Michael

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