Having a go at making a mirror

[Astrobits]

I have not used ceramic ( household wall/floor type ) tiles for just the reason Rusted mentioned: thin hard layer overlying a softer base. I just used 10mm thick glass. I did try 6mm but needed to do another tool when the edge glass tiles got very thin with a risk of them cracking up. As I mentioned earlier I used steel tools with one tool doing many mirrors but always of the same focal length because steel doesn't wear anywhere near as fast as glass. Rusted's mention of using a flat steel plate would produce a curve on the glass but the tool will remain essentially flat so that it would be very tricky to get a smooth curve. Other metals can be used instead of steel, I do know that 2p pieces have been used which I understand worked well but all metal tools must be pre-curved to the radius of curvature wanted in the final mirror.

When grinding glass on glass there will be equal amounts ( volume ) of material taken from both surfaces. Thus using tiles of less surface area than the mirror will use much more thickness of the tiles and they will then need replacing, preferably on a new plaster cast from the now curved mirror. Your tiles are well separated and I suggest that you try to get as many as possible on the tool while allowing sufficient space to clean between them.

Rusted mentioned that he had difficulties cleaning up the tools made of plaster. My tools were varnished after drying and the tiles were put on with hard pitch covering the while surface. The tool was then covered with PVC electrical tape over the bottom and sides and proved very easy to clean. The I used "tiles" were steel "holes" from a local engineering shop. Heavy engineering shops don't always drill holes but punch them out with a big press and I used the waste steel from the holes, most of them were 1/2" thick!

Nigel

 

[Rusted]

We had the advantage of a curve generating "shaper" to put curves on our iron tools.
It was almost as much fun as watching the machines moving the glass to-and-fro.

[barkis]

You'll get a great deal of satisfaction out of this project. As N.E. Howard said in his telescope making book.
It is very rewarding fashioning  a lump of glass into a precise Optical aid. Or words to that effect.
I don't wish to comment in the thread, as you have others experienced in mirror making advising you.
I might be wrong, but the work top your video shows, seems to be flexing somewhat. Might be my imagination,
so apologies if I'm wrong. Probably won't matter in the rough grinding, but could in the polishing/figuring stages.
Best wishes on your journey.
Ron.

[discardedastro]

Thanks again for all the advice. Did another 20 minutes using less grit and slower strokes and I can happily say that there is now a tiny, just-visible gap under my best straight edge.

@barkis you're quite right that my work table isn't so solid - I have better options but need to clear them of other projects! Might end up making something akin to the portable mirror making stands I've seen on the Stellafane site and others.

In lieu of having the ability to pre-generate a curve I think my best option is going to be getting some 10mm plate glass and making my tile tool with that, either cast or laid on top with epoxy - anyone got any tips for suppliers? Can find plenty of places online who will sell me toughened glass but simple plate glass is tricky to find...

[davidc135]

Your local glazier might be able to supply 10mm ordinary float (plate) but there may be a minimum size of sheet. For some 'white' float I ordered it was 50cm square. Would you be able to cut it up?

There's something to be said for using that 21mm thick tool, especially on a first project.

David

Edited October 23, 2019 by davidc135

[discardedastro]

I'd quite like to stick to making the tool - it's a skill I'll have to work out eventually if I want to do any more mirrors. Plus with that 21mm being Borofloat it's not ideal.

I can get tempered or laminated glass online - I do have a local glazier who might be able to rustle something up though, that's a good idea. Will give them a ring tomorrow.

[davidc135]

www.ukglasscentre.co.uk sell 10mm float in 50x50cm for 24.00 plus shipping plus vat maybe but your local glazier might be a bit cheaper. Enough for a number of tools.   David

Edited October 24, 2019 by davidc135

[barkis]

Before I constructed my Grinding machine, I employed the use of an Oil drum
I partially filled it with water to keep it stable on the ground
I fixed a wooden base in the central area on the top. Drilled a few holes it the top
around the wooden platform. placed three wooden cleats at three points around the tool disc.
Not too tight as to prevent manual rotating of the tool  occasionally. 
When you have completed work with the first Grit, the sludge  can be sluice away down into
the barrel. The whole area thoroughly cleaned to prevent any remaining large particles being a pest.
You get to walk around the barrel too, so no need to rotate the tool yourself. 
The old barrel makes a good steady work top.  
You need a place to do this outside the home, as it's not a suitable method indoors.
Ron.
 

[Rusted]

15 minutes ago, barkis said:

Before I constructed my Grinding machine, I employed the use of an Oil drum
I partially filled it with water to keep it stable on the ground
I fixed a wooden base in the central area on the top. Drilled a few holes it the top
around the wooden platform. placed three wooden cleats at three points around the tool disc.
Not too tight as to prevent manual rotating of the tool  occasionally. 
When you have completed work with the first Grit, the sludge  can be sluice away down into
the barrel. The whole area thoroughly cleaned to prevent any remaining large particles being a pest.
You get to walk around the barrel too, so no need to rotate the tool yourself. 
The old barrel makes a good steady work top.  
You need a place to do this outside the home, as it's not a suitable method indoors.
Ron.
 

Great advice on the oil drum.
Though I never found one with my name on it.
I got into making grinding and polishing machines far too early.
Working by hand on a solid base is much quicker and probably much better than a machine.
The random stroke lengths and rotations of hand working must be beneficial to a smoother curve.
I did some hand grinding and polishing using my last machine as a 24" turntable.
I had no room in my shed for a drum or "going round in circles."

[barkis]

1 hour ago, Rusted said:

Great advice on the oil drum.
Though I never found one with my name on it.
I got into making grinding and polishing machines far too early.
Working by hand on a solid base is much quicker and probably much better than a machine.
The random stroke lengths and rotations of hand working must be beneficial to a smoother curve.
I did some hand grinding and polishing using my last machine as a 24" turntable.
I had no room in my shed for a drum or "going round in circles."

👍

[JamesF]

2 hours ago, Rusted said:

I had no room in my shed for a drum or "going round in circles."

I find there's quite enough room just inside my head for going round in circles

I'd love to have a go at mirror-making myself.  I just need the time, really.  A lottery win would help.  Perhaps they could tell me the winning numbers for next week so I could go out and buy the right ticket?

James

[Chris]

Nice progress I look forward to following along Nigel and Barkis are an amazing source of knowledge and enthusiasm for people taking on this challenge so you're in the right place  

Take care of that chamfered edge and all will be well. I really should finish my mirror! it's been on hold at the polishing stage for ages now.  

Edited October 24, 2019 by Lockie

[discardedastro]

Oil drum would be good if I had somewhere to put it! I think my best bet is going to be making a simple wooden frame in the garage and mounting a turntable on it. I've got plenty of stuff to weigh it down with to stop it moving around so long as I make something fairly solid. Time to spend an evening in Fusion 360...

Did some more grinding this evening and the slower strokes/hand positioning works well - getting the hang of it now, I think. The tile tool is definitely looking worse for wear now so I've ordered a 50x50cm plate glass square from ukglasscentre and I've bought some scoring/snapping tools as well as a diamond wheel for my Dremel - I think between that lot I should be able to make some appropriately-sized irregular bits of glass. I think for my first go with glass I'll repeat the embedding-in-plaster approach and see how that goes - other than the ceramic wearing too fast the plaster tool seems to be working just fine and it's easy enough to make. Just need to use a slightly more dilute mix in the first layer to make sure I don't get any voids that'll trap grit. The worst part is a week-long wait with no grinding for the thing to dry!

In the meantime I can at least keep grinding on the ceramic tool and try and get as much material out of the centre as I can. It'd be nice if I could finish the 80 grit before I come to pour the new tool, but that might be a bit optimistic.

[Rusted]

I hope the glass is normal float glass? Or you'll have more facets than you'll ever use in a lifetime.

A new tool, with flat glass facets, will need to be brought to curve.
That will undo whatever curve you have now even if you mould the plaster surface to your mirror.

Wear safety glasses/goggles, dust mask and leather gloves for the glass cutting.
Something to catch all the tiny glass shards too. So it can be safely disposed of.
Kids and pets don't go well with razor sharp, glass powder. Don't put any into the air with a grinder!

Choose a suitable facet size to match the diameter of your tool for maximum coverage. Just as you would do with a pitch lap.
Start with a facet corner slightly overlapping the centre of the tool and allow enough space [6mm?] for cleaning between the glass rows.
The tool facets will remain viciously sharp until you have done some coarse grinding on it. So beware!
And warn others not to go near it out of curiosity. Keep it covered?

[Chriske]

47 minutes ago, Rusted said:

The tool facets will remain viciously sharp until you have done some coarse grinding on it. So beware!

Using a quality glasscutter there's no need to be afraid of cutting yourself. Together with my wife we've cut (literal) kilometres of glass(Tiffany).
Never cut ourselves. Making a cut(from edge to edge) with a quality cutter the perimeter of the glass-sheet is always broken at a perfect 90° angle. Right after making a cut and break it I can go over that edge with my fingers, no harm done at all. No need to use any cutting oil or fluid to make a clean cut btw.
NEVER go twice over a cut, when you do you destroy your cutter.

[Rusted]

20 minutes ago, Chriske said:

Using a quality glasscutter there's no need to be afraid of cutting yourself. Together with my wife we've cut (literal) kilometres of glass(Tiffany).
Never cut ourselves. Making a cut(from edge to edge) with a quality cutter the perimeter of the glass-sheet is always broken at a perfect 90° angle. Right after making a cut and break it I can go over that edge with my fingers, no harm done at all. No need to use any cutting oil or fluid to make a clean cut btw.
NEVER go twice over a cut, when you do you destroy your cutter.

For the benefit of those less experienced how would you define a "quality" glass cutter?

My own experience is only with a humble [dirt cheap] wheel cutter.

[Cosmic Geoff]

When I was a youth I made an 8" f7 Newtonian from scratch, including grinding and polishing the mirror. I remember the grinding tool was squares of 1/4" glass stuck on a round wooden disk with pitch.  IIRC I was working on a conventional workbench and rotated tool or mirror at intervals by hand.

Looking back, I am amazed that I managed it and that the completed instrument worked (I'm not sure how well 🙄).

[Chriske]

Buying a glass-cutter is hardware store is not a good idea.

One golden rule while cutting glass : Do not press to hard on your cutter.

This is not a good cutter. Costs only £3.2
By good I mean : safe.

 

 

These are

TOYO (my favorite) Ergonomically 100% ok
Very good to cut curved shapes

Also very good

I also have this one from 'Silberschnitt'(rather expensive). Never had any failure using this one.

[discardedastro]

1 hour ago, Rusted said:

I hope the glass is normal float glass? Or you'll have more facets than you'll ever use in a lifetime.

A new tool, with flat glass facets, will need to be brought to curve.
That will undo whatever curve you have now even if you mould the plaster surface to your mirror.

Wear safety glasses/goggles, dust mask and leather gloves for the glass cutting.
Something to catch all the tiny glass shards too. So it can be safely disposed of.
Kids and pets don't go well with razor sharp, glass powder. Don't put any into the air with a grinder!

Choose a suitable facet size to match the diameter of your tool for maximum coverage. Just as you would do with a pitch lap.
Start with a facet corner slightly overlapping the centre of the tool and allow enough space [6mm?] for cleaning between the glass rows.
The tool facets will remain viciously sharp until you have done some coarse grinding on it. So beware!
And warn others not to go near it out of curiosity. Keep it covered?

All good tips. Hopefully the tool I've got will work okay. I'm familiar with working with tiny bits of glass in another application (my day job involves lots of fibre optics) so well aware of all the safety precautions to take - hopefully clean cuts will be order of the day!

[Chris]

On 24/10/2019 at 23:01, discardedastro said:

Oil drum would be good if I had somewhere to put it! I think my best bet is going to be making a simple wooden frame in the garage and mounting a turntable on it. I've got plenty of stuff to weigh it down with to stop it moving around so long as I make something fairly solid. Time to spend an evening in Fusion 360...

Edge of a table will do, just keep on turning your position plus the mirrors and tools position. The more random the better as it will average out over time. This is my rig if you want ideas.  

Edited October 26, 2019 by Lockie

[Chriske]

I have two MoM's, but when I'm doing one manually, this is my setup.

 

[Chriske]

This one is just for teasing you all...🤭

Me hogging out a 20" mirror. This one took me about 1.5 hours to finish it to f/5.6
Another one, 20" f/4 was done just over two hours.

This device is nothing more then an angle grinder with a diamond disk.
Two plank with the correct radius(about) roll over 4 plastic wheels.

 

[Robert72]

Nice setup.  I like the idea of the plank radii.  Nothing like the accurate automation of bulk removal.  Kind of reminds me of my pattern router/lathe.  There was once a very good website of a guy that suspended a diamond AG from his garage roof to hog out a large mirror, like a large pendulum.  From what I can remember it was problematic, as you can imagine.  I think the website was poor meadow observatory, or something.  Can anyone remember it?  It was from way back in the early days of the www.

[Chriske]

Sorry to highjack your thread discardedastro, but I couldn't resist, sorry a thousand times...
Won’t do it again...promise....😉

[Astrobits]

To see when you have got to your  curve ( before you get the spherometer) you can try using drills to measure the gap under a strait edge. Rather a rough method as common drills are usually in 0.5mm increments. Another method is to cut a piece of card to the correct  curve and use that to check progress. You can also use a depth measuring vernier with the strait edge and measure the difference between the centre and edge of the mirror.

Nigel