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discardedastro

Having a go at making a mirror

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Indeed, you don't need a spherometer for this. Use a simple ruler and feeler gauge. When I grinded my mirror some 20 years ago I used a protractor/ruler with a 26mm coin ( 5 Slovenian Tolars ) to measure sagitta. Check the photos to see what I mean : http://www2.arnes.si/~gljsentvid10/izdel_tele.html#globina

Just came to my mind would a simple Amazon tyre depth gauge work? 0.01mm resolution? https://www.amazon.co.uk/ULTRATOOL-0-25-4mm-Measurer-Motorbike-Batteries/dp/B0711S1XHY/

Edited by heliumstar
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I third this method! I preferred to use coins after measuring them with a micrometre or Vernier, but you do get more size choices with drill bits :) 

Mirror2.jpg

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Cue hundreds of pictures of spherometers? :smile:

An engineer freind made mine decades ago.
I added two more clamping screws for concentricity and lower friction of the plunger.

I used a mechanical dial gauge for years until I found an affordable, digital one on eBay.
When it comes to measuring long radii [like the rear surface on a doublet] they become increasingly innacurate.
A thou' difference can mean almost anything.

 

P1310300 rsz 600  spherometer.JPG

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15 hours ago, Chriske said:

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

Never going to complain about thread hijacks with actual useful content :) though the angle grinder rig is just showing off!

2 hours ago, Rusted said:

Cue hundreds of pictures of spherometers?

Hah! Well, I have already got a Mitutoyo digimatic indicator much like the photo above - nothing fancy, but enough to measure rough grinding - just need something to mount it in. I think I should have enough bits lying around to make one that'll be pretty stable... but the feeler gauges under a metal rule and variations thereof will do for now I think!

The various mirror/tool stands and clamping arrangements are great to see. I think I can do something on the corner of a table with some big clamps - I also reckon I can mount a set of tool/mirror holding buffers in the bottom of the plastic tray I'm using at the moment which will be ideal to avoid mess.

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Really fascinating thread! Good luck with this project and looking forward to hearing more.

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Well done and good luck.  I made a 8" one 30 years ago and it is still in a telescope not so far from here.  It is very satisfying making your own.  However, financially I am not so sure it makes sense anymore in 2019 when you can buy the mirror and diagonal ready made for about the same price when you consider aluminising etc.

Key tip is to keep the abrasives away from each other at all costs.

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3 hours ago, kirkster501 said:

Well done and good luck.  I made a 8" one 30 years ago and it is still in a telescope not so far from here.  It is very satisfying making your own.  However, financially I am not so sure it makes sense anymore in 2019 when you can buy the mirror and diagonal ready made for about the same price when you consider aluminising etc.

Key tip is to keep the abrasives away from each other at all costs.

So true, I could have bought an 8" Dob with the money I've spent just getting past fine grinding an 8" mirror. It really is about the satisfaction rather than economics. I think things swing the other way once you get to 14" mirrors. 

Yes, defo always store fine grits above course grits and not the other way round. 

 

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Ready made mirrors are fine if you want to make a standard telescope but if you want something special - a more unusual type of telescope - then grind it yourself is the only way.  Chriske has fine examples of these.

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Well, once I've done the "simple" 8" f/5, who knows... :)

The glass has arrived for the glass tool along with the various tools that I hope should suffice for cutting it. Nice big chunky 10mm thick slab delivered in impeccable packaging - tons of cardboard and bubble wrap.

I've had a long day at work so my current level of inebriation precludes glass cutting, but it's looking like it's "on" for the weekend or tomorrow evening. Need to make some space on the workbench and then my plan is to score a reasonable (10cm) strip off the side and then subdivide that strip irregularly - should limit working on the whole sheet to a minimum and if I cock up anything with the small strip the damage is limited. Only ~£25 though, so not the end of the world if anything happens - just potentially very messy!

The economics of it are interesting. Once I have a working mirror I'm happy with I'm definitely going to take the existing mirror on the 200PDS and measure it as well as I can. The objective in my mind is to produce a mirror of better quality than the commercially available mirrors. As for coating I'm currently planning on shipping it off to OOUK for a Hilux coating - seems a sensible enough option and won't break the bank, plus I know OOUK can do a reasonable measurement on their Zygo as a final QC step prior to coating.

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Right - well, glass cutting basically worked! I've ended up with a mix of sizes of chunks of glass which I think should be okay. I've managed to do it in a sufficiently controlled fashion that I've got plenty of undamaged glass left to hack on if I want to take another ago at it all.

The sheet as it started out...

20191031_182805.thumb.jpg.39c9efbe183bbb36fbe36f8bd7ce6412.jpg

Scored it twice, once more firmly than the first time, after I failed to get the score to open with my pliers. In the end it turned out my first score was fine, but the pliers weren't going to do the job on 10mm thick glass.

20191031_184127.thumb.jpg.257aa159d169ca5dc89c829840abf8a9.jpg

Very gentle tap with the pictured hammer did the trick, though.

20191031_184131.thumb.jpg.424c5e2283250febf4eb62b8330cd031.jpg

The break followed the first score after a bit, and the 10cm ish chunk I wanted came away fine. The rest of the glass went back into its packaging for safe keeping.

20191031_184223.thumb.jpg.5e6255f07dc5e1f3ea94e337bed6ed08.jpg

Then it was just a case of repeating a few times. I wanted various sizes of chunk to ensure some irregularity in the tool. I had another 10 pieces or so but this was the set that I was happiest with in terms of fitting everything up on the mirror with few gaps/nothing poking the edges.

20191031_201711.thumb.jpg.cb131c54a6c546b84d5e296df4a303eb.jpg

Mixed up 1.5kg of Dentstone KD with 450mL of water and gave it a good mix, agitated once poured to get the bubbles out, and it's curing now. Used some plain card for the mold walls but didn't tape all the way down and had a little bit of buckling at the joints which won't look pretty but shouldn't be structurally relevant. Now just to wait a week!

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Interesting, I've not seen a broken glass for tool before. I just copied most people with the tile tool. Did you consider using a tile tool? 

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1 hour ago, Lockie said:

Interesting, I've not seen a broken glass for tool before. I just copied most people with the tile tool. Did you consider using a tile tool? 

See earlier in the thread where I made the tile tool and ate through the hard bit of the tiles in less than a week of evenings! Glass tiles I did consider but this approach should last much longer.

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Pro glass cutters I've seen [many years ago] tended to work on firm surfaces.
They'd place a small wooden item under the very end of the cut glass and press down each side to "bend" the glass apart.
The cut would open up away from them with a nice zipping sound.
I've tried the tile pliers myself [on tiles] and found them hard work.
When I was cutting 3/8" glass for tools I'd make long equal strips first and then cut across at intervals.
I only ever used the cheapest form of glass cutter. [with bicycle oil]
Confidence seems to help when cutting glass. Submit it to your will!  :wink2:

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6 hours ago, discardedastro said:

 Glass tiles I did consider but this approach should last much longer.

My glass tile tool has lasted very well FYI

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1 hour ago, Lockie said:

My glass tile tool has lasted very well FYI

How thick are the tiles and where did you get them? All I could find online or at my local tile place was thin (2 or 3 mm) "corrugated" tiles. Bit late for this tool but would be good to know.

3 hours ago, Rusted said:

When I was cutting 3/8" glass for tools I'd make long equal strips first and then cut across at intervals.
I only ever used the cheapest form of glass cutter. [with bicycle oil]
Confidence seems to help when cutting glass. Submit it to your will! 

That all makes lots of sense. My glass scorer was a cheap thing off eBay that looks to be a Silberschnitt clone of some sort, and seemed to do the job ably. I worked on a flat work bench with a towel for the first bits and that worked very well - felt is what I've seen most people use on t'internets as a base but a towel is pretty close mechanically. I tried leaning the cut across the edge of the bench but putting something below the glass and leaning sounds a good plan - the "pro" thick glass cut running pliers look to have some pads arranged on the inside of the jaws to produce that effect.

Anyway, tool now on a drying rack above a radiator to dry out - looks to have come out okay, but a couple of bits of glass "floated" to the edge during the pour (despite my efforts to keep everything still) and there's a ~2cm long section of the side which has the exposed glass chunk as an edge. Shouldn't be an issue once the epoxy goes on, I think.

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On 01/11/2019 at 08:59, discardedastro said:

How thick are the tiles and where did you get them? All I could find online or at my local tile place was thin (2 or 3 mm) "corrugated" tiles. Bit late for this tool but would be good to know.

I think they were around 5mm maybe more? Might have picked them up at B & Q. I've just found some different tiles in the garage which I didn't use and they read as 4mm on my calipers. I can't find any of the thicker tiles, maybe I chucked them when I moved in the summer? 

You can kind of see how thick they are on the pics: 

IMG_20170326_184743.thumb.jpg.12e077b47def8ddd59eac588e2751e7411.jpg

IMG_20170326_190634.thumb.jpg.aeffcb54dccec77d927e42a879a9b0bc.jpg

IMG_20170416_121407.thumb.jpg.d1790903b2cdec899ad1f0bfed582563.jpg

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24 minutes ago, Lockie said:

I think they were around 5mm maybe more? Might have picked them up at B & Q. I've just found some different tiles in the garage which I didn't use and they read as 4mm on my calipers. I can't find any of the thicker tiles, maybe I chucked them when I moved in the summer? 

Yeah, pretty chunky! I had a look at my local B&Q and while they have some glass tiles they were all 3-4mm or so, which I thought would be marginal.

I've just finished the epoxy coat on this tool - it won't have quite as much consistency/glass directly in contact, there's quite a bit of plaster in the overall front surface. Not ideal really. I think I can make smaller tiles. Nonetheless I think I should be able to get some work done with it so I'll start grinding again this weekend once the epoxy is cured. 10mm thick "tiles" should work fine though!

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1 hour ago, discardedastro said:

Yeah, pretty chunky! I had a look at my local B&Q and while they have some glass tiles they were all 3-4mm or so, which I thought would be marginal.

I've just finished the epoxy coat on this tool - it won't have quite as much consistency/glass directly in contact, there's quite a bit of plaster in the overall front surface. Not ideal really. I think I can make smaller tiles. Nonetheless I think I should be able to get some work done with it so I'll start grinding again this weekend once the epoxy is cured. 10mm thick "tiles" should work fine though!

Good progress. You certainly need good ventilation with the epoxy coat don't you, I nearly gassed out my family indoors even though I did the work in the garden! 

I think 4mm is enough for one mirror grind, I still have loads of depth remaining after reaching the polishing stage. 10mm should last several mirrors I should imagine. I don't know if you want to use this trick, but I melted wax in between the tiles on the tool so you can scrape the wax out after each grit size and then re seal it with wax. In theory this should help stop any grit which is lodged in the epoxy coming loose and scratching the mirror once you've moved to a finer grit. 

One of the fine gents on this thread told me this and it has worked well so far :)

Good luck with the next stage, and I personally think it would be better if you can get the glass in smaller bits for more randomisation and extra channels whilst grinding. Things get caked up and stuck sometimes! 

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1 hour ago, Lockie said:

Good progress. You certainly need good ventilation with the epoxy coat don't you, I nearly gassed out my family indoors even though I did the work in the garden! 

Yeah - the respirator I bought at the start of this has gotten a lot of use - dust kicked off while making the tool is plentiful, and as the respirator's got a reasonable degree of organic compound filtering in the A1 cartridges you can't smell a thing while doing the epoxy. It's similarly good when unblocking sewerage lines, as I found out last week! I've got a back porch so I've done it all in there and left it out there - it's bloody cold so it'll take a while to cure, but it'll get there.

I think I'll do one more try at the embedded tool before I give up and epoxy glass to the front of a ceramic tool (or cave in and use the 21mm 8" blank I have as a tool).

The wax trick is neat - for the coarse stuff it's visually pretty clear where it is but at finer grits I can imagine it'd start to be a bit of a nightmare to clean out.

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Well, I'm getting somewhere!

20191114_001648.thumb.jpg.c4614d17bae98195b9ef3562fbd018d5.jpg

Definitely making progress with removing material now. My grinding down of the sharpie marks at the edge suggests I'm pretty symmetrical overall but doing basically no grinding near the edge. One of my glass chunks let go of a bit but this didn't leave a mark on the mirror - the shard just got pushed around a bit and I washed it off.

20191114_001748.thumb.jpg.a148413cb19b279b8c0c351b2eb65910.jpg

So, so far so good, I think... just more grinding to go!

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Your bevelled edge looks a bit small to me, you can very quickly grind that away getting the right curvature.

Regarding the lifetime of a glass tile tool it depends on the relative working area compared to the mirror. You will remove roughly the same VOLUME of glass from both the mirror and tile, after all, the grit cannot distinguish which side to chip away at so will remove from both sides equally.. Consequently with a smaller area of glass on the tool the thickness that will be removed will be greater than the mirror, the less the glass area the more thickness lost.  Different materials for the tiles will have different thicknesses lost depending their relative hardnesses.

I have tried making an 8" f/5 mirror with 6mm thick glass tiled tool and needed a second tool as the thickness of the glass at the edge of the tool got so thin that I was concerned that those edge tiles would break up.

I have also tried using a 10mm thick glass disc as a stand-alone tool but that failed because the curvature of an 8"  f/5 took the mirror edge down to touch the retaining wedges before I got to the correct depth. Mounting it onto a plaster casting worked fine and I subsequently polished the tool to produce a handy 8" dia plano-convex lens with a focal length of about 56".

Nigel

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9 minutes ago, Astrobits said:

Your bevelled edge looks a bit small to me, you can very quickly grind that away getting the right curvature.

Regarding the lifetime of a glass tile tool it depends on the relative working area compared to the mirror. You will remove roughly the same VOLUME of glass from both the mirror and tile, after all, the grit cannot distinguish which side to chip away at so will remove from both sides equally.. Consequently with a smaller area of glass on the tool the thickness that will be removed will be greater than the mirror, the less the glass area the more thickness lost.  Different materials for the tiles will have different thicknesses lost depending their relative hardnesses.

I have tried making an 8" f/5 mirror with 6mm thick glass tiled tool and needed a second tool as the thickness of the glass at the edge of the tool got so thin that I was concerned that those edge tiles would break up.

I have also tried using a 10mm thick glass disc as a stand-alone tool but that failed because the curvature of an 8"  f/5 took the mirror edge down to touch the retaining wedges before I got to the correct depth. Mounting it onto a plaster casting worked fine and I subsequently polished the tool to produce a handy 8" dia plano-convex lens with a focal length of about 56".

Nigel

This is a good point and makes sense. So maybe the only reason my particular tool has lasted well is due to only grinding down around f6.7.

DiscardedAstro - Nig is your man for advice :) 

 

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9 hours ago, Astrobits said:

Your bevelled edge looks a bit small to me, you can very quickly grind that away getting the right curvature.

Regarding the lifetime of a glass tile tool it depends on the relative working area compared to the mirror. You will remove roughly the same VOLUME of glass from both the mirror and tile, after all, the grit cannot distinguish which side to chip away at so will remove from both sides equally.. Consequently with a smaller area of glass on the tool the thickness that will be removed will be greater than the mirror, the less the glass area the more thickness lost.  Different materials for the tiles will have different thicknesses lost depending their relative hardnesses.

I have tried making an 8" f/5 mirror with 6mm thick glass tiled tool and needed a second tool as the thickness of the glass at the edge of the tool got so thin that I was concerned that those edge tiles would break up.

I have also tried using a 10mm thick glass disc as a stand-alone tool but that failed because the curvature of an 8"  f/5 took the mirror edge down to touch the retaining wedges before I got to the correct depth. Mounting it onto a plaster casting worked fine and I subsequently polished the tool to produce a handy 8" dia plano-convex lens with a focal length of about 56".

Nigel

That does indeed all make complete sense! I think from the sounds of it I should get away with it, but I do have a relatively low areal coverage on the tool. Will have to see how I do I suppose. I've practiced my glass-breaking and I can do smaller bits for the next tool, should it be needed.

I'll refine the bevelled edge - I haven't touched it since I put it on initially. Also going to cut some channels in the tool with a Dremel and a diamond disc. Cleaning it for the finer grit is going to be painful - starting to really see the value in making a plaster disc, sticking glass onto it and using wax as a surface, I think.

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