Having a go at making a mirror

[discardedastro]

Had another 20 mins of grinding on 80 grit this evening doing a normal stroke with 1/6th overhang each side and it is slowly making better contact and bringing the curve out to the edge - slow progress now but promising.

My friend-with-a-mill hasn't gotten to making the spherometer body I designed, but work's got an Ultimaker S5 on loan and I'm going to print one off for now - it should be accurate enough. I'll be using M4 bolts with ball bearings epoxied to the ends as measuring feet and the body should be good and rigid; tweaked my original design to give me a pocket to fit a nut for clamping the indicator stem.

[discardedastro]

Well, this worked out okay for a first attempt!

The clamping isn't ideal and is a bit off-center but it's fairly stable - I'll shim the barrel out a bit with some bits of plastic. The "ridges" were an attempt to try and give this otherwise quite thin structure some additional rigidity but it's actually very solid - I printed this with a 30% gyroid infill in ultimaker Tough PLA, using PVA supports for the holes. The clamping just uses a captive M4 nut. I've attached an STL.

My indicator is only a 0.01mm accuracy device so will want to upgrade that sometime. At the moment the feet are just the bolts, but I've got some ball bearings coming which I'll glue onto the bottom of the bolts.

Measurement-wise I saw 0.37-0.38mm at the edges and 0.36mm in the centre.

So plugging this into an Internet Calculator I'm seeing 1773mm ROC - so fairly close to target 1500mm! But I think more 80 grit to go before I switch to finer stuff.

spherometer-v7.stl

[discardedastro]

Now I have a way to measure, even roughly, I've been making good progress - it helps to be able to "see" what you're doing. I've got everything down to about .41-.42mm pretty evenly now, which is about where I need to be I think. So I'm going to clean everything up and start on 180 grit.

My main concern at this stage of things is the tool. There's a lot of small bubbles and voids containing grit. I'm hopeful I can get it all cleaned up, but I might struggle to get it completely clean, in which case I'll either have to attack the tool vigorously with a dremel and a diamond bit, or cast a fresh tool (and I will use epoxied-on glass rather than embedding it this time, and use the wax trick in the gaps). Much rather get things into a sustainable position for fine grinding now rather than later. If I can get it completely clean I might try and do a wax covering to fill all the voids.

I have a better set of vernier calipers on the way to improve my spherometer measurements along with a 0.001mm dial indicator - hooray for eBay, both used-but-good-nick digital Mitutoyos for under £30.

And last but not least I've got all the bits to make a mirror mount for Focault testing, and assembled a rough-and-ready Focault tester stage with some foamcore - I just need to rebuild this in wood/plastic print and then get a little X/Y stage to mount it on.

Edited February 23, 2020 by discardedastro

[discardedastro]

So, here's my next challenge - cleaning out this tool.

I've been going at it with a fine point needle and some running water. The tiny little pockets of grit are the ones giving me most concern - it's very hard to get in there and clean them out. I can get a dremel and start just widening all the holes out etc, but not sure it's the best plan. I can backfill all the pits with wax or epoxy.

There's also a bit of 80 grit being held in the epoxy around the edges - I think some gentle sanding should get rid of that though.

That or I could just pour a fresh tool and make it with thick glass tiles epoxied on top and wax between the tiles so I can effectively clean the whole thing properly between grits without spending a few days digging increasingly tiny bits of grit out of the tool surface.

[Rusted]

I hated dental plaster tools for this reason.

[Astrobits]

That is the problem with tile-cast-in-tool arrangements. Plaster ( and cement ) is fine as long as you stick the tiles on top and fill the space with wax as I do. The plaster tool will have cavities around the edge and underneath as well so I covered the edge and base with PVC electrical tape. The tool is then virtually waterproof  so can be dunked into water for cleaning. Unprotected plaster will absorb water and can possibly swell causing the figure on the tiles to change.

Another disadvantage of the tile-in-tool is that you have less visibility of the thickness of the tiles, particularly at the edge, and cannot easily see whether they are thick enough to withstand the subsequent operations.

My method was to make a plaster cast against the coarse ground mirror. When it was dry arrange the tiles on the curved mirror surface and warm them up with a hot air gun, pour some hard pitch onto the tool in a thin layer and quickly invert the tool onto the tiles. This way the curve of the new tool matched the mirror fairly well. Once you have done the coarse grinding it is possible to use thinner glass as subsequent grits will not remove much glass. This, of course, assumes that you will not be needing the grinding tool for further mirror(s) with the same focal length. When I made the curved plaster cast I always made two, the second one was for the pitch lap. This ensured that I could return to fine grinding should that have been necessary.

Nigel

[davidc135]

Those pits and possible abrasive particles are asking for trouble. Best scrap it and start again along the lines described above. You have to have complete faith in freedom from contamination.  David

[discardedastro]

OK, so starting 180 grit with a new tool that I can clean properly is going to be the right thing.

My thinking is:

• Cast a new plaster tool back on the coarse-ground mirror using the same dental plaster I've been using - the plaster's held up well all things considered, I've been cleaning it through dunking it in water without issue
• Epoxy seal the back and sides as before (possibly with some harder-curing epoxy/resin that won't let things embed quite as easily as this stuff has)
• Once dried, thinly coat the front with epoxy and press the glass tiles in firmly - I think this should preserve the curve fairly well without epoxying the mirror to the tool (I'm not yet ready to get my pitch on)

Rather than try and cut down my really thick glass I'm planning to use these: https://www.amazon.co.uk/MosaixPro-302-Piece-Glass-Tiles-White/dp/B004QTT4RQ/ref=sr_1_56 - they're 10x10mm, 4mm thick, so should last, are small enough they should take the curve on well but won't be a nightmare to clean around.

Then once it's all set I'll melt wax over the lot and sides and crack on.

My more accurate indicator arrived today, so I'll see how the mirror looks with that in the spherometer - might do a last bit of 80 grit tool-on-top if it's needed at the edges.

[Glasspusher]

Over the years I have tried every type of grinding tool and a nut tool is difficult to beat. I use M16 steel nuts set in dental plaster. Cover the concave glass mirror with something like greaseproof paper, place a damn round the edge, place the nuts on the greaseproof paper and carefully mix up the plaster and pour into the mould. Allow to set and that is it. During use if the tool has dried out immerse in water for a few minutes before use.   

Worth thinking about!

Good luck with your mirror!

John

[discardedastro]

12 minutes ago, Glasspusher said:

Over the years I have tried every type of grinding tool and a nut tool is difficult to beat. I use M16 steel nuts set in dental plaster. Cover the concave glass mirror with something like greaseproof paper, place a damn round the edge, place the nuts on the greaseproof paper and carefully mix up the plaster and pour into the mould. Allow to set and that is it. During use if the tool has dried out immerse in water for a few minutes before use.   

Worth thinking about!

With the exception of using some chunky 10mm glass chunks instead of nuts I've done what you've done - how did you clean your tool between grits?

[skybadger]

On 31/10/2019 at 22:36, 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? 

I set my 1" square tiles on edge in cement. Haven't had problems rough grinding at all. Created a new cement form for polishing by casting against the mirror then mounting the pitch.  Those tiles aren't going to wear out. I like the wax idea, though it runs the risk of picking up grit as well as removing it. 

M

[Glasspusher]

To clean the nut tool between grits I scrub with a stiff brush under running water if not satisfied with that I use a wire brush which usually does the job. With tiles or glass chunks water can ingress into the dental stone down the edges which can cause the pieces to become loose and draw in abrasives. You can see this more obviously at the edges of the tool where the  tiles are exposed. Always try to set the tiles in from the edge so that they are surrounded by dental plaster and use the thickest tiles you can find, 4 mm is a little thin in my experience. 

John

[heliumstar]

I follow this with interest since I have a few blanks myself. I made 6" f5.33 20 years ago. There's something to be said for convenience of glass tool

[Astrobits]

3 hours ago, skybadger said:

I like the wax idea, though it runs the risk of picking up grit as well as removing it. 

Unfortunately, any material used for the body of the tool (plaster, cement) will pick up grit as Glasspusher and Discardedastro have mentioned.

The advantage of the wax as I see it is that it is soft and easily scraped out along with any embedded grit. Secondly you can use a hot air gun to melt the wax and seal in any grit as it will sink below the surface of the molten wax. I have found that on initial pouring of the wax the boundaries with the tiles can become a bit depressed (where the wax doesn't wet the tile) which will trap grit if left as is. Going over the tool with a hot air gun can cause the wax to bond well with the tiles removing that little valley.

I, too, think that 4mm glass is a bit thin. Try visiting local glass merchants for offcuts of 6mm at least. Should be minimal cost if not free out of their scrap bin.

I also used steel tools which are great for doing many mirrors as they don't wear much at all. A steel tool will produce consistent focal lengths time after time but a single tool will not produce different focal lengths very easily.

Nigel

[discardedastro]

Bit of a delay what with all this lockdown but I've gotten things moving again. Made a fresh tool and let it sit for a good long while to dry, and epoxy coated the back and front. I'm using 4mm thick tiles from Amazon - they've got some backing material which I've left on as getting it off was a real pain. I set them onto the epoxy on the front while it was still wet to try and take on the shape as accurately as possible.

This was the tool ready for the wax pour, with epoxy good and dry. Tile pattern a bit irregular.

And post-wax. Definitely should've poured with a dam, in retrospect, to try and get some cover in the edges but this'll do for now. I'll scrape the beeswax off the tiles and maybe remelt with a hot air gun but I'm liking the look of this so far.

So in theory, back to grinding tomorrow, moving to 120 grit.

I also took some pictures of the other tool with a stereomicroscope:

I think that this shows that it's definitely feasible to clean these ceramic/glass tools for larger grits. I've had to take the microscope to work (got to use it to do my actual job at some point) but I'm still fairly frequently in the lab so I'm a little tempted to try and compare cleaning strategies, which should be applicable for this tool too.

[Ships and Stars]

Hi there, looking good! I've been wanting to try mirror grinding since last winter, but never took the plunge. I also made my way to Stathis online. His prices seem good. My end goal is to make a very large mirror someday perhaps. Someday...

Since I already have a 12" SW flextube dob, I was thinking about making a 12" f4.9 or f5 copy, so I could just swap out mirrors for star tests etc, and not have to buy a secondary mirror/build another complete dob etc, until I was sure it was right optically.

Then if I got it right enough to put into action, I could perhaps fabricate something like a 12" travel dob etc. 

Could I please ask off the back of your hard-won knowledge - any tips on what to buy from Stathis or what you would do differently regarding materials, tools or testing equipment? 

Wondering if I need 2x 12" blanks, or if I could get away with using a smaller blank as a tool, perhaps an 8" even. I'd buy his grinding and polishing supply kits as well. 

I have a good digital caliper and lots of drill bits, but that's about it. No optical test bench etc. I have an air compressor and angle grinder etc and lots of hand tools/air tools.

I read glasspusher's excellent thread as well, but need a refresh.

Good luck there! Looks like you are on track 👍

 

Edited May 2, 2020 by Ships and Stars

[discardedastro]

11 minutes ago, Ships and Stars said:

Hi there, looking good! I've been wanting to try mirror grinding since last winter, but never took the plunge. I also made my way to Stathis online. His prices seem good. My end goal is to make a very large mirror someday perhaps. Someday...

Since I already have a 12" SW flextube dob, I was thinking about making a 12" f4.9 or f5 copy, so I could just swap out mirrors for star tests etc, and not have to buy a secondary mirror/build another complete dob etc, until I was sure it was right optically.

Then if I got it right enough to put into action, I could perhaps fabricate something like a 12" travel dob etc. 

Could I please ask off the back of your hard-won knowledge - any tips on what to buy from Stathis or what you would do differently regarding materials, tools or testing equipment? 

Wondering if I need 2x 12" blanks, or if I could get away with using a smaller blank as a tool, perhaps an 8" even. I'd buy his grinding and polishing supply kits as well. 

I have a good digital caliper and lots of drill bits, but that's about it. No optical test bench etc. I have an air compressor and angle grinder etc and lots of hand tools/air tools.

I read glasspusher's excellent thread as well, but need a refresh.

Good luck there! Looks like you are on track 👍

 

The 12" approach sounds exactly like what I'm doing - I'm using my 200PDS as I'm already happy changing mirrors on that and I know it's sound otherwise. I bought a 10" kit off Stathis and so far haven't come even close to making a significant dent in the material quantities, so I'd order 1 size above. I ordered two blanks, with the back ground flat on one (the thinking being I would use #2 as a tool if tile tools didn't work out, though in practice I'd have wanted a plate glass tool to avoid chip damage I think). Definitely get the back ground flat, doesn't cost much and makes life easier. Stathis is the only EU provider I'm aware of (to the general public) of Borofloat so definitely the way to go for an inexpensive-but-sound piece of glass. I'd like to work my way up to a bigger piece of glass once I've done this one right - I might go for a 12" for a Dob next, but don't want to get ahead of myself. Still a long way to go on this one! The only bit from Stathis I am not planning to use is the pitch - I'm planning to get some Acculap from the states when I get to figuring. I like the sound of a less-smelly pitch alternate, given I know how bad bitumen smells. I'd start with a tile tool and see how you get on.

I'd definitely get a good respirator if possible in the current climate, makes all the handling concerns go away (though working wet is very safe, more about moving stuff around and cleaning etc - with organic filters it also helps massively with the smell from epoxy et al). Disposable gloves likewise are a godsend for keeping epoxy off hands and making cleanup a whole lot easier. I'm still using an old potting tray as a base, just rested on some anti-slip rubber mats on my dining room table, and that's working OK (though a dedicated stand would undoubtedly be better - there's no clamping in place currently, though I think I could add this easily enough). The potting tray gets some plastic sheets laid into it to make cleaning easier. I use the same tray for epoxy work, again with plastic sheeting laid in.

Dental ceramic I bought via Amazon, it's Dentstone KD - works well but make sure you get a few big disposable buckets and spatulas etc to work with. I'd also say that a "back massager" with some clingfilm over the head and gently held in contact with the surface of the wet pour is an excellent way to get rid of bubbles in the dentstone once it's poured. Sheets of 1mm plastic you can bend around your mirror blank are a must-have for making that process pleasant, too. I got straightforward arts and crafts casting resin for encapsulating the tool, which seems to work just fine; it dries non-tacky and is rigid.

Get the best possible spray bottle you can since it'll be in near constant use. I use this one, which has a metal weight and filter on the end of the hose internally - this stops it ingesting air in use. Don't skimp here. I have a big bucket of water I use to get the bulk of the grit off after remaining grinding sessions.

On measurement tools, I'll say the spherometer gives a huge degree of confidence that you're not screwing anything up, so well worth getting something sorted out for that. I managed to snag a more accurate digital indicator on eBay for £75 which I'll be using in future; the surface plate I got for £80 or so I think (it's small but 00 grade, so pretty flat). 3D printing the body of the tool looks to work fine (online services would be the way to go to get this done if you've not got a printer to hand), and I used M3/M4 bolt stock from Model Fixings (who are magnificent for odds and ends between M1 and M6, and BSF/UNC equivs). I've also added some ceramic spherical balls as "feet" for the spherometer, which helps (if nothing else, helps to stop me worrying about scratching the glass). I've got no optical bench or anything, though I have access to a small optical table at work.

I've also bodged together (truly - rushed while a few gins in and split some of the wood with screws) a mirror test stand but it's a bit puny and I might upgrade that. I've got some bits from Model Fixings to fit levelling feet into it. Once I've got that and a Foucault tester sorted I'll be in good shape for testing.

[Ships and Stars]

2 hours ago, discardedastro said:

The 12" approach sounds exactly like what I'm doing - I'm using my 200PDS as I'm already happy changing mirrors on that and I know it's sound otherwise. I bought a 10" kit off Stathis and so far haven't come even close to making a significant dent in the material quantities, so I'd order 1 size above. I ordered two blanks, with the back ground flat on one (the thinking being I would use #2 as a tool if tile tools didn't work out, though in practice I'd have wanted a plate glass tool to avoid chip damage I think). Definitely get the back ground flat, doesn't cost much and makes life easier. Stathis is the only EU provider I'm aware of (to the general public) of Borofloat so definitely the way to go for an inexpensive-but-sound piece of glass. I'd like to work my way up to a bigger piece of glass once I've done this one right - I might go for a 12" for a Dob next, but don't want to get ahead of myself. Still a long way to go on this one! The only bit from Stathis I am not planning to use is the pitch - I'm planning to get some Acculap from the states when I get to figuring. I like the sound of a less-smelly pitch alternate, given I know how bad bitumen smells. I'd start with a tile tool and see how you get on.

I'd definitely get a good respirator if possible in the current climate, makes all the handling concerns go away (though working wet is very safe, more about moving stuff around and cleaning etc - with organic filters it also helps massively with the smell from epoxy et al). Disposable gloves likewise are a godsend for keeping epoxy off hands and making cleanup a whole lot easier. I'm still using an old potting tray as a base, just rested on some anti-slip rubber mats on my dining room table, and that's working OK (though a dedicated stand would undoubtedly be better - there's no clamping in place currently, though I think I could add this easily enough). The potting tray gets some plastic sheets laid into it to make cleaning easier. I use the same tray for epoxy work, again with plastic sheeting laid in.

Dental ceramic I bought via Amazon, it's Dentstone KD - works well but make sure you get a few big disposable buckets and spatulas etc to work with. I'd also say that a "back massager" with some clingfilm over the head and gently held in contact with the surface of the wet pour is an excellent way to get rid of bubbles in the dentstone once it's poured. Sheets of 1mm plastic you can bend around your mirror blank are a must-have for making that process pleasant, too. I got straightforward arts and crafts casting resin for encapsulating the tool, which seems to work just fine; it dries non-tacky and is rigid.

Get the best possible spray bottle you can since it'll be in near constant use. I use this one, which has a metal weight and filter on the end of the hose internally - this stops it ingesting air in use. Don't skimp here. I have a big bucket of water I use to get the bulk of the grit off after remaining grinding sessions.

On measurement tools, I'll say the spherometer gives a huge degree of confidence that you're not screwing anything up, so well worth getting something sorted out for that. I managed to snag a more accurate digital indicator on eBay for £75 which I'll be using in future; the surface plate I got for £80 or so I think (it's small but 00 grade, so pretty flat). 3D printing the body of the tool looks to work fine (online services would be the way to go to get this done if you've not got a printer to hand), and I used M3/M4 bolt stock from Model Fixings (who are magnificent for odds and ends between M1 and M6, and BSF/UNC equivs). I've also added some ceramic spherical balls as "feet" for the spherometer, which helps (if nothing else, helps to stop me worrying about scratching the glass). I've got no optical bench or anything, though I have access to a small optical table at work.

I've also bodged together (truly - rushed while a few gins in and split some of the wood with screws) a mirror test stand but it's a bit puny and I might upgrade that. I've got some bits from Model Fixings to fit levelling feet into it. Once I've got that and a Foucault tester sorted I'll be in good shape for testing.

This is brilliant, thank you very much for writing all of this up! I'm on a tight budget at the moment, so trying hard not to make any fundamental mistakes on purchases. There will likely be enough other mistakes or wrong turns along the way...

We are on the same page with the primary mirrors. I'd start smaller than 12" but since I have the 300p, I'll try for that. Perhaps a bit ambitious, but we'll see I suppose. 

I didn't even think about the smell of pitch - that could be a big problem at home. I'm lucky enough in that I have a JSP respirator from metal working and sandblasting, and bought some replacement P2 cartridges before they were wiped off the shelves back in early March. They are hot items these days, can't believe the prices.

All the tips on dental ceramic and tools are excellent, especially the spray bottle, that's something I'd find out the hard way in the middle of grinding no doubt. The measuring tools will be another thing to hunt for online and something to ponder for a while. 

There was actually a mirror grinding machine on ABS a while back, wasn't terribly expensive, but looked like it had a few hard miles on it.

I like the sound of the gin-powered test stand build! 

Again, can't thank you enough for writing all of this. I just need to take the plunge and order from Stathis on Monday. I have realised this is an itch that is not going away.  

I hope you are able to get yours sorted in time for autumn, I'm not setting any hard, fast timeline for mine, but I'm hoping to move fairly quickly on it when everything is lined up.

All the best for now - I'll keep an eye out for updates here 👍 

[discardedastro]

Back to the grind! Got everything good and clean and did about 15 minutes of 180 grit (normal stroke, mixed mirror-on-top and tool-on-top). Tool took a couple of minutes to bed in but looked to be making good contact pretty quickly.

Tool after some grinding:

And a moderately unremarkable mirror, now sporting a 180 grit finish. It looks pretty consistent, will probably do another 15 mins or so just to ensure it's all covered after checking with the spherometer.

Edited May 5, 2020 by discardedastro

[discardedastro]

Finally got around to widening the spherometer centre port a little to accommodate this new, slightly-higher-accuracy indicator I wrangled from eBay; this has a data port so if I get bored I might hook up something to do direct ROC readout and datalogging.

ROC is a bit shallow now, so need to do a bit more tool-on-top work to get back into spec, but not a million miles off. 0.42 in the middle, 0.44 at the edge, 0.41 is the target value.

Bevel is a bit small, but increasing it is very slow going. Suspect I'm struggling to get all the glass out of the coarse whetstone that came with the Stathis kit; got a fresh 400 grit whetstone in the post yesterday which is a bit fine really, so might try and pick up another coarse one and see if that helps. I'm definitely not at 3mm, though, more like 1.5mm as it is, maybe 2.

Will spend some more time on the bevel tomorrow to bring it to at least 2.5mm and then do one final pass of 180 grit - checking with a laser and manual inspection I'm fairly sure I've gotten all the 80 grit done. Rather surprised how quickly I've gotten through 180 grit if I have in fact done so - I've only spent 10-20 minutes grinding I'd guess. Will reinspect carefully before I get to cleaning up for the next grit size. The beeswax is definitely catching some of the 180 grit (visibly) around the edges of things; I've got a hot air gun on the way and will try the "simply melt it under the surface" approach for starters, but have also got another half kilo of beeswax just in case (along with some disposable alu trays to use for heating it up - I'm still scraping it off the pyrex jug I used!)

[Chris]

Looking good. You definitely mean business with that spherometer. Posher than my pennies and ruler  

[discardedastro]

1 minute ago, Lockie said:

Looking good. You definitely mean business with that spherometer. Posher than my pennies and ruler  

I'm still using a couple of rulers as a sanity check! 🙂 never rely entirely on the high-tech...

[discardedastro]

OK, so, question for those who've gone before!

I've been working on widening my bevel - realised going into fine grinding it's still a bit small. It's now about 1.5mm wide which I think should be OK for a 8" - it is very slow going to widen it with the 1000 grit or 4000 grit whetstones I have, and I am now nervous about proceeding with the coarser stones as I've had some damage from this, shown below in various angles. It's a chip of approx 0.8mm dia, not very deep, on the inner surface of the bevel.

What's the best way to deal with this? Should I widen the bevel another 1mm to absorb the chip, and if so, how best to do this without risking more damage?

[Astrobits]

The edge bevel definitely should be more than 1.5mm, you will loose a little more in the finer grits which could leave you with 1mm or less as the final bevel if not enlarged. A shrinking bevel size is a common problem in many mirror making efforts especially when more coarse grinding than expected has been needed to get to the curve.

You can try to only grind the edge on the stroke away from the surface, lifting the grindstone away from the glass for the backstroke. This will hopefully leave chips only on the outer edge of the bevel but be prepared to go back to coarser grinding if there are more mirror surface chips.

nigel

 

[discardedastro]

I've definitely got to come up with a better way of beveling. I just spent a full hour with bucket and coarse whetstone and I have increased my bevel by 0.3mm. So that's going to take me... 5 more hours of bevel grinding. I'm fairly sure I'm going to physically wear through my whetstone before I'm done.

Either my technique is terrible or my whetstones are irredeemably clogged (I've been cleaning them after every rotation, though). Grinding away from the surface definitely is lower risk - I introduced a couple of (tiny) chips when I went back to being in contact on both strokes, so quickly swapped back though it's slower going.

Edit: I've bought a few "diamond sanding block" things from Amazon, 60/120/200 grit. They look a bit more resilient than the whetstones and have seen some threads on CloudyNights recommending them or similar things. In lockdown, if I can get the materials, rather tempted to get on with upgrading my grinding stand to, uh, an actual stand. Doesn't look too hard to make something that should be able to start out life as a simple rotary table to make some things and working on the mirror easier and then get upgraded to a fixed post machine if I so desired later...

Edited May 19, 2020 by discardedastro