Members of the ASV's instrument making group have started building a batch of all sky cameras, myself among them, and I thought I challenge myself at making a glass dome for mine, rather than use a commercial plastic one.
Just recently I had built a small electric kiln so I started experimenting with glass slumping. After a number of trials I came up with a two stage procedure to get a roughly hemispherical glass blank that could be ground and polished into a good optical dome.
At the first stage I used a terracotta dish to slump a 99mm diameter glass disc to a concave shape.
At the second stage I slumped the preformed glass onto a 63mm stainless steed dome.
The main problem is that, while the inside is quite spherical, the edge of the glass has thickened to almost 5mm, and the center is 2.7mm thick, down from the original 3mm.
I will try to grind the outside spherical and achieve a wall thickness of at least 2mm.
This is an interesting project. How thick was the sheet of glass you started with?
When you perfect the process I'll have to send you the size of my clock face, a hand made timber clock from Italy in the shape of a ships steering wheel I purchased new in 1981 but when my son was 2 he slammed a door and the clock fell off the wall and broke the domed glass. I've never been able to get a domed glass replacement for it.
Though if I get a kiln (or make a gas kiln) I may have a go at it myself, nothing to lose but some cheap glass.
Thanks for the comments. It's nice to know that there is interest in my little projects.
The rough grinding is now completed, down to #240 grit.
Lateral concentricity is better than 20 microns and wall thickness is about 2.35mm.
All done with hand tools.
If I make another one, I will invest some time in making fixtures so that I can machine away the extra glass.
That looks great Stefan. How long did it take working by hand? Did you use a tile tool?
Rod.
I must have spent something like 6 or 7 hours on it, over several days.
You can see the 3 principal tools on the attached picture, but most of the glass removal was done with a coarse grinding wheel and a big diamond file.
I finished the fine grinding and slumped a couple more discs for making the polishing laps.
I will try to polish the inside surface first because I think it is more likely that I break it while working the inside surface.
Very impressive! I can also see why one would do this: optical glass domes are expensive.
I recall being in an optical surplus shop in Pasadena (think JPL's headquarters) in the pre-digital camera epoc, (that shop has long since closed) and pondered some optical domes...some were 6" in diameter and the asking price was about $US 20 depending on the size.
Very impressive! I can also see why one would do this: optical glass domes are expensive.
I recall being in an optical surplus shop in Pasadena (think JPL's headquarters) in the pre-digital camera epoc, (that shop has long since closed) and pondered some optical domes...some were 6" in diameter and the asking price was about $US 20 depending on the size.
I though..cool..but what would I use them for?
If only.....
Thanks Peter!
I find it very strange that it is impossible to find any prices online, apart from the ones dedicated for underwater photography. And yet there is no shortage of manufacturers but they don't reveal their prices.
Anyway, the polishing is progressing better than expected.
Yesterday as I was about to finish the polishing of the inside surface, disaster struck! I must've got a bit overconfident and at one point I did not engage the driving pin properly. As a result, when I switched on the machine, the contraption instantly went into self destruct mode.
So, back to square one, it seems, but in fact not, because I now have most of the tooling and fixtures needed. Also I will start with a small batch so that I end up with at least two finished ones - one for a replacement spare if I ever need one.
That's the worst thing with glass Stefan.
At least you have everything set up as needed to get a faster start on the new units and the experience.
What was you using to polish the glass?
Was it some sort of sphere with diamond tiles or something?
Back 40 or so years ago I worked in glazing for a not extended period, the boss expecting me to carry big sheets of glass up an 8 metre ladder in rain and strong winds "shove it moron". There were no OH&S regulations back then.
I'd often find myself doing a pencilled edge on large sheets, hold it under the arm and run it evenly up and down the shaped grinding tool (large machine) and whatever you do don't twist the glass. The strangest bit of the entire process was using a cork wheel and belt to polish the ground edges to the nicest sheen. At least I've known how to cut glass when I've needed to.
Leo,
The polisher was plain synthetic pitch with cerium oxide - nothing unusual.
The difficult bit is the extreme curvature which would require a special purpose polishing machine. I had to make a curved driving pin and used pure spin polishing.
Anyway, I have an artisan glass cutter just around the corner and I got half a dozen new discs cut the other day.
I did a lot of glass bending over the last week and now I have 4 good blanks.
I've also done the rough grinding of the inside surfaces.
Next I need to make a fixture for diamond machining the outer surfaces.
Thanks for the link. Interesting find but those domes are huge and it is interesting to see an optional US$258 quality inspection associated with them.
Some months ago I managed to find some suitable sized glass domes on one of the online markets but the price was over $400.
I made a holder for machining the external radius. Not having a suitable size fly cutter, I just used a hole saw to generate the required radius.
For the glass machining I happened to have just the right size diamond plated tool.
For attaching the domes to the holder I used a mixture of paraffin and bees wax.
I figured it out, it's one of those off axis rotational chucks used on milling machines. I've yet to purchase one. I'm yet to weld up a stand for a milling machine which came as an attachment to a lathe. I purchased a separate cross slide table and the milling machine still sits in it's box until I get a suitable stand welded up. I didn't want it bolted to the lathe when I read the notes stating it may either need packing to fit properly or, machining of the milling machine upright mounts. That would have been a big pain so I purchased a separate compound table to set it up as a stand alone machine.
