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Has anyone tried to set up a spin casting kiln?

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#1 Steve Drapak

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Posted 03 December 2012 - 01:52 PM

Being lazy and curious why I shouldn't try (and yes, building a rotating kiln or a rotating platter inside a kiln is probably waaay more work, so let's call it selective laziness combined with masochism), it would be nice to have a mirror relatively well curved prior to grinding, especially if there's more than one being made.

I know there are rotating mercury electrical connectors or rotating couplings of various types that can power a kiln and a rotating platform is easy to set up. Not so sure about rotating something inside a kiln, I did see someone mention running a rod up through the bottom of the kiln but that sounds a little scary.

Why start small when you can have an impressive industrial accident to brag about (assuming you survive)?

#2 glennnnnnn

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Posted 03 December 2012 - 02:44 PM

If its got an adequate thermal barrier you could use a stainless shaft (through the bottom of the kiln where its not so hot.)
BUT.....
To cast a thick piece of glass in a kiln requires that it be held at temperature for a length of time that's probably directly proportional to the thickness x diameter. The bigger you go, the longer you have to play with the temperature: especially the last part, the annealing process that relieves internal stresses and makes it a good piece for a telescope.
Going out of your way to produce a curve won't work as well as you might think because the curve has to solidify and then cool, which can produce all sorts of interesting results that are less than precision.
Having said that, it sounds like a really cool idea if you have the resources. If you could get the curve thing down to a method that worked pretty good, you might be able to make some really short-focus mirrors!
You have to try to do something if you're going to succeed!

#3 frodostang

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Posted 03 December 2012 - 03:19 PM

Do a google search on telescopew mirror galss casting, and there are anumber of articles early in the list, including one on spin casting the Giant Magellan telescope. Might be easier to figure a way to spin the whole furnace than to have the turntable spinning inside the kiln.

http://www.npr.org/2...scope-mirror...

http://www.mdpub.com...anks/index.html

#4 Steve Drapak

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Posted 03 December 2012 - 07:56 PM

Frodostang: Going to be in Tucson later in Decemeber, the mirror lab there is definitely on my list of things to see.

Glennnnn: True, there shouldn't be too massive heat loss if the gap is really small and the spindle can take the heat. Might be a shame to put a hole in the bottom of a kiln but it's all for fun anyway.
Good pooint too, I might be expecting too much from the resulting curve. But definitely it will be fun to try.
I'll post something here if I do start working on it.

#5 kfrederick

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Posted 03 December 2012 - 07:59 PM

Maybe try it with water spin it and freeze it

#6 allardster

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Posted 03 December 2012 - 08:04 PM

Hi Steve, I see you are in Toronto as well. There is a small but active ATM community here. If you're interested I'm more than happy to introduce you. Just PM me.

As for your mirror. Have you considered slumping a blank in to a meniscus shape? Maybe an easier project and the result will be much more rigid.

#7 gregj888

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Posted 03 December 2012 - 10:48 PM

Steve,

The primary reason to spin cast is to reduce the time to hog out the mirror and to save material. On a 10m f/3 spinning saves a lot of grinding and a lot of glass. Even at 1m, you don't save all that much. Diamond generating or slumping will do the job and are much easier to do "at home."

#8 Steve Drapak

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Posted 03 December 2012 - 10:53 PM

Kfrederick: That's a great idea to get the shape down. Probably the viscosity difference of glass wouldn't make too much of a difference.

Allardster: Slumping is something I didn't consider, you're right, much easier if you already have a blank. Definitely worth considering. Large blanks are probably cost effective compared to setting up a kiln like this. Hmmmm.
The one thing I guess it can't do is reduce the mirror weight by adding hex inserts, but you can't win them all.

#9 Steve Drapak

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Posted 03 December 2012 - 11:11 PM

Gregj888: After talking with people I have to agree with you, it's much more work to set a spin caster up than the time it saves unless I'm going for mass production, which I'm not. It's more of just trying it to see if I can. Though it might be better in invest the work in a grinding machine if I really want to minimize the manual labor.

Do you have any pics of the work you're doing on the 20" cassegrain?

#10 allardster

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Posted 03 December 2012 - 11:24 PM

Here is the article that introduced me to the benefits of a meniscus shaped mirror:
http://www.bbastrode...pe_to_the_Re...

R.F. Royce mirrors are conical, like a meniscus a stronger shapen and easier to support.

#11 danjones

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Posted 03 December 2012 - 11:47 PM

I have a kiln identical to the one mike davis has at mdpub.com. i'm in the process now of building a ramp/soak controller to control the temp, soaking times, and for the needed annealing times associated with casting glass. i had the opportunity to test this process in someone elses kiln with good results. I can do up to 18" diameter and whatever thickness I want in mine. I don't plan to do any spinning in it for pre generated curves, but easily done on my home made fix post turn table similar to gordan waite's. waiteresearch.com. good luck, sounds fun.

#12 Steve Drapak

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Posted 04 December 2012 - 01:52 AM

Allardster: Cool! I hadn't heard about these mirrors before. lighter and stronger without hex inserts is awesome. I like this the best so far. A simplified mirror support structure is a bonus.

DanJones: Gord's videos are helpful. Between this and the meniscus mirrors I'm going to be up late tonight...

#13 Aperturefever

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Posted 05 December 2012 - 05:52 PM

Back in the late 80s myself and a couple of others teamed up to have a go at spin casting. It was ages ago now and I can't recall all the technical details, but we tried to cast a pyrex 14 inch at about F4 (I think). It seemed to work but we just couldn't get it to cool without cracking - we tried it a couple of times but it was very fiddly and the project ended up on the shelf.

#14 careysub

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Posted 05 December 2012 - 10:52 PM

...It's more of just trying it to see if I can.


How about experimenting with the process by spin-casting urethane casting resin? Then check it out with a Foucault tester.

#15 FlorinAndrei

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Posted 06 December 2012 - 12:48 AM

I can do up to 18" diameter and whatever thickness I want in mine.


How many kilowatts are you feeding it at peak temperature?

What's the total duration from cold glass to cold glass?

#16 Steve Drapak

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Posted 06 December 2012 - 03:17 PM

...It's more of just trying it to see if I can.


How about experimenting with the process by spin-casting urethane casting resin? Then check it out with a Foucault tester.


I've read about them, some young whippersnapper was playing with the idea. As far as I know it was something still not ready for prime time. That being said, it does sound interesting.

I'm quite envious of Drew with his StarStone business he's been able to start it, it looks like a real breakthrough for very light mirrors.

#17 Al8236

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Posted 07 December 2012 - 02:44 AM

...It's more of just trying it to see if I can.


How about experimenting with the process by spin-casting urethane casting resin? Then check it out with a Foucault tester.

Years ago I did a spin casting with an old record turntable, variable speed control and polyester resin to cast a 14" approx. F/2.5
After it was cast I added a couple Oz. mercury and used the speed control for focus.
It didn't work to bad but of course had all the limitations of a zenith scope!

#18 Steve Drapak

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Posted 07 December 2012 - 11:00 AM

That's really cool! I've seen videos of the mercury telescopes, seems like the least expensive way to make a huge mirror, though definitely limited with where it can look :) Where did you get the mercury from?

#19 careysub

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Posted 10 December 2012 - 12:20 PM

Where did you get the mercury from?


Chemicals of any kind have gotten quite difficult to obtain in recent years. But you can get mercury here:

http://www.elementalscientific.net

$25 for 4 oz.

#20 Norm Meyer

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Posted 15 December 2012 - 03:54 PM

Not a bad idea. I think I would go with the rotating turntable inside the kiln. Make a turtable of refractory cement on top of a round steel plate to which you weld a
shaft. Put the shaft down through the bottom of the kiln.
support the shaft with flange bearings and hook up a DC motor
with rectifier and variac. Put a sleeve of high temp material where the shaft comes through the bottom of the
kiln to keep air out as much as possible. the turntable
could be shut off when the glass has solidified enough to
hold it's shape. then you just let the kiln slowly cool
to anneal the glass. This way you avoid the fancy and
probably expensive slip rings that would be necessary to
supply power to a rotating kiln. There must be a formula for figuring what speed to rotate the glass to generate
proper curve. I'd like to try it but can't afford kiln
right now (or the amps to power it!)

Just my two cents.

Regards Norm






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