Mirror Grinding Machine

[Orion]

Has anyone ever built a mirror grinding machine?

[h0ughy]

ed you are a glutton for punishment

[GTB_an_Owl]

Yes Ed

it is called "George" :whistle:

pretty advanced machine i'm told - it even talks to you :rofl:

and it even works underwater - TRUE - i saw it at Lostock

geoff

[I.C.D]

I saw it too ,by late arfternoon it slow down a bit but still kept on grinding

[Orion]

:rofl::rofl::rofl: I know Iv'e seen the underwater version too:lol:
You should see how he cleans his mirrors, with his tongue! No Joke!
He recons that there is less scratches that way.

There is only one George:thumbsup:

[AstroJunk]

I've not built one, but i've seen a few in action. The mechanics are very simple, especially if you plan to use a tool the same size as the primary.

(and I'm sure you have googled the designs by now)

What they don't do, but my mirror making mentor David Sinden http://www.britastro.org/iandi/mariot04.htm did: Rather than clamp his mirrors in position he attached a threaded plate to the back with pitch. It was very neat, and the 'nut' screwed onto the lower spindle when on the bottom, and captured a peg coming down from the upper arms when on top. I'll draw a diagram!

[snowyskiesau]

Here's a local one called Donkey.

http://users.tpg.com.au/users/daveg/GMACHINE.html

[CoombellKid]

Ed,

Are you a member of either

http://www.atmlist.net/

or

http://groups.yahoo.com/group/zambutomirrorgroup/

they have some good discussions on the topic from time to time.

regards,CS

[Satchmo]

Mirror making machines are diversion all of their own, not necessarily a labor saving device for the inexperienced.

Its probably the case that until anyone learns the subtleties of producing a superb mirror by hand, that they will never make a good mirror by machine . Part of the art of making mirrors on a machine is learning to introduce randomness to mimic the natural smoothing action of handwork ;). I still use an element of hand polishing in my mirrors and will always do so .

[Orion]

Wouldn't the pitch move with the summer heat making the grinding uneven?:shrug:

[Orion]

Thanks for that Geoff I've seen that one before.
He has done a good job of it.

[Orion]

Yep member of both, just curious to see if anyone here had built one.

[AstroJunk]

You can (and will need to) change the hardness of the pitch to suit the temperature. (boil it to make it harder, add turps to make it softer!) The pitch is pretty firm stuff at normal room temp.

Secondly, it's a good idea to do your figuring at least in an environment which matches the temperature that plan to use the mirror. Not such a big deal with low expansion glass, but worth considering.

Finally, a little movement isn't actually a bad thing. It's the randomness of the hand movements that make such fine surfaces, and something machines find difficult to emulate. There should be no great forces involved either - just time and grit.

[Satchmo]

Hi Astrojunk

Could you elaborate there ? The relative mechanical dimensions of a glass blank are the same regardless of its temperature. Are you referring instead to the practise of undercorrecting a plate glass mirror to allow for possible overcorrection condition as the ambient temperature falls and the optics can't keep up, when in use under the night sky ?

[Orion]

Mark when using a machine would you still need to do the figuring by hand or are there some mirror manufacturers that don't bother?

I have never used a machine so I wouldn't know, but I can't see it doing the figuring.

[AstroJunk]

You're right, I suppose the critical factor is the even-ness of temperature and it was probably cheaper to keep the workshop evenly cold rather than evenly warm where I grew up!

As you have reminded me, it was always said to better to under correct mirrors - and back then we used to grind everything to f8 anyway.

[Satchmo]

A great deal of figuring can be done by machine. On smaller mirrors 10" to 12 " of modest asphericity it can actually be done with one polisher and a skilled operator all by machine .

In my experience a smoother zone free finish can be had with hand work and I'd guess I do 50% by machine and 50% by hand on my mirrors, as I like to take the figuring process from engineering science into art and a handcraft.

Large fast pro mirrors ( like the 8meter varieties ) are polished by computer controlled laps which actually warp to fit the shape of the mirror. They are not touched by the human hand.

As a contrast in technologies , the final corrective polishing runs on the 200 inch ( 5 metre ) Mt Palomar were done in 1947 by the late Don Hendrix by hand using 6" polishers. As the rib thickness had come out uneven in the casting, the mirror performed worse in its cell on the sky than it did hanging vertical on the test stand.

The final touches were made by taking `foucault-grams' on a star, where a knife edge null test was made on a real star and captured on a photograghic plate. The final rubbing down of the 1 wave high edge evident from the mirror `in-situ' in the Palomar observatory by Don Hendrix. Thats what I call `flying by the seat of your pants ' ;)

[Orion]

There's a nice bit of history about Don Hendrix. Thanks I didn't know that.

[Orion]

Any comments on the Draper style?

http://www.youtube.com/watch?v=OkT5aCla_YI&feature=related

I was also looking at the Mirror o Matic but I don't think it is for large mirrors.

http://www.youtube.com/watch?v=TEuI1tIh5TM

[AstroJunk]

Wow - that mirror o matic was going fast wasn't it?!

The Draper machine was going at the rate that I remember David Sinden's machines went. He had one of those that he built for 18" plus mirrors, but his beam arm was much bigger - 2 or 3 meters long to minimise side to side movements.

[Vince G]

G'day Ed,

I'm seriously thinking of building or buying a 24 inch mirror.
Would you tell me who is making/made yours please?

Vince G

[Orion]

Hi Vince, PM me.

[DaveGee]

Hey, that's my machine!

here are some videos of Donkey in action,
rough grinding...
http://www.youtube.com/watch?v=DFQ3EkMYnQo

and polishing
http://www.youtube.com/watch?v=HVyyrI49LpQ

Donkey does all the err... donkey work however figuring is done by hand.

[Orion]

Hi Dave,
Nice machine. How long did it take you to build it?

[DaveGee]

Hi Ed,

About a year I guess, off and on.

[netwolf]

I have read on other forums and ATM site of clubs where people have mirror making classes etc. Do any of the Aussie societies or clubs conduct and facilitate such activities?
This is a Belgian group i found and they have levels you pass through to graduation. The have some great information on mirror making.
http://users.pandora.be/telescoopbouw/
An example Polishing with Pitch
http://users.pandora.be/telescoopbouw/polijst%20met%20pek-e.htm


This is the famous American group known as Stellafane.
http://stellafane.org/
Look here for Telescope making information, very good read.
http://stellafane.org/tm/index.html

Regards
Fahim

[ausastronomer]

Ron Royal used to run a mirror grinding/telescope making class at Sydney Observatory. However, I think it ceased about 4 years ago. This was a class which went over about 8 weeks, 1 night a week, where you went and ground your own mirror and built an 8"/F8 scope. It used to cost just over $500 (all materials supplied). It was a great concept but the proliferation of cheap chinese/taiwanese dobs of reasonable quality killed it.

Cheers,
John B

[CoombellKid]

The Queensland Telescope Makers club is still active, if you live in Brisbane.

regards,CS

[astroron]

Fahim what is Infamous about Stellafane:shrug:
Ron

[netwolf]

Ron, i have edited my post. I meant famous.

[Orion]

This might be the beginning of a grinding machine.

I scored these two motors a couple of days ago.
All I know is that the one on the left is from an old washing machine and it has a gear box!:thumbsup:

[cfranks]

I made a machine about 20 years ago when I made my first telescope, a 15' dob for the ASSA, As has been mentioned, manual figuring was essential but the machine's turntable was used all the time. The scope is still in use (I think)

I still have the machine and it is available in Adelaide for anyone who might have a need for it.

Charles

[GrahamL]

great to have you aboard Charles :thumbsup: the incredible knowledge base of this website just keeps on growing:)