The Giant Binocular

[savarin]

The first bit of grinding has started.
The front concave curve of the mirrors need a radius of curvature (roc) of 144", this will be ground in roughly with this abomination of a machine.
The pendulum carries a spinning diamond disk at its end.

These are the aluminium puller plates rough ground at 122.5" roc concave to marry with the back curve of the mirrors via a 10mm thick foam backing.


and a close up of how rough it is.

I used a diamond cutting disk with WD40 but was very surprised at how long it took.
(And the mess)
To set it up I set the pendulum at 122.5" from the centre of the top pivot to the edge of the diamond blade.
I then adjust the three tripod legs so the diamond disk "just" kisses the edge of the aluminium disk all around.
Then raise the legs equally till the diamond disk touches about an inch from the centre and start grinding, the aluminium disk rotates anti clockwise pretty fast and the diamond dish rotates clockwise so it cuts against the direction of the main turntable. Lower the pendulum a little and off again, rinse and repeat.
The arrow is to remind me, if I get the direction wrong the diamond disk rides up onto the aluminium disk too fast to control. No big deal but if it happens on the glass mirrors they have a habit of cracking. (dont ask me how I know)
Next is to kiss grind the main tool at 122.5" roc I've just made. Its a large plaster tool, concave ground with steel dumps epoxied to its surface, not set yet so cant run a cleaning grind yet.

 

[woodchucker]

so I have more questions than you provided answers to.

1. do you manually control the pendulum by holding it and swinging it by hand (held all the time)?
2. does the tripod sit unanchored? (what if it moves?)
3. what kind of grinder? Is it an angle grinder?
4. any video of it in progress

 

[savarin]

so I have more questions than you provided answers too.

1. do you manually control the pendulum by holding it and swinging it by hand (held all the time)?

yes, left hand just above the round yellow guard and right hand clamping the white guard against a length of angle iron clamped against the rear two legs.

1. does the tripod sit unanchored? (what if it moves?)

No, each leg has a length of all thread fixed to it with brackets and lock nutted to the frame. It can be adjusted up or down, the pendulum can be adjusted up or down as well.

1. what kind of grinder? Is it an angle grinder?

No, its a washing machine spin dryer motor. I was only thinking yeterday that an angle grinder would have been more efficient and lighter.

1. any video of it in progress

No, and I doubt there will be as I need both hands to grind with. Video will be available when I return to conventional methods of fine grinding and polishing.

 

[savarin]

This is the tool to generate the curve on the back of the mirrors.
Another tool has to be cast into this to get the curve to finish the aluminium pullers.
I think it will also have to be steel dumps and resin.

These are both the mirrors hogged out (very rough grind) to 144" Radius of curvature.

A tool has to be cast in one of these to do the fine grinding.
This tool can be glass mosaic tiles in plaster.
If I alternate the grinding between the two mirrors I may be able to get away with one tool.
Still heaps to do but were getting there

 

[woodchucker]

from here those look like hell.
they probably photograph worse than they are.. they look like the face on a cinder block..
just saying...
looking forward to the next installment.

 

[savarin]

from here those look like hell.
they probably photograph worse than they are.. they look like the face on a cinder block..

They are exceedingly rough, rougher than if I had conventionally ground them out with 60-80 grit carborundum but the important part is they are both at the correct radius of curvature so its just a journey down the grit sizes with constant checking the roc doesnt change till smooth enough to start polishing.

 

[rwm]

This is fascinating! What is your degree of confidence that you will get optically correct mirrors from this process?

 

[savarin]

I'm reasonably confident that I will get them optically correct but not quite so confident I will get them identical.
I've chosen this route as I believe it will be easier to ensure they both end up the same roc.
I will be spin polishing them to spherical and measuring often to keep the roc stable.
My biggest worry is the edge chipping that occurred on one of the disks but a slightly larger bevel should (I hope) remove the majority of the damage.

 

[rwm]

I saw that chipping. Will that screw up the pull? Are you convinced pulling a spherical curve is better than trying to grind a parabola? I guess the size matters here!

 

[savarin]

I'm a little scared the chips will cause a degradation in the image but at f 6 the puller tension and deformation is actually quite small so hopefully they wont be too big a problem. If that mirror does cause a problem I will have to do another. I have the glass. A bigger stumbling block is I think the maximum wedge in the mirror must be under 0.0006" I dont think I can measure that.
The math shows at f6 they have the possibility of being at the top end of excellent.

I ground the back of an under corrected commercial 8" f3.7 (approx) mirror that had an excellent finish spherical and it pulled that to a superb mirror so thats my dream here.