Ballbars

[Evan]

Epoxy resin has a very low coefficient of linear expansion with temperature, about 50% of steel. It shouldn't cause any problems and should be compatible with the graphite/epoxy layup.

I am sure that the company I linked to could accommodate whatever mounting method you come up with. I don't think it would be too difficult to make a socket to trap one end of the adjuster. I also would expect anything that is adjustable to the 1 micron range to be internally temperature compensated.

[small.planes]

Hmm, I like the idea of a differential adjuster.
I cant for the life of me see how those ones attach, but the principle should work nicely.
As for glue, there is a possibilty of residual stresses in the glue causing movement as it sets up, or if constrained during curing of it moving afterwards.
Its weird that something that small matters...

Interestingly there is more than a little doubt that this can be made to the required spec (which is of course actually even tighter than I thought!), so it would be really nice to do the impossible here

Dave

[Evan]

The glue should not enter into the picture as far as affecting the length. You cut the rod to a rough length and then grind the ends to finish length using accurately positioned stops to achieve accuracy of around +- .01mm. One end is a simple stub fitting that is inserted in the tube with glue. As the glue holds around the circumference of the stub it won't change the length as temperature changes.

The fitting for the other end is special and widely available. It's a differential thread mirror adjuster. The ones in this link are adjustable to 1 micron location accuracy.

http://www.optosigma.com/miva/merchant.mv?Screen=PROD&Store_Code=OS&Product_Code=pg374-375&Category_Code=Accessories

[Peter N]

How about making the hemispheres, attaching them to the end of the CF tubes (plural), then make a micro-adjustable joiner for the 2 assembled tubes to set the length?
Just a thought.

Peter

[small.planes]

Yes, I think the length is quite a significant part of the problem.
It seems that the ball seats can be made to the required spec, and I know we can attach them to a zero CTE CF tube, but the attaching is never going to be that accurate, especially as it involves glue...
So I think that either a more accurate attachment method, or a way of finishing to OA length after attachment is required.

It certainly is an interesting project

Dave

[aboard_epsilon]

OK
I've been talking to Alwyn

and he says that you're talking impossible tolerances ..because of the 1 metre long bar.

and the only place who would be able to verify them once done is

National Physics Laboratory of London .

The plastic you should use should be a plastic called Tuffset

Tuffest is very stable and is even used on the lense positioning of a ZYGO interferometers.

all the best.and good luck ..markj

[small.planes]

Cool video. That would do (in reverse of course), if only we could get a 1 meter long bar inside the machine

TM, Lapping is a possibility, but if we can get close enough using machines that would be nice (and less skilled). And the closer we can get the less lapping would be needed.

Dave

[Teenage_Machinist]

I cannot imagine this not requiring lapping with balls. Not that I know about this.

[Peter N]

Yup, sparking is a form tool process, with the electrode being the reverse from of what you need minus the spark gap (around 2-3 thou' a side).
I don't think sparking would give you enough precision Dave, or certainly not a good enough surface finish to get down to the precision you need, as at that level the peaks and troughs from the sparking would be all over the place.

I just did a quick search and found this short Youtube video showing an EBM machine in action, topically enough its machining a sphere

http://uk.youtube.com/watch?v=fDYuSleApiQ

Peter

[small.planes]

Cheers Peter, Ill look into that.
I did wonder about spark erosion process as well, but Im not sure of the tolerances obtainable, and essentially its a form tool process, so would need new electrodes for each part I imagine?

Dave

[Peter N]

Just had a thought Dave.
You could rough it to near shape, a few thou' or less, using conventional machining, harden it, then send it off for Electron Beam Machining for the finished surface. Capable of nanometer level machining so I've been told.
I've known of the EBM process for some time, but can't point you in the direction of anyone actually doing it.
A friend of mine who works for Dage designing ultra high resolution digital x-ray equipment switched me on to it some time ago.

Peter

[small.planes]

Thanks for the offer, but I hope the Optical Research group at Loughborough Uni has access to the required measuring tools
This isnt a home shop job, but there are a lot of practical knowledgeable people round here to pick the brains of.

Dave

[Evan]

Need to borrow my 1 micron per division dial indicator?
Nah, foget it, nobody can measure to those tolerances in a home shop.

[small.planes]

Evening All,
Ive been pondering on this, and after a little digging about looking into methods of making hemispheres I found a thread here showing how to do it on a mill.
So I knocked up a seat to fit one of my daughters rubber balls:



Ignore the turned finish, it was a scrap offcut of mystery metal, and was like that when I started.
Looks like the seat part *should* be possible.
Of course they'll need to be cut (or at least finished) in situ on the end of the bar, as the glue will have set then...

Dave

[Evan]

The bar isn't metal. It is specified as carbon fibre. If the correct carbon fibre layup is used it will have a nearly exactly zero coefficient of expansion with temperature. This is what I used on my telescope for the truss frame. Pulltruded carbon-epoxy rods have a very slightly negative CLE meaning they expand very slightly as they grow colder. Combined with the positive CLE of the aluminum parts at each end my telescope doesn't change focus over the entire possible range of temperature when taking astrophotos.

Dang, fell asleep for a few minutes before posting this.