Aluminum expansion uniformity -- design advice for avoiding warping? 4

[ttuibk]

Hello,

I am designing a mechanical system which uses an aluminum rod as a temperature dependent length tuning element. With the exception of machinability and long-term stability I do not care about the mechanical properties. The main problem is that I need the end faces of the rod to stay parallel during temperature changes on the order of 30 deg C or so near room temperature. Really parallel. Like on the order of 0.1 arc second. The other design option is a piezoelectric stack, but for large displacements (50um or so) I am not sure things will stay parallel enough, and I do not like the idea of the length being susceptible to fast electronic noise.

Am I totally crazy here? Or am I worrying about this too much. I am going to try it anyway, but what can I do to increase the chances of this working? Some questions:

1. Alloy choice. I have been reading about "artificial aging" and "annealing" for various aluminum alloys. Is this relevant? I chose aluminum mainly for its high expansion coefficient.
2. Shape choice. I chose a rod shape only to save space and weight. Will a rod with a larger diameter maintain parallelism better? The current diameter is 12mm. (150mm long). The cross section is irrelevant but there needs to be a 6mm dia. clearance down the middle, and as mentioned previously the length must be 150mm.
3. Machining. Would it be better to start from some kind of extruded piece and then anneal it? And then machine the ends flat at the very end? Or will machining the entire thing give a similar (thermal expansion uniformity) result. Or should I have everything machined and then anneal it?

Thanks for reading!

 

[Compositepro]

Generally, high precision optical instruments are used in well controlled environments and given a lot of time to stabilize. If there are thermal gradients around, then the only real solution is to use low CTE materials, like fused quartz. Any sort of heat flow results in a temperature gradient, so the goal is to not have any heat flow in the rod.