ATM Spincast analysis of construction and mediums

[alphasig@mail.usa.com]

Spincast construction and mediumd

I have read through most of the ATM archives on spincasting and I wanted to do an 
overview of ideas and suggestions.

I believe David Beach wrote this about using epoxy:
thermal conductivity (terrible), thermal expansion (worse) and internal stresses from the
reaction exotherm (appalling).

Several members went into detail about the difficulties they encountered polishing the 
finished product. The epoxy particles that were ground off then ground scratches into the 
surface.

Another member suggested using proper hardness of abrasives while working plastic/epoxy. 
Glass is 5 to 6 Mohs, and that is slightly less than the hardness of the final abrasives 
used with glass.

One member wanted to use Corion, but no technical data was given for this substance.  

As for the technical side of trying to spincast an optical quality finished surface (as 
opposed to a rough blank), here are the suggestions I found.

1.  Start with a "holographic quality" stable surface.  I believe this may be a bit 
overkill when one looks at the viscosity of the substances we will use (as opposed to  
mercury). This surface can be made by placing a number of halfway inflated inner tubes on 
a concrete surface, placing a large sandbox on top of that to absorb momentum of any 
vibrations, and placing a sturdy worktable on top of that (thick plywood).

2.  Need to work the material in a vacuum to prevent air currents from affecting the 
surface.

3.  Need suitable vibration free bearings. Members mentioned machining their own air 
bearings. Magnetic bearings were mentioned (one of my personal favorites). One thing that 
I thought of that might eliminate the table as well was to suspend the container by a 
number of ropes to a common point and then work out a vibration free pivot for that 
point. A number of vibration dampening rubber cords could be included before and after 
the pivot point. Since we are talking about a few RPM I think this method might have some 
small merit. Force bearings were also mentioned, in which an air-hockey approach is 
taken. One member went into great detail about the need to filter the air and provide 
very precise air flow.

4.  If you are bound and determined to attempt this with epoxy, one method for 
controlling the exothermic reaction stresses would be to hold the epoxy at higher 
temperatures while curing. This might alleviate temperature differentials.

5.  Also, if you are still determined to use epoxy, reducing the amount of hardener to 
about 10% total volume has been reported to reduce shrinkage to below what can be seen by 
the naked eye, but probably still screw up the figure. This also weakens the epoxy to  a 
point where it probably won't withstand aluminizing.

6.  If you still haven't given up on epoxy, don't forget additives to reduce shrinkage, 
usually made up of glass particles which may show up on the surface or be totally spun 
out to the sides depending on the relative density of the epoxy to the glass.

7.  If you are still going to use epoxy, use one that will stand up to the aluminizing 
process. EPON 828 has been mentioned.

8.  Epoxy will require the use of mold release to keep it from sticking.

9.  One epoxy oriented method that I liked a lot was the use of successive thin films, 
like the dipping of a candle. This method could be applied to successive layers of epoxy, 
or to the top of another spincast material that would act as a base for the epoxy (if 
using non-epoxy base, keep the epoxy thin to prevent bi-material warping)

10.  There was some discussion about spincasting glass. At first I found this idea 
appealing, then I read on about the huge kilns and precise temperature controls required 
(all while maintaining a vibration free environment). I am afraid this is beyond my 
means.

If you can get past the vibration (as they have done for liquid mirrors) then you still 
have to deal with the shrinkage/warp/temperature problems of the medium you choose to 
work with.

I started this review by reading all the posts and writing down the merits and 
recommendations of each person. I was bound and determined to work with some form of 
spincasting. Now I have decided that at best I can get a rough parabolic surface which I 
would then have to finish somehow. My final analysis is that spincasting is beyond my 
abilities.


---------------------------------------------------------------
Get free personalized email from USA.com at http://mail.usa.com