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Parabolic Troughs for DIY High-Temperature Projects - Page 2

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Posted by Jeff on November 27, 2006, 6:20 am
 
Morris Dovey wrote:


Not sure about what "CNC" is, but I did some parabolic troughs years
ago. What I learned was the reflective properties of the trough are
crucial and was beyond what I had on hand (polished aluminum printing
plates). I don't think that has changed.

   Jeff

  My


Posted by Morris Dovey on November 27, 2006, 1:34 pm
 
Jeff (in Ssvah.3675$sf5.2934@newsread4.news.pas.earthlink.net) said:

| Not sure about what "CNC" is, but I did some parabolic troughs years
| ago. What I learned was the reflective properties of the trough are
| crucial and was beyond what I had on hand (polished aluminum
| printing plates). I don't think that has changed.

Jeff...

CNC is an acronym for "Computer Numeric Control". In my shop the
controlled machines are routers hooked up to a PC controller. My
larger machine has 3-axis control of a 5HP router with a precision of
+/-0.001" for all three axes over a 96"x48"x6" workspace, which allows
me to produce reasonably accurate wood, composite, and (occasionally)
metal parts.

The beauty of CNC technology lies in combining the precision of tool
motion with the speed and repeatability of computer control.
Programming can be hand-written or generated directly from CAD
drawings. I wrote a parabolic rib application that calculates the
points with 0.010" spacing and runs the cutting tool at 90"/minute -
performance that a human would be hard put to match.

There are, of course, a number of options for reflector material -
ranging from reflective foil applied to plywood or plastic to highly
polished/plated metal sheets. I've been experimenting with 'silvered'
plastic sheets, as you've probably already seen on my web page.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/StirlingProject.html



Posted by SJC on November 27, 2006, 4:58 pm
 I have had good luck with the silvered mylar. The MIT guys appear to have
used polished stainless steel sheets. The last time I priced those, they went
for more than $00 for a 4" x8" sheet about 15 mils thick. Seemed a bit pricey,
but if they last a long time, maybe not.



Posted by Duane C. Johnson on November 27, 2006, 5:31 pm
 Hi SJC;



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Posted by Duane C. Johnson on November 27, 2006, 6:45 pm
 Hi SJC;


 > I have had good luck with the silvered Mylar. The
 > MIT guys appear to have used polished stainless
 > steel sheets. The last time I priced those, they
 > went for more than $00 for a 4" x8" sheet about
 > 15 mils thick. Seemed a bit pricey, but if they
 > last a long time, maybe not.

I wouldn't use stainless, or for that mater nickel,
chrome, or polished aluminum.

Stainless, nickel, or chrome are only about 65%
reflective no mater how nice they look.

And aluminum corrodes easily unless it is really,
actually very pure. Alclad aluminum aircraft sheets
are in this class. Alclad has about 1 mil of pure
aluminum plated onto the structural base aluminum.
This stuff is used on airplanes that are non painted
aircraft. Kind of expensive though.

Reflective Mylar is a better choice as it is so cheap.
However the Mylar stuff requires periodic replacement
every few years depending on the local pollution.

BTW, this square dish was also built using a
CNC router:

http://www.redrok.com/electron.htm#vdish1

Duane

--
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