[simpits-tech] Display walk-around.
cyplesma at aol.com
cyplesma at aol.com
Wed Jun 8 07:29:58 PDT 2011
On Wed, 8 Jun 2011, Brian Sikkema wrote:
>> The current setup requires nearly the full draw of the Fein Turbo III dust
>> collector that I'm using and even with my 60gal air compressor going full
>> tilt, I can't approach that amount of volume. The Fein will pull a 105"
>> water column which is roughly equivalent to about 8in HG.
> Just spitballing here, but IIRC there's a fair amount of space behind
> the mirror, no? It seems if you could reduce that volume that maybe your
> vacuum requirements would be eased.
> Just thinking about our sim and how the mirror is set up, and the
> backing behind it matches the shape of the mirror pretty closely. I'm
> sure there are multiple reasons, but it stands to reason that one of
> them is to minimize the amount of volume you're drawing the vacuum on.
It's not so much the volume as it is the vacuum required to stretch the
Mylar in the desired shape. Commercial systems have a smaller space
because they build the Mylar framework as a single monolithic assembly
that is perfectly air tight. They can use a much smaller vacuum inlet
that feeds a much lower CFM, but much higher draw vacuum pump. Because
our system is very (comparatively) "leaky", it takes a pretty stout CFM
rating to keep the mirror formed properly, but not that much actual draw
(if that makes any sense). The max the Fein can pull is about 7.7PSI,
which in the world of vacuum pumps is pretty weak. The tiny vacuum pump I
bought to test with can pull 25 inHG (Fein can do about 8 inHG) or roughly
12psi. Unfortunately the small vacuum pump has such a low CFM rating that
it can't overcome the system losses AND hold the mirror shape.
We're going to be adding reinforcing ribs to the frame on Saturday to
avoid deforming the back and we'll do some caulking & adjusting at that
point to drop the losses as best we can.
If I could afford it, I'd just build one out of fiberglass. That would be
a pretty expensive undertaking though - especially when working with a
plug mold that size. :)
How about using fiberglass with this method:
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