RE: [ATM] Carbon Fiber for Spider Vanes

["Peter" <peter@kitgear.com>]

Hi,

You are probably correct that 3 layers of glass is sufficient. The surface
does seem hard and does not compress after testing as you suggest so I'll
try it without inserts then change if it seems to be a problem.

The next task is to make the rocker box. I'd like to build a low profile
version of the Kriege design so I wanted to ask advice how to attack this
part of the project?

I planned to laminate all flat surfaces including the edges in the same
manner as I made the alt bearings. Once cured I would epoxy the parts
together at the contact edges. The inner contact seams would have another
layer of glass applied to provide added stiffness to the structure.

Thanks,


Peter


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf
Of Dream - Telescopes & Accessories, Inc.
Sent: Saturday, April 09, 2005 1:48 PM
To: atm@atmlist.net
Subject: RE: [ATM] Carbon Fiber for Spider Vanes


Peter,

I am currently working on the back plate for a Mak-Cass.  It is carbon
fiber skins over a sandwich core.  I can clamp it down to work on it with
conventional clamps.  I can also apply the same amount of force that I
would if I were working with wood or metals.  It doesn't compress the core
and/or dimple the skin's surface.  A simple Dob needs no where near this
kind of force to hold it in place, unless you're talking about making a
300-400 lb telescope....

Again, do some tests on the material you are using.  Make up a scrap piece
that uses the same number and type of skins that you plan to or did use on
your bearing.  Lay it up the exact same way: vacuum, no vacuum,
whatever.  Hold as many variables consistent as possible so you're testing
a sample that is as close to the other part as possible.  Then test
it.  Unless you have some very weak foam from home depot, it should stand
up extremely well.

The total mass of your telescope is expected to be under 50 lbs so I'm not
sure why you think your bearings need to take enormous clamping
force.  Maybe I'm missing some design feature.

Epoxy has, in general, the strongest bond of main resin types: polyester,
vinyl ester, phenolic, etc..  It is used on carbon fiber and especially
Kevlar because these two base composite goods don't like to stick to
things.  I've never tried bonding to Delrin.  Just do a test.  Clear the
surface thoroughly and then bond it.  Or contact the resin company and ask
them.  That rarely answers the question though.  Just as in metals,
composites, concrete, etc., there are a great many variations of what
appears to be one product.  Take aluminum: 2024, 7075, 6061 and on and
on.  Even within each type there are subtle differences, from batch to
batch.  Differences a metallurgist needs to know about but not differences
that affect most machine shops or the like.  Each material is like
this.  One may bond well within the same "Delrin" name but another
variation may act like Teflon (x model number/batch...).

Vacuum bagging is not a panacea.  It can actually cause a great many more
headaches than you might think you are facing now.  Try low resin content &
vacuum bagging and things really start to get frustrating, especially
surface quality.

There are vacuum bags that stretch more than typical ones.  Would they
stretch enough to completely wrap around those cutouts, to push the
fiberglass laminate against those inner walls?  I don't know.  There are
other problems that would occur with that anyway.  Wrinkles.

Rules of thumb in composites are few and far between, other than
basics.  When it comes to making parts, especially more elaborate parts, it
is a black art.  Years of experience give a fabricator past experiences to
draw upon.  To choose potentially the best fab method to make the current
part.  There are no set it stone ways of making parts though.  Elaborate
parts are a lot of work unless you want to injection mold them with either
30% fiber or 25% carbon fiber.  That process though is usually very
expensive because of the tooling costs.  Plus, note the resin contents on
what I just listed.  They are 70 and 75% resin, by weight of the final
part!  This high resin content tends to be needed so the material can flow
into the mold and all corners.

Sincerely,
Shane Santi
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833

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