RE: [ATM] Carbon Fiber for Spider Vanes

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

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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