Random Order of Tips and comments.....
1) Cloth and Paper carriers
2) Sheared Cloth, How to and why.
3) Carbon Sanding (Health hazard)
4) Composite Sanding (Jay Decker)
5) Kevlar Sanding (Avoid the Fuzz)
6) Carbon Composite Cutting
1) Cloth and Paper Carriers
Lightweight cloth cut on the 45/45 bias (as it is mainly used in DLG
construction) will stretch very easily. Backing it with a paper layer (template)
will help it hold it's shape until needed, then the paper can be removed.
The immediate problem is if you stick the paper too well it won't come off,
so then the cloth gets pulled out of shape. The solution was 3M77 (the de-facto
glue for 99% of DLG work) but too much of this would cause problems. The new
solution is 3M Post It Glue Stick (described as much less messy, no over-spray
and the right amount of stick) but these don't seem to be in the UK yet.
Mark Drela -15th Dec 2003 - SALGlider
5044
Another way is to use plain light paper, like
newsprint, and apply Post-It glue stick around the perimeter. Let dry a bit,
then stick down on the Kevlar, and cut out the shape. Roll resin onto the
mylar, put down the kevlar/paper, and peel off the paper. Roll on more resin
to wet out fully.
I've used both methods, and the Post-It
method is much better. The problem with 3M-77 here is that spraying is always
messy, and it's easy to spray too much which will prevent easy release. The
Post-It glue stick consistently has just the right amount of tack.
Mark Drela - - SALGlider 5050
"Post-It" adhesive is intentionally very low
tack. In glue-stick form it looks, feels, and smells a lot like soap. When
applied it's somewhat slippery, while normal glue sticks are more sticky and
gooey. Post-It also does not dry out.
It's made by the US company 3M, which also makes
Scotch-brand tapes. Although it says "made in Korea" on the stick. I
don't know if it's available in the UK. It says the following on the stick,
which might help you to locate it:
3M IPC
Cat. 6307
34-8506-0787-9
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2) Sheared Cloth
Basically you pull the edges so that the fibres no longer cross at 90 degrees
but have more of a directional component. The terms 45/45 and 20/20 are often
used. The 45/45 denotes un-sheared cloth with the fibres at 45 degrees to
the axis of the part (Normal wing skins, etc) which gives good torsional
stiffness. The 20/20 denotes sheared cloth with the fibres running more along
the axis (used in Tail Plane fronts to give greater bending stiffness)
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3) Carbon Sanding
A recent thread (as at 25th Feb 04) on SALGlider and RCSE explains the health
hazard of sanding carbon fibre. Mainly due to small sharp particles becoming airborne. So the advice is to wet sand where possible and wear a proper face mask
if you have to dry sand. This is only a synopsis, you should read and understand
the full thread.
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4) Practical home shop safety guidance
for sanding composites
Jay Decker -
17th Feb 2004 - SALGlider
5524
1. Just to be clear, any type of small particles can be bad
for your health. Small particles in any significant quantity that get in your
lungs can be detrimental for your health, whether they come from carbon fibre
(CF), Fibreglass (FG), Kevlar, polystyrene foam, or balsa dust. People who
smoke are also more susceptible to having problems.
2. Wet sand where you can. When working with composites,
particularly CF, I look for ways to shape the composite by wet sanding or
cutting wherever I can. For example, I tried wet sanding with my Permagrits
(tungsten carbide brazed on to steel strips, sold by Hobby Lobby and others),
it works very well and I do it all the time now.
3. Perform dry sanding tasks, e.g., rounding the CF corners
of “Drela spars”, outside if possible.
4. Use a filtered vacuum for dust collection when using
power tools. I clamp the hose of Shop-Vac to collect as much dust as possible
when using cutting CF tools like the Dremel cut-off wheel.
5. Make big “particles” first if possible. When I do
things like hog down gross flashing, I start with a tool, e.g., a Surform,
that cuts small “chunks” off, rather using the power sanders and making a
lot of dust.
6. Plan ahead and sand less. The more I get into composite
construction I find that I can reduce or eliminate sanding composite
materials. For example, when bagging CF skins on to foam cores I cut the CF so
that the edge of the material lands on leading edge material and doesn’t
overhang. Yes, I have to plan it out before hand and it does take a little
more time to cut the fabric, but it also saves me more time post bagging. I am
now to the point with CF wings where I can peal the Mylar, trim the trailing
edge with a knife and just do a little touch up wet sanding.
7.
House keeping helps. Wipe up the dust you create so that you do not re-suspend
or track dust somewhere else to be re-suspended. When cleaning up composite
dust, I wipe up CF dust with a wet paper tower and I use a filtered Shop-Vac
for balsa, FG, or Kevlar dust. This also helps keep me in the good graces of
the local purvey of pleasure and pain. :o)
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5) Kevlar Sanding
Here the problem is more aesthetic than health related (but don't go
breathing Kevlar or any other dust for that matter!). The Kevlar tends to fuzz
when sanded and the main tips are:
a) Don't sand it.... i.e. work out you construction so that you do
not need to sand Kevlar. Place a thin glass layer outside the Kevlar if the area
will need a finish sanding (i.e. on Leading edges).
b) Sand with light pressure and hit the area with foam safe CA if
it starts to "Fuzz Up"
c) Use 3M....grade... sanding paper.
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6) Carbon Laminate
Cutting
Mark Drela - 13th Aug 2003 - SALGlider 4358
Re: Carbon laminate handling ideas please....
In response to Kevin Sharbonda's comments that :-
Seem to be fighting trying to trim the .014 laminate for
spar caps with the taper! Relish any ideas or guides for handling this stuff
and trimming it to required tapers. Sure don't want any nasty splinters or
cracked edges.
Mark Wrote :-
You need a good metal straightedge, preferably steel. Put
pieces of double-stick tape on it to prevent sliding on the slick carbon --
one piece of tape at each end and another one in the middle works OK for
30" sparcaps. Add more tape if in doubt.
First, press the straightedge down near the edge of the
carbon stock and make a truing cut. I use a fresh single-edge blade, and make
repeated light cuts until I cut through. The first pass is the most important,
since it makes the initial score which tends to be followed by the subsequent
passes. The carbon chews up the blade very fast, and I normally use a fresh
blade corner to start each new cut. A snap-off knife would probably be more
economical here, but single-edge blades aren't too expensive.
To cut a tapered sparcap, I carefully mark off the spar
width at each end by pressing down with a razor blade to make a nick, using a calliper
to measure off the width. To position the straightedge, I hold a blade in the
nick, and bring the end of the straightedge up to it. Repeat on the other end.
This may disturb the end I just set, so a few iterations between the two ends
is usually needed. Once both ends are aligned with the nicks, I press down
firmly on the straightedge so that the double-stick tape grabs onto the
carbon. Then I make the cut using the light multiple-pass technique.
When cutting at a shallow angle the blade strongly wants to
follow the fibres. To keep it on track, it is necessary to firmly press the
blade against the straightedge, with the fingers down very low on the blade
(my blade-holding thumb actually slides on the straightedge). While making
such a cut, you also need to very firmly press down on the straightedge itself
close to the blade to prevent it from slipping, even with the tape. I'll slice
for only a few inches at a time right next to the press-down fingers.
When finished cutting, I'll wet-sand the edges lightly in
the sink with a 220 sanding block to eliminate most of the jaggies.
I suggest that you cut the caps several inches too long. If
they end up slightly mismatched in width, you can minimize the difference by
trimming the wider cap on the wide end, and the narrower cap on the narrower
end.
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