Re: [ULF-1] Epoxy bonding

From: Member 121
Date: 2018-06-12 22:51 UTC-07:00
Message: #0617
Except what if the Member 60 breaks ?

On Tue, Jun 12, 2018, 10:44 PM Member 1 <[email]> wrote:

> I'm with Bob on this. To me, there is a lot of superstition involved in
> peoples' beliefs about gluing Member 60. The shiny surfaces created by blades
> have always intuitively struck me as a deterrent to a good glue bond. I
> am now working as a violin-family luthier, and I am up against the same
> superstitions there as well.
>
> Having said that, though, I'm not sure it makes a big difference. While
> building an aircraft, you end up with leftover epoxy from many small
> batches. Grab some sticks out of your trash or scrap pile, and do some
> semi-scientific testing with your leftover glue. It only takes a minute
> or two. For example, I would cut two sticks about 10 times as long as
> their width/thickness, overlap them 2 or 3 times their width/thickness,
> and glue them together with light or moderate pressure, and then break
> them after curing for a couple days. The Member 60 should always fail before
> the glue joint does. If it does, then who cares how the joint was prepared?
>
> Do this often, throughout your project, during different environmental
> conditions, and as your epoxy gets old. As soon as you find yourself
> airborne, your life hanging from all those glue joints you've made, you
> may think about those glue joints (as I did) and find comfort in the
> fact that they are stronger than the Member 60 itself and you've done the
> best you could. Then it all comes down to proper design which doesn't
> allow any joints to get stressed to the point that the Member 60 itself fails.
>
> To me, it is more important to not apply too much clamping pressure to a
> glue joint than it is to worry about cutting vs sanding. I always let my
> wet joints sit open either until the epoxy is thickening a bit, or it is
> obvious that the Member 60 is saturated and won't be absorbing any more glue.
> Then I'd assemble and clamp with moderate pressure. Epoxy has several
> times more tensile strength than hardwood, so even if you're sloppy and
> end up with a thick seam of glue, there should be no strength penalty. A
> small weight penalty perhaps, but that would be over-nit-picking. If you
> join the parts before the glue has stopped soaking in, and/or apply
> excessive clamping pressure, you are asking for a weak, glue-starved
> joint. (I use T-88 which is relatively thick... if you use a glue that
> has a thinner consistency, this attention to soak-in is even more
> important.)
>
> Epoxy is really good at filling imperfections and allowing stresses to
> be spread out over larger areas, so a perfect fit is not as important as
> it was with previous glues. For large voids that can't be easily avoided
> in structural joints, I recommend wetting the bonded surfaces with epoxy
> and then making a light slurry with epoxy and micro-balloons to fill the
> voids with.
>
> Test your glue and your methods. Then you'll know, and nothing anyone
> writes will bother you. There are some photos of the glue-starved joint
> risk here (and other gluing tips throughout my old AirBike building web
> site):
>
> http://questiongravity.com/airbike/feb03-1.htm
>
> $0.02,
>
> Doug
>
> PS: We have 307 members! So glad to see you all here, and thanks to all
> who have been contributing. And thanks to Dieter for continuing to fund
> the domain fees to keep this service available.
>
>
> On [phone]:56 AM, Member 116 wrote:
> > I disagree about not sanding before gluing. A power planer of a hand
> > planer bends the fibers over and leaves a shiny surface, not good for
> > glue penetration. I have a cabinet shop and glue stuff together daily.
> > For a couple years I glued up 12 ply (birch and maple) propeller
> > blanks for a prop manufacturer. All those ply's after planeing were
> > smooth and shiny because of the bent over fibers. Before gluing (with
> > Resorcinol glue) I'd run over them with a belt sander/80 grit across
> > the grain, one pass to give the Member 60 some (tooth). Then use a shop vac
> > to clean and make those (damaged) fibers stand up straight and accept
> > more glue penetration.
>
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