On Wed, 20 Oct 2010 12:34:28 -0700 (PDT), robgraham
wrote:

Ditto - and it does explain why I've had Al joints fail despite
cleaning, abrading, etc. Interestingly I have not had a failure with
PU glue on Al joints, admittedly in low stress situations.

What does a 'precise gap' mean; do it take it that firm clamping is
also not recommended ? Are you suggesting that the bond layer should
have minimum thickness and not a minimal thickness ?

Clamping is good but you need to maintain a certain thickness in the
glue line whilst keeping the parts concentric or spaced by a certain
distance depending on the adhesive used. This ought to be engineered
into the design of the joint but often isn't.

A typical epoxy, and indeed some polyurethanes ideally require a joint
gap of around 0.05 - 0.2 mm or about 2 - 8 thousandths of an inch.
With some consumer products, and with repairs you'll often have much
bigger gaps than that and hence more reliance on the gap filling
properties and thus a greater tendency for joint failure.

When joining two flat surfaces thin shims would be fixed in a grid
pattern to the surface with appropriately distributed clamping in
order that a consistent glue line is maintained.

On circular parts you would ideally have a recess in one or both of
the parts such that there was almost full annular contact between the
two parts you were joining (and thus almost no glue) whilst in the
recess would be the glue that joined the two parts together. Vent
paths for air and glue are then needed when assembling these tightly
fitting joints.

Fitting pins across the joint to lock the two parts together is a
bodge that really isn't needed unless required for fail safe purposes.



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