Odd Jobs
I find myself, driven by a need to keep the wolf from the
door, doing some odd things from time to time. I describe
myself as an “Industrial consultant” which helps
put the chargeout rate up a bit but I tell my friends that
I am an “Odd job man”.
One of the more interesting “odd jobs that I have
had of late is helping a friend rewrite some boating industry
training guides. Guides that needed to catch up with the technical
and social changes affecting the training of boatbuilders.
An interesting job and one in which there is a lot of crossover
between my life as a designer, and my Industrial Consultancy
work, which is mostly in the field of wood remanufacturing.
One of those guides covers the making of laminated components
from wood. Simple? Slice up some bits of wood thin enough
to go around the curve, make a jig of some sort, smear on
the glue and clamp it all into place until its not sticky
any more.
Yeah! Simple! Fight with a dozen wobbly thin sticks all slippery
with epoxy, all trying to escape , trying to clip you behind
the ear as they spring loose and covering your best Levis
( if I get this done now before we go out Dear, it will be
ready for me to fit in the morning) with permanent stripes.
Actually it's worse than that. I, as mentioned, do a lot
of work in the industrial end of the wood business, and commercial
lamination is a large part of that work..
When building a boat one hopes that the glue holds, as the
result of failure may lead to wet feet, wet right up to your
topknot! But a failure in a structural beam in a critical
part of a several story high structure can endanger large
numbers of people, and failure cannot be countenanced so there
is a quite rigid set of testing requirements that go with
the certification that a manufacturer must have.
A part of the process that few give consideration to, even
among the big manufacturers, is the actual method by which
glue bonds wood and the effect upon that bond of the surface
preparation of the glueline.
I have on several occasions been engaged to trouble shoot
situations where the glue tests within specification, where
the process (such as mix ratios, clamping pressures, curing
times and temperatures) is fine but there are occasional test
failures.
Glue manufacturers are often blamed, and must go to every
effort to try and find the source of the problem to avoid
liability. That’s where I get involved.
Over the years I have learned a lot about this subject, and
when I came across this very issue in the training manual
that I am currently working on (or should be, it’s a
couple of days past the deadline but I know that the guy dealing
with it hasn’t finished with the previous one I sent
him so I’m ok) I though that I would put it into column
form for Duckworks readers to mull over.
It is in “training manual “ language so is a
bit pedantic, but otherwise, I hope it is helpful.
Preparation of surfaces for glueing
* Note, a “Lamell” is a strip of wood to
be included in a laminated component.
Surface preparation of the glued faces is critical to the
success of laminating. Practically all of the structural
glues used are mechanical bonding agents which means that
the resins and their reinforcing materials penetrate the
open cells of the wood to be glued, then set hard so keying
the resin to the wood on each side of the joint. Surface
preparation should be of a nature that preserves the openings
into those cells cut during the preparation of the lamell.
Scientific tests have shown that the best surface for glueing
is a freshly hand planed surface cut with a very sharp edge,
however sawn surfaces cut with a very sharp circular saw
blade, planed with a very sharp power planer ( buzzer, jointer
or thickness planer) are almost as good.
Hand sanding or sanding with hand held power
sanders is a less reliable but acceptable method of preparation
but there are some common methods outlined below which are
not satifactory and which may lead to failure of the glueline.
Wood purchased dressed or machine planed four sides has
normally been dressed in a “four sider” or Planer
Moulder. If run past its optimum knife life this type of
machine, as with a “buzzer” ( jointer) or “thicknesser”(
planer) heats and burnishes the surface of the wood leaving
a very smooth glossy surface with a very good appearance.
This appearance can deceive the user as it may be that the
burnishing and heating has caused the lignin that binds
the wood cells together to flow closing or filling the cut
open cells preventing the glue from penetrating and making
an effective key. Note that this is a known and serious
problem in the production of industrial laminates such as
structural beams.
Note also that in wood with a high resin content, (many
common softwoods are in this category) resin may bleed into
those open cells causing a reduction in the strength of
the bond. It is good practice to prepare laminating feedstock,
in fact to prepare any wood surface to be glued as soon
prior to glueing as is practical. Certainly no longer than
24 hours prior to the application of the glue.
This means that it is good practice to resurface the outside
edges of machine finished stock before inclusion in a laminate.
Industrial weight power sanding machines of
the type used for finishing panels may also produce a surface
not ideal for the penetration of glues. Much the same issue
arises, that of blunt cutting edges which by virtue of massive
power and pressure are able to produce a surface which looks
acceptable but which may not glue reliably. This is particularly
common with plywood and any such surfaces should be carefully
checked and possibly prepared with cabinet scraper, random
orbital sander or some other suitable means before inclusion
in a glued structure.
