How to bend
Laminate
Bending Laminates
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hi there I'm Tim Johnson senior editor
at American wood
magazine and I'm going to talk a little
bit about bent lamination which is a
process that allows you to bend wood to
create curved shapes they can be a
simple arc they can be an s-shape
undulating whatever you want now here's
a shape a curved shape and you might say
why the heck would you want to go
through a process that sounds difficult
bent lamination when all you really need
to do is go to the bandsaw make a couple
cuts clean it up with sandpaper and
you're done well here's the reason when
you saw this shape you create what's
called short grain and it's right down
there and short grain is weak and it's
likely to split especially if the wood
is put under any kind of stress
now what we've drawn here is the foot
for a shaker style trestle table now
when you imagine all of the way to the
table pushing down on the top of this
cut out piece it's going to create
stress on this short grain and it may
cause it to fail a bent lamination
solves that problem because you take a
number of laminations which are thin
strips of wood and you bend them to the
shape and by doing that you can see that
it eliminates the short grain problem
because the grain actually flows in an
arc so let's talk a little bit more
about the process of bent lamination you
need two things you need the laminations
the thin strips and we'll talk about
them in a minute but you also need a
form and the form has to have the same
shape as the end shape that you want to
get as a result now your form has to be
thick thicker than the laminations
and it has to be stout so I've created
the form using three pieces of
three-quarter inch MDF which I glued
together into a big rectangle after that
I wanted to create this arc shape so I
used a router with a trammel style base
and simply made up
with a straight bit on to keep create
the outside edge of the top arc and then
I moved the position of the indexing pin
on the trammel to cut the inside art and
after I'd routed about halfway down and
went to the bandsaw sawed out the waste
here it is and then I took these two
pieces flip them over inserted a flush
trim bit in my router and cleaned up
this area of waste to get two nice
surfaces now that the form for bending
can consists of just these two parts
alone and the idea is that you clamp one
of them down you put the laminations in
the middle and then you push this part
up to smash them all into this shape but
because you're gluing them you're going
to get glue over all over everything you
don't want to get it on my workbench so
I've added underneath this piece of
melamine so the glue won't stick to that
and to help control this piece as it
slides in before I cut it in into two
pieces I made three passes with the
dataset to create these grooves on the
bottoms of both pieces and into those
I'm going to insert these splines which
are just made out of hardboard and
they're going to guide the second part
of the lamination as I pull it together
and I'll demonstrate how that's going to
work in just a second without these
guides this piece could go virtually
anywhere but now you can see that it's
somewhat controlled and as it gets
closer and closer it's controlled until
it's going to go straight in the last
thing that I've added are these two
strips on the top of this piece that's
going to get clamped to the surface now
these pieces are going to keep this
section of the form from twisting up due
to clamping pressure and this is a
problem that you have if you don't find
a way to control it so once I'm under
the
section of this piece I am NOT going to
move anywhere so this is going to make
it much easier for me to clamp these two
pieces together alright let's talk about
the laminations no as I said you start
out with a board like this and we're
going to joint one face of the board and
joint one edge so it's 90 degrees to the
face now we're going to take this board
and run it through the bandsaw using the
fence and a resaw blade and if your
bandsaw is set up for wrestling
accurately set up and has a good blade
in it you can cut the wood very thin and
you can almost go from cutting directly
to laminating the thickness of the
laminations that you need to cut depends
on a couple of things the radius of the
curve you're going to bend and the type
of wood that you're using some types of
wood are more flexible than others and
so to increase the flexibility of the
board of the lamination you have to make
it thinner now when you resaw the
laminations you've got a jointed edge
and after you've sawn it you have a sawn
edge to cut the next lamination yury
joint the surface of the board and then
make a second pass so what you end up
with are a series of laminations that
have one jointed surface in one sawn
surface and as I said if you have a
resaw that's set up properly for reefs
on you can get away with stopping there
but oftentimes you have to clean up the
back surface of the board and the way
that you can do that there are a couple
ways one of them is to run them through
your planer and when you're milling
lumber this thin that's pretty risky
you're likely to get a lot of failure so
you have to use a support that allows
you that's a thick piece of MDF kind of
like this that supports this thin strip
as it's being sent through the planer
underneath the pressure roller so that
they can't cause this piece to flex this
is
when you run into problems one other
thing that you should do with these
pieces is to flex each one of them to
make sure that you don't have a section
of wood where there's a fracture
something that will cause it to crack as
it's bending and even though the board
looks good you may find some pieces that
have these problems this board right
here there's a knot and I don't know
what that's going to look like inside
the board so when I saw through the
surface it there may be something really
bad going on down in here which may
cause that particular lamination to fail
so it's not unusual to lose some of the
laminations as your as you're cutting
them you may have noticed that I've got
a couple lines drawn on the face of this
board now these are lines that are going
to help me reorient the laminations
after I saw them so I put them back in
the same orientation that they came out
of the log so I haven't flipped them
over and have the grain running in
opposite directions this will just help
me get a more uniform Bend on the
laminations another thing that we need
to know about the laminations is that
you have to start out thicker than your
end thickness is going to be and that's
because as the laminations get crunched
together in this form they have a
tendency to swim around a little bit so
the surface won't be dead flat so you're
going to have to joint and plane these
two phases after the pieces been
laminated so you're going to lose a
little bit of thickness so the piece
that we were talking about with the big
arc shape that we're going to make will
be an inch and 3/4 thick so we're
starting out with two inch thick
material now another thing that that
these little runners do is to raise each
lamination off the bottom of my form and
that's going to allow the glue to kind
of get out it won't get smashed onto the
face so that gives me a little bit of
clearance and it also serves to raise
this lamination up into the center of
the jig so it's it's going to help it's
going to limit the amount of movement
that I'm going to get in that piece as I
glue it together
now these laminations I talked about how
you can you can make them thinner and
make them more pliable and another thing
that you can do to make them more
pliable is to run a sponge with damp
water or with water on them so you
dampen them and then let them they'll
look wet let them dry out they'll still
have a lot of moisture in them but when
you add water it increases the
flexibility and there's not a problem
with adding water if you're going to use
polyurethane glue to glue the pieces
together which is what we're going to
use because polyurethane glue draws
moisture from the air and from the piece
to cure if you're going to use another
kind of glue an epoxy or a different
type of resin glue or even a yellow glue
you'll need to let those pieces dry
overnight before you glue up if you
dampen them let's take these pieces over
to the form and these pieces are exactly
two and a half inches thick when I press
them together and that's the thickness
of the piece that we want so I'm going
to place them in position on the form
and I'm going to do what's called a dry
fit just to make sure that everything
works and this is you know when you're
gluing up a complex assembly it's always
a good idea to do a dry run so
I'm going to use quick grip clamps for
this which I've discovered are excellent
for this type of process and I'm going
to use three of them
and I'll use them in sequence I'm going
to start out I'm getting these guys set
up and I really don't need to I'm going
to start out with Center one and I'm
just going to put some pressure on to my
laminations you can see them start to
flex and now I'm going to apply the
other two clamps and now I'm going to
draw the two outside ones this will
loosen the inside clamp I'm going to go
back to the inside clamp draw it tight
collect my outside clamps and I'm
watching the laminations Bend that I'm
checking to make sure that they don't
rise up here they've rised up a little
bit and I'm listening to hear if any of
them crack if they fracture we're going
to have to stop and take them back out
pulling this one up in the middle is
kind of like my safety and getting to
the point now where the laminations are
going to move into the curve portion of
the jig everything's still going well
I'm going to tighten the center one one
more time and I'm getting close to the
end now and I really do want to keep the
pressure on these outer laminations I'm
going to draw them tight to the curves
first and that kind of automatically
draws the center to the curve I'm
looking things over and it looks to me
like I've drawn nice and tight
I didn't hear any fractures and what
that means is that these laminations and
this process is going to work so my next
step is to unclamp these and then start
applying glue okay now we're ready to
glue these laminations together now the
first step is to protect your workbench
table with wax paper or using a roofing
paper here something so that the glue
isn't going to mess up the surface of
your workbench and then I've laid out my
laminations individual individually and
sequentially starting from the one
that's going to be the bottom and then
working all the way to the one that will
be just underneath the top this is the
top piece and I'm not going to put glue
on that what I'm going to do is roll
glue onto this entire section and then
I'm going to stack the pieces so we'll
have glue on one side of the joint
sometimes some glue manufacturers
recommend using glue on both sides of
the joint we're just going to do it on
one side for the sake of this
demonstration the glue I'm going to be
using as I mentioned earlier is a
polyurethane type glue now you're
probably familiar with gorilla glue
they've made made their name with
polyurethane glue but there are many
other manufacturers of polyurethane on
the market polyurethane glues have a
real advantage over yellow glue and a
bent lamination because they're rigid
they don't creep and a yellow glue over
time because it's a little bit elastic
you can have the laminations move ever
so slightly and so when you run your
finger down the edge of the table or of
the leg you'll you'll see you'll be able
to feel ridges maybe it's not a huge
deal but if you're a perfectionist using
polyurethane glue which is more rigid
will give you a
slightly better success another
advantage of polyurethane glue as I
mentioned earlier is that it actually
draws moisture out of the air and out of
the wood to help the joint cure so that
a lot gives you the opportunity to
dampen your laminations to make them
more flexible now you should dampen them
let them sit for about an hour so they
dry off so they look like they're dried
they'll still be moisture on the surface
and then proceed to gluing now I've put
some of my polyurethane glue into a
little roller pan and I'm simply going
to roll it on all these surfaces this
isn't rocket science it's kind of
tedious as a matter of fact just want to
get a uniform coat of glue on every
lamination okay now I've got the glue
spread on all the laminations a nice
even spreading one of the advantages of
polyurethane glue is that it has a 30
minute open time which means I've got
plenty of time to roll all these pieces
out and take a look at them and make
sure I've got a nice uniform coat of
glue on all of the layers as you know
yellow glue gives you about five minutes
do that'd be much much more difficult I
also mentioned earlier that some
manufacturers tell you to put glue on
both sides of lamination some say you
only need to do it on one side so if you
wanted to put glue on both sides of
these laminations
since we start this is our topmost
lamination the one that we didn't put
any glue on and it's going to go on this
piece so the way I would do both
laminate both sides is to put glue on
the bottom of this piece so I'm going to
do that and put it on the bottom side of
this piece
I'm almost there and then I'm simply
going to flip over flip it on top of
this lamination which is the second one
now I flip these two back over and I'm
ready to put another coat of glue on
this side and then I'll flip this piece
over and we'll keep going
as I said I'm only going to put glue on
one side of the laminations and another
thing that this manufacturer recommends
is that if your wood is really dry the
conditions are really dry or the wood is
dry they recommend just misting the
glued surface with a little bit of water
and here you're you're adding some
moisture to help the glue cure so we've
added just a little tiny bit and now my
process for stacking is to start at this
back-end and just literally stack these
pieces
again this would take if I was gluing
both sides of these pieces it would take
longer but again we've got plenty of
open time you can see how the glue is
starting to foam up a little bit if
you're familiar with polyurethane glue
you're familiar with the fact that it
foams as it cures it's one of the
downsides of poly urethanes it used to
be problem than it is now manufacturers
have worked really hard to to reduce the
foaming and it's actually a an
advertising point for many of these
glues I've got a nice stack and I'm
checking to see all my glue lines and I
want to make sure I put this piece on
correctly and now we're ready to move
make sure these are all nice and flat
get them started right we're ready to
move back to the jig and one thing that
I want to make sure to mention is that
because of this sticky glue you want to
make sure that you've got all the
surfaces coated with wax so that the
glue won't stick so the things won't
stick because as you know this is this
is pretty powerful glue I have no glue
on the top of the top piece and no glue
on the bottom of the bottom piece it's
time to position them in our form and
now we're going to pull out our handy
clamps
and get ourselves started I've got the
laminations in place and I'm starting to
draw them together and I'm kind of
watching these ends to make sure that I
go they stay symmetrical from end to end
so I've got a hamper to help me with
that and again I used the two outside
clamps and then come back with the
inside clamp it's kind of my safety soon
as I feel the outside clamps release I
go back to them okay we'll draw this one
again you can see that the polyurethane
glue is started to foam going to make
sure that I'm staying nice and flat on
my runners keep those laminations as
uniform as I can so far so good
we're going to clamp from the outside
instead of the center because this is
the toughest part of the curve to draw
close to and you watch as I draw in on
the outside will draw tight to the
center there we go one last pull there I
can see that I've drawn nice and tight
all the way around we're done
