bend

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