How to Build a Torsion Box

BYS Torsion Box 5F00 lead How to Build a Torsion Box

How to Build a Torsion Box

It’s amazingly
strong, light. . .
and cheap!

By Alan Schaffter

Torsion Boxes–the Real Story

“How can something made from such thin wood be so stiff?” That’s what everybody
asks when they first meet a torsion box. The engineering principles behind a torsion
box are pretty simple; even so, there’s a lot of misleading information about torsion
boxes, particularly on the Internet. Without getting too technical, here’s what you
should know:

• A torsion box works like an I-beam (see right). It’s almost as strong as it would be if it
were made from solid material–but it’s much lighter and less expensive.

• The thicker the torsion box, the stronger it will be–by a lot! If you increase a torsion box’s
thickness by 25%, for example, it will be 100% stronger. If you doubled the thickness, it
would be eight times stronger. Strength increases by the thickness cubed.

• The stiffness of the skins isn’t that important. A thick material, such as 1" plywood,
doesn’t necessarily make a stiffer box than using thin material, such as 1/4" plywood.
Thickness does matter in another way, though: thin material will deflect more easily
where it’s unsupported, in the spaces between the web pieces. If you’ll be pounding on a
torsion box, clamping things to it, or setting heavy objects with narrow feet on it, a thick
skin is better than a thin skin. Or you could space the web pieces closer together.

• The thickness of the web pieces isn’t all that important, either. They can be relatively
thin and lightweight, as long as they resist stretching and compression. For an MDF box,
such as the one in this article, I usually space the web pieces 6” to 8” apart. The web
pieces must be well-glued to the skins to prevent the skins from buckling, though. That
means that the web material shouldn’t be too thin–it must be thick enough to have a
sufficiently wide glue surface. You don’t need to use a special glue to assemble a torsion
box–a PVA (yellow) glue works fine. And to clear up two misconceptions on the Internet:
the webs don’t have to look like miniature I-beams, nor do you have to cut dados in the
skins to receive the webs. Torsion boxes don't need to be that complicated!

Click any image to view a larger version.

Strong, but light

BYS Torsion Box 5F00 strong but light How to Build a Torsion Box

This huge torsion box is 8 ft.
long, but it easily holds 300
lbs. of bricks and deflects less
than 1/2". It weighs only 30
lbs., and is made from inexpensive
3/16" hardboard.

How an I-beam works

BYS Torsion Box 5F00 i beam works How to Build a Torsion Box

When you press down on a solid
beam, you actually compress its top
section and stretch its bottom section.
These compression and tension
forces are neutral at the beam’s center.
In an I-beam, most of the center
is removed, to save material and
weight, but the forces still act in the
same way.

A torsion box is a
remarkable piece of
engineering. It’s stable,
light and uses a minimum
amount of material, yet it’s extremely
strong. The idea has been around for
years, and you probably have one in
your house: hollow-core doors are torsion
boxes.

Making a torsion box doesn’t
require any specialized equipment
or skills–a guy with a small shop and
a decent tablesaw can easily handle
the job.

When I needed a flat, solid, durable
and inexpensive top for an assembly
table, I planned on making it as a torsion
box. I researched the subject and
found a lot of conflicting information.
I also talked to some experts–and
learned a lot.

In this article, I’ll show you how I
built that assembly table top, but you
can use this method to build a torsion
box of any size or composition. If you
stretch your imagination a bit, you can
find many ways to use torsion boxes
in furniture making: they can be desk
or dining table tops, pedestal ends,
shelves and, of course, doors.

 

Materials

A basic torsion box is composed of
two types of parts: skins (the top and
bottom) and webs (which form an
internal grid, and include the sides of
the box). Skins are usually made from
an engineered material, such as plywood, MDF or hardboard. The webs
and sides may be made from engineered
material or solid wood. The
web pieces do not need to interlock,
as mine do, but it is quicker and easier
to make the grid that way. Plus, the
box will be stronger.

I used 1/2" MDF for all three parts.
Though heavier than plywood, MDF
has a more consistent and uniform
structure. It is flat and stays flat, if
properly stored. It has no internal
stresses, resists compression and tension
along the surface, machines easily,
holds glue relatively well, and is
relatively inexpensive. Using the same
material for all the parts minimizes
the amount of sheetstock you have to
buy, and gives you more flexibility in
cutting it up.

MDF that’s 3/4" thick would work,
too, but there’s no significant advantage
to it (see Torsion Boxes–The
Real Story, above). Material that’s
1/2" thick strikes just the right balance,
I think–it’s thick enough so you
can safely nail into its edges, but thin
enough to make a large table that's
light enough to move.

 

Cut the web pieces

1. You will need only two sheets of
1/2" MDF to make the skins (A), web
pieces (B and C) and sides (D and E).
Break down the sheets into manageable
pieces (Fig. E), then cut all the
parts to exact dimension (see Cutting
List, below). Rip the web, sides and
leg socket reinforcing parts (F) at the
same time to ensure they’re exactly
the same width.

2. Set up your tablesaw to cut halflap
joints in all the web pieces (Photo
1). First, adjust the width of a dado set
to just a little bit over 1/2", or whatever
it takes so the parts will easily
slip by each other. Make an indexing
jig to space the notches (Photo 2
and Fig. B). The jig is just a fence for
your miter gauge with a small notch
cut into it, plus a pin to fit the notch.
Cut the notch after you’ve adjusted
the width of the dado set. Set the
distance between the indexing pin
and the blade (for this web, it’s 7-1/4"). Raise the blade to cut a notch 1/32"
deeper than half of the pieces’ width
(for these 2" wide pieces, make the
notches 1-1/32" deep). You don’t want
the notches bottoming out when the
web is assembled.

3. Cut the half-lap joints (Photo
3). Make the first cut with one end
against the alignment pin. Slip the
first notch over the pin and cut the
next one. Repeat the process until
all notches are cut in all web pieces.
The distance between the last notch
and the end of the web piece may
be slightly different than the spacing
between the other notches, but
this is not a problem. Mark an "X" on
all pieces at the end you started cutting
from. Place all the "X" ends in the
same direction when you assemble
the web.

 

Build a construction platform

4. To make a torsion box that’s
flat, you must build it on a flat surface.
The best strategy is to make a
temporary construction platform that
you can adjust to become absolutely
flat (Photo 4). All you’ll need is a pair
of sawhorses and some straight and
knot-free 2x4s (Fig. C). Joint one edge
of all the 2x4s and run them through
the planer so they’re all the same
width. Place the sawhorses approximately
3' apart. Secure them to the
floor with screws, nails, hot melt
glue–whatever–so they won’t move.
Add bracing, if necessary, to make
them stable and rigid.

5. Place the long 2x4s on the sawhorses,
then use hot-melt glue to
attach the short 2x4s on top. Make a
pair of winding sticks by jointing and
ripping two thin boards that are 2" to
3" wide by 4 ft. long. Paint one board
white and the other black. Place the
winding sticks on the first and last
supports. Sight across the winding
sticks (a white backdrop helps) and
insert shims under the long supports
until the top edges of the winding
sticks are parallel. Without disturbing
the platform, securely attach the long
2x4s to the horses with hot-melt glue.

6. Place both of the skins (A) on the platform–this will give you a large,
flat area to glue up the web and sides.
Cover the topmost skin with thin poly
sheeting to protect it from glue drips.
Stretch the sheeting tight and anchor
it with tape or thumbtacks.

 

Assemble the web and sides

7. Join one short and one long side
(D and E) with glue and brads (Photo
5). Align and clamp these two sides to
the edges of the skins. Place the long
webs on the skins with the notches
facing up. Slip one or two short webs
in place to prevent the long webs
from falling over.

8. Glue the web, starting from one
end (Photo 6). Make sure the tops of
both pieces are flush at each joint.
Continue working towards the opposite
end; apply glue and add one short
web at a time. Place weights on the
web as you complete each row.

9. Once all the webs have been
glued, slide the grid away from the
sides. Apply glue to the ends of the
webs and position the grid back
against the sides. Shoot a few brads
through the sides and into the ends of
the webs. Attach the remaining two
sides with glue and brads. Remove
any glue from the top of the webs,
check that the grid is square, and let
it dry.

 

Build leg sockets

10. This is an optional step. There
are many ways to attach legs to a torsion
box top–I chose to house them in
sockets built right into the top. Begin
building the sockets by adding reinforcing
pieces (F) to the web’s sides
(Photo 7).

11. When the glue is dry, temporarily
place a leg in the socket and glue
and clamp the remaining reinforcing
pieces (Photo 8). Remove the leg
before the glue dries.

 

Add the skins

12. Draw layout lines on the bottom
skin for attaching it to the grid
with brads (Photo 9). First, remove
the grid and both skins from the platform.
Replace the skin with plastic on it and put the grid on top. On the
grid’s sides, mark the centerlines of all
the web and reinforcing pieces. Mark
the centerlines of each leg socket on
the sides as well (you don’t want to
shoot brads here). Place the remaining
skin onto the grid and align two
adjacent edges of the skin with the
sides of the grid. Transfer your marks
from the sides onto this skin, then use
a straightedge or large square to connect
the marks. This skin will be the
bottom of the torsion box.

13. Glue the skin to the grid (Photo
10). To begin, remove the skin and
place masking tape inside the leg
sockets, to prevent glue from sticking
here. Apply a liberal amount of glue to
the edges of all sides, webs and reinforcing
pieces. Work quickly and don’t
be concerned about drips or using too
much glue. Place the bottom skin on
the grid and align the same sides you
used when laying out the nailing lines.
Shoot 1-1/2" long brads, spaced every
3", along the layout lines and along
the sides (Photo 11). Place weights on
the skin to keep it flat.

14. Drill and rout the skin to open
up the leg sockets (Photo 12). The top
I’m making also required additional
pieces (G) to accommodate bolts that
secure a set of leg braces. To install these anchor pads,
turn over the top and glue them to
the bottom skin. After the glue is dry,
drill holes through the skin and pads
and install T-nuts in the pads.

15. Remove the plastic from the top
skin and mark it using the same procedure
as you followed for the bottom
skin. Glue the skin to the grid. Use a
router and flush-trim bit to make both
skins flush to the sides all around.

 

Add the facing and laminate

16. The torsion box is basically
complete at this point, but I added
hardwood faces (H and J) to the sides
and plastic laminate (K) to the top.
You can butt the faces together, miter
them, or make box joints, as I did. I
attached the facing to the torsion box
with glue and brad nails (Photo 13)
and trimmed it flush to the top.

17. Glue on the plastic laminate
(Photo 14). If you want to avoid working
with laminate, you could make
the top skin from 1/2" MDF with
one melamine face. It’s not quite as
durable as laminate, but better than
plain MDF.

18. Sand and finish the facing and
the bottom skin.

 

Add the legs

19. The legs that I designed for
this table are removable. They’re fastened
to the top with bolts that pass
through the box’s faces and reinforcing
blocks. The bolts thread into
T-nuts on the back side of the leg.
Make a drilling jig (Fig. D) to position
the bolt holes and to ensure that the
holes are perpendicular to the legs.
The jig has two holes, one for each
side of the leg. Before drilling, mark
the holes’ locations. Counterbore
the holes so the bolts’ heads won’t
protrude. Then drill the holes for the
bolts, going into the legs (Photo 15).
Finish drilling the holes through the
legs at the drill press.

Cutting List

BYS Torsion Box 5F00 cut list How to Build a Torsion Box

Fig. A: Exploded View

BYS Torsion Box 5F00 fig a How to Build a Torsion Box

Fig. B: Web Notch Layout

BYS Torsion Box 5F00 fig b How to Build a Torsion Box

Fig. C: Gluing Platform

BYS Torsion Box 5F00 fig c How to Build a Torsion Box

Fig. D: Drill Guide

BYS Torsion Box 5F00 fig d How to Build a Torsion Box

Fig. E: Plywood Cutting Diagram

BYS Torsion Box 5F00 fig e How to Build a Torsion Box

BYS Torsion Box 5F00 1 How to Build a Torsion Box

1. Inside this
torsion box

there’s a grid
of interlocking
web pieces.
Adjust the
width of your
dado set to
make these
pieces easy to
fit together.

BYS Torsion Box 5F00 2 How to Build a Torsion Box

2. Make an
indexing jig
to
space the web
piece’s notches.
Adjust the distance
from the
jig’s pin to the
dado set, then
fasten the jig
to your miter
gauge.

BYS Torsion Box 5F00 3 How to Build a Torsion Box

3. Cut equally spaced
notches

on the web
pieces. After
cutting each
notch, pick up
the piece and
reposition it on
the indexing
pin.

BYS Torsion Box 5F00 4 How to Build a Torsion Box

4. Make
a dead-flat
assembly platform
to build
the torsion box.
Use a pair of
winding sticks
to check for
twist. If the top
edges of the
sticks are parallel,
you’re good
to go.

BYS Torsion Box 5F00 5 How to Build a Torsion Box

5. Place the
top skin
of the
torsion box on
the platform
and cover it
with plastic,
which prevents
glue from sticking
to the skin.
Assemble the
box’s frame
with a brad
nailer.

BYS Torsion Box 5F00 6 How to Build a Torsion Box

6. Construct
the grid
by gluing
one row at
a time. Place
weights on the
completed sections
to ensure
the grid stays
flat as the glue
dries.

BYS Torsion Box 5F00 7 How to Build a Torsion Box

7. This torsion
box
has builtin
sockets at
the corners for
removable legs
(see Adjustable
Height
Assembly
Table, page
48). Reinforce
the corners
with additional
pieces of hardwood.

BYS Torsion Box 5F00 8 How to Build a Torsion Box

8. Temporarily
set a leg
in
each corner. Fit
additional reinforcing
pieces
around the
leg and glue
them in place.
Remove the leg
before the glue
hardens.

BYS Torsion Box 5F00 9 How to Build a Torsion Box

9. Place the
bottom skin
of the torsion
box on the
grid. Mark the
centerlines of
the web pieces
inside the box
to guide your
nailing in the
next step.

BYS Torsion Box 5F00 10 How to Build a Torsion Box

10. Apply a
liberal amount
of glue to the
entire grid, the
frame and the
leg reinforcing
blocks. Work
quickly so the
glue does not
harden before
you install the
skin.

BYS Torsion Box 5F00 11 How to Build a Torsion Box

11. Place
the skin
on
the grid, align
its edges and
attach it with
brad nails.
Using a router
and flush-trim
bit, trim the
skin so that it's
even with the
sides.

BYS Torsion Box 5F00 12 How to Build a Torsion Box

12. Open the
leg sockets.

Drill an access
hole, then use
a router with a
flush-trim bit
to define the
edges. Square
the corners
with a chisel.
Turn the assembly
over and
glue and fasten
the top skin.

BYS Torsion Box 5F00 13 How to Build a Torsion Box

13. Fasten
hardwood
faces
on all
four sides of
the box. Use a
block to align
the top edge of
each face with
the skin. Trim
the faces flush
with the bottom
skin using
a router and a
flush-trim bit.

BYS Torsion Box 5F00 14 How to Build a Torsion Box

14. Apply
plastic laminate

to the top
skin. This makes
an ideal working
surface–
laminate is
smooth, resists
scratches and
is more durable
than melamine.
You can pop off
dried glue from
it with ease.

BYS Torsion Box 5F00 15 How to Build a Torsion Box

15. Drill
holes
through
the faces and
reinforcing
blocks for bolts
that will fasten
the legs to the
top. The bolts
thread into
T-nuts in the
legs.

This story originally appeared in American Woodworker December/January 2010, issue #145.


AW145 5F00 cover How to Build a Torsion Box


December/January 2010, issue #145


Purchase this back issue.

 

 How to Build a Torsion Box

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