I have a project on my hands where I have to create a smooth curve. I
plan to sandwich a number of MDF pieces together to achieve depth.
However, for structure, I also need to sandwich a number of 2x4's
(Pine) in there (facing edges will alternate between pine and MDF).
Imperfections and joints will then be bondo'd to achieve a
consistently smooth surface and later painted. I'm hoping to reach
the end result of a smooth contemporary piece that looks like it came
out of a mold.
Is this plan bound for cracks in bondo/paint as humidity changes and
the piece is moved around? Would I be much better off with hardwood?
I'm aware that wood breathes, but I have no clue how much. I plan to
use wood screws and "General Project" glue (brown caulk-like
container) for assembly.
Thanks for any and all advice!
Have you considered bending wood? It is a product made for cabinet
makers and is in stock or can be ordered from plywood dealers. Cabinet
shops use it to build curved casework such as reception desks. I've
never done it, but I presume you can laminate it with P-Lam to achieve
the look you want. I think the bondo idea is asking for trouble.
more information please.
how much strength do you need? MDF can be pretty strong, in some
what are you making?
is your curve more than 8' long? if so, the face ply can be one piece-
no need for filler beyond fastener holes.
Ah, somebody brave enough to attack the problem at the root :) I'd
greatly appreciate some feedback on the design. The curve is def more
than 8' long. The table needs to be 40" tall with a 50" span - so
figure 130" total (depth of 25")
Imagine this glass table (
but painted black and consistent thickness of ~6". The top will need
to be semi-hollow as I will be sinking audio equipment into it
(~150lbs worth). Because the table will be so top-heavy and possibly
prone to tipping, I'm reconsidering option 1 in favor of option 2:
1. Make box frame out of 2x4 for legs and top. Then use metal
brackets to attach top to legs. Finally, cover frame in 1/2 or 3/4"
2. Top frame remains a box from 2x4's. But legs are made out of many
sandwiched MDF pieces already with the curve. For structure, I will
need to embed a couple 2x4's in the sandwich and use same metal
brackets to attach legs to top.
Google "torsion box" and make the whole thing from plywood. Standard
3/4" stuff for the frame, and bending plywood for sheeting. The shape
can be constructed of one-piece plywood girders attached to
crossmembers, wich form the tosion box. The whole shebang then gets
sheeted with bending plywood, chreating a strong, relatively light
structure. Think "flying model airplane" or "plywood boat".
Forget the 2x4's and MDF. On another note, 2x4's are not properly
dried for furniture, so you'd have to store them an awfully long time
before you use them.
Bending plywood looks interesting and a possibility. Though, I was
hoping to accomplish my goal with commonly available material. Plus,
isn't the surface fairly rough? I would need something for the frame
though as the top span will deflect too much under weight. Is
hardwood a sufficient alternative to Pine in this case?
Irrelevant, but long ago I actually designed and built a bridge based
on the torsion design. This was done out of balsa using crazy glue.
At the competition, not a single stick fractured but the bridge
twisted and snapped at the glue joints :-/
I improperly pictured the proposed solution when I replied. Now that
I better understand the cross-sectional design, I think it's an
excellent idea. So essentially, I take two sheets of (any particular
wood type?) 3/4" plywood, cut out U shapes from each sheet to form the
front and the rear cross section. Then lay bending plywood on top and
bottom? What kind of load will this design be able to support with
only a 6"-wide cross section? Will the corner in the bend be strong
enough and not tear apart without reinforcement?
What are my options for joining the cross-sections together? I will
need to center three 50lbs (each) components inside the table top.
Each will be about L15"xW15" and need to be placed/removed from top of
table. This means that whatever flooring I use for the table top
span, will need to be sturdy.
Got it. I'm thinking two cross-sections, three inches or so apart, on
either edge of the table (four total. maybe one in the center as
well? - though it will be interrupted shortly after the curve).
Reinforce (glue and screws ok?) with cross-members as seen in this
. Am I close?
Can I use screws as well as nails to secure the cross-member to cross-
sections? And where do I notch the other side of the cross-section?
Exactly across or shifted a bit? Please excuse my lack of
terminology, I'm learning as I go. So in the above image, if I have
another horizontal cross-section to join, would I shift the notch or
THEN can I use Bondo??? :) Or will P-Lam take it's place.
You might try looking on diynet.com, for a "Freeform Furniture" episode
where AMY (host) makes some curved nesting tables using bending plywood.
You may be able to get some ideas about your tables construction. If
not...oh well, Amy's kinda cute.
here is a link to the site...
message Got it.
I'm thinking two cross-sections, three inches or so apart, on> either edge of
the table (four total. maybe one in the center as> well? - though it will be
interrupted shortly after the curve).> Reinforce (glue and screws ok?) with
cross-members as seen in this> image -
. Am I close?>> Can I use screws as
well as nails to secure the cross-member to cross-> sections? And where do I
notch the other side of the cross-section?> Exactly across or shifted a bit?
Please excuse my lack of> terminology, I'm learning as I go. So in the above
image, if I have> another horizontal cross-section to join, would I shift the
notch or> not?>> THEN can I use Bondo??? :) Or will P-Lam take it's place.>
transfer the weight of the components to the top skin. think of a tray
that the electronics sit in, bonded to the underside of the top skin.
you can bond it to the bottom skin too, if you want, but the important
loading is to the top.
the inturrupted rib isn't helping much. a lighter rib right on either
side of the component tray will probably be better.
screws into the edge of MDF are pretty much useless.
don't notch the long ribs (the ones that turn the corner and bear the
weight of the top) very much. don't make them from MDF either. the
transverse ribs can float between the long ones. it will be convenient
for assembly to fasten them into a frame before applying the skin, but
not necessary for the strength of the finished article. try to either
limit yourself to fasteners that don't displace a lot of wood (like
finish nails) or have correct pilot holes for the screws you use.
shifting the notches (staggering the transverse ribs) is fine, and
will help avoid weak spots lining up with each other. better is to not
this assembly is similar to a hollow core door. in a door, the
function of the ribs is taken up by some lightweight, more-or-less
continuous material like foam or cardboard. you need to bear weight
against the panel, so you will need a thicker top skin and some ribs
running the long way. other than that, though, it can be a lot like a
hollow core door. the cross ribs can be replaced with shaped blocks of
foam, if you want. the bottom skin, the harder one to assemble, can be
thinner that the top. if you can, the cool way to make the ribs is to
either steam bend or laminate hardwood.
Thanks to all for great advice! I've shifted my plan and will be
using the suggested torsion box design with 3/4 ply. I just need to
iron out a few more kinks before I start.
Bridgerfafc -- I've been convinced that I won't be using MDF for this
project. Unfortunately, I won't be able to transfer the weight to the
top skin since the components don't have an edge that they can rest
on. What I was thinking though is that instead of doing the entire
bottom-center skin out of 3/4 ply, I could use the thin bendable ply
for the bottom skin and then use separate 3/4 ply sections under each
component for support. Each section would be the width/height of the
individual component and would be screwed into the edge through the
long ribs and the cross-members. Though I'm curious about the
strength of this method. Now that I re-read your suggestion for the
third time, is this exactly what you're suggesting?
So don't bother with the center long rib? This means that I will have
the ribs spaced as such: 0"--5"--20"--25". Sufficient span for
If I don't have to, I'd rather not notch the long ribs. Especially
since this will weaken them to some extent. So, standard butt-joint
with glue and wood screws (with pilot hole) it is.
Another puzzle that I'm trying to figure out is this: I want to round-
off the corner edges for a sleek look. However, how do I do this if
the top/bottom skins will be screwed onto the edge of the long ribs?
The only solution I can come up with is to double up on outside ribs
(doesn't have to be a continuous piece since it's not for support -
per say) and use one rib for attaching the skin, and the other for the
rounded edge. Any better suggestions?
it's close, and it will work, if you make suru the 3/4" bottom panels
are well attached 4 sides to a rib and that that rib is well bonded to
the center long rib will still help stiffen the curve from top to leg,
so it's not a total waste. make sure it terminates at the top into a
cross rib, and that that connection and the ones between that cross
rib and the long ribs IT terminates into are strong joints.
build the frame, apply the skins a bit oversize, trim, apply the outer
rib a bit oversize, trim and round over.
As Bridger points out, the entire skin is structural. If properly glued
to the sringers, it'll be incredibly strong.
Good quality plywood stringers will be far stronger around the corners
than hardwood. Plywood has many plies with alternating grain direction
to carry the load. A hardwood stringer would have some directions that
aren't as strong as the others. The stability of the plywood will also
help the joints and bonds last.
So screws driven between the plies will not compromise plywood's
strength to any significant extent and will not separate the plies
from the weight, right?
I'm very glad I asked this group before I started the project! The
torsion box design you guys suggested is MUCH better than the amateur
one that was brewing in my head.
Is Bondo still my friend to fill in fastener holes, joints, and other
voids before painting?
I'd imagine there would be some flex where the skin meets the ribs.
Also, since I won't be able to get a long-enough skin, I'll have to
stitch it half way. This would also have to be filled. If not bondo,
what should I use?
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