MDF, Pine, and Bondo

[fourrings]

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!

[GROVER]

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. Joe G

[bridgerfafc]

more information please.

how much strength do you need? MDF can be pretty strong, in some applications.

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.

[fourrings]

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" MDF.

1. 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.

[B A R R Y]

How about bending plywood?

You can get it with many different facings, and it's pre-kerfed to facilitate bends. Check with your local hardwood dealer, as it's usually a special order item. Plywood wholesalers usually stock it, so the special order won't take long to get.

You MIGHT be able to order it from a home center, as they often deal with the local plywood wholesalers, but you'd need to know exactly what you want, as the home center won't.

[B A R R Y]

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.

[GROVER]

Many carcases similar to your description are made by constructing a framework of 3/4" plywood. The plywood pieces are cut to the cross- sectional shape you want and then layed out in the desired pattern. Its something like the wings of a balsa wood airplane model. Then the bending wood is applied, giving you the smooth curve. This becomes the substrate to lay up the P-Lam. This method is very strong and should hold your equipment Joe G

[fourrings]

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?

[B A R R Y]

Not if you buy it with a smooth surface. You can even get a smooth MDF or laminate surface, if that's what you want.

That couldn't be more wrong. Too bad you didn't actually read up on torsion boxes. 8^(

Do you realize that 747's and major bridges are built with torsion boxes?

[fourrings]

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.

[B A R R Y]

Use more than two girders. Think of your recessed gear on top, as you'll need to work around it.

How are rails held together in any torsion box? Notched crossmembers of the same material.

3/4" cabinet grade plywood (the kind with lots of plies) is very strong. You could sheet the center section with that, and bending plywood on the curve, with internal stringers to bridge the seams.

[fourrings]

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.

[MikeG]

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.

[MikeG]

Sorry about the link...try this

[Lee Michaels]

"MikeG" wrote

Amy is cute. But she makes atrocious furniture.

If they gave out awards for ugly furniture, she would be a constant winner.

[bridgerfafc]

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 notch.

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.

[fourrings]

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 bendable plywood?

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?

[bridgerfafc]

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 top.

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.

sounds right. build the frame, apply the skins a bit oversize, trim, apply the outer rib a bit oversize, trim and round over.

[B A R R Y]

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.

[fourrings]

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?