Woodworking question - how do I hold these pieces of wood up?

Factual Questions
#1

I’m making a table suspended between two low bookcases for a friend - and I’m trying to find some sort of hinge to hold them up in the middle. I’m trying to somehow take 2 pieces of 4’x1’x5/8" medium density fiberboard (MDF), with the hinge in the middle, and make it straight. ASCII art below. I’m trying to find something at point ‘A’ below that will hold up the table without putting a leg down, and hopefully without involving another piece of world. The horizontal lines are the table pieces (which I want to be able to remove, and fold in half for storage), and the vertical lines are the bookcases. My trip to the Big Orange Home Store yielded nothing, as all the hinges open too far (beyond 90 degrees). Any ideas?

  |             A            |
  |                           |
  |                           |
#2

ahh the ASCII art screwed up - the spaces got compressed between the bookcases.

#3

Put the ASCII art inside a {code} {/code} block.
like this:



----------------                      --------------- 
                          | A | 
| |                                                | |
#4

Just checking to see if anyone has made a joke about keeping wood up.

#5

FINALLY! After all this time, a question in MY field of expertise. I was beginning to feel like I have nothing to contribute besides wisecracks. Stand back, teeming millions, and let me field this one.

Ready for a professional answer?
Um, I don’t know. :smack:

But seriously, the material that you use will dictate how you build this shelf. The truth is MDF is not very good for making a shelf, unsupported. Without legs (or braces on each end) AND some sort of support in the center or along the 48" span, MDF will absorb moisture from the air and begin to sag, even with no weight on it, and uncut between the two bookcases.

If you use plywood instead, it’ll hold up much better for a longer period of time. If you use solid wood it’ll last even longer, but then there are maintenance issues to deal with (depending on how much your humidity levels change in a year - In what part of the country do you live?) so that it doesn’t warp or split. I would advise plywood.

Now, how did you intend for this shelf to be attached to the bookcases? If your idea was to have it hinge in the center because you want the shelf “removable”, I’d recommend making it one long piece, sitting on a couple of cleats. Then the whole thing could be lifted up and stored in a closet or under a bed or something.

However, even 3/4" plywood will sag over a 48" span with even a little weight over time. I’d recommend a solid edge to your shelf or a vertical cleat running under it from one end to the other.

So I guess I need to ask five questions.
1: What area do you reside?
2: Why MDF instead of plywood or solid? (Cost? Already have it?)
3: Does it HAVE to hinge in the middle?
4: Do you have the ability to make modifications to your wood, or are you simply cutting and screwing it in?
5: What’s the shelf for? Decoration? Cat’s to sleep on? A place to do homework and look out the window? A spot to hold your 50 gallon fishtank?

#6

any run of 8’ of mdf will hold ziltch. it will sag under its own weight, and whatever fasteners you use to secure a hinge will pull out.

I used 3/4 x 18 hdf for shelves (max unsupported run = 4’) it worked

if the ends are really, really square, a piano hinge would be best.

but first, come up with another design - either add a center support or attach 1x2’s to the bottom of the front and back edges (or use something like 1" to 1 1/4" oak, and the hinge will work)

#7

Well thank you for your thoughts and questions: I actually think I figured out a novel idea - I made 5/8" tall X 72" long (the length of the bookcase) X 7" deep “pockets” on the top of each bookcase, into which each leaf can slide in. It works very well. I’m concerned about the use of MDF as a material now. My plan was to prime, paint, and seal everything with a lacquer or polyureathane. I guess I could buy some ply…

Answers for questions:

  1. I live in L.A.
  2. I am a very amateur woodworker and have only a table saw and a jig saw to deal with (and they were bought at a pawn shop). I chose MDF because it is easy to work with, cheap, and it looked a lot less wobbly than CDX, etc.
  3. Well it doesn’t have to fold, but the pieces can’t be any more than 4’ long to fit in the drawers.
  4. See #2.
  5. The shelf will be used as a big table for eating, working, etc.

Do a google search for ‘card table hinge’ - will something like this work to even up the slight differences I have now?

#8

Go to your local hardware store and pick out some 3/4 plywood. If you’re going to paint it you can get “paint grade” which is relatively cheap. If you’re going to stain it, you probably only need one good side. Pick a wood you like, and ask them to cut it in half for you. Ask if they have any half sheets of anything, too. You’ll only need about 1/2 a sheet. Most stores with plywood have a panel saw on which they can cut sheets in to manageable pieces for you. (If you can transport and carry around a full sheet, you still may want to cut the sheet in to smaller pieces by hand before wrestling with it on the tablesaw.)

Cut two pieces 48" x 12". Then rip two and a 1/2 inches off of each piece’s width, and glue and screw it back on at a 90 degree angle to that same edge (predrill for the screws so that they don’t split the plywood). This will help keep the ply from sagging under use. This is the front edge upon which your elbows will rest, so you may want to sand or ease this edge a bit.

What you have now is a pretty strong shelf that should hold your dinner and books just fine. But you need to hold up the shelf. It’s nearly impossible to have it unsupported over a 96" span, regardless of how it’s attached in the middle. It’s actually not really recommended that it be unsupported over a 48" span, either, but that’s a judgement call.

I’d advise a permanent cleat screwed in to the side of each bookcase that the shelf can be set on, and removed. You can use a piece of 3/4 ply 8" x 2" for each. Place the top edge about 29 and 1/4 up fom the floor for a “normal” table height. You can put it higher or lower depening on your chair.

Now, you need something in the center. I’d go with a third cleat on the wall halfway between the two bookcases, and a leg in the front. That way the shelf is supported on both sides, as well as each end. Make your leg about 3 or 4" wide in order to support both shelves, and the same length (29 1/4?) as your cleats are from the ground. The more supports you put between the bookcases the better. Three legs would be better than one. Cleats on the back wall between the bookcases would be better, too.

All the points where the shelf meets the cleats and the leg should be screwed rather than just setting the shelf on top, but that’s up to you. It’ll be noticably wobbly without screwing it together. Fine for eating pizza and reading comics, not so good for soup and Shakespeare.

Not to sound like your mother or anything, but please, for the love of Pete be careful with power tools which are new to you. It’s very easy to lose an appendage on a tablesaw, or lose a testicle from kickback.

#9

OUCH!

So much for keeping my wood up.