Edge joining large pieces of plywood

[Matt Schroeder]

For 40 years my parents have used a 4x8' sheet of 1/2" plywood as a table extender for large parties. Dad is finding it too heavy to move safely alone now, and doesn't always have help available to move it when he wants to get it ready for gatherings. It goes on top of a 72" by 28" table and is quite stable; leaning on any edge does not lift the opposite edge.

He asked about cutting it into 2, 4x4' sections with some sort of joint that will maintain its overall stability when installed. He would like to stick with the 1/2" thickness for weight, and would like the pieces to be independent (not hinged) so the weight is manageable. I have a thought, but wanted to throw it out for the collective wisdom to improve.

I am thinking of installing dowels about every 6" along the edge, gluing every other one into the same side (so each half of the board has half of the glued joints) while the other end of each dowel is kept to a firm slip fit. I would place two table leaf fasteners on the joint, like http://www.leevalley.com/US/hardware...=3,43586,43588, a type that not only pulls the edges together but also provides some vertical shear support.

I have the new style Jessem dowel jig for 3/8" dowels. It would be great for drilling and aligning the holes, but I am wondering about the stability of 3/8" dowels in a 1/2" (or so) edge. I could purchase the 1/4" kit for the jig, and may use it again on other projects but so far I found the 3/8" to work well for everything else I have wanted to do so I am guessing it would just gather dust after this. I was hoping that by alternating the glued sides of the dowels it would reduce the tendency of the edge to blow out by giving a solid connection on alternate sides, and by having 8 (or more) across the 4' span there would be enough points to spread the load if someone leans on the end of the piece.

I also have a biscuit jointer (seems too flimsy) and a slot cutting bit for my router, so I could think about a continuous spline or several smaller splines, either backed up with the table leaf fasteners.

Has anybody tried something like this and could share what they learned? Any strong feelings about the best way to do this? They like the one large table approach, as opposed to adults and kids tables set up, so I would really like to make this work for them.

Thanks for any and all ideas!

Matt

[George Bokros]

3/'8 dowels are too large for 1/2" stock of any kind in my opinion. They would leave less than 1/16" to the surface. 1/4" would be better.

[Jim Dwight]

Inserted leaves for the tables I've owned and seen have used dowels but they are usually about an inch thick. I think 1/4 inch dowels, centered by a good jig, might work but you will only have 1/8 material outside of them, if it is true 1/2 (which I don't see any more). That isn't much strength. A spline or biscuit would leave more material in the plywood. I think you would find biscuits a bit difficult to get to slide in the cuts after the pieces sit. They come compressed and expand some with exposure to moisture. So adding that all up, I would probably use a thin spline - like 1/8. I used to find door skin plywood sometimes, that would seem ideal.

I wouldn't mess with a fastener until trying the joint without it. I think it may have enough resistance to coming apart that the fastener isn't needed. That will also make it somewhat challenging to get together, however. If you open up the slot, then the fastener would seem necessary.

I think there are also slide together metal pieces meant to be put into a biscuit slot. I've never used them but you might want to look for them. They might be your best option.

[mark mcfarlane]

Maybe a 1-1.5" lap joint would be adequate. You could add some latches on the underside if you want to ensure the joint stays tight, but the weight may be enough.

[Jamie Buxton]

You need to retain the 1/2" thickness only for the middle 28", where the 4'x8' sheet sits on the smaller table. You can build up the thickness outside that central 28" by adding wood to the bottom face. With that added thickness, you can do any peg or tongue technique you want.

[Al Launier]

Since the plywood sheet(s) will be resting on top of the 28"x72" table there will be only about 10" overhang on each side, so I don't think you would need to be concerned with shear strength at the joint, only how to connect the two sections together. I think the 1/8" wall left after using 1/4" dowels would be plenty adequate given that alternate dowels are glued, although the 3/8" would serve as well, even with the 1/16" wall, keeping in mind that the glue would adhere around the dowel adding to the overall strength. If i were in your place & had only the 3/8" dowel jig I would go with the 3/8" dowels.

[John Lankers]

A series of 1/4" splines either from BB ply or better hardwood could work (think floating tenons).

[Jim Dwight]

I didn't think of the 10 inch wide edges that will hang off the table earlier. A second layer of 1/2 inch spaning the joint, glued on one side and bolted using flat top bolts into t-nuts would be a very secure joint, it would take a lot of stress, and it would be simple to assemble. You could glue the second layer to one side, after the glue dries you could drill through holes, and then take the pieces apart to expand the holes on the bottom, insert the t-nuts, and countersink the top. Then you can bolt the pieces together and have a solid top. A couple bolts in each side should be sufficient. I'd probably use 1/4 inch. I doubt whoever sits at the joint would notice the added thickness. You might want to put a dowel or two in the center part to keep things aligned.

[Lee Schierer]

Since the plywood sheet(s) will be resting on top of the 28"x72" table there will be only about 10" overhang on each side. Make two pieces of 1 x 5 9" long out of poplar or maple to fit under the two half pieces of plywood as shown in this sketch. Get eight 1/4 -2 pand head screws and eight 1/4-20 X 1/2" T-nuts and insert them so the flange is on the top surface of the table. Put four T-nut into each side so they can hold the two half sheets together as shown in this sketch. Capture s.JPG

[Matt Schroeder]

Thanks to everyone for the ideas. I think we will go with the idea as outlined by Lee (thanks for the picture!) and others to use a thicker backer board and some easy to install hardware (t-nuts and screws). We'll see how the center aligns and may add a spline if needed.

Thanks everyone!

Matt

[Glenn de Souza]

Hi, you've received some good ideas. Here's another to throw in the hat:

Since part of the objective is to make the system manageable for your parents, I would cut the sheet into four approximate 2x4 sections. Then make two 95" long rails out of whatever straight stock you like. Maybe they could be 1.5"x2" or 2x2. Drill and countersink holes in the 2x4 panels at the locations that will mate with the rails on either side of the 28" wide table. Lay all four panels on the table tightly together with an even overhang, and clamp the rails to the underside on either side of the 28" table. Then you can use the countersunk holes to mark the rails for mating holes that will have a tee nut on the bottom to receive pan head machine screws. If you don't care to mess around with tee nuts, you could use wing nuts and washers to capture the screws.

Basically what you have is a knock down system of four 2x4 panels, two rails, and a ziploc bag for the screws. This should be easy to store and set up for one person. Plus the two rails will stabilize the top and prevent it from sliding off the sub-table even if someone leaned or fell against it.

This is essentially a variation on Lee's idea.

[Keith Westfall]

Cut it length wise, and put a couple of strips across at the end of the table, (use a countersunk bolt with a wingnut on the bottom for easy assembly/disassembly) and maybe a short spline/dowles/? in the middle to keep it aligned.

Throw a table cloth on, and good to go.

As your original request was to 'lighten the load' so to speak for your parents, I would want to avoid adding doublers, strips and other pieces just to make a 'feasable' joint in the middle.

[Marshall Mosby]

I was looking for a solution to a similar kind of problem. Now going to try Lee's Idea.

[Rich Engelhardt]

Why reinvent the wheel?
You can buy T&G subfloor or roof sheathing in 1/2" thickness.

[Jim Bowers]

Dovetail joints would work very well. Lock and unlock with little trouble. Both sides would be very stable when connected