Ive glued up a 34″ wide table top out of some especially wide oak boards.Ive put a bread board end on them. I have a 1″ long tongue by 1/4″ with total top thickness of 7/8″. But Ive and heard various ways of attaching the breadboard end and still allowing movement..”glue here..not here”…; Norm said to dowel the bottom of the joint in 3 places with some kind uv elongated hole in tongue or bbrd?
Whats best place to go here?
Replies
Drill 3 holes the size of dowels through the bread board end on a drill press. dry fit the breadboard end in place and with the same drill bit in each of the holes tap the bit to make a center mark. pull the bread board end off and get your combination square. Mark each of the new centers about 1/32" more toward the center of the table. this makes them drawbored, that is the dowel will bend slightly as the joint is assembled pulling the joint home. also mark the outer two holes as slots. this centers the breadboard end no matter how much or often it moves. glue the center only and your good to go.
In addition:
Plane or otherwise make a slight, gradual dip (no more than 1 - 1.5mm in the centre)along the inside edges of the breadboard. When those drawbored dowels are tapped home they will pull the gap closed and there will then be less chance of the gap opening along the unglued portions of the breadboard-to-top join out towards the edges of the table.
You can put dowels out towards the ends of the breadboards as well, with elongated holes in the tongue to allow the differential movement (probably 6mm or 1/4 inch of elongation is enough but a little wider won't hurt). The aforementioned "gradual dip" tends to make these "outrigger" dowels redundant however.
To ensure the breadboard can move easily on the top-tongue, should differential expansion occur, I also wax the tongues on their outer thirds. This also ensures a spot of errant glue on the outer portions of the tongue will not inadvertantly stick the breadboard where it should not be stuck.
Lataxe
Depends. The top WILL move as related to the bb end, no matter how you attach it. It will shrink in dry seasons and expand in wet. Nothing you can do will stop that. That said, you should plan on how you want this movement to affect your piece. If it is going to be up against a wall, for example, you might want all the movement to be on the end against the wall. In that case, tightly pin (and glue if you want) the front end, and elongate the other holes.
If you want to split the difference, so the movement shows up the least amount possible, pin/glue the middle, and elongate the other holes.
If you try to restrain the wood from moving, you'll end up with a split table top. As they used to say in the commercial, "It's not nice to fool Mother Nature", so don't even try.
Mike Hennessy
Pittsburgh, PA
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