bird's-mouth joint

Mark E Smith

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190
Location
Arkansas
Just got me an MLSC bird's-mouth router bit set, going to make some octagon planters, and maybe a few bird houses, never made these and a little confused. Every picture I see the routed edge is along the grain meaning when put together the grain will be running up/down, wouldn't this put enormous strain on all these joints since expansion and contraction will be side to side. Shouldn't the routed edge be on the end grain, then Exp/Cont is up/down and the side to side and the glue joint (which I thought was the whole point of (birds mouth joint) would be stable, or does it make a difference since each piece is so short, just curious since I have no experience with these, never knew these were around, I have always just did the math cut both sides of the board then went through the labor intensive task of putting dowels in each joint...probably way I rarely made these things...lol...also how do you determine how long each piece needs to be for a given diameter of final product.
 
Just got me an MLSC bird's-mouth router bit set, going to make some octagon planters, and maybe a few bird houses, never made these and a little confused. Every picture I see the routed edge is along the grain meaning when put together the grain will be running up/down, wouldn't this put enormous strain on all these joints since expansion and contraction will be side to side...

Assuming all eight pieces expanded/contracted at the same rate (although it's unlikely) then there'd be no strain on the joinery, since the angles would remain the same. Since equal movement is unlikely, you're right - the joinery would be strained by the changing angles.

That said, for a bird house, the pieces would be narrow enough that movement would be very slight, so there'd likely be no problem. The planters, I'm assuming, would be made of wider stock, and would be in some nearly constant state of wetness, so you may well have a problem there.
 
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