Expansion movement across grain

[Jay Buckler]

So I am putting a headboard with an 11"W tennon in a a mortise in 3x3 legs. How much longer does the mortise slot need to be for expansion? How much of the tennon should I put glue on? It is made of 3 boards so I was thinking of putting glue just on the first board width?

thanks

Jay

[Paul Murphy]

Assuming your project is built during a dry winter and the 11" tenon expands during a humid summer, a reasonable expectation for most domestic hardwoods would be the 11" tenon expands to 11-1/4" or less. I would glue about 4" and float the rest of your tenon. Many folks pin the floating portion of the tenon in an elongated hole.

[John TenEyck]

If you put horns on the ends of your tenons you can make the mortise extra long and not have to worry about the expansion. So, instead of planing for 1/4" expansion, as mentioned above, make the mortise 1/2" long and then you have plenty of buffer. The horns also will help against racking forces, whether or not you pin the tenons.

John

[Ted Calver]

What are "horns"??

[John TenEyck]

What are "horns"??

I probably should have used the word shoulder instead of horn. Does it make sense now? Sorry for any confusion.

John

[Ted Calver]

Thanks John...I'm easily confused:)

[Joe Angrisani]

So I am putting a headboard with an 11"W tennon in a a mortise in 3x3 legs.....

Shouldn't a tennon that wide be split into two? Like you'd do on a breadboard end.

If I had that situation, I'd cut the center 2" out of the tennon (leaving a short stub tennon in the middle), pin/lock one big end solidly, and use an elongated hole to pin the other big tennon.

[Howard Acheson]

I would allow a 1/4" or so for the mortise and glue the first 3-4 inches of the tenon. I would also probably "peg" the joint with a dowel inserted from the non-money side. For that matter, as a bed gets lots of strains, I would probably "peg the lower part of the tenon with a slotted hole (no adhesive) to allow movement.

[John TenEyck]

Shouldn't a tennon that wide be split into two? Like you'd do on a breadboard end.

If I had that situation, I'd cut the center 2" out of the tennon (leaving a short stub tennon in the middle), pin/lock one big end solidly, and use an elongated hole to pin the other big tennon.

Yes typically I think you would. That's usually done so as to not weaken the mortised part. In this case, however, the leg is 3 x 3" so I doubt it matters how long the mortise is. Just my thoughts.

John