Wood movement?

[SteveCogs]

I have a M+T joint that measures 7" with 2x2" tenons. I question wether or not I need to leave a little extra room to move in the mortises. Maybe 1/16 at each end? Also, should I to glue just one or both? They are all pretty snug. Actually surprised myself with that one.:gar-Bi
This is my first try with the M+T.



Thanks
Steve

 

[Bas]

By an extra 1/16", you mean the depth of the mortise? That won't help with wood movement, wood expands along the grain, i.. your tenon would get wider, not longer. But you shouldn't have to worry about that, because both the mortise and tenon will expand equally, since it's the same wood.

Having said all that, making your mortises a little deeper is good practice. It gives the glue a place to go. The end grain on the end of the tenon won't add a lot of strength anyway, so if there is a small gap (1/16" - 1/32") between the bottom of the mortise and the end of the tenon it won't make a difference. Better too deep than too shallow.

 

[DaveO]

Hmm :eusa_thin:eusa_thin I think that you are right at that breaking point where wood movement should be considered the design of your M&T joints. A 7" wide, let's say table apron, joined by M&T into the legs is going to give you a cross grain situation. The apron will expand mainly in width and the legs will move ever so slightly in the opposite direction. Red Oak, which your pictures look like to me, has a very high level of movement in service. For every 1% change in MC you can have a 3/64" change across the width of a 12" panel. That translates to you as ~1/32" movement for every 1% change in MC.
The question you have to ask yourself is how is the environment your working in relate to the environment the piece will reside in. And will the piece's final home stay at a pretty consistent humidity level. Also is leaving room in the M&T joint worth compromising strength for movement freedom.


Sorry I didn't answer your question, but only you can with the right information.

Dave

 

[SteveCogs]

Thanks Bas and Dave.
I think I will have a movement problem. The changing table will be in the new nursery which I'm sure the LOML will keep very different from my [strike]garage[/strike] shop. I'm going to glue the top only which should be fine as there are 2 other M&T cross pieces on the sides.

I'll post the finished product for all to see my first attempt at a piece of furniture.

Steve

 

[Gofor]

When building my table, I did a lot of research on the web (take that for its dubious credibility), but it seemed the general consensus I found was that the max safe length of a glued tenon in hardwood was 3 1/2 to 4", and that was species dependent. (Most of the references I found were articles about making frame and panel exterior doors). As you have two both 2" long with a space between, that means the outside dimensions of the two would probably be closer to 5"+. If it were me, I would glue one, and lengthen the mortise for the other (or trim the tenon) to allow 1/16" shrinkage and 1/8" expansion. If it is an apron, I would glue the top one also. I have glued 3" ones in pine for a workbench that sits in the garage and have had no problems after 2 years.

Just my negligible value's worth

Go

 

[Bas]

Ignore what I said, should have looked at the picture better. Though the expansion for for M&T would be in the same plane. Imagine that, clicking on thumbnails to get a bigger picture