Why every doweled seat joint fails.

[Dan Lindberg]

For those curious about why every doweled seat joint fails, here is an explanation,
(that was posted on another wood based site.)
I'm not sure why or how this may or may not effect a M&T joint but...

Dan


For those who haven't read Hoadley's experiments with dowel joints, here is how it works:

The round hole that the dowel goes into has a fixed diameter. It doesn't change size with changes in humidity (at least not measurably). the dowel, however, does change with humidity changes. In times of high humidity, the dowel wants to expand. It is restricted, however, by the hole. As a result the dowel's fibers get crushed against the confines of the hole. Then when there is a time of low humidity, the dowel shrinks. It is now smaller than the hole, and the glue joint fails. It is the same problem that causes axe and hammer handles to become loose. Drive that wedge in deeper, it won't matter. Next winter, just when you want to split wood with it, your axe handle will be loose again without fail.

 

[Rollin Thurlow]

Of course the type of wood and grain that are used will effect how the joint will move. All things being equal, the member with the hole will be effected by moisture as much as the dowel. The hole will become smaller as the wood around it expands and the dowel will expand as it gains moisture. However in a well made joint the wood will accommodate any of this movement.
I believe the twisting and stress on the seat as its being used, that get transferred to a fairly small joint, have a lot more effect on the joint than the humidity around a properly made joint.
There is very little jointing surface compared to the stress that a 180lb butt causes when its twisting the seat for four hours trying to find a comfortable position for each butt cheek!

 

[chris pearson]

So in conclusion, its "Butt Check Fatigue"

 

[Dan Lindberg]

If you think the stress is high with a little guy on the seat, just think that it's like with a big guy.

"180lb"



But, ya, I not sure what to make of this, except that Hoadley is the recognized expert on wood and using wood. I did some quick searching on him and this book, and the passage also includes M&T joints as being affected by this mechanizam.

I think I'll get his book, "UNDERSTANDING WOOD 2nd EDITION - By R. Bruce Hoadley" and check it out further.

From Wiki:
R. Bruce Hoadley is a professor of Building Materials and Wood Technology in the Department of Natural Resources Conservation at the University of Massachusetts Amherst.[1] He specializes in wood identification and wood properties, has written several books, and is a contributing editor and technical consultant for Fine Woodworking magazine.

 

[Max Peterson]

The weakness in a dowel joint, whether it be in manufactured furniture or canoe seats, is the tiny amount of long grain to long grain gluing surface in the joint. As the radius turns on the dowel and the hole, the long grain quickly shifts to end grain. End grain, as we all know, has almost no gluing strength. Dowels are rarely a good solution and are most of the time one of the weakest joints available. They are highly desirable in manufacturing however, because they are quite easy and quick to set up for automatic machining, and this is why they have been used in manufacturing for over a century. When all of the frame members are good in an old canoe seat, I knock it apart, rout a matching slot in each piece, and fit a loose tenon. It will probably outlast the canoe.

 

[pklonowski]

Dowel joints

Okay, I have the seats taken apart from my 1918 OT HW. The dowel joints failed, no surprise. When setting up for the M&T, what do you do with the dowel holes? Obviously, the old dowels come out, but once I take out the 5/16" dowels, I only have about 3/16" left of the seat frame for the M&T. I typically would use a 1/4" tenon on a 3/4" frame, so my options are to fill the dowel holes with new dowels, cut flush, and then cut the 1/4" mortises, or cut for thicker (5/16"+) tenons. The larger mortises don't leave as much thickness in the frame, which is my concern. Any thoughts?

 

[Max Peterson]

Use a thicker tenon. Your seat frames are probably 13/16" or 7/8" in whch case 1/4" isn't thick enough. 3/8" would be about right.

 

[pklonowski]

The frames are between 3/4 & 13/16... some parts are closer to 3/4, others closer to 13/16. I'll go with 5/16. Thank you!

 

[Bongocruiser]

I have rebuilt so many seats I now use brass dowells or hide a stainless steel finishing nail into the joint.