I am making two small benches out of 1.5″ pine. They are about 18″ tall and 11.5″ wide and 23″ long. I did this to try my hand at mortise and tenon joints. The mortises and the tenons came out beautifully, and went together well. I doubt they will ever come apart. However, I would like to put pins through them and am wondering what size pins to use for joints this size. The mortises are 1/2″ x 3″. The mortises are 3/4″ from the edge, so a pin would have to be 3/4″ plus 1 1/2″ long, just to make it through the tenon. I figure that the length of the pin should be about 2 3/4″ or 3″.
My question is, what diameter dowel is appropriate for a M&T joint of this size with wood of this size???? 3/8″, 1/4″ ???
Using pins here is not for practical reasons, but strictly for gaining experience. Besides, it is possible that someone weighing over 600 lbs could come along and want to jump up and down on my stools. They will be used outdoors where heat and cold and rain and snow will cause the wood to shrink and swell a good deal. I used Titebond III glue. I have read widely about M&T and pins. I have learned a lot about pin placement and the use of 1 vs 2 pins, but have not found anything on the general issue of the appropriate size of pins for specific applications. My application is probably most like the construction of doors for a cathedral.
I will try to attach a photo.
Thank you.
Replies
I'd probably use 3/8" or maybe even 1/2" for this (probably two in each joint) -- whatever looks better with your design. However, for this application, I would have probably located the joints closer to the vertical face. No big deal - the pegs will still function fine, it's just that, to my eye at least, it'll look a bit "odd" as built. Kind of like the pegs have no relationship to the joint since it's so far away.
Mike Hennessy
Pittsburgh, PA
Hi Mike,
Thank you for your advice. After I posted my question, I continued to search for information on the use of pins in M&T joints for outdoor items. I found an article in FWW by Ben Davies. He recommends against using pins in M&T joints for entry doors because "Seasonal movement and stress may split tenon along grain, making holes into slots".. He never said whether he came to this conclusion analytically or whether he actually experienced the problem.
I believe that I will continue my learning experience with the stools and will do an experiment. I will use two 3/8" pegs on the M&T joints on one side of one stool, with single pegs in the joints on the other side, and the second stool will have no pegs. Lets see how they hold up on the porch over the years.
I appreciate your taking the time to give me some advice. By the way, I see you are from Pittsburgh. My son and his wife both got Ph.Ds in robotics from Carnegie Mellon University. My son, also named Mike, spent his entire nine-year college career at CMU, so I got to know your fair city pretty well. I love Pittsburgh!!! My wife and I met at Penn. State. For your amusement, I have attached a photo of a "Penn State" bench I made that has a cartoon of Joe Paterno on it.
The beat goes on. Thank you.
96, (may I call you by your first numbers?) ;-)
The traditional "reinforcement" for through tenons, as you have here, would be wedges, driven into the ends of the tenons near their top and bottom edges, instead of pins.
A 1/4" square peg will be inclined to break off before being driven that deep, so I'd recommend 3/8". Larger than that will tend to weaken the relatively thin mortised member, with a likelihood of splitting it, esp if you use two pins in the joint.
Regards,
Ray Pine
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