Mortise & Tenon Frame

As mentioned in the previous post, the two different construction types for the green oak frames were Mortise & Tenon and Simpson connections. The Mortise and Tenon connection has been used for thousands of years in woodworking to connect pieces of wood at 90° angles. It is comprised of a mortise hole and a tenon piece. The tenon, often referred to as the rail, slides into the mortise hole and can be glued, pinned, or wedged into place to lock the wood together. We have briefly discussed purchasing or renting a mortise machine to assist in cutting the wood to make production quicker; we are planning on building test connections not attached to the frame for a quick look at how the green oak responds to different connectivities. 

Mortise & Tenon used for a fence or gate

Mortise & Tenon used for lateral stability for
a custom barn wood table

Fabrication:  The beginning process we conducted with the green oak was milling it down into exact sizes.  The cants themselves came at different dimensions, even though they were all supposed to be around the same. At times, the difference could range up to half of an inch.  During milling, we were able to see just how much water was contained inside these pieces; you could feel the water dripping from the core.  Milling also allowed us to experience just how heavy green oak is.  With a moisture content much higher than the average piece of wood, even a four foot piece was hard to maneuver. We then went to work figuring out the best way to create each section.  Tenons were the easier parts of our fabrication. Measuring the end dimensions, we cut around with a bandsaw.  Mortise holes were the most time intensive and hard to measure correctly.  Because we did not have an appropriately sized mortise machine at the FabLab, we had to drill holes to take as much of the wood out, then manually chisel the mortise to be square.  As much labor as this was, we do know that there are machines that could do the job much faster and more efficient than we did.  Also, simply cutting and chiseling into this wood was an endeavor in itself.  We are dealing with the heart of the tree; it is one of the most dense parts. Physically pushing the wood through table saws and jointers was an exerting task.

Assembly:  We made up the time we lost figuring out how to cut the mortise holes on assembly.  Each piece had a specific place and the appropriate mortise or tenon were fitted before assembly.  When on site, the frame was fitted together quickly with a mallet and some extra hands.  The only problem that we ran into was putting back together the predrilled bird mouth joints that had wood dole rods running through them. It was user error; we did not remember to label which hole goes with which tenon so alignments were skewed and not lining up correctly.  The holes had to be adjusted on site due to bad planning on our part.  Our team got the frame together many hours earlier than the Simpson frame team, though we spent many more hours in the shop before assembly than they did.

Post-observations:  After a month in the kiln, we observed many positive results. Though some of the tenon edges pulled away from the mortise holes, the structure as a whole stayed together quite well. We have assumed that this is because the structure as a whole shrank together internally because there was no hardware or fasteners like in the Simpson construction. We now understand the labor and skill it requires to build such a frame and are prepared to build it with ease and skill due to the success of the mortise and tenon frame. Different viable options include a hybrid with hardware and prefabrication.

You can see the checking on the support and the joint raised up

Beam B shown pulled away from the column  

You can see the checking on the joist and the gap space between it and the column

This shows the significant gap between pieces after drying

Piece E as also pulled away from the column and a check runs down the middle

The bottom of piece G did not stray or shrink away from the column
but the top experience bowing and shrinking

The knot in the wood got worse after drying
Piece I shrank significantly; you can see the tenon piece revealed from the gap space

The amount of bowing and shrinking depends on each different piece of wood
and the different properties and you move along the piece
Piece K had bowing at the top and bottom