Marking out and cutting basic Mortise and Tenon joints
Before marking out either the mortise or tenon, select a face on each piece of timber and mark them as the datum faces - make all measurements from these faces. Mark one face (with the point to one edge) and the other (with the down line towards the point on the other face).
A tenon needs to be a snug fit into the mortise, too tight and it may split the timber around the mortise, too loose and the joint will be weak.
Marking out the mortise and tenon
Mark all timber faces, sides and ends using the same gauge setting.
On the face edge (the side which will butt up against the tenon rail) of the mortise rail, mark the length of the mortise, this will usually be the same as the width of the piece of timber in which the tenon is to be cut. With a pencil and carpenters square mark lines where the joint is to be made, around all four sides of mortise timber. And using a gauge mark the position of the tenon.
With the tenon, mark the length of the tenon around the four sides and scribe the thickness of the tenon on the sides and end.
To mark the thickness of the tenon, measure the thickness of tenon rail, divide this by three and set a mortise gauge (right) to that measurement.
Release the thumbscrew holding the pointer and stock, adjusting these so the distances between each and the fixed pointer are one third the thickness of tenon timber.
If a mortise gauge is not available, a gauge with one scribe point can be used but this requires the adjustment to be made separately for each side of the tenon. Mark the first line on both the mortise and tenon timbers before adjusting for the second line. ALWAYS USE THE GAUGE ON THE DATUM FACE - NEVER TRY TO SCRIBE THE TWO LINES FROM OPPOSITE FACES OF THE TIMBER.
When using the gauge, keep the stock of the gauge up against the datum face side of the timber.
Cutting a mortise
Check the width of the chisel being used to cut out the mortise, its blade width must be less than the required thickness of the tenon. Start by using the chisel to cut the wood grain on the sides and ends of the mortise.
It is sometimes easier to drill out most of the timber from the mortise hole, clamp the mortise timber in a vice and, either using a jig or carefully hand drilling, make a series of holes in the area previously marked. Use a drill bit narrower than the final width of the mortise required and place a piece of scrap wood behind the workpiece to prevent tearing of the timber as the drill breaks through.
With a G-clamps secure the timber to a workbench, making sure you put a piece of scrap wood underneath and padding between the clamp and the work. Chisel out the mortise with the correct width blade.. If wedges are being used to secure the tenon, angle the mortise hole slightly to cutaway the sides of the mortise to allow the tenon to expand when the wedges are fitted.
Cutting a tenon
To cut out the tenon in the timber, place the timber at an angle in a vice and cut with a tenon saw at an angle down both sides of the tenon to the depth line already marked. Use pieces of scrap material to protect the timber from the jaws of the vice.
Turn the wood round and repeat the procedure.
And then adjust the timber in the vice so that it is vertical and saw to the depth line on both sides of the tenon.
Secure the timber horizontally (either in the vice or by G-clamp onto a bench) and remove the waste from both sides of the tenon by cutting along the depth lines.
When wedges are being used, cut two thin tapered wedges from matching wood the same width as the mortise. And put two saw cuts in the tenon to about two thirds of its length.
When assembling the joint, apply woodworking adhesive to the tenon and the inside of the mortise and fit them together. Ensure that the shoulders of the tenon rail meet the mortise rail squarely and without any gap.
If wedges are being used, apply adhesive to the wedges and insert them in the saw cuts. Use a mallet or a piece of wood under a hammer to drive the wedges alternately to keep the tenon straight in the mortise.
Clamp the joint between padding until the glue is cured.
When the glue has gone off, saw off the ends of the tenon/wedges and smooth the joint with fine glasspaper or a block plane.