Technical - Workshop Notes
“Mortise and Tenon joints - what's this all about?”
Whilst only doing a few joints like these myself, a recent discovery at the KDWC clubrooms, of a fine pair of tenon jigs, that few knew about, took my eye. How do they work? What are they best used for?
Typically found in furniture, like tables and chairs where beams need to be joined at right angles, these joints comprise a slotted hole (the Mortise), and the other piece has a tenon (inversely shaped spigot) that fits neatly into the mortise, joined with just glue. The slotted arrangement, coupled with the right glue, makes for a mighty strong joint.
Many a woodworker attempts to end-drill out the slotted hole with a power drill, and then tackle the making of a tenon on the other piece of wood, using a table saw, set low. But for repetitive joints, a jig makes life easier, and more accurate. Plus, making a slotted Mortise hole, is made easy with a square drill. What? Making a square hole? Yes, more commonly known as a "Square Hole Mortise Chisel Drill Bit Tool", the bit comprises a square ended set of chisel cutters, with an internal drill bit that has a flat end to it. Like counterBORING a hole with a flat ended drill bit, mentioned in last month's article on pocket hole tools.
People know of dowel joints, where a small round dowel is glued in a tight hole in two joined pieces of wood. Well, a Mortise and Tenon takes that concept to a bigger joint, using more timber in the hole, so to speak, and utilises only the timber that is part of the two pieces being joined. This is unlike the addition of a third piece of wood, as is the 'dowel' approach.
A Biscuit Joint is like a dowel joint, except the third piece of wood is a wafer thin slither of wood (the 'biscuit') that is inserted into a shallow slot, made identically into the two pieces of timber being joined.
Both of these are not strong enough for furniture that needs to take weight, like tables and chairs.
Preparing the mortise is best done first, once all measurements are known, and the selection of the right sized Square Hole cutter applied. For instance, making a 17mm wide slot, 55mm long. into which, a tenon will fit later. The depth of the hole made, needs to match the length of the tenon yet to be made.
The club uncovered a pair of massive cast iron tenon jigs with no labels recently. Turns out, they fit the pair of heavy duty router tables in the sanding room. They allow a piece of wood, lets say a piece of 75mm x 37mm dimension, to have one end of the wood shaped to have a spigot formed out of the inner section of the wood, by removing the outer edges. As mentioned, this removal of the outer edges could be performed on a table saw, set really low (say 10mm) so that the small protruding saw teeth could be run across the base of the piece time after time, reducing the size to an inner spigot of let's say 55mm x 17mm in the end. Such a process would require lots of movement of a fence, and very careful measuring to leave the right amount of desired tenon.
By comparison, the tenon jig, on a router table, can be set to the desired removal of 10mm all round the end, at a desired length too, and hold the piece in place. Then the job is passed over a router bit to complete, in four pases, for the four sides of the tenon that remains.
Angles and compound angles
Sometimes, two pieces of timber need to be joined at a slight angle, for design purposes or simply for aesthetics. No problem, the tenon jig can be adjusted for a lateral angle in one direction as well as a side angle if a complex compound angle is needed overall.
Many TENON JIGS these days are sold with a mitre foot, to run in a mitre track on either a bench saw table, or a router table. But the two Carba-Tec router tables at the club, have a pair of bolt holes for fitting to a sliding rail facility just behind the fence. Other than that, the principle of setting the angles (or leaving at 90 degrees), and the positioning of the job over the router cutter, all remain the same. The length of the tenon is determined by the amount of exposed router bit, and the amount of removed material around the tenon, is determined by the circumference of the router bit in conjunction with its distance fom the fence. A simple wind up clamp allows multiple pieces to be cut in identical manner by easy insertion/removal of the next one to cut.
The proof is in the tasting as they say. The setup required removal of the wooden fence plates from the sliding fence support, and the adding of the tenon jig with a single bolt to the swivel point on the round support. Just be sure to align the bearing over the casting, and not on the adjacent ruler, as is shown incorrectly in on of the photos. The bearing wheel runs along the top of the moving fence. Any pushing on the handle when in motion, tends to want to cock the whole jig in its lateral movement along the round support. But when pressure is applied at that point of connection to the round support, the jig moves freely. This calls for the addition of a 2nd vertical handle as part of the connecting bolt process. Possibly why modern models for saws on mitre tracks, have two handles.
Tear out becomes the enemy. After some consultation with fellow Woodies, the routing sequence was improved, by taking away a small amount out of the shoulder around the ultimate tenon first. Then, moving the fence to make the final router cut, yeilding the desired thickness of tenon. Multiple passes are suggested, raising the router bit progressively, rather than trying to cut the ultimate tenon length in one pass. So, in reality, there is need to move the job in the jig quite a number of times as each of the four sides of the tenon are cut away.
Possibly from this this experiment, it is now clear as to why these jigs have been stashed for so long! But it was a good opportunity to test out the new mortise square drills in the vertical borer too, so the day wasn't totally wasted.
Happy woodworking! .... Gary Pope 0408994799