Learning Mortise and Tenon Joints

The Mortise and Tenon Joint [courtesy Wikipedia]

Sooner or later every woodworker must learn how to make mortise and tenon joints. These joints are required for a variety of situations, especially in many kinds of framework. Most shutters, doors and windows are made using this joint. It is a very fundamental joint and every woodworker must learn to make it.

I had been dawdling for some months, putting off my first mortise and tenon joint. Frankly, I was intimidated. For, the joint is a potentially tricky matter because of the precision required. It may look easy but unless the alignment of the tenons and the mortise slots are perfect, the joint will not come together properly or will be horribly misaligned. 

A quick search on the Internet provided hundreds of results on how to make the joint. One particularly simple one was at technologystudent.com (url: http://www.technologystudent.com/joints/morten1.htm)
I also watched several YouTube videos on the subject and eventually convinced myself that it did not seem that hard.

To watch British master woodworker and author John Bullar make mortise and tenon joints watch this YouTube video: http://www.youtube.com/watch?v=t3jByNHeGxs&feature=player_embedded
The other YouTube video worth watching is the one by Paul Sellers on hand cut mortise and tenon joinery: http://www.youtube.com/watch?feature=player_detailpage&v=LPBkO2chZxk

I watched these great men cutting mortise and tenon joints and it looked so simple that I was inspired. I sharpened my chisels and got out my Stanley cross cut saw and launched forth.  

My first few attempts were extremely disappointing; one problem was that my chisels were not sharp and I was making the mistake of using pencil lines to make the cuts. Master woodworkers warn against using pencil lines because they are thick and imprecise. It is imperative that a marking gauge and marking knife be used for marking the cuts. These cuts are fine and precise and translate into greater accuracy. In fact, I have stopped using pencils for marking any kinds of cuts. The marking knife is the best choice.

Mortise gauge. Good ones like this can be purchased at any carpentry shop for about Rs 250. I bought this one at Chawri Bazar in Delhi.

Tenons marked in stock with a mortise gauge. The width of the tenon is determined by the width of the chisel that is going to cut the mortises.

However, even after marking the cuts accurately I found that my mortise and tenon joints turned out poor and rattlly. 

The joint in my first few attempts turned out loose and rattly. I was terribly disappointed.

I realised one problem was the saw: its kerf was too thick and I needed to compensate for that. Also, my mortises were not as clean and straight as they should be for a tight fit. Moreover, cutting tenons cleanly and absolutely straight with a saw required a level of skill I clearly lacked. It was frustrating.

I finally decided that cutting mortise and tenon joints by hand was not my cup of tea. I would try doing them with a router and a jig. Cutting tenons with a router is relatively easy because it is basically a slight variation of the method for cutting trenches and dadoes. I can cut dadoes, grooves and so on quite accurately using a router fitted with a bearing guided mortising bit such as the one shown below.

Mortising bit with bearing guide. The guide runs along the straight edge.

To cut a tenon by this method, first depth the cut by precisely lowering the router bit so that its bottom aligns with one edge of the tenon, then place a piece of wood with a straight edge along the shoulder line of the tenon and make the cut. Repeat the procedure for the other cheek of the tenon. This works well provided the router is held steady and does not tip even slightly during the cut. The slightest tipping will result in a furrow along the tenon cheek.

Alternately, there are many simple tenon cutting jigs for use with a router. Most depend on a guide bush to guide the router. One is this simple but effective jig such as this one featured in the Shop Notes magazine:

This jig was featured in Shop Notes magazine No. 97. To view the jig and instructions on how to make it go to http://www.woodworkingseminars.com/wp-content/uploads/2010/04/WS97_SimpleTenonJig.pdf

Another great little article can be found at the Trend Machinery Company website at this url: http://www.trend-uk.com/en/JA/trend/content/content_detail.php?record_type=Knowledge&id=44
In short there are many methods and many jigs to cut tenons with a router and I would recommend this path to aspiring woodworkers without superb skills in using hand tools.

My Mortising Jig

I also made a simple jig with MDF to cut the mortises. This jig is based on my DeWalt 625 router and has a slot cut which matches the diameter of the guide bush supplied with the router. In other words, the router glides within the slot and does not move sideways. Fitted with a long straight bit, preferably a solid carbide spiral up cut bit, very clean mortises can be quickly routed. The jig has to be clamped to the workbench and the stock clamped perpendicularly to the jig against a sliding attachment.

The stock is clamped from below
The stock is aligned with the edge of the slot in the jig

The key to this method is to ensure that the stock aligns perfectly with the edge of the slot or else the cut will not be parallel with the sides of the stock and the tenon stock will not align properly. The jig also has two adjustable stops for setting the beginning and end of the mortise. This is a very simple jig that can be built in a couple of hours. The width of the slot in the jig will depend on the diameter of your router’s guide bush.

Frame made with mortise and tenon joint

With a bit of adjustment, trial and error, the combination of tenon and mortising jigs can produce very well fitting joints. I was satisfied with my attempts and managed to make a frame for a medicine cabinet door I had been working on. The sides with rounded off with a round over bit. This came to good use as the shutter in my new medicine cabinet.

Medicine Cabinet: Plywood and Saal with Shellac finish
 Making mortise and tenon joints has proved to be a useful skill which I plan to develop over the next few weeks.

The Medicine Cabinet in use

Indranil Banerjie
14 February 2013