How to make wooden coffee tables, making the coffee table corner joints.

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Details of Wooden Coffee Tables Corner Joints.

diagram showing proportions of mortise and tenon joints

The top diagram on the right shows a typical wooden coffee table corner joint, (a haunched mortise & tenon joint). The haunch is square in section looking from the top. Looking from the side the haunch is one third & the tenon two thirds of the total depth.
I normally set the face of the rail about 6mm from the face of the leg.
The mitre should be cut to give 4-6mm clearance between the ends of the tenons when they are both fully inserted into the mortices, it is not a mitre joint, but merely allows a longer glue face on the outside face of the tenon giving more strength, especially when the leg is not very big in section.

The top view of the tenon shows the tenon as being one third of the thickness of the rail. These are the correct proportions for a tenon when the stock containing the mortise is of equal thickness to the rail, splitting as it does the thickness and therefore the strength equally between the three parts. As however, the coffee table leg is normally of much greater thickness than the rails, it is possible to increase the thickness of the tenon until it matches the dimension in from the outside of the leg to the mortice thus making a stronger joint.

In fig 7 I have shown the mortice & tenon in 3D but before the mitre has been cut and with only one mortice cut for the sake of clarity.
The 4-6mm cut off the bottom of the tenon allows the mortice to be made slightly oversize at the bottom allowing "wriggle" room when you are dry fitting the tenon as the recess in the rail will cover the gap when the tenon is fitted.

TIP: The ends of the tenons and the haunches should have a chamfer taken off either with a chisel or small plane to ease the fitting of the tenon into the mortise.

Some of the Methods of Making Mortises & Tenons.

MORTISING

BY HAND - There are dedicated mortise chisels available with a meatier shaft than usual to withstand the leverage needed to break out the waste wood, but it is perfectly possible to do this with a normal set of chisels. Does require a good eye and craftsmanship to keep the chisel at 90 degrees to the workpiece.
PLUNGE ROUTER - You can either make a jig to guide the router, (best option if you have a fair number of mortises to cut), or use some accurate setting up with the fence. Produces a mortise with rounded ends which either have to be squared off by hand, or you need to cut the tenon only to the squared length of the mortise and leave an overlapping shoulder on the ends of the tenon to cover the gaps.
VERTICAL BOX CHISEL MORTISER - The commonest and easiest method. Comprising of a hollow box chisel with a long series drill bit inside fitted to a lever handle drill press, they vary from heavy duty machines with fully adjustable table worked with hand rotating wheel handles to cheaper lightweight machines where one has to slide the workpiece along by hand for subsequent cuts.
CHAIN MORTISER - I have never come across one of these in the workshops I have worked in, but from the illustrations I have seen in woodworking books, they appear to consist of something akin to a mini chain saw mounted on a lever handle press.

TENONING

Lightweight Vertical Box Chisel Mortiser.

Homemade Router Jig for Tenoning.

BY HAND - Using a tenon saw.
CROSS CUTS ON CIRCULAR SAW TABLE - The tenon is cut by setting the saw blade to the correct height to remove the waste stock and making parallel cuts close together by moving the rail in towards the fence about 1/8" (thickness of the saw blade) repeatedly after each cross cut until the rail reaches a block clamped to the fence set at the correct distance in from the saw blade for the shoulder cut. The rail is then turned over and the process repeated on the other side. The height of the blade is then adjusted for the end shoulder cuts and the process repeated again. This process is not possible on some saws where the riving knife protrudes higher than the blade and cannot be adjusted downwards. On a saw where the crown guard for the blade is fixed to the top of the riving knife the process is only made possible by removing the crown guard which is not recomended as it involves breaking health and safety rules. This is a fairly slow method of cutting tenons.
VERTICAL CUT ON CIRCULAR SAW TABLE - This involves parallel saw blades (combination or rip but not crosscut only) divided by a spacer on the spindle the same thickness as the tenon. The rail is held vertically in a sliding jig with a handle well away from the saw bade. The two blades cut the two sides of the tenon accurately. Can also be done with single saw blade by turning the rail after the first cut but accuracy will not be quite as good as you will not be working from one face only or with a fixed width spacer.
The shoulders have to be cut separately by other means.
FIXED TABLE MOUNTED ROUTER - Using a bench mounted router with a fixed fence and sliding jig to push the rail over the router cutter at 90 degrees, if there is a lot of waste to remove, it is better to adust the cutter height up in increments of about 3 - 4mm rather than trying to make deep cuts in one pass. Shown on the right is my version of such a jig. In this case I cut the shoulders of the rails on the circular saw first to minimise break out of the grain at the end.
PLUNGE ROUTER - The workpiece is held secure in the jig and the router is passed over the rail by means of a guide jig to cut the tenon.
TENONER - This is more of an industrial method used in larger workshops or factories. A tenoner is in fact a spindle molder with dedicated set of cutters for tenoning and a sliding table to which the rail is clamped. The tenon has both sides cut with one pass and it is therefore a very fast and efficient process. The one drawback is that, due to the large amount of waste wood being removed in one cut, there can be quite a lot of break out of end grain on the shoulders.
BANDSAW - This is my preferred method for the comparatively small tenons required in the making of a coffee table. The blade needs to be a combined rip and cross cut blade and of sufficient size to cut without deviation, minimum about 16mm but wider if possible. It is also important to ensure that the blade is sharp as a blunt blade will wander off line. Because of the length of time it takes to change and set up a new blade on a bandsaw, I keep my larger machine permanently set up for tenoning with a large blade and have a smaller bandsaw with a thinner blade for making curved cuts. The bandsaw can be set up, with the aid of a cross cutting jig and the use of stops, to make all the cuts for a haunched tenon.

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