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MAKING THE JOINTS FOR A WOODEN COFFEE TABLE - Mortice & Tenon Joints - Methods of Mortising.

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Details of Wooden Coffee Tables Corner Joints.

diagram showing proportions of mortise and tenon jointsdiagram showing mortice and tennon

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.

MORTISING

Lightweight Vertical Box Chisel Mortiser.
Lightweight box chisel mortiser.
  1. 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.
  2. 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.
  3. 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.
  4. 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.

Researched & written by Nick at TheCoffeeTable.co.uk - Copyright © TheCoffeeTable.co.uk - Telephone: 01420 474862

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