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The Chair – Mortices

I was advised by a number of wiser heads to complete the mortices before turning. However I have been conservative in this regard because I was not sure how to deal with the angles, in particular the curved front and rear stretchers. The side stretchers are straight, however the two sides of the chair taper. The front and rear mortices were simply too difficult to calculate beforehand. Point-and-shoot was the more predictable option, albeit it with more handwork at a later stage.

As mentioned in the previous chapter, the tenon used in The Chair looks like this. The mortice is a stepped design to enable two legs to intersect the same section of the leg.

Here is a side stretcher with completed tenon. Note that this has yet to be coped. Also, the ends have been left square since the mortice is to be chopped out with a chisel. The factory tenons in the photo were created with a sander and fit mortices created with a router.

The thickness of the tenon is 9.5mm (3/8”), each is 60mm wide and 25mm long.

Here is the factory mortice ..

A jig for chopping a round leg

The ends of the legs had been left square for registering them when chopping the mortices.

The “jig”, for want of a better description, is made of three sections of MDF and a few small wedges.

The MDF section (at the rear) has a 3/8” wide through mortice that runs about 150mm/6” in length. This is a template for marking the mortices.

Before using this, the legs needs to be marked for the mortice.

Begin by marking the centre of the leg at each end.

Draw a line at the centre along the length of the leg …

The “jig” is built up around the leg. Simply, the leg is sandwiched by MDF one each side and stabilized with wedges ..

Enclose the other side and clamp together. The leg is now immobilized. Clamp this to the bench.

Mark off the position and length of the mortice, and use the template to mark the 3/8” width …

Score the outline of the mortice as deeply as possible with a heavy duty knife.

Chop out the top layer of wood. This is to create a flat surface.

The mortice is divided into two each sections, one that is 9.5mm deep and the other that is 25mm deep.

The mortices at the sides will be deeper at the top half, while the mortices at the front and rear will be deeper in the lower half. This will enable them to intersect at the same level.

The plan is to drill out the mortice to each of the step levels (half at 9.5mm and half at 25mm). The diameter of the leg in this section is 40mm.

To facilitate vertical drilling, a reference is taken from the clamp at the far end.

I started drilling with a Stanley depth stop for the auger …

However this marked the wood as it reached the end of its travel, and I returned to using trusty old blue tape.

After drilling out, a chisel was used to remove the remainder of the waste.

The fit is good for both mortices completed so far. The shoulders sit at the edge of the leg as they still need to be coped. I plan to finish all the mortices first, then cope in one hit.

Everything is nice and square. Side on, there is no twist.

So far so good.

Two down, and six to go.

Regards from Perth


January 2014