How to Cut a Mortise
A drill stand, combined with a special mortising bit and a yoke holder, allows mortises to be cut cleanly, accurately and quickly with a power tool.
The mortising bit itself consists of a twist bit rotating inside a hollow square chisel. As the drill removes the waste in the centre, the chisel is able to cut downwards under pressure from the lever on the drill stand.
It is essential to have a strong, rigid drill stand for use with a mortising bit — lightweight ones tend to whip. The stand is suitable for this sort of work and has the additional advantage of being able to accommodate most makes of.
To cut a mortise, mark off the mortise positions on the work in the usual way, using a try-square and mortise gauge.
Carefully set the work under the holder, aligning the marked-out mortise area with the mortising bit, and then clamp the work down firmly to ensure that there will be no movement or twisting when drilling starts.
If a through-mortise is being cut, drill to just over half the depth from one side, then complete the cut from the other side. This will reduce inaccuracies and the splitting which is bound to occur if an attempt is made to drill right through from one side.
Start cutting at the left-hand end of the mortise, cutting out about 5 mm (¼ in) at a time until you reach the required depth. Withdraw the bit after each cut to clear away waste. Next start the right-hand side, again with a series of shallow cuts.
The centre portion can then be sunk, working from left to right and going down to the full depth required. This is now possible because the waste is automatically cleared to one side without fouling the bit.
Although care and attention are required at all stages, the power tool and mortising bit method is certainly quicker than hand cutting and, except for the expert craftsman, far more accurate.
When a number of mortises have to be cut, mark them all up first, then cut them with the mortiser one after the other.
The comb or box joint used in drawer and tray making can now be made with a power drill attachment. The combined drill and attachment must be firmly fixed to a worktop. Once it is set, only minor adjustments are necessary to produce many joints on varying thicknesses of timber.
The basic principle on which the tool works is a simple one. The circular blade is held at an angle to its arbor or spindle by a pair of matched wedge-shaped washers so that, when rotating, the saw oscillates and cuts a slot wider than the true width of its teeth (the wobble saw used in grooving works in exactly the same way).
Remember that such joints can be cut only on end grain, ie., at the ends of lengths of timber, that comb jointing ofquickly wears the cutting edge of the saw so that frequent adjustments become necessary between batches of work, and that the cut-out width must equal that of the timber left between them if the joint is to fit.