The Router Mortise Box - Woodworkers Institute

The Router Mortise Box:
If you want good strong joints the traditional mortise and tenon is the way to go. Here’s how to machine them using a router and a mortise box.

If you want good strong joints the traditional mortise and tenon is the way to go. Here’s how to machine them using a router and a mortise box

You can use a router and its fence with a straight cutter, but it is very easy for the router to stray off course and spoil the mortise. The amount of cutting effort involved when the cutter is deep in the wood is enough to make this rather traumatic when the cutter jumps sideways. It’s far, far better to make a simple, reliable device called a mortise box. This holds the workpiece securely and controls the movement
of the router.

How to make
The size of the box isn’t critical, but all the components need to be square-edged and parallel. It must be big enough to accommodate the largest intended components.
The box is nothing more than an inverted ‘U’ shape. To fix it down you can either screw or bolt it to the bench or alternatively, fix a batten underneath so it can be clamped in vice jaws. In the past it has been necessary for a router to straddle the width of the box, but this can limit the width of the box. It also needs longer than average fence rods and a second fence to avoid sideways movement.
My solution has been to add a ‘tray’ on top of the box. It sits perpendicular to the mortise box and needs to be the width of the base so the router can slide sideways smoothly and without any play. The tray has two little fillets of wood glued underneath and on the outside of the box, which ensures they slide smoothly along the box.

Another view showing the workpiece clamped to the side. Note the router tray has two infill blocks preventing it moving sideways

To control the width of the mortise, infill pieces of the appropriate size are placed in each end of the tray. Different width pieces will alter the mortise width and position.

Two blocks dropped in which allow a limited amount of sideways movement so the mortise width can be increased

The workpiece can be straight or shaped like a rear chair leg for example. It will generally be clamped to one side of the box and sit on packers to bring it right up under the tray. Begin by marking out the mortise, then position the chosen straight router cutter against the width lines and make and drop in infill pieces so they act as sideways machining stops.
Now, move the cutter to each end of the mortise markings and add clamping pieces to the top of the box at one side, or screwed right across the box top edges so they prevent the router moving beyond those fixed points. If you do a series of identical mortises just line the workpieces up to a datum mark so you get exactly the same mortise position each time.

Cutting tips
Mortises are cut with straight cutters. There are different types but a standard plain twin-bladed straight cutter works well. Alternatives are pocket, stagger tooth and disposable tip models, although these are more expensive types. Make a series of passes to final depth, and don’t take heavy cuts as it will strain the cutter. If the router has been given freedom to move sideways to enlarge the width of the cut, always travel so the router and cutter moves into the rotation of the cutter, i.e. clockwise.
Chippings extraction is problematic as the tray will sit right over the mortise and your vision of the cut is limited, although you can open up the tray base quite a bit. Lift the router and tray off between passes and use a vacuum and nozzle to clear the chippings out.