The mortise and tenon joint is a fundamental in furniture making. It has been around for ages and has stood the test of time. The joint consists of two parts – the mortise, which is the hole, and the tenon, which fits into the hole.
What makes this joint so strong is the large gluing area. The joint can also be reinforced with wedges and pins. Examples include the tusk tenon and the draw-bored tenon. When cutting a mortise and tenon joint, the mortise is usually cut first, then the tenon is made to fit the mortise. This is because it is much easier to trim the tenon to fit into the mortise than vise versa.
Traditionally, the mortise was cut using a mortising chisel. Modern, powered methods use some sort of bit to evacuate the mortise. They all work, but each has it’s own quirk. Using a drill press and a forstner or brad-point drill bet makes quick work of removing the bulk of the material, but leaves round corners and ridges along the inside of the mortise to be cleaned up, often with a chisel.
A mortising attachment is available for the drill press and consists of a square, hollow chisel with a drill bit inside. This essentially allows the drill press to bore a square hole. However, both the bit and chisel need to be kept very sharp to work well and they have to be precisely aligned with each other. It is requires a special attachment to hold the hollow chisel and the purchase of a different size hollow chisel and drill bit for each width of mortise cut. A very similar tool is the dedicated mortiser.
The router is another commonly used tool used to create mortises. Paired with an up-cut spiral bit, the router is very effective at quickly cutting mortises. A plunge router equipped with a fence works well as long as there is enough of a work surface to keep the router stable on. To solve this problem, you could add a second fence on the other side of the bit and/or build a mortising platform which is clamped to the work (or vise versa). Visibility is a little difficult, peering through the router and its (hopefully) transparent base plate.
Or you could invert the router in a table and use the fence to position the work. This solves the stability issue, but visibility is non-existent. Another variation of the router table is the horizontal router table, where the bit is oriented horizontally as opposed to vertically. This gives a much better view, does require a horizontal router table and some way of controlling the feed, which is often done with a sort of cross-slide fence. An industrial version of the horizontal router is called a slot mortiser. All of these options leave rounded corners at the ends of the mortise.
Today, I experimented with another way to cut mortises. I chucked a 3/8″ up-cut spiral bit into my drill press and set up my drill press fence to approximately center the bit on the 2×2 scrap piece of maple I was testing on. I began by plunging the bit to the full depth of the mortise at either end, then continued wasting away the center portion by plunging the bit, overlapping the previous hole by about 1/16″.
Next, I engaged the quill lock on the drill press and lowered the bit about 3/16″ into the wood and slid the work across the bit, from left to right. I returned the work to the left and lowered the bit again and made another pass, continuing until I had reached the depth, as set by the drill press’s depth stop. The first attempt left a rough face along the edge opposite the fence, so I clamped another fence against the stock and tried again and was rewarded with a much better result.