Multiple Methods for Making a Mortise

The mortise and tenon joint is about as old as woodworking itself. Woodworkers tend to be an innovative lot, which might explain why there are about as many ways to make a mortise as there are woodworkers. In this post, I'll summarize some of the more common ways of cutting a mortise.

Chisel and Mallet

The traditional way to make a mortise, before power tools came along, was with a mortise chisel and a mallet. Using a thick chisel, designed to take the stresses of being beat with a hammer, you literally chop out the material to form the mortise. Typically you start in from the ends of the mortise and remove the majority of the material. Lighter paring cuts are then made to bring the mortise to it's correct length.

Drill and Chisel

Another common method involves the use of a drill (powered or otherwise) and a chisel. After laying out the mortise, you use a drill equipped with a bit slightly smaller than the width of the mortise to drill a series of holes along the length of the mortise. A forstner bit is a good choice for this method since it leaves a mostly flat bottom and makes it easier to overlap the holes.

Once the holes are drilled, use a sharp bench chisel to pare the mortise to the layout lines. The idea here is that the drill removes the bulk of the material and the chisel cleans up the mortise.

Hollow Chisel Mortising Machine

At some point, someone came up with a mechanized, one-step version of the previous method, inventing the hollow chisel mortising machine. A square chisel with a drill bit inside plunges into the wood to make a square hole. The drill bit removes the majority of the material and the chisel squares up the hole. A mortise is made by making a series of overlapping plunges into the wood the entire length of the mortise.

There are dedicated machines for making mortises as well as attachments for your drill press. The former typically have a longer arm to get the leverage needed to plunge the chisel into the wood, but they take up additional space. The mortising attachment for your drill press uses a machine you may already own, but it's harder to plunge the chisel. In addition, it's not trivial to switch between mortising and drilling operations with the mortising attachment

Router

The router can be used to make just about every joint in woodworking, and the mortise is no exception. Using a plunge router to make a mortise involves some kind of jig or fence system to guide the router along a straight line and, in some cases, limit it's travel to the length of the mortise. An upcut spiral router bit it usually the best choice here since it tends to eject the chips from the mortise.

The basic technique, regardless of the jig used, is to first make a full depth plunge cut at each end of the mortise. The remainder of the material is removed with multiple passes, increasing the depth of the mortise (about 1/4 inch or so) each pass. Once the mortice is cut, you can square up the corners with a chisel or make your tenons with rounded corners.

The jig to guide your router can be as simple as an edge guide mounted to your router and a way to hold the router stable on the workpiece. If you want something fancier, there are no shortage of plans for more elaborate jigs to make mortises with your plunge router. Of course, there are also, commercial jigs you can buy. Typically, as the speed and accuracy of making the mortise goes up, so does the price.

These are just some of the ways to make a mortise. What is your favorite way to make a mortise? Are there any tips you have for making mortises quickly and accurately?

1 comment:

  1. Good to see some one collate information and publish like this. Good work man. Great info to put out in the public.
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