Photos by Carl Duguay; Mortiser Courtesy of King Canada
There are many approaches to cutting mortises. When you only have a few to make, or you have a lot of time to spare, you can cut them completely by hand, using a mallet and mortising chisels. If you need to cut a lot of mortises you can speed up the process by using a combination of hand and power tools. One option is to drill out the mortises with a hand drill or drill press. Another option is to mill the mortises with a hand-held router or table mounted router and a mortising jig; with either of these methods you may want to square up the walls of the mortises with a mallet and chisel, though it’s not imperative.
A hollow chisel mortiser allows you to bypass the mallet-and-chisel work, yet still end up with square-ended mortises. These machines also make quick work or mortises once set up. They come in both stationary and benchtop formats. Unless you work in a production environment, churning out dozens or hundreds of mortises every day, a benchtop machine will do just fine. If you typically cut mortises under 5/8″ wide, then a model with a 1/2 HP motor that accepts 5/8″ shank bits will do nicely. For mortises 3/4″ and larger you’ll want a model with a 1 HP motor that accepts 3/4″ shanks.
You can think of them as specialized drilling machines that use a unique two-part bit – the hollow mortise chisel and bit. The chisel is a square hollow metal tube with beveled facets on all sides, which squares up the hole that’s created by the bit (aka ‘auger’) that telescopes through the tube.
Just like on a table saw or router table, the quality of cutting tool you use is of primary importance, and the key to cutting consistently clean mortises with a benchtop mortiser is using premium chisels. A good set of four basic chisels (1/4″ to 1/2″) will cost a couple hundred dollars but is well worth the investment. You can easily sharpen the chisels with a cone sharpener, or send the bits out to a reputable sharpening service.
There are two key things to keep in mind when setting up a benchtop mortiser. The first is alignment. Ensure the fence is square to the table, and the chisel/bit is square to the fence in all directions. I recheck these details every time I use the mortiser. Second, when installing the mortising chisel, ensure the bit extends slightly below the tip of the chisel so the two components don’t rub together, and there’s room for the chips to pass up through the chisel and out the side windows.
Don’t ram down the swing arm when you start the cut – allow the bit to do its job of drilling into the stock, then increase downward pressure advancing the chisel into the evacuated hole. And don’t skip lunch – you’ll need to put a lot more arm muscle into each stroke than you do on a drill press.