How to Use a Benchtop Hollow Chisel Mortiser

Shop Skills: With a benchtop mortiser, you really can drill square holes.

How to Use a Benchtop Hollow Chisel Mortiser

How to Use a Benchtop Hollow Chisel Mortiser



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 tablemounted 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.
 
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Proper Alignment – Ensure that the fence is square to the work table – if not, you may have to shim the table.
 
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Square to the Fence – When installing the chisel always check that it's square to the fence. Orient the chisel windows to face sideways, to help with chip ejection.
 

Good bits are crucial

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.
 
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Set the Offset – Place a 1/16"-thick spacer at the top of the chisel, push the chisel up against the bushing, and then tighten the chisel locking screw.
 
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Flush the Tips – Using a piece of wood lift the bit all the way up and secure it in the chuck.
 
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Create the Bit Offset – Loosen the chisel locking screw, push the chisel up as far as it will go, and then re-tighten the chisel locking screw. Now the bit should protrude 1/16" below the chisel.
 

Setting up a mortiser

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.
 
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Set the Mortise Location – Position the fence so the tip of the bit registers on the center line of the mortise.
 
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Depth of Cut – Adjust the chisel and bit set to cut at least 1/8" below the bottom of the mortise to allow for glue build-up during assembly.