A hollow-chisel mortiser makes short work of cutting mortises. Here’s what you need to know to get the best results.
When you only need to cut a few mortises for a small project, drilling out the waste and squaring the sides with a chisel works great. But when you’re faced with the task of cutting a large number of mortises then you need a method that’s fast, accurate, and reliable. A hollow-chisel mortiser just might be the answer.
SQUARE HOLES.
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| Hollow chisel and auger bit. |
A hollow-chisel mortiser is designed to “drill” a square hole. (I’ll talk more about that later.) It does the job of removing the waste and squaring up the walls of the mortise all in one stroke. That not only makes the work go faster, but helps you get consistent results every time.
AFFORDABLE.
Benchtop mortisers are available at prices ranging from $200 to $450. So when you figure out the time you can save in a big project, they start to look pretty attractive. But like any tool, there are a few things you’ll need to understand before you can consistently get good results.
HOW IT WORKS.
The business end of a mortiser is the hollow chisel and auger bit combination, like the set shown in the left margin. They’re available in several sizes to cut mortises from 1⁄4" up to 3⁄4" wide. When drilling a mortise, the auger bit spins inside the chisel, cutting the hole and clearing the waste. The hollow chisel squares up the sides of the mortise as you plunge it into the workpiece.
THE MOTOR.
A 1⁄2-hp, direct-drive motor is the standard on most benchtop mortisers. But in terms of drive speed, you’ll find 1700 and 3400 RPM models. Unlike a drill press, the speed isn’t adjustable.
Each type has its own advantages. The faster motor cuts more aggressively and is less likely to bog down during a cut. But the speed can also generate more heat. Too much heat can quickly dull your chisels and bits. The lower-speed models cut slower, but they also help reduce the heat build-up.
HANDLE.
The strength of a mortiser lies in the rack-and-pinion plunge assembly. To take full advantage of this system, all mortisers have a long handle to drive the pinion and provide plenty of leverage to plunge the chisel into the workpiece. You’ll really appreciate this mechanical advantage when you’re chopping mortises in oak and other hardwoods.
CHUCK AND CHISEL LOCK.
As I said earlier, most of the waste is removed by an auger bit that fits inside the hollow chisel. This bit fits into a standard drill chuck. Some models include an extralong chuck key to reach inside the case. A chisel lock holds the hollow chisel in position around the bit.
THE TABLE.
While some mortisers have a cast iron table, an MDF table is more common on benchtop models. There’s no real drawback to an MDF table, but it’s important to make sure it’s square with the chisel. If it’s out of square, you can correct the problem by removing the screws that hold the table and adding a shim. Usually, a couple pieces of masking tape are all it takes to square the surface.
FENCE.
Sitting just above the table is the adjustable fence. The fence keeps the mortise properly spaced on the workpiece. Cast iron is the norm for fences. Once again, you’ll want to make sure it’s square to the table. To keep chips from piling up between the fence and workpiece, most mortisers leave a gap between the fence and table.
HOLD-DOWNS.
Since the hollow chisel fits tightly against the walls of the mortise as it cuts, the workpiece has a tendency to lift off the table as you raise the chisel after a cut. For this reason, all mortisers have some form of hold-down. The example at right shows a common arrangement. A post is incorporated into the fence and the hold-down is adjusted vertically.
DEPTH STOP.
To control the depth of cut, all mortisers include a depth stop. Like depth stops on drill presses, how they work varies from one machine to another. One of the simplest and easiest to use is an adjustable rod, like the one shown on the model at right.
An Alternative: Mortising Attachment on a Drill Press
Another option for hollow-chisel mortising is to add an attachment to your drill press. A mortising attachment kit is relatively inexpensive (around $75) and kits are available for most types of drill presses. The nice thing about a drill press kit is you can adjust the bit speed.
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HOW IT WORKS.
The way the kit works is pretty simple. To hold the chisel, a yoke fits over the quill of the drill press and locks in position with a collar. With the yoke holding the chisel, the drill drives the auger bit. The photo at right shows the fence and hold-downs that are included in a typical kit.
DOWNSIDE.
But this arrangement has some drawbacks. First, setting up and removing the attachment is time consuming. And the handles of a drill press can bend under the torque needed to plunge a chisel into hardwoods.
TUNE-UP TIPS.
Before you can start cutting mortises, it’s important to have all the components of your mortiser tuned-up properly. And this starts with the chisel and bit.
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| A quick spritz of dry-lube spray helps reduce friction and heat build-up for smooth cutting. |
SHARPENING THE CHISEL.
Just like any other woodworking tool, the cutting edges of the square chisel and auger bit need to be sharp to keep them cutting efficiently. But sharpening the hollow chisel is a little different than what you may be used to.
I start by working on the beveled, inside faces of the chisel. I use a specially designed diamond cone. Refer to Sources on page 51 for more information. Once you’ve honed the inside bevel, the next step is to work on the outside faces. As you can see in the center photo below, I like to use a stone to polish each of the four faces of the chisel. Keep in mind that sharpening changes the width of the chisel slightly.
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| A diamond cone makes short work of sharpening the bevel on the inside of the chisel. |
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| The four outside faces of the chisel are easily honed using a waterstone. |
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| Hone the inside faces of the cutting lips of the auger bit with a fine-grit diamond paddle. A few strokes is all it takes to get a sharp edge. |
AUGER BITS.
Since the auger bit does most of the work of cutting a mortise, it’s important to keep it sharp. For this task, I turn to the paddle-style diamond sharpeners. The important thing to remember here is that you only want to sharpen the flat, inside edges of the bit.
INSTALL THE CHISEL & BIT.
After sharpening, you’re ready to install the chisel and bit.
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| When set up properly, the auger bit should extend just beyond the chisel. |
When properly set up, the auger bit extends beyond the cutting edges of the chisel. Without adequate clearance, chips can clog and burn in the chisel. But if the bit extends too far, it could break. Fortunately, there’s an easy technique for installing the bit and chisel correctly.
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| A penny is just the right thickness to set the clearance between the end of the chisel and bit. |
You can set the right clearance by using a penny as a temporary spacer when you install the chisel. With the chisel in place (using the coin as a spacer), install the auger bit in the chuck with the spurs of the bit even with the tips of the chisel. Finally, loosen the chisel lock, remove the coin, and raise the chisel. Close the gap and seat it firmly against the mortiser, then tighten the lock. All that’s left to do now is to align the chisel with the fence.
ALIGN THE CHISEL.
A successful mortise and tenon joint relies on a flat-walled mortise. That means the chisel must be square to the fence. To square the chisel I lower it until it almost touches the table. Then loosen the chisel and rotate it until the back face is parallel to the fence and retighten it. Now you’re set to start cutting.
CUTTING TECHNIQUE.
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| The top mortise was cut with a misaligned chisel, while the one below was dead-on. |
With the chisel and bit sharpened and your mortiser now properly set up, you’re ready to start cutting mortises. And this is where proper technique makes a difference.
SET THE FENCE.
After you’ve marked the mortise locations on the workpiece, the next step is to set the fence. This will determine the location of the mortise relative to the edge of the workpiece.
If you need to cut a mortise centered on the thickness of a work-piece, there’s a simple technique you can use. With the motor off, lower the bit until it creates a small dimple in the workpiece. Then, flip the workpiece end-for-end and repeat this process to create another dimple near the first one you made. Now adjust the fence and repeat this process until the dimples match in the center.
CUTTING STRATEGY.
A common mistake many woodworkers make when using a mortiser is to simply cut overlapping holes. The problem with this technique is that the chisel is apt to deflect toward the unsupported side of the cut. The resulting mortise may not have square walls.
The section below illustrates a more reliable technique for cutting square-sided mortises. I like to start by cutting a hole at each end of the mortise first. Then make a series of cuts, leaving a narrow “bridge” between each one. A final clean-up pass removes the remainder of the waste and smooths the walls.
How-To: Cutting a Mortise
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| Start With the Ends. Make sure the hold-down is tight against the workpiece, then make a cut at each end. | Staggered Holes. Next, remove more of the waste, leaving a small ”bridge” of material between each hole. | Clean-Up Pass. Finally, go back and make one last pass to remove the waste left between cuts and clean up the mortise. |
HEAT & SMOKE.
Even with a well-tuned machine, you might get some smoke when you cut mortises. Most of the heat is a result of chips not ejecting properly, so you’ll want to stop and clear the chips often. A little bit of discoloration is common when chisels overheat, as shown in the center photo at right.
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| The chisel can become packed with chips that cause heat build-up. | Discoloration caused by heat build-up doesn’t affect the edge-holding ability of the steel. | The discoloration can be easily removed by honing. |
As you can see, getting clean, square mortises is easy with the right setup and technique. In the end, that means stronger joints and better-looking projects.
