A hollow-chisel mortiser is one of the fastest and most accurate ways to cut clean, square mortises for mortise-and-tenon joinery. Instead of drilling away the waste and squaring the mortise by hand, a hollow-chisel mortiser does both jobs at once.
But getting good results takes more than simply pulling the handle. The hollow chisel and auger bit need to be sharp and properly adjusted, the chisel needs to be aligned with the fence, and the sequence you use to make the cuts can have a big effect on the finished mortise.
Here, we'll walk through how to set up and use a hollow-chisel mortiser, from installing and aligning the chisel and bit to cutting clean, straight-sided mortises. We'll also cover sharpening, avoiding heat buildup, and getting the best performance from your mortising machine.
What Is a Hollow-Chisel Mortiser?
A hollow-chisel mortiser is designed to cut square-sided holes in wood. It's particularly useful when a project calls for a large number of mortises and you want them to be fast, accurate, and consistent.
The business end of the machine consists of two parts: a square, hollow chisel and an auger bit that spins inside it. The auger removes most of the waste and carries away the chips. As you plunge the assembly into the workpiece, the four cutting edges of the hollow chisel square up the walls and corners of the mortise.
The result is a clean, square-sided mortise in a single operation.
Understanding the Parts of a Hollow-Chisel Mortiser
Most benchtop mortisers operate on the same basic principle, but understanding the major components makes setup and adjustment much easier.
Motor & Chuck
A direct-drive motor powers the auger bit through a standard drill chuck. Unlike a drill press, most benchtop mortisers operate at a single speed.
Higher-speed machines cut more aggressively and are less likely to bog down, but they can also generate more heat. Slower machines don't cut quite as quickly, but the lower speed can help reduce heat buildup at the chisel and bit.
Plunge Handle
One of the advantages of a dedicated mortiser is its rack-and-pinion plunge mechanism. The long handle provides the leverage needed to force the hollow chisel into the workpiece.
That mechanical advantage becomes especially noticeable when cutting mortises in dense hardwoods such as oak.
Table & Fence
The table supports the workpiece while the adjustable fence controls the mortise's distance from the edge of the workpiece.
Before using the machine, check that the table is square to the chisel and that the fence is square to the table. Small alignment errors here can translate into poorly aligned mortises and joints later on.
Hold-Down
A hollow chisel fits tightly against the sides of the mortise. As you raise the chisel after a cut, that friction can lift the workpiece off the table.
That's why a properly adjusted hold-down is important. It should keep the workpiece against the table while still allowing you to reposition the workpiece for the next cut.
Depth Stop
The depth stop controls how deeply the chisel plunges into the workpiece. This makes it easy to cut a series of mortises to a consistent depth—an important part of creating accurate mortise-and-tenon joints.
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| Hollow chisel and auger bit. |
How to Set Up a Hollow-Chisel Mortiser
Before cutting a mortise, spend a few minutes tuning up the machine. A sharp chisel and auger, correct bit clearance, and proper alignment make the mortiser easier to use and produce cleaner results.
Sharpen the Hollow Chisel
Like any cutting tool, a hollow chisel works best when its cutting edges are sharp.
Start with the beveled surfaces inside the chisel. A diamond sharpening cone makes it easy to hone these surfaces while maintaining their shape.
After honing the inside bevels, polish each of the four flat outside faces on a sharpening stone. You don't need to remove much material—a few careful strokes are usually enough to refine the cutting edges.
Keep in mind that sharpening the outside faces gradually changes the dimensions of the chisel, so remove only as much material as necessary.
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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. |
Sharpen the Auger Bit
The auger bit does most of the actual waste removal, so keeping it sharp is equally important.
A fine-grit, paddle-style diamond sharpener works well for this job. Hone only the flat inside faces of the cutting lips. Again, you're looking to restore the cutting edge rather than reshape the bit.
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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. |
How to Set the Chisel & Auger Bit Clearance
One of the most important steps in setting up a hollow-chisel mortiser is establishing the correct clearance between the auger bit and the chisel.
The auger should extend slightly beyond the cutting edges of the hollow chisel. If there's too little clearance, chips can become trapped inside the chisel, creating friction, heat, and burning. But if the auger extends too far, the bit can be damaged.
Fortunately, there's an easy way to establish the clearance.
Place a penny between the shoulder of the hollow chisel and the mortiser as a temporary spacer. With the chisel held in this position, install the auger bit in the chuck with the spurs of the bit even with the tips of the chisel.
Next, loosen the chisel lock, remove the penny, and slide the chisel all the way up until it's firmly seated against the mortiser. Tighten the chisel lock.
Removing the penny effectively raises the chisel in relation to the auger, leaving the bit extending slightly beyond the chisel—the clearance needed for efficient cutting and chip removal.
Align the Hollow Chisel with the Fence
Before cutting, make sure the hollow chisel is square to the fence.
Lower the chisel until it's just above the table. Loosen the chisel lock slightly and rotate the chisel until its back face is parallel with the fence. Then tighten the chisel securely.
This small adjustment matters. A mortise-and-tenon joint relies on straight, flat mortise walls, and a misaligned chisel can produce a mortise that's noticeably out of square.
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| The top mortise was cut with a misaligned chisel, while the one below was dead-on. |
How to Set the Fence for a Mortise
After laying out the mortise on the workpiece, adjust the fence to position the hollow chisel the correct distance from the edge.
If the mortise needs to be centered on the thickness of a workpiece, there's an easy way to dial in the fence.
With the motor turned off, lower the auger until its point makes a small dimple in the workpiece. Flip the workpiece end-for-end and make a second dimple.
If the two marks don't line up, adjust the fence and repeat the process. When the two dimples coincide, the mortise is centered on the workpiece.
Once the fence is set, adjust the depth stop for the required mortise depth and position the hold-down close to the workpiece.
Now you're ready to cut.
How to Cut a Mortise with a Hollow-Chisel Mortiser
It may seem logical to simply work from one end of the mortise to the other with a series of overlapping cuts. But there's a better approach.
When one side of the hollow chisel enters an existing opening, the unsupported chisel can deflect toward that opening. This can leave the mortise walls uneven or out of square.
Instead, use a three-step cutting sequence.
1. Cut Both Ends of the Mortise
Start by making a plunge cut at each end of the mortise. These first two cuts establish the finished length of the mortise.
Make sure the workpiece is held firmly against the fence and the hold-down is properly adjusted before making the cuts.
2. Make Staggered Cuts
Next, remove the waste between the two end cuts, but don't make consecutive overlapping cuts.
Instead, leave a narrow bridge of wood between each plunge. These small sections of material support both sides of the hollow chisel and help keep it from deflecting.
3. Make a Final Cleanup Pass
Once you've removed most of the waste, go back and plunge through the narrow bridges left between the previous cuts.
Because there's very little material remaining, this final pass cleans up the waste and leaves the mortise with straight, square walls.
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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. |
How to Prevent Heat, Smoke & Clogged Chips
Don't be surprised if you occasionally see a little smoke while using a hollow-chisel mortiser. The important thing is to determine what's causing it.
Heat is often the result of chips becoming trapped inside the hollow chisel. As the chips accumulate, friction increases and the chisel gets hotter.
Stop periodically and clear away packed chips, especially when cutting a deep mortise. A light application of dry lubricant can also help reduce friction and heat buildup.
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| A quick spritz of dry-lube spray helps reduce friction and heat build-up for smooth cutting. |
Some discoloration on the chisel can occur from heat. The original tests shown here found that the discoloration could be removed by honing the chisel.
If the mortiser is consistently difficult to plunge or produces excessive heat, check three things: make sure the auger and chisel are sharp, verify the clearance between the bit and chisel, and make sure chips are ejecting freely.
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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. |
Can You Use a Hollow-Chisel Mortising Attachment on a Drill Press?
If you don't have a dedicated mortising machine, another option is a hollow-chisel mortising attachment for a drill press.
The attachment uses a yoke that fits around the drill press quill to hold the square chisel in place while the drill press chuck drives the auger bit. A fence and hold-down position and secure the workpiece much like they do on a dedicated mortiser.
One advantage of this setup is that the drill press allows you to adjust the bit speed.
There are tradeoffs, however. Installing and removing a mortising attachment takes time, and a drill press doesn't provide the same plunging leverage as a dedicated mortiser. The original Woodsmith testing also noted that drill press handles can bend under the force required to plunge a hollow chisel into hardwood.
For occasional mortises, a drill press attachment can be a practical alternative. If you routinely build projects with mortise-and-tenon joinery, the convenience and leverage of a dedicated hollow-chisel mortiser become much more valuable.
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Getting Better Results from a Hollow-Chisel Mortiser
A hollow-chisel mortiser is a fairly simple machine, but small setup details make a big difference in the quality of the finished joint.
Start with a sharp hollow chisel and auger bit. Set the proper clearance between them, align the chisel with the fence, and secure the workpiece with the hold-down. When it's time to cut the mortise, resist the temptation to simply make overlapping cuts from one end to the other. Cutting the ends first, leaving small bridges between the remaining cuts, and finishing with a cleanup pass helps produce straighter, cleaner mortise walls.
Once the machine is properly tuned and you've developed a good cutting routine, a hollow-chisel mortiser makes it easy to produce clean, consistent mortises for strong, accurate mortise-and-tenon joints.
